EARTH CONDUCTIVITY DETERMINATIONS EMPLOYING A SINGLE SUPERCONDUCTING COIL

Abstract

A low‐frequency airborne electromagnetic prospecting method has been developed which exploits the inherent low resistance of a superconducting coil. Changes in the input resistance of this coil are monitored in the presence of the conducting earth. The response of the system, the change in the input resistance, is proportional to the quadrature secondary magnetic field at the transmitter, although unlike two‐coil systems, the response does not decrease with increasing frequency. This research has demonstrated that superconducting wires, large scale nonmetallic cryostats, the requisite measurement circuitry, and an appropriate data acquisition system are realizable in a practical flight configuration. The unicoil presents the following significant advantages in electromagnetic prospecting: 1) The measurement sensitivity is not limited by the relative coil motion experienced by two‐coil systems. 2) Ample field strength may be supplied to override ambient noise. 3) Optimum frequencies for specific geologic sections are easily implemented in the range of 10 to 2000 Hz. 4) Maps of ground conductivity may be obtained because precise thermal stability is maintained and the measurement, therefore, is absolute. 5) The point source observation minimizes analytic complexity. 6) The combination of the foregoing features with multiple frequency operation, yields a system of potentially high sensitivity and unprecedented depth of exploration. The unicoil system also possesses some disadvantages: 1) An operational complexity results from the cryogenic procedures required in the field, and 2) the heavy sensor requires a large helicopter.