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Development of a gamma logger to work in conjunction with the CPT for geotechnical and environmental… Singha, Sandeep 1997

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DEVELOPMENT OF A GAMMA LOGGER TO WORK IN CONJUNCTION WITH THE CPT FOR GEOTECHNICAL AND ENVIRONMENTAL APPLICATIONS BY SANDEEP SINGHA B.A.Sc, The University of British Columbia, 1993 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Civil Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August 1998 © Sandeep Singha, 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of U \ \ f ( l C O Q J p > P - g f ~ ( O O The University of British Columbia Vancouver, Canada Date L 10,, iqqft DE-6 (2/88) Abstract 11 The piezocone (CPTU) is a commonly used instrument to assess soil parameters and soil type in geotechnical investigations. However, it is sometimes difficult to distinguish between two soil types that are only subtly different such as fine sand and silty sand with the CPTU. In these cases an alternate method of determining soil type may be required. In order to assess soil type, we may measure the fines content of the soil. One method of assessing fines content within a soil is to measure the natural gamma radiation decay for the common clay mineral constituents potassium and thorium. A gamma logging module has been developed that fits the aforementioned cone penetrometer and records natural gamma data while the CPTU is performed. This test has been labelled the Gamma Cone Penetration Test(GCPT). Since emitted gamma radiation increases with increasing fines content, the instrument can be used to qualitatively distinguish soil type and fines content based on local correlations. Another application of the GCPT is for the measurement of insitu soil density. The back scatter from a module containing a small Cesium source mounted below the GCPT can be correlated to soil density. The GCPT also has environmental applications. It can be used locate radioactive contamination based on a gross gamma count log. Once the contamination is located, the GCPT can measure the gamma energy spectrum in order to identify the radioactive isotope. Since the GCPT is a penetration tool, there are no drill cuttings brought to the surface and worker exposure to potentially hazardous radiation is minimized. IV TABLE OF CONTENTS Page ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES vi LIST OF FIGURES vii ACKNOWLEDGMENTS viii 1. INTRODUCTION 1 1.1 Rationale for Gamma Cone Development 1 1.2 Applications and Objectives 4 1.3 Scope of Research 8 2 . BACKGROUND 12 2 . 1 Cone Penetration Test (CPT) 12 2.2 Natural Gamma Radiation 14 2 . 3 Scintillation Gamma Ray Detectors 18 2 . 4 Radioactive Contaminant Logging 23 2 . 5 Nuclear Density Logging 24 3. EQUIPMENT AND PROCEDURES 2 6 3 .1 Gamma Logger 2 6 3.2 Nuclear Density Logging 31 3 . 3 Data Acquisition System 33 3. 4 Software 35 4 . SITE DESCRIPTION AND STRATIGRAPHY 36 4.1 Regional Geology.- . ... . 36 4 . 2 Massey Tunnel Site' 36 4.2.1 Site Description 36 4.2.2 Stratigraphic Profile 38 4.3 KIDD 2 Site 38 4.3.1 Site Description 38 4.3.2 Stratigraphic Profile 40 4 . 4 Coal Port 40 4.4.1 Site Description 40 4.4.2 Stratigraphic Profile 42 5. PRESENTATION AND INTERPRETATION OF RESULTS 44 5. 1 Massey Tunnel 44 5.1.1 Natural Gamma Log 44 5.1.2 Nuclear Density Log 47 5.2 Kidd 2 4 9 5.2.1 Natural Gamma Log 49 5.2.2 Nuclear Density Log 53 5 . 3 Coal Port 53 5.3.1 Natural Gamma Log 53 6. DISCUSSION OF RESULTS 58 6.1 Natural Gamma Logs For Soil Type assessment 58 6. 2 Nuclear Density Logs 61 6. 3 Environmental Logging 62 6. 4 Logging Speed 62 7 . CONCLUSIONS AND RECOMMENDATIONS 64 7 .1 Soil Type Assessment 64 7 . 2 Environmental Evaluation 65 7.3 . Density Logging 65 7 . 4 Radio-Active Source Activity 67 7 . 5 Resolution 67 7 . 6 Logging Speed 68 7 . 7 Future Considerations 69 REFERENCES 72 Appendix I - Calibration 74 VI LIST OF TABLES Table Page 1 Natural Gamma Logging Windows 83 vii LIST OF FIGURES Figure Page 1 CPT Soil Type Interpretation Charts 13 2 Potassium, Uranium, Thorium Peaks and Windows... 16 3 Gamma Profile with Soil Stratigraphy 18 4 Photo Multiplier Tube(PMT) 20 5 Cobalt 60 Energy Peaks .' 22 6 Cone Penetrometer with Gamma Module 27 7 Active Gamma CPT Probe 32 8 Cesium Energy Spectrum 34 9 Test Locations 37 10 CPT Plot of Massey Tunnel 39 11 CPT Plot of KIDD 2 41 12 CPT Plot of Coal Port 43 13 Massey Tunnel Gamma CPT Plot 45 14 Massey Tunnel Gamma Ray Count and Fines Plot.... 4 6 15 Massey Tunnel Density Plot 48 16 Kidd 2 Gamma CPT Plot 50 17 Kidd 2 Gamma Ray Count and Fines Plot 51 18 Kidd 2 Density Plot 54 19 Coal Port Gamma CPT Plot 55 20 Particle Size Distribution 60 Al Thorium Energy Spectrum 77 ACKNOWLEDGMENTS vm I wish to thank: Dr. Campanella and Dr. Howie for their comments and suggestions on the written part of this project. ConeTec Investigations Ltd. and Gregg In Situ for funding this project. Ron Dolling for his work in developing the circuitry for the Gamma probe. Ilmar Weemees for programming software for data acquisition. The Geological Survey of Canada and the Atlantic Geoscience Center for providing natural gamma logs and grain size data from borehole tests. Dr. Robertson for Providing Density data acquired for CANLEX and for proof reading the written part of this project. Michelle Lefebvre for his comments and suggestions on the final draft of this thesis. 1 1. INTRODUCTION 1.1 Rationale for Gamma Cone Development The Cone Penetration Test(CPT) has become one of the most useful insitu tests for delineating soil stratigraphy. The basic test consists of pushing an instrumented probe into the soil and measuring the tip resistance, sleeve friction and penetration pore pressure at predetermined depth increments. These three measured parameters have been correlated to soil type, density and other soil properties(Robertson and Campanella 1986). This subject is discussed in further detail in section 2.1. Interpretation of the soil type is based on charts using the three measured parameters(Robertson and Campanella 1986). Occasionally a soil may lie near the border of two different soil types on a soil classification chart. It is important to accurately distinguish soil type such as between a silty sand and a loose sand in situations such as liquefaction studies. In these cases it would be useful to have another soil property to assess the soil type. Gamma logging is a well established method of qualitatively assessing soil or rock type and determining boundaries between different stratigraphic units in the oil industry(Schlumberger 1989). Gamma logging is generally performed in a cased hole. 2 The cost of casing installation makes it impractical to perform gamma logs for the purpose of geotechnical site investigations. The difficulty related to the use of traditional borehole geophysical gamma logging is that there are many variables that must be accounted for(Schlumberger 1989). These variables include the following: • the diameter of the borehole. This influences the amount of radiation reaching the probe. A larger borehole will reduce the gamma ray count measured by the probe. • the casing material. Denser casing materials such as steel will tend to attenuate the gamma rays when they pass through the casing. PVC plastic casing will have a much lesser effect. • the casing wall thickness. Thicker casing wall thickness will tend to attenuate the gamma rays to a greater extent. • the grout density. The density and amount of grout surrounding the casing will affect the incoming gamma radiation. The type of grout used will also affect measurements. • vertical soil movement during drilling. During drilling soil may move up or down in the borehole. This may 3 cause the probe to measure a gamma ray count at a depth that has soil from another depth present. • the probe position in the borehole. The borehole probe will not always be in the same position in the bore hole. It may be in the center or it may be near the casing walls. The probe position relative to the surrounding soil will affect the gamma ray count measured. • the drill hole Diameter. The diameter of the drill hole cut before the casing is placed will affect the gamma count. A larger diameter hole means that the gamma rays have to travel further and thus the probe will measure fewer gamma rays. The hole diameter will be influenced not only by the cutting bit size but also the mud circulation rate, and the erodibility of the soil. A correction factor for the above stated variables must be used in order to produce a repeatable gamma ray profile in a borehole. If the borehole effects could be eliminated, the process would be much simpler and gamma ray logging could be a more reliable and economical tool for geotechnical investigations. A fast and cost effective method of performing a gamma log is needed. 4 Thus, a module was constructed that could be used in conjunction with the CPT to measure the gamma emissions of a soil. The unit consists of a conventional CPT probe with a gamma logging module attached above. The configuration of the GCPT is further discussed in section 3.1. 1.2 Applications and Objectives The GCPT has three principal applications. These include soil grain size characterization, environmental site characterization, and nuclear density logging. The first application is estimating soil grain size by logging natural gamma radiation. In general soil having a greater fines content will have a higher natural gamma ray count than soils with a lower fines content. Fines are defined as all soil passing the #200 sieve(<74 urn). Since the mineralogy of fines varies from site to site, and fines of different mineralogy will not have the same natural gamma ray emissions, it is not possible to directly correlate fines content to natural gamma ray count for all situations. Instead, the gamma ray count profile should be used to qualitatively distinguish soil of varying grain size and fines content in order to minimize the number of samples required on a site. Sample sites are discussed in section 5. 