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Magnetics of Bowie Seamount Michkofsky , Ronald Nick 1969-06-02

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MAGNETICS OF BOWIE SEAMOUNT by RONALD NICK MICHKOFSKY B.Sc, Carnegie Institute of Technology (Carnegie-Mellon University), 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of GEOPHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH April, 1969 COLUMBIA 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 representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver 8, Canada Date ii ABSTRACT Using the oceanographic ship, the Endeavor; the University of British Columbia undertook a study of Bowie Seamount (53°18', 135°41') during the summer of 1968. Receiving a proton precession magnetometer for the cruise from PNL, a magnetic field survey was included in the study. The magnitude of the observed anomaly was about 850 gammas. The regional was determined by data taken in an airborne magnetic survey done in 1958 by the Dominion Observatory. Corresponding to the topography, the contour plot of the magnetic field showed a strong linear trend from the SW to the NE. In addition, despite fairly symmetrical bathymetry, the observed anomaly is decidedly unsymmetrical, implying a non-uniform intensity of magnetization. This seems to be confirmed by the use of a three dimensional,model - assuming uniform magneti zation - developed by Manik Talwani. Using the least squares best fit intensity of magnetization, a large dis crepancy was found between the model and the observed anomaly. In light of the fact that the earth's magnetic field has reversed directions many times in its history, the above discrepancy may well be due to the fact that the lava flows of the seamount span at least one time boundary separating a normal and a reversed magnetic period. This is given some credence in that an age determination was made on one sample yielding an age of about 100,000 iii years, a normal period in the earth's magnetic history. This together with the fact that the magnetic anomaly over most of the seamount is negative yields the above conclusion. iv TABLE OF CONTENTS Page ABSTRACT ii LIST OF TABLES vLIST OF FIGURES vii ACKNOWLEDGEMENTS ix CHAPTER I INTRODUCTION 1 1-1 Nature of Cruise1-2 Proton Precession Magnetometer 2 CHAPTER II RUN TO THE SEAMOUNT 10 CHAPTER III SHIP'S PATTERN 15 CHAPTER IV BATHYMETRY 9 IV-1 Hand Contoured Maps 19 IV-2 Computer Contoured Map 22 CHAPTER V PHOTOGRAPH AND ROCK SAMPLING 2 7 V-1 General Comments 27 V-2 Remanence and Susceptibility 29 V-2-1 Rock Magnetism 2V-2-2 Magnetic Properties of Samples 31 CHAPTER VI ORIGINAL MAGNETIC DATA 34 VI-1 Data Recording 3VI-2 Diurnal Corrections 35 VI-3 Contour of the Original Data 38 V Page CHAPTER VII 4 2 CHAPTER VIII THREE DIMENSIONAL ANALYSIS 4 7 VIII-1 Nature of Terrain Correction 47 VIII-2 Theory and Method of Talwani's 48 Program VIII-3. Applications of the Model 55 CHAPTER IX CONCLUSIONS 63 BIBLIOGRAPHY 66 vi LIST OF TABLES Table Page I Instruments Used on the Cruise 3 II Specifications of Varian-4937 Proton 8 Precession Magnetometer III Magnetic Samples 32 IV Averages of Samples (CGS Units) 32 V Magnetic Stations 37 VI Regional Data 44 VII Model Results 58 vii LIST OF FIGURES Figure Page 1 iMain Deck Outline of the Endeavor 4 2 Buoy 5 3 Schematic of the Varian-4937 Proton 7 Precession Magnetometer 4 Geographical Setting 11 5 Magnetics and Bathymetry of the Run to 12 Bowie Seamount 6 Topographical Setting 14 7 Bowie Seamount, Ship's Track 16 8 Bowie Seamount, Ship's Track Over Summit 18 Area 9 Bowie Seamount, Topography 20 10 Bowie Seamount, Topographical Detail of 21 Summit Area Above 200 Fathoms 11 Computer Contoured Bathymetry 24 12 Perspective of Bathymetry 26 13 Bowie Seamount, Sample Locations 28 14 Total Magnetic Field of Bowie Seamount 39 15 Perspective of Total Magnetic Field of 40 Bowie Seamount 16 Airborne Magnetic Survey 43 17 Observed Total Anomaly of Bowie Seamount 46 18 Coordinate System and Topographical Feature 49 Q 19 Contour at Depth z and Its Polygon Repre- 52 sentation 20 Polygon System A 56 21 Polygon System B 7 22 Calculated Anomaly 60 Vlll Figure 23 Residual Anomaly 24 Magnetic Reversals ix ACKNOWLEDGEMENTS I wish to thank Dr. W. Slawson for his patient supervision and encouragement. Additional thanks are given to Bernard Caner for the time he spent discussing the data, especially with regard to diurnal corrections. Special thanks are given to John Steele for his work in making the measurements on some rock samples, for remanence and susceptibility. John did this work in conjunction with E. Irving of the Dominion Observatory in Ottawa, whom of course I also thank. I would also like to thank R, Herzer who did the geological interpretation of Bowie Seamount and whose consultation was invaluable. The personal acknowledgements were possible only because of the magnetic survey. Thus thanks are given to PNL for its loan of the V-4937 Proton Precession Magnetometer and to the Graduate Research Committee for its grant, 68-1212, which supported my research. 1 CHAPTER I INTRODUCTION I- 1 Nature of Cruise In the summer of 1968, the University of British Columbia undertook a study of Bowie Seamount (53°18'N, 134°41'W). The origin of this cruise was a similar one that occurred the preceding' summer. On that cruise only echo-sounding and bottom sampling were done. Unfortunately, the cruise did not adequately accomplish these two tasks. The reason for this is that the navigation was done by dead reckoning with respect to a buoy anchored to one of the shallower (20 to 30 fathoms) parts of the seamount. Originally, it had been planned to navigate by radar re flection, that is a metal reflector was to be attached to the top of the buoy; however, the rod joining the reflec tor to the buoy broke. The resulting poor navigation led to an inaccurate topographical map. With the shortcomings of the 1967 cruise, the 1968 one was undertaken. Navigation was accomplished by means of a radar transponder mounted on an anchored buoy. With this set up, navigation was good to about .2 of a mile radially and .5 of a degree azimuthally. Besides bottom photography and sampling, a bathymetric and a mag netic survey were completed. Data obtained during the cruise is the basis for the M.Sc. theses of R. Herzer of the Department of Geology 2 of the University of British