Electromagnetics for Geologic Structure (M-4)
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Q: How does it work?
A: A strong alternating magnetic field on the surface of the earth induces eddy currents in earth materials. The terrain conductivity (analogous to apparent resistivity), an average conductivity for nearby geologic materials, is measured.
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Q: What is the geologic model?
A: Earth resistivity (inverse conductivity) is effected by (in approximate order of importance) the amount of pore water (including fractures) total dissolved solids in the pore water, resistivity of rock matrix, amount of clay minerals present, shape and connectivity of pores, and temperature.
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Q: What are the requirements?
A: Lateral variation in conductivity (Frequency domain electromagnetic methods, FDEM) or vertical variation in conductivity (Time domain electromagnetic methods, TDEM) which correlates with geologic structure or lithology.
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Q: What are the pitfalls?
A: Low interference: Stay Away from:
grounded fences or utilities
power lines or electric fences
tanks, used car lots, or curbs with rebar
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Q: What logistics are needed?
A: Crew size is usually one or two persons-the specific instrument determines the number. However, another person can often help with gridding or other tasks which make the crew more efficient. A grassy field is ideal but thick brush and steep slopes simply require more effort.
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Q: What are the deliverables?
A: Plan maps of station locations, profiles for two dimensional investigations, profiles of soundings for one-dimensional soundings, inversions of the data are done but the ratio of observations to parameters is usually low.