5 Vibroreplacement is a method of compacting granular soil by means of lowering a vibrating probe into the soil and subsequently filling the void it creates with gravel. This procedure is repeated several times at a given location, on a grid spacing of a few meters. It is important to have a good knowledge of the grain size characteristics of the soil for vibroreplacement as the application of this technique is limited to soils of less than 20 percent fines. In addition fines slow the removal of excess ground water which is required for compaction(Hunt 1986). Typically, few CPTs are performed prior to compaction. Therefore, a site may not be well characterized before commencing ground improvement. Sites that have been modified using this method tend to have elevated lateral pressures(Howie 1997) . This causes the friction on a CPT log to be abnormally high thus tip resistance and friction based soil type interpretation charts may erroneously classify the soil type. A soil that was classified by the CPT as a sand before compaction may be classified as a silty sand after compaction. This can cause problems between the densification contractor and the geotechnical engineer since the tip resistance specification that the contractor must provide is often based on the friction ratio. The friction ratio(Rf) is defined as the CPT side 6 friction divided by the tip resistance. In general finer soils will have a greater friction ratio. If the friction ratio is higher than expected, the densification contractor may argue that the soil is finer than it really is and that the tip resistance specified is not attainable in a particular soil. In such cases it may be possible to locally correlate the gamma ray count to the fines content to more accurately determine soil type without the need for direct sampling. This can be done by performing a GCPT nearby in an area where compaction has not been performed and then comparing the soil type classification and gamma ray count to that from the compacted area. A knowledge of non-plastic fines content is also necessary for the proper assessment of liquefiable soils. The CPT alone cannot always determine the grain size of a soil accurately. The GCPT has the ability to qualitatively measure fines content and thus better determine soil type. This reduces the uncertainty of the CPT results and thus reduces the amount of sampling that must be performed at a site. The second application of the GCPT is for the investigation of radioactive contamination in the ground. In the past, sites contaminated with radioactive isotopes could only be 7 investigated by sampling and then taking the samples to a laboratory for analysis. Without any modification, the GCPT logger can also be used to locate underground radioactive contamination. The greatest advantage of using the GCPT for environmental work is that it is a penetration tool. Thus, there are no cuttings and worker exposure to harmful radiation is minimized. The GCPT can provide a continuous gamma ray log of the soil stratum in question with less expense and in less time than sampling. In an attempt to identify radioactive contaminants, the GCPT can be stopped at depths of interest and a spectralog can be produced. A spectralog is a chart of gamma energy. All radioactive isotopes emit gamma rays at a characteristic energy on the spectralog. The characteristic energy should not be confused with the activity of an isotope. The activity is related to the rate of gamma ray emissions. The activity of an isotope will decrease with time although the energy of each emission will not change. Sample spectralogs are further discussed in Appendix I. The third application of the GCPT is for the measurement of insitu soil density. This is accomplished by placing a small radioactive source below the gamma module and measuring the 8 gamma back scatter from the soil. The back scatter can be correlated to the soil density. The objective of this research was to develop and evaluate a Gamma Cone Penetration Test(GCPT). The GCPT was used to look at natural gamma radiation profiles to better characterize soil type. The second purpose of this research was for the utilization of the GCPT for characterizing sites with radioactive soil contamination. Although it was not possible to obtain access to such a site, the procedure for testing and the expected results are discussed. The final purpose of this research was to assemble and calibrate a nuclear back scatter density logger using the GCPT. 1.3 Scope of Research There is much published technical literature on the use of scintillation detectors for gamma logging using borehole devices (Schweitzer 1991, Schlumberger 1989) but none refer to gamma logging using a penetration tool. However, much of the knowledge available from borehole devices can readily be applied to a penetration tool. Most of the literature discusses the use of borehole logging for the purpose of locating underground oil reserves. This is accomplished by utilizing the gamma ray tool to locate porous soils with hydrocarbons free of clay minerals 9 that make it difficult for the hydrocarbons to flow. Development involved several steps to research, construct, and calibrate the GCPT. The steps are summarized as follows: Step 1 - Literature Review The first step in completing this project was to gain an understanding of the subjects involved. This step included selected readings and discussions with "experts in the field". This step also included researching current literature on the following topics: • Natural gamma logging and natural gamma ray spectrometry; • Natural gamma correlations to soil grain size; • Characterization of sites with radioactive contamination; • Correlating active gamma ray back scatter to soil density. Step 2 - Gamma Logger Development Preliminary tests were carried out using a down hole gamma logger supplied by EG&G Ortec. EG&G Ortec is a company based in Oakridge Tennessee that produces equipment for the measurement of radiation. Tests were carried out in order to ascertain if 10 it would be possible to perform a gamma log with the type of equipment available and to test the logging software. Once it was established that the equipment and the software worked properly, equipment miniaturization could commence. The gamma loggers currently available were of the wire line type that are lowered down a cased borehole. The gamma logger for this project had to be reduced in size sufficiently to allow it to be encased in a cylindrical module above the CPT tip. Step 3 - Field Testing and Correlation Borehole logs including gamma logs are readily available for the Fraser River Delta region and thus field tests were carried out adjacent to these boreholes at 3 locations. The grain size data from the borehole logs were compared to the gamma ray logs from the soundings adjacent to the boreholes. The effects of logging rate were also investigated. This was done to determine if the standard CPT logging rate of 2 cm/s gave the gamma logger adequate time to gather a statistically sufficient gamma ray measurement. Step 4 - Contaminated Site Investigation Many attempts were made to gain access to a site with low level radioactive contamination. However it was not possible to get 11 onto such a site. The insitu test procedure and the expected results if access had been gained to one of these sites is discussed. Step 5 - Report Writing The final task was to compile all of the data obtained from field tests into a format that clearly displays the information in a manner that is easily understandable and usable by others for reference material. Discussion of the test results and conclusions are presented as well as possible future research considerations. 2. BACKGROUND 12 2.1 Cone Penetration Test The cone penetration test(CPT) has become one of the most commonly used geotechnical and environmental insitu tools for soils(Lunne et al. 1997). The test consists of pushing an instrumented probe into the ground. The probe, is cylindrical with a cross sectional area of 10 cm2. The tip of the CPT consists of a cone with a 60 degree internal angle. Directly above the tip is a porous element where penetration pore pressures are measured. Above the element is a friction sleeve with a surface area of 150 cm2. As the probe is pushed into the ground, tip resistance(qc) , sleeve friction(fs) , and penetration pore pressure(u) are measured at predetermined depth increments. The depth increment in practice is typically 5 cm or less. Charts have been established to determine soil type from the above three measured values. Charts are based on tip resistance and either of sleeve friction or penetration pore pressure. Both types of charts are illustrated in Figure 1. Tip resistance and sleeve friction based charts will have the friction ratio(defined in section 1.2) along one axis and tip resistance along the other. Tip resistance and penetration pore pressure based charts will have the pore pressure ratio(Bq) 0 0.2 0.4 0.6 0.8 1.0 1.2 Pore pressure parameter Bq 2 3 4 5 6 7 Friction ratio (%) Zone: Soil Behaviour Type: 1. Sensitive fine grained 2. Organic material 3. Clay 4. Silty clay to clay 5. Clayey silt to silty clay 6. Sandy silt to clayey silt 7. Silty sand to sandy silt 8. Sand to silty sand 9. Sand 10. Gravelly sand to sand 11. Very stiff fine grained* 12. Sand to clayey sand* * Overconsolidated or cemented. Figure 1 CPT Soil Type Interpretation Charts (From Robertson and Campanella 1986) 14 along one axis and tip resistance along the other. Pore pressure ratio is calculated as follows: u -u Bq= max ° ^T-°vo where Bq is the pore pressure ratio umax is the penetration pore pressure u0 is the static pore pressure qt is the CPT tip resistance ovo is the overburden stress In addition to soil type, the three measured parameters have been correlated to numerous other soil parameters including shear strength and density (Campanella and Robertson 1986). It is possible to include a module directly above the cone in order to measure other soil parameters such as bulk resistivity, pH, or in this case, soil gamma activity. 2.2 Natural Gamma Radiation All soils tend to emit a small amount of natural gamma radiation due to radioactive elements within the soil. In general finer soils will emit more radiation(Hunter et al. 1994)since they have greater amounts of radioactive elements bound within their matrix. Coarser soils emit less radiation due to the absence of 15 radioactive elements. Radioactive elements tend to wash out of large grained soils leaving them with a lower gamma activity. Mineralogy also plays a role in the radioactivity of a soil. The fact that finer grained soils will have a greater gamma ray count can be used to qualitatively identify different soil types. The gamma ray log can also be used to determine grain size deposition trends in a soil stratum. Sources of natural radiation in soil include thorium, potassium, and uranium(Serra et al. 1980). Different isotopes will emit radiation of different energy. The gamma logger used is capable of logging and differentiating radiation over a large energy range. Generally when a natural gamma log is performed, the energy spectrum will be divided into three windows, one for each of thorium, potassium, and uranium. The total number of counts in each window will be summed to estimate elemental concentration of each. Figure 2 illustrates the energy peaks produced by these three natural sources and their respective energy windows. The counts in the entire spectroscopic plot divided by the time taken to develop the spectroscopic plot gives the gross gamma count in units of counts per second(cps). Details of these energy windows are discussed in Appendix I. In general, fine grained soils will emit a greater amount of 16 W1 I W2 W3 W4 | W5 Energy (MeV) | Schlumberger Figure 2. Potassium(K), Uranium(U), and Thorium(Th) Peaks and Windows (from Schlumberger 1989) 17 natural radiation than will coarse grained soils. This is demonstrated in Figure 3 which illustrates a typical gamma ray and soil stratigraphy borehole log from the Fraser River delta. The gamma ray count is relatively high, approximately 80 to 90 counts per second(cps) in the silt near surface to a depth of 2 metres. As the silt becomes sandy below, the gamma ray count drops off significantly. Silt interbeds in the sand at a depth of 4.5 to 5.5 metres cause the gamma ray count to increase. Below is a sand formation to a depth of 17.5 metres with a gamma ray count varying between 40 and 70 cps. The count is higher in the fine sand and lower in the medium sand. Below 17.5 metres is a deposit of sandy silt and clayey silt. The gamma ray count in this silt is higher than in the sand above. A more detailed discussion of natural gamma logging is presented by Hallenburg (1984), Schweitzer (1991), and Schlumberger (1989). 