Columbia and the author of this report, R. N. Michkofsky. Mr. Herzer is to inter pret the bathymetry, the bottom samples, and the photo graphs; I will interpret the magnetic field measurements. The main instruments used on the cruise are listed in Table I, along with available model numbers. Fig. 1 is a schematic of the main deck of the Endeavor, the oceanographic ship used on the cruise. The ship, which has a permanent crew of 40, is approximately 250 ft. in length. The lab's radar was located about 50 ft. above the water level. Along with the radar transponder and its corresponding tower, Fig. 2 is a schematic of the buoy used. 1-2 Proton Precession Magnetometer* To measure the magnetic field, a Varian Proton Magnetometer was used. The basic theory of the instrument is simple. The proton has a precise magnetic moment. On applying a sufficient field, the magnetic moment tries to align itself to the applied field. As a consequence, the proton's axis (direction of the magnetic moment) precesses about the external field. The frequency of precession is proportional to the strength of the field, i.e. H (gammas) = *Varian Associates, Data Sheet 3 TABLE I Instruments Used on Cruise Instrument Edo Echo Sounder (12kHz Transducer) Alden Precision Graphic Recorder Gift Depth Recorder Transducer (beam width 17°) Alpine Geophysical X-Band Radar Transponder Decca Radar (Beacon Receiver) Varian Associates Marine Proton Precession Magnetometer E.G. and G. International Camera Light Source Ocean Research Equipment Pinger Twelve Inch Pipe Dredge Petterson Grab Sampler Dietz-Laford Snapper Model Number 185 419 427-D 828 V-4937 205 206 250B Custom made Custom made 4 GALLOWS (A-FRAME) Figure 1 Main Deck Outline of the Endeavor RADAR TRANSPONDER RADAR TRANSPONDER TOWER AVERAGE WATER LEVEL BUOY ANCHOR (DISCARDED RAILROAD WHEEL) SCALE: ' 1 IN. EQUALS H FT. Figure 2 Buoy 6 23.4875 fp (cycles per second) where fp is the preces sion frequency. The proton precession magnetometer uses a hydrocarbon fluid to provide a source of protons. By means of a coil that surrounds the fluid, a strong field is applied to the fluid at an angle to the earth's field, causing a polarization of the protons. This field is about two orders of magnitude greater than the earth's magnetic field. The field is cut off and the protons, already orientated, try to align themselves.with the earth's field. The precession around the earth's field induces a voltage in the coil at fp . This signal is used to measure the earth's magnetic field. To avoid the possibility of the polarizing field being parallel to the earth's field, two perpendicular coils are used. The induced signal is amplified, is squared, and is entered into a phase detector. The output of the phase detector controls a voltage controlled oscillator which generates a signal about 32 times fp, the precession fre quency. To help insure that the output remains at exactly 32 fp, a feed back system is used. A digital frequency counter then counts exactly 23.4875 times fp. A schematic of the above is in Fig. 3. In addition, a few of the instrument's specifications are in Table II. To finish off, I would just like to add that to avoid the magnetic effects of the ship, the sensor head illustrated in Fig. 3 7 A SWITCHING CIRCUITS LIMITER VOLTAGE PHASE CONTROLLED DETECTOR OSCILLATOR —> AMPLIFIER SENSING HEAD POWER SUPPLY 1/32 FREQUENCY DIVIDER ELECTRONIC COUNTER D/A CONVERTER ^ GRAPHIC RECORDER Figure 3 Schematic of the Varian-4937.Proton Precession Magnetometer 8 TABLE II Specifications of Varian-4937 Proton Precession Magnetometer Range Accuracy Sensitivity Sampling Rate 20,000-100,000 gammas ±1 part in 105 (proton constant, 23.4875, is known to 8 ppm) ±1 gamma in all ranges 2, 6, 10, 60, 100, 300 seconds per count Reference Frequency Stability 2 ppm per month 9 was towed about 600 feet behind the ship or over twice the length of the ship. 10 CHAPTER II RUN TO THE SEAMOUNT ' On May 28 the cruise began from Victoria, British Columbia. The final loading of the ship occurred in Vancouver. Then the Endeavor travelled north between Vancouver Island and the mainland. The voyage from the entrance into the Queen Charlotte Sound to the seamount is illustrated in Fig. 4. Navigation on the voyage was done by sunshots and dead reckoning. In the figure the circles show the places where the ship's positions were recorded from the bridge. We see that our destination, Bowie Seamount, is approximately at 53°18'N and 135°41'W. On this run magnetic and bathymetric measurements were made. A magnetic measurement was taken every 60 seconds; bathymetric measurements were made for all practical pur poses continuously. Starting at 132°W, positions were recorded on an hourly basis. A plot of the magnetics and the topography versus distance from the main peak of the seamount is shown in Fig. 5. Having looked at the original records, one observed that there were several crests and troughs in the magnetic and the bathymetric records bet ween the hourly positional readings. The positions of these crests and troughs were estimated. These extreme magnetic and bathymetric values are the circled ones in Fig. 5. The region in which the cruise covered included 56700 Figure 5 Magnetics and Bathymetry of the Run to Bowie Seamount 13 the magnetic lineations published by Raff and Mason (1961) ; these are shown in Fig. 4 by letting the positive anomalies be crosshatched. Fig. 5 illustrates the published Raff and Mason lineations. As the ship proceeded from this area to the seamount, the alternate positive and negative lineations seem to continue. However, the situation is complicated by the fact that the topography becomes irre gular. Thus it becomes difficult to tell whether the observed cyclic highs and lows are due to topographical features or to the continuation of the Raff and Mason lineations. The importance of this becomes more than academic when one determines a regional gradient. Note that on approaching the seamount, one still has oscilla tions of the order of 200 to 300 gammas. To go along with the geographical setting, Fig. 6 shows the topographical setting. This map is based on U.S. Naval Oceanographic contour maps (1410N and 1411N) that have recently become unclassified. In the region of inte rest, the navy had sounding lines every 5 to 10 miles. The original maps were contoured every 100 fathoms. The main observation