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Power transmission harmonic current and its use in geophysical exploration

University of British Columbia

Commercial electrical power transmission lines can be used as an economical reconnaissance method of geophysical exploration. The advantages of the technique include the remote source aspect of the method, plus the strong, multi-frequency signal that arises from the existing current in the transmission line. The main limitation is that the area must be isolated from extraneous sources of commercial 60 Hz signals. However, the proposed technique would be suitable for developing areas, in which a power transmission line is commonly built through an isolated region well before urbanization of the area begins. A 500 KV B.C. Hydro transmission line extending in an East-West direction from Nicola to loco, B.C. (near Vancouver) was utilized in this thesis to determine the viability of the proposed method of electromagnetic exploration. Access to monitor the transmission line harmonic current was made through Meridian substation, near loco, B.C. and yielded quantitative information on harmonic current strength. In addition, the correspondence between natural geomagnetic field activity and quasi-DC geomagnetically induced current (GIC) in the transmission line was determined. A theoretical discussion of the relation between GIC and harmonic current generation is given, as well as illustrative data showing the effect of GIC on harmonic currents. An extensive EM survey was undertaken in an isolated region situated to the north of the Nicola-loco transmission line. The magnitude of the total vertical magnetic field component, at four source frequencies (60 Hz, 180 Hz, 300 Hz, and 420 Hz) was measured at 66 stations within a 120 km2 area. The results of the experimental survey were qualitatively interpreted in terms of a uniform conducting earth model. One aspect of the model is that the vertical ambient field component exhibits an inverse relation with distance near the source, and begins to fall off as 1/r3 at greater distances from the source, the distance of this transition being dependant on source frequency and half-space conductivity. The data supports this model, and some theoretical calculations on the transition distance are given. In addition, different geologic regions were found to correlate with features of the data, indicating conductivity contrasts associated with the geologic regions. Thus the proposed method of electromagnetic exploration is shown, by the results presented in this thesis, to be a viable and practicable technique for mapping geologic conductivity contrasts, and could be a successful, inexpensive method of reconnaisance exploration for orebodies.

Text

2010-04-19

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http://hdl.handle.net/2429/23864

Geophysics

University of British Columbia

Graduate