2.3 Scintillation The theory underlying gamma ray logging by means of a scintillation detector is well established and has been in practice for many years(Hallenburg 1984). The scintillation detector consists of a crystal composed of a relatively dense material which possesses the ability to convert energy from 18 GEOLOGICAL SURVEY OF CANADA FD94-5 NATURAL GAMMA (cps) 50 E^  X I -Q_ LU Q GENERAL LITHOLOGY ZONE 10 5441750 N 488100E - 0 SAND f.lo m. interbed & interlam. with silt SAND fine to med. T U T SILT laminated itiizoconcretions SILT sandy to clayey laminated, some v.f. sand SAND f. to med. interbed. & interlam with silt SAND f. to v.f. silt interbed. & laminate organic laminae — 10 "SAND f. to med. concretion clasts shells silt laminae SAND fine interbed. of silt SILT sandy to clayey interbed. & interlam. with v.f. sand D m TJ — 20 Figure 3 TOTAL DEPTH 23.5 M Typical Gamma Profile with Soil Stratigraphy (GSC 1994) 19 incoming radiation into light. The crystal has a cylindrical shape.and is encased in a sealed aluminum casing. As the gamma rays enter the crystal, they lose their energy in the form of a light pulse. An opening at one end of the crystal housing allows light to exit and travel towards a sensitive light detector. In the GCPT logger this light detector is a photo multiplier tube(PMT). There is a photo cathode in the PMT that converts the incoming light into photoelectrons. The electrons are then accelerated through an electrical field towards a series of dynodes. The dynodes are a series of charged plates each having a charge such that the incoming electrons are multiplied in number as they travel from one dynode to the next. Finally the electrons strike the anode. The number of electrical pulses determines the gamma ray activity and the amplitude of the pulses determines the energy emitted by the material in the vicinity of the crystal (ANSI N42.9-1972). Figure 4 illustrates a typical layout of a photo multiplier tube. The result is that a gamma ray emission that strikes the crystal will emit light that is proportional to the energy of the incoming particle. The more intense the light produced, the greater the number of electrons that strike the anode. The current pulses are carried up-hole through a cable to a PC based data acquisition system. The data acquisition system 20 TRANSMISSION MODE PHOTOCATHODE ELECTRON LENS FIRST DYNODE ANODE SOCKET ELECTRON RACKET PHOTOCATHODE, TRANSIT TIME ELECTRON MULTIPLIER TRANSIT TIME •vl-.-m / OUTPUT CONNECTOR m OUTPUT STRUCTURE. DELAY TIME DEVICE TRANSIT TIME Figure 4 Photo Multiplier Tube(PMT) (from ANSI N42.9-1972) 21 measures the current in each pulse and increments the pulse count in one of 2048 "energy bins". The energy of a gamma ray is measured in units of electron volts (eV) . The units of an electron volt are 1.60xl0~19 kg(m/s)2. Units of kg(m/s)2 are those of energy(Joules). The higher numbered "energy bins" are incremented for higher energy gamma ray strikes. The "energy bins" are plotted with the bin number along the x-axis and the number of occurrences along the y-axis. Over a period of time, an energy spectrum appears with distinct peaks. The energy spectrum will appear sooner for higher activity gamma sources. Figure 5 illustrates an energy spectrum for the isotope Cobalt 60. Cobalt 60 has two distinctive energy peaks, one at 1170 keV and the other at 1333 keV. If a gamma source of known energy is placed next to the detector, the gamma ray energy that corresponds to the bin on which the energy peak(s) lay can be determined. Two or three energy peaks can be used to calibrate the energy spectrum. With the energy spectrum calibrated, unknown gamma ray sources can be brought close to the detector and the energy peak(s) that are produced can be used to identify the isotope. 22 mm m CO > * o CN c in 3 , Ql Is si CO QJ *- c Hi o <l O 0-1 > S a) "3 5 a. R I T- a) «~ c HI 0000 L lunoo ys EG IT) m i _ _ j O) LL E 3 i _ C.J CD Q. CO >* O) 1— a) c HI o CD *; m Q O O 23 Further technical information regarding the components of a gamma logger are available in ANSI N42.9-1972. Calibration of the gamma logger is discussed in Appendix I. Further details about gamma ray spectroscopy are discussed by Gilmore et al. 1995. 2 . 4 Radioactive Contaminant Logging Much work has been done in the past for delineating radioactive contaminants using borehole gamma detectors. This method was used to analyze low level radioactive waste at the Port Hope landfill by the Geological Survey of Canada(Killeen et al. 1993). The United States Geological Survey has also used this method for assessing the presence of radioactive elements in ground water in the early 1980's. The above stated studies involved gamma logging using a borehole and as discussed in section 1.1, gamma logging without a borehole is far more desirable. The procedure is identical to that of natural gamma logging but if radioactive contamination is encountered the measured activity would be higher than the background activity. The radioactivity could be correlated to isotope concentrations by taking soil samples for analysis and comparing the concentrations from the samples to the activity measured by the GCPT. In addition, the radioactive isotope 24 could be identified using gamma ray spectroscopy. Gamma ray spectroscopy for the purpose of evaluating radioactive contamination is discussed further by Gilmore (1995), Killeen (1993), and Pflug et al. (1993). 2.5 Nuclear Density Logging The use of borehole nuclear density tools is very common. Nuclear density logging involves using a gamma ray detector and an active source set at a fixed spacing and shielded to prevent direct gamma ray strikes. Higher density materials will impede the gamma rays to a greater extent. Using calibration materials of varying densities, a calibration curve of nuclear back scatter verses density can be established. A detailed description of the calibration procedure is presented in Appendix I. There has been some work done on creating a penetration tool for density logging in the past. Sully and Echezuria (1988) developed a penetration tool similar to the GCPT but it was strictly a density tool and did not work in conjunction with the CPT. This tool required different calibration curves for sands, silts and clays. It is believed that this was needed to account for the varying amount of background radiation in the different soil types. The procedure followed when using the GCPT for 25 performing nuclear density logging accounts for background radiation. Similar work done in the past using a borehole includes that of Plewes et al. (1988) who discussed the utilization of a borehole density tool for the analysis of sand mine tailings. Text books such as Hallenburg (1984) cover this procedure as well. 26 3. EQUIPMENT AND PROCEDURES 3 .1 Passive Gamma Logger Before miniaturization of existing gamma logging equipment began, preliminary tests were performed utilizing conventional gamma logging equipment. The equipment used in the preliminary development consisted of an EG&G Ortec ScintiPack Model 296 PMT Base coupled to an EG&G Ortec Crystal-PMT assembly. The crystal, was composed of sodium iodide, has a diameter of 50 mm and length of 50 mm. The ScintiPack base contained all of the down-hole electronics including the photomultiplier tube(PMT) and the power supply. After the completion of preliminary testing the down-hole assembly had to be miniaturized to a maximum outside diameter of 25 mm. This would be housed in a steel casing with an inside diameter of just over 25mm and an outside diameter of 43.7 mm. This required that a smaller crystal and photomultiplier tube be used. Figure 6 displays the completed passive gamma logger attached to a cone penetrometer(GCPT). The GCPT consists of the gamma logging module attached to a conventional CPT probe. The difficulty which arises with utilizing a smaller diameter Gamma Ray Excitation Load Cells Porous Filter Element Gamma Module Photo Multiplier Tube Csl Crystal Cone Penetrometer Inclinometer (I) Thermistor (T) Friction Sleeve (Fs) Pore Pressure Transducer (U) Cone Tip (Qc) Figure 6. Cone Penetrometer with Gamma Module (FromConeTec 1997) 28 crystal is that the incoming gamma rays have less distance to lose energy as they pass through the crystal. Many of the incoming gamma rays will strike the crystal and exit after having lost only part of their total energy. The result is that they will be registered as gamma ray strikes of lower energy. This causes spectroscopic energy peaks to be less pronounced and skewed. To partially rectify this problem a higher density cesium iodide crystal, verses a lower density sodium iodide crystal was used. The higher density cesium iodide crystal results in a greater amount of energy from the incoming gamma ray being converted to light, therefore, giving a more accurate indication of its energy content. Sodium iodide crystals tend to be extremely brittle and temperature gradient sensitive. The cesium iodide crystal used in the downsized module was slightly malleable and was not as prone to breakage and thermal shock as the sodium iodide crystal. This is an important consideration for long term reliability of the gamma module as a brittle crystal may easily be damaged during handling. The crystal in the GCPT is coupled to a 0.75 inch photomultiplier tube(PMT). The reduction in size of the PMT also reduced the quality of the data slightly. The smaller PMT 29 tends to have a smaller quantum efficiency and a higher dark current. A PMT with a smaller quantum efficiency