to make is that Bowie Seamount has the same lineations as do the other seamounts or for that matter the same lineations of any of the shown contours, i.e. lineations in a southwest to a northeast direction and a corresponding one that is approximately perpendicular. Figure 6 Topographical Setting 15 CHAPTER III SHIP'S PATTERN The Endeavor reached the seamount on May 31. At 1400 P.S.T. the first attempt to anchor our buoy was made; it failed. Due to a subsequent storm another attempt was not made until the early hours of the 2nd of June. The above illustrates some of the headaches of oceanographic work. However, by 0430 P.S.T. all was ready for the taking of magnetic and bathymetric measurements. To get adequate coverage of the whole seamount as quickly as possible - in case of equipment failure -the ship first undertook the star pattern shown in Fig. 7. Perhaps a more primary reason for the planning of the star pattern was that there existed an uncertainty in how far from the buoy the ship could be and yet pick up the buoy on radar. Thus it was felt that if some dead reckoning had to be done, radial paths to the buoy would give the most accurately known tracks. However, the radar navigation worked for the extent of the coverage. In order to get cross-overs a type of spiralling square pattern was super imposed on the star pattern. Unfortunately, at all cross overs the two coincidental points are separated in time by at least 24 hours. Thus it was impossible to correct for daily magnetic variation from the data itself. Of course with the very large gradients and with a position ing error of -.2 mile, one probably could not do so even 17 if the cross-overs were close in time. What was needed was a continuously recording magnetometer mounted on the buoy. While making the star and the square patterns, the bathymetry was continuously recorded, but measurements of the total magnetic field were made at 20 second inter vals. Position readings were usually taken at intervals of less than .5 of a mile.* In addition to the two prin cipal patterns, a detailed pattern - F.ig. 8 - was done around the buoy; however, only bathymetric measurements were made on this run. The method of taking data was quite routine, a necessity for long periods of ship work. When one read a position aloud another recorded the range and azimuth. At the same time the magnetic and depth charts were marked. Then a data sheet was filled out giving the station number, time, range, azimuth, depth, and magnetic field. Altogether there were 741 stations in the star and square patterns and 160 in the small detailed run. *A mile is being defined as 6,000 feet, for it was in such units that the navigational system measured the distance to the buoy. KILOMETRES rrTTTTTTm i i i 10 12 5 19 CHAPTER IV BATHYMETRY IV-1 Hand Contoured Maps Looking at Figures 7 and 9, one sees the bathy metry of Bowie Seamount contoured at 100 fathom intervals*. First, one notes that the ship's tracks just cover the seamount. Second, one sees that the seamount is approxi mately elliptical around the center of the seamount. The major axis is along a southwest to a northeast direction. The southwest corner is closed but at the northeast end one notes that the ellipse breaks into a ridge. In the transi tion to the ridge there seems to be several subsidiary volcanic cones. In addition, one notes in the northwest corner a ridge perpendicular to the semi-major axis. By looking at Fig. 6 this ridge can be seen to lead to another seamount called Hodgkin. As for the depth to the abyssal plain, it appears to vary from 1600 to 1800 or 1900 fathoms as one travels from east to west. Fig. 10 is a detailed bathymetry map of the top of the seamount. The map is contoured in increasing in tervals of twenty fathoms starting from the 200th fathom contour. One notes that there are four areas in which there are 40 fathom contours. Thus one has a seamount whose altitude is of the order of 10,000 feet or roughly *Figures 7,8,9 and 10 were drawn by R. Herzer. O I1S"00'\\ WSO'yj IJS'SO' ^ US'10' \<f Figure 10 BOWIE SEAMOUNT TOPOGRAPHIC DETAIL OF SUMMIT AREA ABOVE 200 FATHOMS CONTOUR INTERVAL. - 20 FATHOMS 120 CONTOURS CONTROLLED BY SOUNDING LINES < 1/2 MILE APART — — CONTOURS CONTROLLED BY SOUNDING LINES 1-3 MILES APART SCALE NAUTICAL MILES ' I i I 1 1 1 -L-1/2 I 11/2 KILOMETRES crrrxnxrij r I 0 I 22 the size of Mt. Baker in the Northern Cascades of Washington. Among the important features of Fig. 10 are the two substantial terraces that are located between the 40th and the 60th and between the 120th and the 140th fathom contours. The shallower terrace could be explained by-wave action during the Pleistocene. It has been estimated that during the Pleistocene the level of the sea was lowered at times by more than 400 feet - Curry (1965). At present, there is no hint as to the age of the second terrace. However, due to the surfaces of sample rocks and to the underwater photographs both terraces appear to be wavecut. Another prominent feature in Fig. 10 is the very steep gradient- in the southeast corner which could well be a relatively recent lava flow. IV-2 Computer Contoured Map To contour the total magnetic field measurements it was useful to use a computer program - Soup - written by W. J. Coultard (1968). The program takes the x, y, and z coordinates of the raw data to be contoured and gene rates values at the intersections of a square grid; x and y are the position coordinates of the field data and z is the height that corresponds to that position, i.e. magnetic field, depth below sea level, etc. To calculate the grid the program divides the region surrounding an intersection, 23 grid point, into octants and the nearest field point in each octant is selected. However if there are no points in any three adjacent octants, it is then arbitrarily decided that there is insufficient data to calculate the grid point and its value is set equal to -1030. If there are no such adjacent octants, the value of the grid point becomes the weighted average of the selected points using a weighting function of 1/d2, where d is the distance from the nearest octal point to the grid point. The number of grid lines in the x-direction is chosen by the user. Along with this number the maximum and the minimum values of x- determine the size of each square. The number of grid lines in the y-direction is