Method using a d.c. source for determining resistivity distribution of the earth as a function of depth

Abstract

An electromagnetic survey method for geophysical exploration, in which the variations in the earth's magnetic field are measured in two, non-parallel directions at one point in the survey area. Simultaneously, the variations in the earth's electrical field parallel to the survey line are measured at a number of points along the survey line. These measured variations are transformed to the frequency domain, and then the horizontal component of the magnetic field orthogonal to the direction of the measured electrical field is calculated. The impedance at each measurement point on the survey line is calculated as a function of frequency, and weighted averages of the impedance for predetermined frequencies using a zero phase length weight function corresponding to a low pass filter applied to the electric field are used to calculate the subsurface conductivity distribution.