Adaptive noise cancellation for time‐domain EM data

Abstract

A typical time‐domain electromagnetic (TDEM) survey is carried out by recording the transient response of the target area due to currents induced by the abrupt turnoff of the current in a large ungrounded loop at the surface. The depth of exploration is controlled by the strength of the transient signal at late times, when the natural fields become comparable in amplitude to the controlled‐source response (Spies, 1989). Nichols et al. (1988) claim that it should be possible to reduce the natural magnetic fields in controlled source electromagnetic data by 40–60 dB because the fields are coherent over large distances, thus reducing the needed transmitter power and averaging time dramatically. This study tests a method using adaptive filtering of remotely measured magnetic fields to improve the signal‐to‐noise ratio for the late‐time response.