determined by the maximum and the minimum values of y and by the size of the square. On calculating the intersections, the program then yields a contour and a perspective plot of the calculated grid. For the perspective plot, the maximum value of the z component is arbitrarily set equal to .3 of the distance between the maximum and the minimum values of x . Unless otherwise specified the perspectives are drawn looking down at an angle of 45 degrees and rotated about 20 degrees. Fig. 11, a computer contour plot of the bathymetry of Bowie Seamount, illustrates the use of this program. Looking back at Fig. 9, one notes that, despite the fact that the contour interval of the computer plot is twice 24 Figure 11 Computer Contoured Bathymetry 25 the value of the hand contoured plot, there is good agree ment between Figures 9 and 11. Though some people may still prefer to use the hand contoured plot for bathymetry, it would seem quite adequate to use the computer's ver sion for geophysical data. For interest, the perspective plot of the bathymetry is reproduced in Fig. 12. 30 MILES 27 CHAPTER V PHOTOGRAPHS AND ROCK SAMPLES V-1 General Comments Looking at Fig. '13*, one notes that there were 4 underwater camera stations. Altogether there were about 500 well developed photographs. The success of the pho tography was due to the fact that a pinger was attached to the camera setup. A pinger is merely a device which sends accoustical signals in all directions. Hovering over the setup, the ship picks up the signals directed straight up and the ones reflected straight up from the ocean floor. The time delay between the two signals yields the height of the pinger and thus the camera setup above the topography. Rock samples were only taken by means of a dredge and a grab bucket, though a core was unsuccessfully attempted. On the 1968 cruise there were 3 dredge and 2 grab stations; while on the 1967 cruise there were 6 dredge and 7 grab stations. The distribution of the sampling stations can be seen in Fig. 13. A detailed analyses of the photographs and the rock samples will be presented by Herzer (1969). The gene ral characteristics of the rock samples are their basaltic *Fig. 13 was drawn by R. Herzer. DETAIL OF SAMPLE LOCATIONS ON SUMMIT 1 1 1 : I I 1 29 composition and their volcanic origin. The volcanic origin is indicated by flows in the photographs and by the macroscopic textures of the rock samples. Of a more specific nature, one finds that the rocks - fine grained, even glassy - become more vesicular as one approaches the summit of the seamount. This is to be expected as the hydrostatic pressure lessens with lessening depth, allow ing the escape of gases and thus leaving small cavities in the lava. An exception to this is a rock picked up at station 67-7. This rock, located at the southeast side of the 150 fathom contour, is crystalline and non vesicular leading to the possibility that this rock appeared on the surface after erosion occurred, i.e. the eruption took place underwater and this rock was subsequently exposed by erosion. This is inferred because in this way the sample would not have been quickly cooled, thus allowing crystals to form and the slow escape of gases leaving, it nonvesicular. In fact one might surmise that it is part of a dyke or perhaps even a volcanic vent. V-2 Remanence and Susceptibility V-2-1 Rock Magnetism In the presence of a magnetic field, i.e. the earth's magnetic field, a rock becomes magnetized by induc tion. The strength of the induced magnetization of the rock depends on the susceptibility of that rock. When the external field is removed most of the magnetization 30 disappears. Besides the induced magnetization of the rock, there is its remanent magnetization designated by NRM, natural remanent magnetism. NRM is often called fossil magnetism, for it was acquired in its past. There are three primary ways of achieving NRM. First, TRM, thermoremanent magnetism, is obtained by cooling rock material from its Curie Point in the presence of a mag netic field, i.e. the earth's magnetic field. TRM is the primary source of NRM in igneous rocks. Second, CRM, chemical remanent magnetism, is obtained by chemical processes, again in the presence of the earth's magnetic field. Finally, DRM, depositional remanent magnetism, occurs in sedimentary rocks by the orientation of small grains of magnetic material. For all three cases, the direction of NRM is;dependent on the direction of the earth's field at the time of the magnetization of the rock. If the earth's magnetic poles were reversed at the time of the rock's magnetization, the rock's NRM will be in the opposite direction to the rock's induced mag netization. Theoretical methods have been published on how a rock could have a reversal in its magnetization independent of changes in the earth's main magnetic field. However these self-reversals.are rare in nature -Irving (1965). 31 V-2-2 Magnetic Properties of Samples . Among the many rock samples obtained from Bowie Seamount a few were large enough and sturdy enough so that cores could be taken. Thus 7 samples were sent to Ottawa to the Division of Geomagnetism of the Department of Energy, Mines, and Resources. The susceptibilities of three of the samples and the NRM of all 7 were made by E. Irving of that group and by John Steele of the Geophysics Depart ment of the University of British Columbia. Unfortunately, the orientations of the samples were not known; thus the direction of the remanent magnetism could not be indicated. The results of these measurements are made in Table III. Besides taking the normal average and standard deviation, the logs of the measured values were taken, averaged, and then the antilog was found. This was done in compliance with a paper by Irving (1966) in which he felt that such a statistic should be given in all future work stating susceptibilities and remanences. This feeling arose from the fact that Irving found that for the Torridonian Sand stones histograms of the number of samples versus the log!o of the intensity of magnetization shows a good fit to the logarithmic normal • function. In calculating these statistics, the value of the sample from Hodgkin Seamount has been deleted. The results are in Table IV. Consider ing the changes in order of magnitude of remanence in differing basalts, the two types of mean for the remanence TABLE III Magnetic Samples Sample Station 14 8 14 7 Year 1967 1967 1968 1968 1968 1968 1968 *error in susceptibility measurements **error in remanence measurements ±51 General Description Diabasic basalt dyke rock, Approx. 