will produce less current at its anode for a given light pulse received from the crystal. Dark current is the amount of current at the PMT anode in the absence of any gamma radiation. Another concern with the smaller crystal is that it has a much smaller surface area and volume and will therefore measure a lower activity. This problem was overcome by using slower logging speeds and thus allowing the logger to collect more data during each depth increment. The activity level measured with the smaller crystal was then compared to that of a conventional size logger for logs performed at the same location. The activity level measured with the smaller crystal can then be factored to coincide with the larger crystal. Borehole natural gamma ray detectors used in the oil industry are standardized for count rate according to the American Petroleum Institute(API) standardization method(Hallenburg 1984). This involves placing the detector in the API calibration chamber and reading the gamma ray count measured by the detector. This reading can be compared to the standardized reading for the calibration chamber. Future gamma ray logs using the detector can then be factored by an amount calculated 30 from the measured reading and the expected reading in the calibration chamber. Many downhole gamma loggers utilize a system that digitizes collected data down-hole in order to eliminate any degeneration of the signal as it is transmitted uphole. This could not be accomplished due to the space limitations of the GCPT unit. In order to cope with this problem, a differential line driver was put into the GCPT module. The gamma ray signals transmitted up-hole consist of small current pulses. The differential line driver sends the amplified analog data through two different wires that carry the same current signal but with opposite voltage. The size of the current pulse is dependent on the gamma energy of the incoming gamma ray. Any degeneration of the signal while it is transmitted up-hole will occur to both of the signals in both wires equally. When the two signals reach the receiving unit, they are subtracted and the original signal is again generated. The procedure used to deploy the GCPT is similar to that utilized in the deployment of the CPT. The probe is pushed into the ground and the software driven data acquisition system takes gamma readings at predetermined depth increments. The only difference is that the probe is advanced slower than the 31 standard CPT testing rate of 2 cm/s to produce a smoother gamma ray profile. The gamma profile is smoother due to the statistical nature of gamma rays. A longer data collection time allows for a more representative measurement. 3.2 Active Gamma Density Logger The active gamma CPT logger(Active GCPT) consists of the gamma module described previously however the- CPT probe is replace by a steel module that houses an active Cesium 137 source. The probe is illustrated in Figure 7. The logging procedure involves performing a passive gamma ray log to determine background radiation levels. This is done in order that background radiation can be subtracted from the gamma-gamma log. Conventional downhole gamma-gamma probes do not need to account for background radiation as the radioactive sources used in the probes are of such high activity that the background radiation is negligible. After the passive log is completed, the CPT probe with the active source is used to log the same hole opened during the passive log. The same hole is used in order to reduce the pushing force required to deploy the probe. A reduction in pushing force is desirable because it greatly reduces the risk 32 Gamma Ray Excitation Photo Multiplier Tube Csl Crystal Active Source Figure 7. Active Gamma Probe (From ConeTec, 1998) 33 of breakage. Pushing an active source into the ground is a large liability and every precaution has to be taken to reduce the risk of breakage. For this reason there is a large factor of safety on the maximum force used to deploy the active GCPT. This is accomplished by limiting the maximum pushing force used to deploy the active GCPT. 3.3 Data Acquisition System Data was acquired using a Micro-Ace data card from EG&G Ortec. This card receives current pulses from the downhole module and measures the current of each pulse in order to evaluate the gamma energy of the incident gamma ray. A gamma ray spectroscopic plot consists of gamma ray counts of a specific energy on the y-axis and gamma ray energy along the x-axis. A typical gamma ray spectrum is illustrated in Figure 8. The data acquisition card stores the x-axis as a series of 2048 energy bins. Initially, the energy bins do not directly correspond with any gamma energy and so must be calibrated using isotopes of known energy. When the data acquisition card receives a current pulse, it measures the current pulse and increments one of the 2048 energy bins. The energy bin that is incremented is dependent on the voltage across the photo multiplier tube and the gain setting of the data acquisition card. Details about the calibration of the energy bins is discussed in Appendix I. Maestro - Buffer f 1.93 g/cm*3 barite mud } File Calculate Services ROI Acquire Display o o o o 660 keV Energy Peak Bin 1 Figure 8. Cesium Energy Spectrum Display <~MCB f1* Buffer MCBJM SEG#1 ^ Full ^Expand Vert: 4096 Horz: 2048 ROI: Off Bin 2048 35 3.4 Software Data acquisition for gamma ray spectroscopy was performed using Maestro 3.0 for Windows(EG&G Ortec 1992). This software package displays the data with gamma energy on the x-axis and the number of counts for a given gamma energy along the y-axis. Maestro 3.0 can be used to start and stop data acquisition and to clear the "energy bins" on the data acquisition card. Maestro can also save a new spectrum and retrieves previously stored spectra. A background spectrum can be saved and later subtracted. Energy windows where energy peaks of known isotopes are expected can be summed. These energy windows are used to perform conventional potassium, uranium, and thorium logging. Maestro 3.0 does not have the ability to log a borehole. It only acquires data over a predetermined time and then the data must be saved and the data buffer cleared manually. In order to log a hole, a program called GAML0G4 was written. GAMLOG4 signals the data acquisition card to begin collecting data. When a depth increment signal is received, it stops the data acquisition card, sums up to 4 energy windows, and writes the data to disk. Then it prints the collected data to a printer, clears the data buffer and restarts data acquisition for the next depth increment. 36 4. SITE DESCRIPTION AND STRATIGRAPHY 4.1 Regional Geology Testing of the GCPT was carried out at three locations in Richmond and Delta, south of Vancouver British Columbia. The regional geology consists of Fraser River deltaic deposits characterized by clays, silts, sands and peat underlying the surface crust. These deposits reach a depth of approximately 180 metres near the north and middle arms of the Fraser River. Below this lies a Pleistocene deposit approximately 80 meters in depth. Tertiary bedrock lies approximately 250 metres below the ground surface(Blunden 1975). Figure 9 illustrates the locations of the three sites. 4.2 Massey Tunnel 4.2.1 Site Description The Massey tunnel is part of Highway 99 connecting Richmond to Deas Island which in turn is linked to Delta with a bridge. The test site is located on Deas Island at the south eastern end of the tunnel on the east side of Highway 99. The Massey tunnel test site is part of the Canadian Liquefaction Experiment(CANLEX) and is a very well documented site geotechnically. The test at the Massey Tunnel site was carried out 1 metre east of the CANLEX freeze sampling circle(Wride 37 PACIFIC OCCAM MfTHH [ *».TA. COLUMBIA^ P u.SJL ROBERTS BANK SUPERPORT 0 10km 1 • • • • ' • • • • ' Figure 9, GCPT Testing Locations (from Monahan et al. 1995) 38 1997). The freeze sampling circle is an area that was used to retrieve high quality undisturbed frozen samples of sand. 4.2.2 Stratigraphic Profile An interpreted CPT Profile is presented in Figure 10. The site consists of fill to a depth of approximately 3.0 metres. The fill is underlain by silt and sandy silt to a depth of 5.0 metres. From a depth of 5.0 metres to 16.75 metres is fine sand with occasional medium sand inter-beds underlain by a medium sand with inter-beds of fine sand. The test was stopped at 24.35 metres due to the required pushing force becoming excessive. 4.3 Kidd 2 4.3.1 Site Description This site is located at the north end of Number 4 Road in Richmond B.C. approximately 500 metres south of the north arm of the Fraser River. The test was performed on B.C. Hydro property known as the Kidd 2 Substation. This site is one of the designated Canadian Liquefaction Experiment (CANLEX) test sites and thus the soil stratigraphy, density, type, etc. are very well documented(Wride 1997). The test at the Kidd 2 site was carried out 2 metres South-East Gamma Site: GCPT96-03 Locatbn: MASSEY TUNNEL Cone: 20 TON A 041 Date: 11:08:96 12:53 0.0 -5 .0 -10.0 a a -15.0 -20.0 -25.0—^ Max. Depth: 24.35 (m) Depth Inc.: 0.05 (m) Figure 10. CPT Plo t of Massey Tunnel Fs bar Rf % 0.0 1.0 0.0 5.0 I I I I I I I I I TTT U2 (in) SBT 0 25 0 12 SBT: Soil Behavior Type (Robertson and Campanella 1900) JSZ. = " Estimated Phreatic Surface of the CANLEX frozen sample circle. 40 4.3.2 Stratigraphic Profile Figure 11 displays a CPT plot from the Kidd 2 site. The site consists of gravelly fill to a depth of approximately 1 metre. Below to a depth of 4.5 metres is clayey silt. Below this to a depth of -8.75 metres is fine sand. Between a depth of 8.75 and 9.5 metres is a layer of silt. From 9.5 metres to 19.40 metres is sand of varying density. The test was terminated at a depth of 19.40 metres due to the required pushing force becoming excessive. 4. 