The multicoil II airborne electromagnetic system

Abstract

A helicopter‐towed electromagnetic (EM) system has been developed with two orthogonal transmitter coils. Units currently in service have either two or three orthogonal receiver coils. The associated software yields in‐phase and quadrature channels which are generally free of the conductive response of overburden and of the magnetic induction response of magnetite. These geologic noise sources can mask the response of bedrock conductors for all previously developed airborne EM (AEM) systems. The new technique involves energizing conductors with two orthogonal transmitter coils, both operating at approximately the same frequency (e.g., 900 Hz). The subtracting of the secondary field components of one maximum‐coupled coil pair from the other yields in‐phase and quadrature difference channels. These channels may contain as much as an order‐of‐magnitude increase in the signal/noise (S/N) ratio for bedrock conductors in a geologically noisy environment. The new system can also indicate whether a steeply dipping conductor is thin (e.g., width less than 3 m) or thick (greater than 10 m). The thickness parameter provides a new criterion for conductor sorting, complementing the usual parameters of conductance and strike‐length. The geophysical data are digitally recorded, and the profile records and maps are plotted by computer. The traces include the basic recorded data, the EM difference channels, a channel of resistivity, and a channel of conductance. The conductance channel essentially is an automatic anomaly selector calibrated in mhos. The usual application of the system is for metallic mineral prospecting. However, the system has also been used for sand and gravel detection using two well separated frequencies. For such applications, apparent resistivities are computed for each frequency and are displayed as channels on the profile record and as contour maps.