150 fathoms, Southside Vesicular block, Central peak near 45 fathom terrace, Northside Finely vesicular basalt block, Approx. 500 fathoms, Northeast flank Vesicular flow, Approx. 500 fathoms, Northeast Flank Vesicular basalt block, Hodgkins Seamount, Approx. 450 fathoms Location same as 67-14 (at buoy) Pillow fragments, Southwest side seamount, Approx ±5% 800 fathoms Susceptibility* (c.g.s.) 1,414 x 10"6 1,263 x 10"6 95.3 x 10"6 Remanence** (c.g.s.) 7.74 x 10"3 1.63 x IO"3 12.15 x 10"3 5.0 x 10"3 .62 x 10'3 6.98 x IO"3 11.0 xlO"3 TABLE IV Averages of Samples (CGS Units) mean standard deviation Susceptibility Remanence 1339 x 10"6 8100 x 10"6 106.8 x 10"G 14.4 x IO"3 antilog of mean log. 1336 x 10"6 6860 x 10"6 33 are quite similar. According to Irving, "It is perfectly clear that whatever anomaly there is over Bowie Seamount is due to the remanence". 34 CHAPTER VI ORIGINAL MAGNETIC DATA VI-1 Data Recording When the magnetic survey was in operation, there were 741 position stations. At the time of these stations, the analog recorder was given a hand time mark. There was also a digital recorder which similarly recorded measurements at twenty second intervals. However, the oscillator controlling the instrument's clock was inac curate, giving 61 seconds to the minute. Actually, the crude test done on cruise indicated that it was slightly variable;' however, the tests were not accurate enough to give an estimate other than 61 seconds to a minute. At erratic intervals the personnel would correct the instrument's clock to the ship's time but without record ing their deed. Further, in order to get magnetic read ings synchronous to bathymetric ones, hand time marks were put on the analog and the digital outputs and on the depth recordings at each time a position was recorded. For some reason the time marks were more shoddily put on the digital output than on the analog, introducing a larger error in the digital output as to which recorded value actually corresponded to a time mark. Consequently, it was decided to use the analog recording in analyzing the magnetic data. Though the manufacturer claims an accuracy of one gamma on the digital and its corresponding 35 visual output, the analog seemed to agree with the visual output to 3 gammas. The analog data was hand digitized, and then the station number, time, range, azimuth, depth, and the magnetic field was punched on computer cards. For the sake of convenience the cards were repunched converting the originally recorded hours and minutes to hours and using x, y coordinates where the origin is at the buoy and where the positive x and y axii are respectively pointing to the geographical East and North. Along with the original analog and digital output, a permanent deck of the above is to be placed at the disposal of the Department of Geophysics of the University of British Columbia. VI-2 Diurnal Corrections Due to atmospheric effects, the magnetic field at a given spot will vary. If possible this variation should be removed from the observed data. Ideally, one would want the total field being recorded by a magneto meter at some fixed point in the region of interest, i.e. on top of the buoy. This would not only remove the daily or diurnal variation but also the local magnetic varia tion. However, there was no such recording and to remove the magnetic variation, magnetograms from May 27th to 36 June 5th were obtained from stations of the Dominion Observatory at Victoria, British Columbia and Meanook, Alberta. From Table V one can calculate that the dis tance from Bowie Seamount to Victoria is about 720 miles and to Meanook is about 880 miles. With such distances one could not directly use the magnetograms from these stations to correct for local magnetic variations. Thus it was decided to take the records from two or three of the quietest days in this period, average their readings, and then use the averages to correct for daily variation. The daily variation is global and is primarily dependent on geomagnetic latitude. In calculating the diurnal, the records were hand digitized at half hour intervals. To correct for the diurnal two time corrections were needed to be made on the averaged variations. First was the trivial correction of converting the universal time to the ship's time. Second was the correction re sulting from the daily variation at a constant geomagnetic latitude travelling westward with time, due to the fact that the daily variation is primarily solar in origin and that the earth rotates. Then a program was written which subtracted from the original data the total diurnal varia tion using the time corrected data from Meanook, from Victoria, and from combining the data from Meanook and Victoria by means of a linear interpolation with respect 37 TABLE V Magnetic Stations Ge'ographical Geomagnetic Station Latitude Longitude Latitude Longitude Bowie 53.30°N 135.68°W 56.4°N 276.8°E Seamount Victoria 48. 52°N 123.41°W 54.3°N 292 . 7°E Meanook 54.62°N 113.33°W 61.8°N 301.0°E I 38 to geomagnetic latitude. Then with the approximately 60 cross-over points in the ship's pattern, the program calculated the discrepancy in the magnetic measurements by taking the value due to the star pattern minus the value due to the spiralling square. The program then found the square root of the average square of the dis crepancy. This was done for the original data and for the original data corrected with the data from Victoria, from Meanook, and from the linear interpolation between the two; respectively, the results were 45.3, 44.5, 42.0, and 43.5 gammas. A surprise is that the data corrected from Meanook gave the smallest value but Victoria is much closer to Bowie Seamount in geomagnetic latitude than Meanook. However, the results are not significant and the original data was used for the analysis of the total magnetic field measurements. VI-3 Contour of the Original Data Entering the original data into the contouring program mentioned in Chapter IV, one gets the contour map shown in Fig. 14 and the perspective in Fig. 15. Similar to the topography, there is a strong SW to NE lineation and there is also a less distinct trend from the buoy towards the NW. There is a relative high around the origin and it extends to an extensive region in the East; however, there is a deep low to the immediate SW of this 39 40 Figure 15 Perspective of TOTAL MAGNETIC FIELD OF BOWIE SEAMOUNT /v 41 high. On the whole the total magnetic field plot does not show as much symmetry as the topography. 