On the airborne transient EM response of a magnetic anomaly

Abstract

Occasionally, airborne magnetic anomalies can have amplitudes as large as 70 000 nT and, for shallow dyke-like bodies, it is possible for the amplitude to vary by more than 70 000 nT over a distance of about 200 m. In such cases, the horizontal spatial gradients can be as large as 700 nT/m. Moving an induction coil sensor through a zone where the spatial gradient is large will, by Faraday’s law, result in a voltage being induced in the coil. Because airborne transient electromagnetic (ATEM) systems normally use induction coils, it is possible for voltages as large as a few volts to be induced when a system crosses a very large magnetic gradient. For a sensor height of 70 m, the wavelength of this type of voltage anomaly is typically about 240 m, which corresponds to a frequency of 0.25 Hz for a flying speed of 60 ms-1. The stacking algorithms employed in ATEM systems are specifically designed to reject signals with frequencies this low.In order to assess the efficacy of ATEM stacking algorithms, the response of a thin shallow highly magnetic body has been simulated and processed using the signal processing algorithms implemented in the GEOTEM ATEM system. The stacking algorithm essentially removes the low frequency signal. The minuscule residual signal that is not removed is at least an order of magnitude smaller than the current noise levels. Hence, flying an EM system through a large horizontal spatial gradient will not have an observable effect on the stacked EM response that is measured by the GEOTEM system.