Geophysical prospecting with collimated magnetotelluric fields

摘要

An electromagnetic survey system for geophysical exploration comprising a reconnaissance method and a survey line method. The reconnaissance method finds the principal electrical axis of conductivity of anomalous zones of conductivity by measurement of variations in the earth's magnetic field in a grid pattern over the survey area, calculation of Tipper vectors for each site, and plotting the vectors to scale. The conductivity axis is determined as being perpendicular to a set of the largest parallel vectors, one or more of which reverse direction, and passing between the direction reversal. A Survey line may be collimated with the set of enlarged vectors to make three dimensional effects more symmetrical so they are attenuated. In the survey line method the horizontal variations in the earth's magnetic field orthogonal to the survey line are measured by magnetic sensor(s) in the vicinity of and perpendicular to the line, and variations in the earth's electrical field parallel to the survey line are measured at a number of points along the line. Simultaneously, a collimating electric field measurement is taken by a dipole parallel to the survey line, preferably spanning the group of electrodes being measured, or a collimating magnetic field measurement is taken by a horizontal magnetic sensor(s) perpendicular to the survey line and remote from the noise environment of the group of dipoles being measured. Data from all measurements is converted to the frequency domain and stacked in a cross-power matrix. The impedance of each electric field measurement point along the survey line, for either collimating field measurement, is first calculated as a function of frequency as the ratio of two elements in the cross-power matrix. The numerator is the cross-power of the dipole electric field with the collimating field. The denominator is the cross-power of the survey line magnetic field with the collimating field. The impedance is next smoothed by the weighted average of adjacent dipole impedances in the calculation of the distribution of conductivity with depth. The method automatically attenuates noise in the impedance calculations, directly measures variations in the magnetic field perpendicular to the electric field for determining the impedance vs. depth under the survey line, attenuates three dimensional and near surface conductivity effects, and provides for the calculation of signal independent of noise. Data offsets are normalized.