42 CHAPTER VII OBSERVED ANOMALY Except for the run to the seamount, the magnetic field measurements did not really extend beyond the topo graphical limits of the seamount. Therefore the regional magnetic field, i.e. the total observed field minus the effects of the seamount, could not realistically be deter mined by the data obtained on the cruise. To determine the regional, data was taken from an air-borne survey done by the Dominion Observatory in 1958. The plane's tracks are shown in Fig. 16, where the area covered by the data used to determine the regional is marked off by x's and the actual data is given in Table VI. To calculate the regional a method of least squares was applied to the data. In doing so the regional was assumed to take the form of a plane, i.e. z = ax + by + c where x and y are expressed in miles and are respectively positive in an east and north direction from the buoy and where z is in gammas. Converting the positions of the data into x^ , y^ coordinates with respect to the buoy, setting f(a,b,c) equal to (ax^ + by^ + c - z^)2 , taking the three partial derivatives of f(a,b,c) , equating the derivatives to zero, and finally solving the resulting equations for a h b , and c ; one gets z = ((5.8x + 6.2y)/miles + 56105) gammas . Upon substitution of a=3.8 , b=6.2 , and c=56105 into the following TABLE VI Flight Station Latitude (i) 25 1 52°37'N 2 52°40'N 3 52043'N 4 52°46'N. 5 52°51'N 6 53°07'N 7 53°10'N 8 53°11'N 6 9 55°00'N 10 54°48'N 11 54°36'N 12 54°25'N 13 54°13'N 14 54°01'N 15 53°50'N 16 53°38'N 17 53°27'N 18 53°16'N 19 53°05'N 20 52°53'N 21 52°42'N 22 52°32*N 2 3 52°20'N 24 52°09'N Regional Data Longitude Total Magnetic Field (gammas) 137°52,W 55440 137°37,W 55620 137°191W 55690 136°52'W 55770 136°20 *W 55860 135°10MV 56060 134°33MV 56230 134°04MV 56230 137°16'W 56520 137°08MV 56480 136°58'W 56370 136°48'W 56320 136°391W 56240 136°29'W 56260 136° 20'IV 56300 136°10'W 56120 136°02'W 56010 135°S3'W 56090 135°46'W 55990 135°37MV 55890 135o30'W 55830 135°22'W 55870 135°13MV 55850 135°05*W 55730 Magnetic Magnetic Declination Inclination 70°34 N 70°40 N 70°59 N 71°12 N 71°21 N ... 71°26 N 71°22 N 71°31 N 27°24 E 72°34 N 27°12 E 72°42- N 27°04 E 72°40 N 27°00 E 72°33 N 26°54 E 72°20 N 27°18 E 72°12 N 26°36 E 72°08 N 26°18 E 72°05 N 26°36 E 71°59 N 26°30 E 71°36 N 26°30 E 71°26 N 26°24 E 71°23 N 26°18 E 71°27 N 26°06 E 71°12 N 26°00 E 71°06 N 25°42 E 70°59 N 45 expression: i s£ ((axi + byi + c - z±)2)"2 hi 24 the average residual is found to be 42.36 gammas. The next step was to subtract the regional from the observed magnetic values, yielding an observed anomaly. In the substraction the original field points were used and not grid points derived from the earlier contour map. The contour of the observed anomaly is in Fig. 17. The main observation is that the magnetic field pattern has hot been drastically changed from that in Fig. 14. How ever,' one should note that Bowie Seamount has a basically negative anomaly. This implies that at least most of Bowie Seamount is reversely magnetized. <+7 CHAPTER VIII THREE DIMENSIONAL ANALYSIS VII I-1 Nature of Terrain Correction With the regional and thus the observed anomaly determined, one is ready to subtract the effect due to the terrain itself. Obviously, if one has chosen the regional correctly and one makes an exact correction due to the seamount, then the residual anomaly will be zero. There are several reasons why this will not happen. First, an approximation must be made of the seamount's shape. Second, if one does not assume a uniform intensity of magnetization -or at least uniform in discrete portions of the seamount, then the calculations-will be very dif ficult. Third, one probably does not know the suscepti bility and/or remanence of the terrain any better than a factor two^ much less its spatial distribution. In addi tion, if from a tremendous amount of samples one knew the surface distribution well, one still would not know how the magnetic properties varied with depth and one would not have information as to whether the terrain was com- • posed of rocks some of which were reversely magnetized while others were normally magnetized. However, one would hope to make some guesses about some of these unknowns on the basis of a few simple models. For this study the simple models were made using a three dimensional analysis 48 program written by Talwani (1965). VIII-2 Theory and Method of Talwani's Program The following has been abstracted from "Compu tation with the Help of a Digital Computer of Magnetic Anomalies Caused by Bodies of Arbitrary Shape" by Talwani (1965). Let us look at Q , a topographical feature, in Fig. 18. For the volume element AxAyAz the scalar magnetic potential - B = -vV - is given in cgs units by* (1) V = HI* ' Pv3 where u is the magnetic moment of the volume element and R is the indicated distance vector. Let J , the intensity of magnetization of Q , be uniform, making u equal to JAxAyAz and thus yielding V to be given by J 'X + J «y + J «z (2) V = — 1 - AxAyAz R3 where Jx , J , and are the components of J What one could call the components of the magnetic field anomaly due to Q is *Reitz, J. and Frederick J. Milford, Foundation of Elec tromagnetic Theory, 1962. 49 Figure 18 Coordinate System and Topographical Feature Q 50 4Y --fffw dxdyd:  (3> 12 = -Jff IT dxdyd: Substituting equation (2) into equations (3) yields AX = J V] + J V? + J V, x 1 y 1 z 6 AY = J V, + J V„ + J V5 x z y 4 z 5 (4) AZ = JxV3 + JyV5 + JzV6 where VL = / / / 5x2 ^ R2 dxdydz V2 = / / / lxy_ dxdydz V3 = / / / dxdydz V„ = / / / 5y2 ~ R2 dxdydz Vs ill ^ ^ (5) V6 = / / / 3z2s' R2 dxdydz 51 To evaluate Vj thru V6 , a double integration was done analytically in the xy plane and a numerical integration in the z direction. of Vi through V6 . Q which produces the anomaly is cut up into a discrete number of depth contours, see Fig. 19. A given depth contour is approximated by a polygon, i.e. KLMNTDFK. The anomaly is calculated at the point P(0,0,0) and P(0,0,z) is the point in Q that lies in the plane KLMNTDFK. Then the surface part of the integrals are obtained by analytically integrating equation (5) at the given depth, the value of z of the polygon, yielding The following is a discussion of the evaluation 3x2-R2 dxdy - - ^ ( gjyi+i-z2tanei i+i 3xydxdy R5 52 P(0,0,OV TOP M L TOPOGRAPHICAL FEATURE Q K(xi,yi,z) z BOTTOM K(xi>yi,z) L (xi+i>yi+i«z) -Si / -^y •/v / / 7 / Figure 19 Contour at Depth z and Its Polygon Representation (after Talwani, 1965) . 