An E.M. method for earth resistivity measurements using power line harmonic fields.

Abstract

An experiment was carried out to infer ground conductivity by making use of magnetic fields created by a power line carrying electric currents as source signals for E. M. exploration. The experiment was done in an area, 10km×10km approximately, which is under an overwhelming influence of a major 3 phase a. c. power line. It runs generally east-west and south of the area. The magnetic field measurement showed that the 60Hz, 180Hz, 300Hz, and 420Hz signals were the four strongest harmonic components. A plot of the field measurements along a northsouth line, normalized to the value at a reference station, shows that the vertical magnetic field strength decreases inversely with the distance from the power line in the vicinity of the source and to the cubic inverse of the distance further from the source. The crossover distance (between the 1/r relation and 1/r3 relation) is dependent on frequency. Extension of an existing theory predicts this trend with the crossover distance √2 times greater than the skin depth, providing that the Earth is considered a uniform conductor. The effective resistivity is inferred to be of the order of 104Ωm from the crossover distance for 300Hz and 420Hz. No crossover is seen for 60Hz, and for 180Hz, the fall-off ∝ 1/r3 begins to take place near the edge of the survey area (-10km in distance from the power line). A deviation from the general trend occurs at the same location for all four frequencies. A change in geologic material in the subsurface is mapped in this locality.