4 Coal Port 4.4.1 Site Description This site is located on the west shore of the SuperPort coal loading facility in Delta, approximately 20 km south of Vancouver. The site is not as well documented as the Kidd 2 and the Massey Tunnel sites, but it was chosen because it has a greater variation in soil stratigraphy than the aforementioned sites. The test was located on the west shore of the coal port where previous CPT tests have been performed for the Atlantic Geoscience Centre. Gamma Site: GCPT96-2 Location: KIDD 2 Cone: 10 TON A 027 Date: 10:09:96 09:28 0.0 -5.0 -10.0 a a) o -15.0 20.0 -25.0 Max. Depth: 19.40 (m) Depth Inc.: 0.05 (m) F i g u r e 1 1 . CPT P l o t of Kidd 2 Fs bar Rf % 0.0 1.0 0.0 5.0 I I | I I I l Drilled Out U2 (m) 0 25 TTTTTTT illed Ou SBT 0 12 £ mi PSa wm S«ff in TTTTT SBT: Soil Behavior Type (Robertson and Campanella 19RH) SZ. Estimated Phreatic Surface 42 4.4.2 Stratigraphic Profile Figure 12 displays a CPT plot from the Coal Port. The site consists of gravelly fill to approximately 4.5 metres. Below is a silt layer 0.5 metres in depth. Below that is silty sand to a depth of 9.0 metres. From 9.0 to 11.0 metres lies silt with a small clayey silt layer at 11.0 metres. Below 11.0 metres to 15.0 metres is a sand layer with a small silt layer from 13.0 to 13.5 metres. The soil below 15.0 metres consists of silt to 21.6 metres where the sounding was terminated. GAMMA Site: 97-GCPT-04 Location: COAL PORT Cone: 20 TON A 044 Date: 03:21:97 11:22 Qt bar 0.0 -5.0 5. -IO.O a a) o -15.0 -20.0 -25.0 Max. Depth: 21.60 (m) Depth Inc.: 0.05 (m) Figure 12: CPT Plot of Coal Port Fs bar Rf % 150 0.0 1.5 0.0 5.0 I I I I Drilled Out i rrr Drilled Out U2 (m) SBT 0 40 0 12 T V I I I I I I I I I Drilled Minium ,**S J SBT: Soil Behavior Type (Robertson and Campanella 19B0) =^ Estimated Phreatic Surface 44 5. PRESENTATION AND INTERPRETATION OF RESULTS 5.1 Massey Tunnel 5.1.1 Natural Gamma Log Figure 13 illustrates a CPT profile with the gamma ray count of the Massey Tunnel site and Figure 14 shows a plot of fines content and gamma ray count verses depth. The fines content was obtained from an adjacent vibro core performed by the Geological Survey of Canada and is defined as the percent passing the #200 sieve(<74 urn). At this site, the soil stratigraphy does not change very much with depth. Below the 2.2 metres of fill lies a sand layer approximately 1 metre thick. This sand layer is picked up by the GCPT as having relatively low gamma ray counts. The gamma ray counts drop to a minimum of 58 counts per second(cps) in the middle of this layer. The gamma ray counts are not as low as they are in the sand below 6 metres because gamma rays from the silt layers surrounding this thin sand layer are picked up by the GCPT. From 3 to 5 metres is a silt layer with a high gamma ray count of approximately 80 cps. A sample taken in this silt at 4.5 metres shows a fines content of 96 percent. The fine sand layer between 5 and 16.75 metres had a gamma ray count averaging about 55 cps. There is a gamma ray count peak Gamma Site: GCPT9B-03 Location: MASSEY TUNNEL Cone: 20 TON A 041 Date: 110896 12:53 0.0 -5 .0 -10.0 a a) o -15.0 -20.0 -25.0 Max. Depth: 24.35 (m) Depth Inc.: 0.05 (m) Figure 13. Gamma CPT Plot of Massey Tunnel U2 (m) 0 25 "111 f 1111 ~-\ 171 1 V GR Count(CPS) SBT 0 100 0 12 SBT: Soil Behavior Type (Robertson and Campanella 19BH SZ Estimated Phreatic Surface 46 Fines Content and GR Count G.R. Counts(CPS) & Fines Content(%) Q. 0) Q Figure 14 Massey Tunnel Fines vs GR Count November, 1996 • Fines Content (%) Gamma Counts Fines Content is % < #200 Sieve From Vibro Core 47 at 6.5 metres where the counts increase to 65 cps. A sample taken at this depth shows a. fines content of 10 percent. The CPT does not clearly pick up this subtle change in soil type from the surrounding soil. Below 6.5 metres to a depth of 22 metres is a medium sand where the gamma ray count drops to approximately 50 cps. The gamma ray count increases to 70 cps just below 22 metres due to an increased presence of silt. This is evident from the CPT friction ratio(Rf) as well as the penetration pore pressure. The soil behaviour type interpretation is silty sand/sand. This is the same as it was in the fine sand above but the increased gamma count confirms that there is a greater amount of silt present. This is also confirmed by examining the CPT penetration pore pressure which displays the generation of excess(negative) pore pressure. A soil sample taken at a depth of 22 metres had a fines content of 22 percent compared to an average of about 1 or 2 percent in the fine sand from 5 to 16.75 metres. 5.1.2 Massey Tunnel Nuclear Density Profile The density log performed at Massey Tunnel along with the densities from the frozen sand samples is shown in Figure 15. The undisturbed sampling data is part of the CANLEX project. At depth the densities measured by the active GCPT are in good agreement with the densities measured from the frozen sand Gamma-Gamma Site: 98-300 CPT-5 Location: MASSEY TUNNEL Cone: 20 TON A 059 Date: 0401:98 11:02 0.0 -5.0 -10.0 a 0) o -15.0 -20.0 Rf % U metres 0.0 5.0 0 25 I I I I I I I I -25 .0 Max. Depth: 21 .80 (m) Depth Inc.: 0.05 (m) Figure 15.' Density Plot of Massey Tunnel rrr i u 111111 Den(g/crrT3) SBT 1.5 2.5 0 12 i — i — r Triangles show Density from Undisturbed Samples SBTi Soil Behavior Type (Robertson and Campanella 1980) "^ •r Estimated Phreatic Surface 4^ OO 49 samples. Near the surface the densities measured by the active GCPT seem too low. This is likely the result of the soil being disturbed during the first penetration to log background gamma radiation. Another factor to the low density is that the soil is above the water table and thus the bulk density truly is relatively low. 5.2 Kidd 2 5.2.1 Natural Gamma Log Figure 16 illustrates a CPT profile with the gamma ray count from the Kidd 2 site. Figure 17 illustrates a plot of gamma ray count from the GCPT and fines content from GSC samples. The gamma ray count at the Kidd 2 site is relatively high to 4.5 metres. This is expected due to the high presence of fines. Counts range between 70 and 90 counts per second. Grain size data was not available at this shallow depth. From 4.5 to 5.1-metres, the gamma ray count drops to 40 cps. This is due to a sand layer at this depth. Based on the gamma ray count, this sand is likely coarser than the sand immediately below it. The gamma ray count increases to approximately 50 cps to a depth of 6.6 metres indicating a layer of soil with slightly greater fines or a finer sand. The Campanella and Robertson(1986) CPT soil interpretation chart interprets the soil as a sandy silt. However, the penetration pore pressures are close to hydrostatic Gamma Site: GCPT96-2 Location: KOD 2 Qt bar Rf % 0.0 -5.0 -10.0 a o -15.0 - 2 0 . 0 200 0 . 0 I I I I | I I I I Drilled Out -25.0 Max. Depth: 19.40 (m) Depth Inc.: 0.05 (m) Figure 16.' Gamma CPT Plot of Kidd 2 Cone: 10 TON A 027 Date: 10:09:96 09:28 U2 (m) GR Count(CPS) SBT 0 25 0 100 0 12 TTT Cuts y L nTTTT §§ia ml— HI SBT: Soil Behavior Type (Robertson and Campanella 19BH) = ^ Estimated Phreatic Surface O Fines Content and GR Count 51 G.R. Counts(CPS) & Fines Content(%) 0 10 20 30 40 50 60 70 80 90 100 1 V •£ 10 - f t *— 5 10 CL a> O 12 ! 16 18 20 fr i Figure 17 Kidd 2 Site Fines vs GR Count November, 1996 • Fines Content (%) G.R. Count (cps) Fines Content is % <#200 Sieve From GSC Vibro Core Samples 52 indicating that the soil is free draining and not a silt. It is more likely a silty sand or a fine sand. This is confirmed by the low gamma ray count. All of the soil from 4.5 to 8. metres is interpreted as a sandy silt to silty sand by the Campanella and Robertson(1986) interpretation chart but the low gamma ray count and the hydrostatic penetration pore pressures indicate that it is a sand. The low tip resistance indicates that it is a very loose sand and mistakenly being interpreted as a sandy silt. However, grain size data is not available above 8 metres to confirm this. The CPT picked up a relatively soft layer at 8.8 to 9.3 metres. This layer was also interpreted as a sandy silt. In this layer the gamma ray counts show a sharp increase. There is also a large drop in penetration pore pressure at this depth as expected with a more silty soil. We can thus conclude that this layer is indeed a sandy silt. This is confirmed by a sample taken from this depth with a fines content of 22 percent. From 9.3 metres down to the end of the test hole at 18.6 metres, the gamma ray count remains relatively low, between 40 and 50 cps. The soil is interpreted as sand or silty sand except for a small layer at 14.5 metres which is interpreted as a sandy silt. 53 There is no difference in the gamma ray count at this depth from that above and below it. The penetration pore pressure does not show any change from static levels. It would be a reasonable conclusion that this is not a silty layer but rather a loose sand layer. 5.2.2 Kidd 2 Nuclear Density Log Figure 18 shows a CPT plot from the Kidd 2 site along with the active gamma density profile. The densities calculated from the CANLEX frozen samples have also been plotted over the density profile. Again the densities measured from the active GCPT compare very well with the frozen samples. Again the soil density seems low near surface. 5.3 Coal Port 5.3.1 Natural Gamma Log Figure 19 displays the CPT profile and the natural gamma log from the Coal Port. Of the three sites tested, this is the most interesting as there is a much greater variation in soil type and in the gamma ray count. The test hole at the Coal Port was drilled out to a depth if 4.5 metres due to the presence of gravels. The soil below the drill out to a depth of 6.8 metres consists of a silty sand. The Gamma-Gamma Site: 98-300 CPT-B Location KDD2 a 0) Q 0.0 - 5 . 0 -10.0 -15.0 -20.0 -25.0^ Max. Depth: 17.65 (m) Depth Inc.: 0.05 (m) Figure 18. Density Plot of Kidd 2 :TTT Cone: 20 TON A 059 Date: 04:01:98 14:22 U2 (m) 0 25 i i ib 111111111 Den(g/cnT3) SBT 1.5 2.5 0 12 Triangles show Density from Undisturbed Samples i i r w 1111 SBTi Soil Behavior Type (Robertson and Campanella 101)11) =~ Estimated Phreatic Surface Gamma Site: 97-GCPT-04 Location: COAL PORT Cone: 20 TON A 044 Date: 03:21:97 11:22 Qt bar Rf % -10.0 x: a CD Q -20 .0 -15.0 -25.0 Max. Depth: 21.60 (m) Depth Inc.: 0.05 (m) Figure 19. Gamma CPT Plot of Coal Port U2 (m) 0 25 GR (cps) SBT 0 100 0 12 M I N I M I Drilled Out III II Mil •mi _1 SBT: Soil Behavior Type (Robertson and Campanella I mill) = " Estimated Phreatic Surface 56 Campanella and Robertson(1986) CPT soil classification interprets the soil as a silty sand/sand. The gamma ray count for this soil averages 60 cps which is what would be expected in a silty sand considering that a clean sand has a gamma ray count of approximately 40 cps and a silt has a gamma ray count of approximately 70 cps. The penetration pore pressures show a dilative response as commonly seen in silts. From 6.8 metres to a depth of 9.0 metres the gamma ray count drops to about 45 cps. The soil CPT behaviour type is interpreted to be a silty sand/sand as is the soil just above 6.8 metres. However, from the low gamma ray count, we can determine that there is not a high presence of silt. The expected gamma ray count in a clean sand would be approximately 40 cps. Since the gamma ray count here is only slightly greater, there is likely just a very small amount of silt present. From 9.0 metres to a depth of 12.0 metres is an inter-bedded silt and sandy silt. The gamma ray count varies between 55 cps and 70 cps except for a small layer at 10.9 metres which has a very high gamma ray count of 90 cps. This layer is interpreted as a clay from the CPT. The high gamma ray count agrees with this interpretation. 