53 s _ f /"3xzdxdy 3 ./.; R5 ^~ z2cos26^ f yj+isecfe-i +gjtan6j i z2+P,2 V R. , 1+1 yjsec2ej-«-gitan8i R^ (3y2-R?dxdy = sin26-j / cjxj+i - z 2cot 6 j i +1 JJ R5 i z2 + P,2 V . R• 51 ~± ( i z2+Pi2 v cixi~ z2cote j~ R. l =rriZi dxdy = X Zsin2ei / xi+icosec2ei+ciCote: JJ R5 i z2 + Pi2 1 R. 1 + 1 x-cosec28i+cicot6i R. l R5 i z2+P-2 \ R.X1 I X 1+1 rjCOS gj Ri where e. , ^ , y. , r± , r.+1 , R. , R.+]> , P^ , , and are all defined in Fig. 19, The. trigonometric functions and the lengths - c^, P^, g^, r^, 54 ri + l' Ri' an(1 Ri + 1 " can a11 ^e exPressec* ^n terms x^, y^-K- and Y^+i'^- The above integrals are evaluated for all the polygons. To find , S^dz is integrated numerically where the limits, zt0p an& zDottom' are defined as in Fig. 19. The integration is accomplished by fitting parabolas to successive points, values of Shifting the origin successively to the other field points and referencing the corners of the polygon to those points, one obtains corresponding values for V, thru V6 .If the values of J , J , and J i b x ' y ' z are known, then the values of AX , AY , and AZ are attained by substituting the values of Vi thru V5 in equation (4). Assuming the anomaly to be small compared to the undisturbed field of the earth then the total field can be assumed to be in the same direction as the undis turbed field. Therefore AT , the total anomaly, is given by the sum of the projections of AX , AY , and AZ in the direction of the undisturbed field. Thus the fol lowing is obtained (7) AT = AXcosDcosI + AysinDcosI + AZsinI where D is the declination and I the inclination of the earth's magnetic field. To get AT in terms of J , J , and J , substitute equation (4) into equation (7): y z 55 AT = J (VjeosDeosI + V2sinDcosI + V3sinl) + (V2cosDcosI + VssinDcosI + V5sinl) (8) + Jz(V3cosDcosI + V5sinDcosI + Vesinl) From equation (8) one notes that the coefficients of J J , and J depend only on the geometry of the topo-graphical feature and the direction of the earth's mag netic field. If the coefficients are determined for a given feature and for a given direction of the earth's magnetic field, then the total intensity anomaly for different magnitudes and directions of the total magneti zation vector is obtained by multiplication. Conversely, if the total intensity anomaly is known from a survey, one can obtain a least squares solution of J^., J , and Jz, and hence determine a best fit model using the least squares solution of J VIII-3 Applications of the Model In applying Talwani's program*, two systems of polygons were used: See Figures 20 and 21. Table VII gives the results of the model studies. In the Table I, *A Fortran II version of the program was obtained from Talwani. Contoured in tenths of a mi (x 103 to get fathoms) * MILES gurc 20 Polygon System A Figure 21 Polygon System B TABLE VII MODEL RESULTS Model No. Polygon System Fig Fig. 20 Fig. 21 no 50 fathom contour Model Characteristics Mean Difference (gammas) B* Fig. 20 Fig. 21 no 50 fathom contour Fig. 20 Fig. 21 no 50 fathom contour Fig. 21 Anomaly due to induc tion only Anomaly due to induc tion only Anomaly due to rema nence plus induction, j* assumed Anomaly due to rema nence plus induction, j* assumed Anomaly due to rema nence plus^induction (best fit J) Anomaly due to rema nence plus induction (best fit J) Anomaly due to rema nence plus induction (best fit J) 314. 2 313. 2 605 597. 5 302.7 304 . 4 304. 7 26.5°W 71.5°N 26.5°W 71.5°N 26.5°W 71.5°N 71.5 26.5°W 71.S°N 71.5 26.5°W 71.5°N -47.7 26.5°W 71.5°N -33.4 26.5°W 71.5°N -24.6 180.0 180.0 2.8 41. 5 44 . 6 *A is positive if measured down from the horizontal to the intensity of magnetization vector, J. **B is positive if measured clockwise from North to the horizontal projection of the magnetization vector. 59 D, A, and B are respectively the earth's magnetic inclina tion - at Bowie Seamount, the earth's magnetic declination, -a the inclination of J - the intensity of magnetization due _^ jk to remanence and induction, and the declination of J. J is either assumed or the calculated least'squares value. The term mean difference is merely the mean absolute dif ference between the observed and the calculated anomalies. It is readily apparent that none of the models have a low mean difference; the best fit models give values approximately one third the value of the total magnetic anomaly shown in Fig. 17. This was to be expected for the assumed uniform intensity of magnetization did not conform to the asymmetric observed anomaly. Thus the model studies imply that Bowie Seamount is not uniformly mag netized. A contour plot of a best fit calculated anomaly, . Model 7, is shown in Fig. 22. The calculated magnetic anomaly reflects the approximate symmetry of the topography and is' in contrast with the observed anomaly. This dif ference is directly related to the assumption of uniform magnetization. Fig. 23 is the contour plot of the observed minus the calculated anomaly, i.e. the residual anomaly. As one sees it does not significantly change the observed anomaly. 60 62 In contrast to my results the reader may be interested in an application of Talwani's program that gave a closer fit. Schimke and Bufe (1968) were able to get models for the Chautauqua Seamount (22°11'N, 162°54.5'W) that had mean differences of the order of 25 gammas. This implied that the Chautauqua Seamount can be usefully represented by a model assuming uniform intensity of magnetization. \ 63 CHAPTER IX CONCLUSIONS It follows from Chapter VIII that the data of the total magnetic field yields only one strong statement: If the regional has been calculated to a sufficient degree - a plane - Bowie Seamount cannot usefully be represented by a model assuming a uniform intensity of magnetization. There are at least two clear possibilities. First, the oscillations - Fig. 5 - of the magnetic field observed on approaching the seamount have their origin located under the abyssal plain. Consequently this might be the cause of the asymmetry of the magnetic field anomaly. Never theless, from Fig. 5 one would predict that Bowie Seamount should have a positive magnetic field anomaly and yet one observes a basically negative anomaly. This implies that the seamount is primarily reversely magnetized. Second, if one assumes that the calculated regional is reasonable, topographical fluctuations cause a good por tion of the oscillations seen in Fig. 5, then Bowie Seamount must have a very non-uniform intensity of magnetization. Let us look at some of the major pieces of evi dences for saying that part of the.seamount has resulted from a flow of normally magnetized lava. First, most of the seamount is reversely magnetized. Second, Figures 17 and 23 illustrate a sizeable yet rather local positive anomaly in the central and the eastern part of the seamount. 