57 From a depth of 12.0 metres to 15.0 metres the soil consists of clean sand with a small silt layer in the middle. The sand has a gamma ray count of 40 cps. There is a dramatic increase in the gamma ray count in the thin silt layer to a value of 70 cps as expected. The soil below 15.0 metres consists of silt with a gamma ray count of 70 cps. This continues down to 21.6 metres where the test was terminated. 58 6. DISCUSSION OF RESULTS 6.1 Natural Gamma Logs For Soil Type Assessment At all three sites, the GCPT was able to distinguish subtle differences in soil type that the CPT alone could not detect. As illustrated in Figure 13, at the Massey Tunnel site the GCPT was able to distinguish a greater presence of fines at 6.5 metres which was not clear from the CPT log. The GCPT was also able to distinguish the small silt layer picked up at 22 metres. Figure 16 illustrated a GCPT log from the Kidd 2 site. Here the GCPT was able to better distinguish the soil at a depth of 4.5 to 8 metres by showing that it was not a sandy silt as the CPT interpretation indicated but instead a loose sand. At a depth of 8.8 to 9.3 metres an increase in the gamma ray count indicated an increase in fines content that was not picked up by the CPT friction ratio based classification chart. A CPT pore pressure ratio based soil type interpretation chart would have picked this up however as there is a large decrease in the penetration pore pressure. At a depth of 14.5 metres the GCPT was able to distinguish a loose sand layer from what the CPT interpretation chart indicated was a sandy silt. The GCPT was able to pick up the presence of a very small amount of fines from a depth of 6.8 metres to 9.0 metres at the Coal 59 Port(Figure 19). Otherwise, the CPT gave a very good indication of soil type at this site. At all locations, the GCPT data could be used to get a good relative assessment of the fines content. It was found that coarse clean sands produced a gamma ray count of approximately 40 cps in the Fraser River delta deposits, and silts such as those found below 15 metres at the Coal Port site tend to produce a gamma ray count of 70 cps. Gamma logging was found to be useful when used in conjunction with CPTU data to assess soil type. However, fines will emit a different amount of gamma radiation depending on fines mineralogy. For this reason it is not possible to accurately define an equation relating natural gamma radiation to fines content. Instead the GCPT data should be used as a supplement to the CPT data. Figure 20 displays the particle size distribution curves for three samples taken at the Massey Tunnel site from a bore hole. The first sample from a depth of 4.15 metres has a fines content of 96%. This sample had a gamma ray count of approximately 80 cps. The second sample from 18.5 metres had a fines content of 0.57 %. This sample consisted almost entirely of fine to medium 1 0 0 I uu 90 -80 S 70 -| 60-2 50 | 40 -S. 30-20 -10 -n U O.C Figure 20 Grain Size Distrifc Samples from M< Particle Size Distribution 01 jution of assey Tunnel A:\5 m Depth / 96.4% fines / 21 3 m Dep "21:9"%-firre uji 5 It 18.5 0.5; kf-m Depth ' % fines i 0.01 0.1 1 Particle Size (mm) 10 100 ON o 61 sand and had a gamma ray count of less than 50 cps. The third sample from 21.3 metres has a fines content of 21.9 % and has a gamma ray count of about 65 cps. This would indicate that even a small amount of fines will produce a dramatic increase in gamma ray count. The change in gamma ray count between two soils with 0% and 21% fines is the same as the difference between two soils with 21% and 96% fines. Another factor that hinders the possibility of correlating fines content to gamma ray count is that of vertical resolution. The crystal in the gamma ray detector is 100 mm long and thus in order to accurately measure the true gamma ray count of a soil layer, the layer would have to be larger than the vertical resolution of the gamma ray tool. The vertical resolution of the tool is difficult to assess as the gamma rays not only enter the crystal from a horizontal direction but also from above and below. The gamma ray count recorded in a thin layer of fine soil will not be as high as in a thick layer of the same. soil. The influence of the soil above and below diminishes exponentially with distance thus it is difficult to account for it. 6.2 Nuclear Density Logging At both the Massey Tunnel and the Kidd 2 sites, the active GCPT 62 was able to quickly and cost effectively measure the density profiles. At depth, the densities compared very well with those measured from high quality frozen samples as illustrated in Figure 15 and Figure 18. The densities near surface seemed too low likely due to a combination of soil disturbance and the soil being unsaturated. Any inconsistencies at depth in the densities can be partly attributed to the tests being performed several meters away from the locations of the frozen sampling in order to reduce the possibility of testing disturbed soil. 6.3 Environmental Logging While it was not possible to gain access to a site with radioactive contamination, it is apparent that on such a site, a gross gamma log could be performed as would be done on a clean soil. A gamma ray spectrum could be measured to attempt to determine the contaminant type. Typical gamma ray spectra are illustrated in Appendix I 6.4 Logging Speed The penetration rate used during testing ranged between 1.2 to 1.7 cm/s. This is slower than the ASTM recommended speed of 2 cm/s. The ASTM specification has a relatively large tolerance on the logging speed and its believed that the slow speed did not greatly effect the CPT results. At logging speeds much 63 faster than 1.7 cm/s, there is a large scatter in the gamma data with depth. The reason for this is that gamma ray emissions are of a statistical nature and thus over a short period of time there may be a large variation in the number of incident gamma rays the logger records. A slower logging speed gives the logger time to collect more data and average it over a longer time period. There will be less variability in the measured radioactivity if each measurement is taken over a longer period of time. 64 7. CONCLUSIONS AND RECOMMENDATIONS 7.1 Soil Type Assessment Soil sampling and laboratory testing for geotechnical purposes can be a costly and time consuming task. The use of natural gamma data as a supplement to CPT data makes it possible to better assess the soil type before sampling. Therefore, a more informed decision can be made about sampling depths and fewer samples can be taken saving time and money. The GCPT was able to rapidly and cost effectively measure a gamma ray profile. It took only slightly more time to perform a GCPT test than a conventional CPT test. The GCPT showed a marked increase in gamma ray count when a layer of increased fines was encountered. In clean sand the gamma ray count remained at about 40 to 45 counts per second(cps). As an increasing amount of fines was encountered, the gamma ray counts increased to a maximum of approximately 90 cps in a clayey soil. In pure silts such as those below 15 meters at the Coal Port, the gamma ray count remained at about 70 cps. It would be expected that the gamma ray count should be greater for soils with even higher clay content. A good knowledge of fines content for performing a liquefaction assessment is important as two soils with similar CPT tip 65 resistances may have vastly different liquefaction properties. A silty sand with a similar CPT tip resistance as a clean sand is less likely to liquefy than the clean sand. 7 . 2 Environmental Assessment Although an environmental testing site was not available, the GCPT clearly has the potential to delineate and identify radioactive contamination in soil. There are many sites in the United States of America where nuclear isotopes have been used in tests or have been placed in landfills. With more stringent environmental regulations, it is important to identify these sites and attempt to remediate them. These sites can be quickly characterized with minimum exposure to workers and without having brought to the surface any contamination that would require disposal. 7.3 Density Logging Likely the most useful application for the GCPT is for density logging. By simply removing the CPT module from the gamma module and replacing it with a radioactive tip, it is possible to perform gamma ray back scatter density logging. Nuclear density logging using a penetration tool is far superior to that using a borehole tool for many reasons including: • no attenuation of gamma rays as they pass through the 66 borehole casing material; • no washing of soil as the hole is drilled; • no mud cake effect on the density measurement. Likely the most significant advantage is that a borehole is not required. The density logs performed at the Massey Tunnel and the Kidd 2 sites gave results which were in good agreement with undisturbed samples taken for the CANLEX project as shown in Figure 15 and Figure 18. As with all penetration tools, there is the possibility of downhole breakage if an obstruction is encountered. The concern with the Active GCPT is that if this was to occur, the active source must be retrieved. In order to reduce the possibility of the active probe breaking off, it is always pushed down the pre-pushed hole used for the background gamma log. This greatly reduces the required pushing force which reduces the likelihood of the probe breaking off. In addition there is a large safety factor in the maximum pushing force used to advance the active probe. 67 7 . 4 Source Radio-Activity The source currently used in the probe is many orders of magnitude less active than those commonly used in the oil logging industry. This activity level was chosen to reduce operator exposure to radiation and to reduce the increased liability involved with a source of higher activity. However, a higher activity source would likely eliminate the need to take a background measurement of natural gamma radiation as background radiation would be insignificant when compared to the high gamma ray counts emitted by the higher activity source. The logging speed could also be increased with a higher activity source as it would not take as long to get a representative gamma ray back-scatter count. A higher activity source would also eliminate the need for background correction during calibration. It should be noted that the activity of the gamma source will decrease with time based on its half life. This would have to be accounted for in the back-scatter verses soil density relationship. 