64 Third, on the basis of the detailed topographical map -Fig. 10 - it was stated in Chapter IV that there might have been a recent lava flow in the region of the posi tive magnetic anomaly. Fourth, a possible feeder dyke in the area of the positive anomaly was indicated in Chapter V on the basis of a non-vesicular sample, 68-7. Fifth, the age of a rock sample at station 67-14 (Fig. 13), located in the vicinity of the positive anomaly, was found to be .075 million years (+0.1 or -0.075 million years)*. This implies (see Fig. 24 which gives the time scale for geomagnetic reversals - Cox (1969)) that the rock is normally magnetized. The conclusion of the author is that the sea mount was primarily formed during a time when the earth's magnetic field was reversed, but then there was a lava flow in the region where there is a positive anomaly at a time when the earth's magnetic field was normally mag netized. Let us assume that the seamount formed in one normal and one reversed period of the earth's magnetic field. Further if one chooses a simple model of a sea mount forming continuously, then Bowie Seamount was formed between .89 million years ago and the present**. *Harakal, J. (1969), K-Ar methods used for dating. **See Fig. 24. The Laschamp event has been ignored in the conclusion because it is short-lived and is not well documented. In fact, it is solely based on the reversely magnetized lavas of the Laschamp Volcanic Cone in France. 65 4.38 >-too CC H < 2 _J UJ O > CL UJ LASCHAMP EVENT JARAMILLO "EVENT G1LSA 'EVENT \OLDUVAI / EVENTS KAENA /EVENT MAMMOTH EVENT C0CH1TI "EVENT NUNIVAK "EVENT h-cc x < o _l o O CL 0_ LU cc o x z o o c_ oo LU UJ or < Q LU 00 cr LU > .LU cc < < >-< o o CL LU < CC o x 2 O „ o 00 Q_ § LU < Q LU 00 cc LU > LU -r-K O i_ o cr LU CO _l o LU Figure 24 Magnetic Reversals (from Cox 1969) 66 BIBLIOGRAPHY Bassinger, B.G. , Marine Magnetic Study in the Northeast Chukchi Sea, J. Geophys. Res., 7_3, 683-687 , 1968. Blackett, P.M.S., Lectures on Rock Magnetism, Jerusalem Weizmann Science Fress of Israel, 1956. Budinger, Thomas F. and Betty J. Enbysk, Department of Oceanography, University of Washington, Tech nical Report No. 60, Cobb Seamount, A Deep-Sea Feature Off the Washington Coast, Office of Naval Research Ref. 59-6, 1960. Bullard, E.C., The Removal of Trend from Magnetic Surveys, Earth Planet. Sci. Lett., 2, No. 4, 293-300, 1967. Coulthard, W.J., XPAND, A Preliminary Write Up, University of British Columbia, Computing Center, 1968. Cox, Allan, Geomagnetic Reversals, Science, 163, 237-245, 1969. Curray, Joseph R., Late Quaternary History, Continental Shelves of the United States, The Quaternary of the United States, Princeton University Press, 7T3^7T5, 1965. Doell, Richard R. and G. Brent Dalrymple, Geomagnetic Polarity Epochs: A New Polarity Event and the Age of the Brunches-Matuyama Boundary, Science, 152, 1060-1061, 1966. Doell, Richard R., G. Brent Dalrymple, and Allan Cox, Geomagnetic Polarity Epochs: Sierra Nevada Data 3, J. Geophys. Res., 71, No. 2, 531-541, 1966. Gay Jr., S. Parker, Standard Curves for Magnetic Anomalies over Long Horizontal Cylinders, Geophysics, XXX, No. 5, 818-828, 1965. Grossling, Bernado F., The Internal Magnetization of Sea-mounts and Its Computer Calculation, Geological Survey, Professional Paper 554-F, United States Government Printing Office, 1967. Harakal, J., Personal communication, Department of Geology, University of British Columbia, 1969. 67 •Herzer, R. Unpublished M.Sc. thesis, Department of Geology, University of British Columbia, 1969. Irving, E., Paleomagnetism and Its Applications to Geo logical and Geophysical Problems, John IViley and Sons, 1964. Irving, E. , L. Molyneux, and S.K. Runcorn, The Analysis of Remnent Intensities and Susceptibilities of Rocks, Geophys. J. R. astr. Soc, lfJ, 451-464, 1966. Irving, E., Measurement of Polarity in Oceanic Basalt, Can. J. Earth Sci., 5, No. 5, 1319-1321, 1968. Nagata, Takesi, Rock Magnetism, Maruzen Company Ltd., 1961. Prindle, Robert 0., Ship-Towed Magnetics in Petroleum Exploration, Quantum Electronics Division, Varian Associates, 1967. Raff, Arthur D., Boundaries of an Area of Very Long Mag netic Anomalies in the Northeast Pacific, J. Geophys. Res., 71_, No. 10, 2631-2656 , 1966 . Raff, A.D. and R.G. Mason, Magnetic Survey Off the West Coast of North America 40°N Latitude to 52°N Latitude, Bull. Geol. Soc. Am., 72, 1267-1270, 1961. Reford, M.S., Airborne Magnetometer Survey for Petroleum Exploration, Canadian Aero-Service Limited, 1964. Reitz, John R. and Frederick J. Milford, Foundation of Electromagnetic Theory, Addison-Wesley Publishing Co. Inc., 1962. Richards, M.L., V. Vacquier, and G.D. Van Voorhis, Cal culation of the Magnetization of Uplifts from Combining Topographic and Magnetic Surveys, Geophysics, XXXII, No. 4 , 678-706 , 1967. Schimke, Gerald R. and Charles G. Bufe, Geophysical Description of a Pacific Ocean Seamount, J. Geophys. Res., 7_3, No. 2 , 559-569 , 1968. Talwani, Manik, Computation with the Help of a Digital Computer of Magnetic Anomalies Caused by Bodies of Arbitrary Shape, Geophysics, XXX, No. 5, 797-817, 1965. 68 Varian Associates, Quantum Electronics Division, Proton Magnetometer, V-4937 Data Sheet. Vine, F.J. and R.F. Macnob, A Program for Fitting the Magnetic Effect of a Three Dimensional Structure to an Observed Anomaly by Calculating the Re quired Magnetization Vector, BIO Computer Note 67-7-C, Bedford Institute of Oceanography. 


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