7.5 Resolution With the vertical resolution of the GCPT it is difficult to accurately assess fines content of thin soil layers. The GCPT detects thin layers of fine soil but the lower gamma ray count 68 from the soil above and below these thin layers is believed to reduce the gamma ray count measured in the thin layer of fine soil. However, as seen at the depths of 9.0 and 14.9 meters in the GCPT log from the Coal Port(Figure 19) it is seen that there is a good gamma ray response between the interfaces of sand and silt layers. The probe reacts fairly quickly to changes in soil type' and in most cases vertical resolution should not be a concern. It may be possible to improve the vertical resolution by shielding part of the crystal in the GCPT. This would result in a decrease in the count rate of the logger. To compensate for this, the logging speed would also have to be decreased. 7.6 Logging Speed It is important that the test be performed slowly enough that the gamma ray logger has sufficient time to collect a representative gamma ray count. A longer data collection time improves the accuracy and repeatability of the results. The logging speed used for measuring the gross gamma speed was slightly slower than the ASTM recommended speed for a CPT of 2 cm/s. This produced a gamma ray profile that was relatively smooth with few spikes. These spikes occur due to the statistical nature of gamma rays. Longer counting times will 69 result in a smoother gamma ray profile. This has to be weighed against the hourly cost of performing the test. 7.7 Future Considerations There are still many topics that can be looked at to improve the soil characterizing properties of the GCPT. As mentioned earlier, one of the most useful applications of the GCPT is for insitu density measurement. Due to the low activity of the radioactive source, it is necessary to record and account for background radiation before the active gamma logging is performed. A higher activity gamma ray source could be utilized to eliminate logging the background radiation and the complete process could be accomplished with only one logging. The drawback to only having to perform one log is that the active probe is penetrating unknown soil whereas when the background radiation was measured any obstructions would be encountered with the passive gamma probe. As with any penetration tool, if advancement of the active gamma probe is not stopped immediately if an obstruction is encountered, there is the possibility of the active gamma source being broken off at depth. Another aspect of the active GCPT that may be refined is the 70 source to receiver spacing. If the spacing was to be decreased, it would give the probe better vertical resolution. The problem that may be encountered in doing this is that the probe measures density of the surrounding soil to a distance that is half of the source to receiver spacing away from the probe(Plewes 1988). If the source to receiver spacing is too small, the probe will be greatly influenced by the disturbed soil surrounding the probe. If the spacing is increased, the probe will give a more precise measurement of the insitu soil density because it will measure back scatter from a greater horizontal distance away from the probe. For this study only a limited number of soil samples were studied for the purpose of comparing natural gamma radiation and fines content. All of the soils studied were from the Fraser River delta and thus had similar mineralogy. To improve the GCPTs soil characterizing ability, further studies could be conducted on the natural gamma radiation of soils of different fines content and with different mineralogy. As discussed earlier the vertical resolution of the passive GCPT is a function of the length of the crystal within the GCPT. In order to improve the vertical resolution of the GCPT, part of the crystal could be shielded with lead to improve the vertical 71 resolution. Doing so would decrease the activity measured by the GCPT and the logging speed may need to be decreased further. There is much potential for utilizing the GCPT for the purpose of environmental logging. This ability could not be explored during this study as a test site could not be located. However if such a site could be located, the possibility of calibrating the GCPT to contaminant concentration could be investigated. This may be accomplished by constructing molds with known concentration of the contaminant in question and inserting the probe into the molds. An easier method would be to perform a GCPT log at such a site and then take samples next to the location of the GCPT log. The concentrations measured in the samples could then be correlated to the GCPT measured gamma ray count. REFERENCES 72 ANSI N42.9-1972/IEEE Std. 398-1972, IEEE Standard Test Procedure for Photomultipliers for Scintillation Counting. Blunden, R.H., 1975, Urban Geology of Richmond British Columbia, Adventures in Earth Science Series. Bre.ck, W.G., Brown, R.J.C., McCowan, J.D., Chemistry for Science and Engineering. 1988, McGraw-Hill Ryerson Limited. Campanella, R.G., Robertson, P.K. 1986, Guidelines For Use, Interpretation and Application of the CPT and CPTU, Soil Mechanics Series 105, Department of Civil Engineering, The University of British Columbia. ConeTec Investigations Ltd., Vancouver, B.C., Various Promotional Diagrams. Craig, R.G., Soil Mechanics, Fourth Edition, 1990, Chapman and Hall. EG&G Ortec, 1992, Maestro 3.0 For Windows Manual, 100 Midland Road, Oak Ridge, TN 37831-0895 U.S.A. Geological Survey of Canada(GSC), 1994, Personal correspondences. Gilmore, Gordon., Hemingway, John D. 1995, Practical Gamma-Ray Spectrometry, Wiley, Chichester, New York. Hallenburg, James K. ,. 1984, Geophysical logging for mineral and engineering applications, PenWell Books, Tulsa, OK. Howie, J.A., Lateral Stress and Penetration Resistance In The Specification of Ground Improvement. Short Course Presented October 1, 1997. Hunt, R.E. (1986), Geotechnical Engineering Techniques and Practices, McGraw-Hill Book Co.' pages 239-243. Hunter, J.A., Luternauer J.L., Roberts, M.C., Monahan, P.A., Douma M., Borehole Geophysics Logs, Fraser River Delta. 1994. Killeen, P.G., Pflug, K.A. et al, 1993. Nuclear Geophysics, Volume 7, No. 4, pages 501-514. 73 Lunne, T, Robertson, P.K., Powell, J.J.M., 1997, Cone Penetration Testing in Geotechnical Practice. Blackie Academic & Professional. Monahan. P.A., Luternauer, J.L., and Barrie, J.V., 1995, The Geology of the CANLEX Phase 2 Sites in Delta and Richmond British Columbia, Proceeding of the 48th Canadian Geotechnical Conference, Vancouver, B.C. Pflug, K.A., Killeen, P.G., and Mwenifumbo, C.J., 1993, Application of Spectral Gamma-Ray Logging to Low-Level Radioactive Waste Management Studies, Proceedings of the 5th International Symposium of the Mineral and Geotechnical Logging Society: Tulsa. Plewes, H.D., McRoberts, E.C., Chan, W.K., 1988, Downhole Nuclear Density Logging In Sand Tailings, Geotechnical Special Publication, n.21, Published by ASCE, New York, NY, USA. Schlumberger, 1989, Log Interpretation Principles/Applications, Schlumberger Educational Services, Houston Texas. Schweitzer, J. S. (1991) Nuclear Techniques in the Oil Industry Nuclear Geophysics Vol. 5, No 1/2, pp. 65-90, Great Britain. Serra, 0., Baldwin, J., Quirein, J.,1980 Theory, Interpretation and Practical Applications of Natural Gamma Ray Spectroscopy, SPWLA Twenty-First Annual Logging Symposium. Sully, J.P., Echezuria, H.J., 1988, Insitu Density Measurement With Nuclear Cone Penetrometer, Penetration Testing 1988, ISOPT-1, DeRulter (ed.), Balkema, Rotterdam. Wride, C.E., Robertson, P.K., 1997, CANLEX Phase 2 Report, University of Alberta. 74 APPENDIX I C a l i b r a t i o n APPENDIX I CALIBRATION 75 Introduction It is necessary to recalibrate the software and the hardware if the gain or voltage across the PMT is altered. Before a calibration is performed, the gamma logger must first be permitted to warm up for at least an hour. This will ensure that there is not a large gain shift that would cause the energy spectrum calibration to drift. Hardware calibration The first step in calibrating the gamma logger is to adjust the bias voltage across the photo multiplier tube. This is done to allow the zero energy range of the spectrum to coincide with the left end of the spectrum and the maximum expected gamma energy with the right side of the spectrum. The energy of a gamma ray is measured in units of electron volts(eV). The units of an electron volt are 1.60xl0"19 kg(m/s)2. Units of kg(m/s)2 are those of energy. The GCPT was adjusted in order that the 0 keV energy was at the first energy bin and the 2900 keV energy was at the 2048th bin. This range should be sufficient for measuring most of the commonly encountered gamma energy sources. 76 Software is required which can read data from the data acquisition card and display it on the monitor. The program used was Maestro 3.0 for Windows. The first gamma energy source used for hardware calibration was thorium 241. Thorium gives a distinct energy peak at 2614 keV. This peak should appear around energy bin 1800 on the gamma energy spectrum. This will be about 90% of the maximum energy displayed. The thorium source is placed next to the gamma module and data is collected for several minutes after which a clear 2614 keV peak is seen. The complete thorium spectrum should be compared with a previous thorium spectrum in order to confirm that the correct peak has been chosen. The spectrum collected as part of this study is shown in Figure A-l. There are often additional, less pronounced peaks on the energy spectrum. These originate from the daughter products of the isotope. Daughter product is the term given to the series of different isotopes that a given isotope transforms into as it decays. As each daughter decays, it will emit gamma rays of a specific energy and it will produce at least one additional peak on the energy spectrum. There are many such peaks on the thorium spectrum illustrated in Figure A-l. After the peaks of interest are identified, the bias voltage Maestro - Buffet ( Thorium Source, GCPT Logger } l e Calculate Services ROI Acquire Display o o o o c o O a CD B i n 1 2614 keV Energy Peak '!;! ' -Display— CMCB € Buffer MCB#1 SEGJM OFull 0 Expand Vert: LOG Horz: 2048 ROI: Off Bin 2048 Figure A1. Thorium Energy Spectrum 78 should then be adjusted to bring the 2614 keV peak to the required position at approximately the 1800 energy bin. The data acquisition system is then used to again develop a spectrum. The new position of the 2614 keV peak is then located and readjusted if needed. These steps are repeated until the peak is at the desired position. The gain adjustment in the software can be used as a fine adjustment to position the thorium peak. The second step in calibrating the gamma logger is to bring the zero energy position of the spectrum to the first energy bin. This is done by adjusting the zero position dial on the data acquisition card. In order to determine the zero energy bin, two or more radioactive sources are needed with known energy peaks. Thorium and Cobalt 60 were used. The energy.bins for the 2614 keV and the 1173 keV peaks must be determined by separately placing the two energy sources near the gamma module. Figure 5 illustrates energy peaks for the Cobalt 60 source. With the energy of two bins having been established, the position of the zero energy bin can be extrapolated from this data. The zero position adjustment on the data acquisition card should then be adjusted in order to move the zero energy level to coincide with the first bin. 79 An Americium 241 source has a very low energy peak of only 60 keV and thus would be a better low energy source for this calibration. However, it was not possible to use Americium 241 with the GCPT as the low energy gamma rays cannot penetrate the steel housing thus a higher energy source had to be utilized. The positioning of the zero energy bin may not be as precise because a much larger extrapolation must be used to locate it. Software Calibration The third step in calibrating the gamma logger is to calculate precisely which bin number on the data acquisition card corresponds to which energy level. This is commonly done using a three point calibration(EG&G Ortec 1992). The calibration is performed by positioning the first radioactive source near the gamma logger and measuring an energy spectrum. After a few minutes, one or more energy peaks will appear on the energy spectrum. The number of peaks and the position of the peaks will depend on the gamma source being used. The shape of the spectrum can be compared to that from a reference book to confirm which peaks correspond to what gamma energies. Once this has been done, the peak energy and the energy bin at which the peak appears are noted. 80 The same procedure is followed for two more gamma ray sources. With the gamma energy corresponding to three energy bins known, a binomial calibration can be made with the form: Energy = A(energy bin)2 + B(energy bin) + C The A term will be extremely small as the calibration will usually be linear. It is possible to use more than just three points but the non-linearity is so small that it is not needed. If sufficient time is given for the GCPT to collect an energy spectrum, the peaks will always lie in the same place for a given isotope. If the unit is permitted to warm up sufficiently before calibration, the calibration should not drift during testing. If, however, there is a need to check the calibration of the unit while it is in the ground, the gamma logger should be stopped in one position for a few minutes to allow a gamma ray spectrum to develop. In a natural soil that is clean of radioactive contamination, this spectrum should have one or more peaks associated with potassium, uranium, or thorium. If the calibration has drifted, the gain should be adjusted through the software in order to correct for the drift. 81 Stripping Ratios As discussed in section 2.2, natural gamma data is usually collected in three windows. These include the potassium, uranium, and thorium windows as shown in Figure 2. The thorium window has the highest energy of the three. As seen on Figure A-l the thorium source contributes to the potassium and the uranium windows. Similarly the uranium window will contribute to the potassium window. In order to correct for this the ratio of the counts in the thorium window and its contribution to the uranium and potassium windows must be calculated. Similarly the ratio of counts in the uranium window and their contribution to the potassium window must be calculated. These ratios are commonly referred to as "stripping ratios". When a natural gamma log is performed, the contribution of the higher energy windows to the lower energy windows can be "stripped" using the stripping ratios to correct the log. The contribution to the lower energy windows happens for two reasons. The first is that the isotope may decay with a series of energy releases. This is seen in Figure A-l with thorium which has several peaks of lower energy than the main 2614 keV peak. These lower energy peaks will contribute to the lower energy potassium and uranium windows. The second is that due to the relatively small size of the crystal, all of the energy of 82 all of the incident gamma rays will not be lost within the crystal. A ray may enter the crystal and lose part of its energy as light and then exit the crystal. This would cause the ray to be recorded as one of lower energy than it actually is. This is less likely to occur with a larger crystal. This is the reason why energy peaks have steeper slopes for a larger crystal. Another less likely possibility is that two or more rays enter the crystal at the same time and these rays get recorded as one event of a greater magnitude than the individual events. These phenomenan would cause errors in measurements of potassium, uranium, and thorium in the soil due to overlapping of the energy regions in the energy spectrum. In order to correct for this, the overlap has to be measured and a correction has to be made for it. The energy windows for potassium, uranium, and thorium and for measuring gross gamma counts are given in Table 1. Table 1 : Logging Energy Windows 83 Window Potassium Uranium Thorium Gross Gamma Lower Bound 1.38 MeV 1.66 MeV 2.4 4 MeV 0.15 MeV Upper Bound 1.5 6 MeV 1.90 MeV 2.7 7 MeV 2.7 7 MeV Since thorium has the highest energy window, there should not be significant counts from lower energy elements in the thorium window. Thus it is assumed that only background radiation contaminates the thorium window. Both the Uranium and the potassium windows measure gamma radiation originating from a thorium source for which a correction must be made. This is done by exposing the detector to a thorium source and measuring the increase in activity in the uranium and potassium windows of the energy spectrum. Similarly, it is assumed that Uranium does not contaminate the Thorium window but only the lower energy Potassium window. The spectrum from a Uranium source can be used to correct the counts in the Potassium window. The gamma ray counts in the three energy windows when exposed to Uranium and Thorium can be used to calculate the stripping ratios as follows: 84 CX(U) C,(X) C2(K) a=— 6=—• v = CATh) CATh) C2(U) Where : C^U) is the reading in the uranium window caused by the thorium source. Cx(Th) is the reading in the thorium window caused by the thorium source. C^K) is the reading in the potassium window caused by the thorium source. C2(U) is the reading in the potassium window caused by the uranium source. C2(K) is the reading in the uranium window caused by the uranium source. The Potassium, Uranium and Thorium log can then be corrected by multiplying the gamma ray count in each window by the corresponding stripping ratio. Gross Count Calibration The gross gamma count was calibrated by performing a GCPT next to a borehole previously logged by the Geological Survey of Canada(GSC). The GCPT gamma count values were then multiplied 85 by a ratio calculated from the GCPT and the GSC gamma count values. As discussed earlier the gross gamma count could be calibrated using the American Petroleum Institute(API) calibrator. However, the cost and time involved could not be justified for the purpose of this project as the facilities are located in Houston Texas. The main reason that the GSC and the GCPT measurements are different is due to the fact they both have different crystal sizes. The Atlantic Geoscience Center module uses a 25 mm by 100 mm inch crystal which is a standard size for borehole gamma loggers. This crystal would not fit inside of the GCPT module however and a smaller crystal had to be utilized. Since the smaller crystal has a smaller surface area and volume, it will detect fewer gamma rays. A difficulty which arises with trying to calibrate the GCPT to measure gross gamma counts is that available calibrated radioactive sources emit gamma rays from a single point. However the GCPT detects gamma rays from all directions. It is not possible to account for the incident gamma rays from all directions using a point source. The gamma logger measures gamma rays from 360° degrees horizontally and almost 90° degrees above and below the horizontal. The incident rays from above and below the horizontal travel through different thickness of 86 metal housing depending on the incident angle. The gamma rays tend to be attenuated to a greater extent when they pass a longer distance through the steel housing. Nuclear Density Logger (Gamma-Gamma Probe) Calibration The density tool is calibrated by placing it inside of at least 3 materials of known density. The first material used was a 24 inch long, 12 inch diameter aluminum cylinder. The aluminum cylinder had a hole bored through its centre to accommodate the gamma ray tool. The logger was placed inside the aluminum without the active source to get a background gamma ray activity measurement. Then the active tip was placed on the logger and the gamma ray activity was measured. The same procedure was followed with a tank of water instead of the aluminum block. The third point was a sand with a known density of 1.85 g/cc. From these three points, a binomial equation was calculated that correlates gamma ray back scatter to density(Hallenburg 1984). A mixture of barite muds of different densities gave a similar correlation as that obtained from the above materials. The gamma probe measures the electron density of a material and not directly the bulk density of the material. The Z/A ratio of 87 the calibration materials can be used to account for this. Z is the atomic number and A is the mass number of an atom(Breck 1988). The average Z/A ratio for elements found in soil is approximately 0.50 (Hallenburg 1984). However the Z/A ratio for the aluminum block used in the calibration is 0.4818. To correct for this an apparent density of 2.60 g/cc is used to calculate the density verses gamma ray back scatter curve instead of the bulk density of 2.70 g/cc. Similarly an apparent density of 1.111 g/cc is used for water instead of 1.00 g/cc. Since the calibration material densities are corrected for the Z/A ratio and it is known that soil has a Z/A ratio 0.50, the bulk density should be equal to the apparent density. If it is known that the Z/A ratio of a soil is not 0.50 such as may be the case for mine tailings with high metals content, the correct Z/A ratio for the tailings could be calculated and used correct the density profile. The Z/A ratio of a soil is based on the Z/A ratios of the individual constituents and the proportion of each constituent. The procedure for calculating the Z/A ratio of a soil is described in detail by Hallenburg (1984) . 


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