Development of grounded electrical source airborne transient EM (GREATEM)

Abstract

We describe a new airborne EM system which uses a grounded electrical source and an airborne magnetic field receiver to increase the depth of investigation. Using a grounded source, we can apply a large source moment with a long transmitter-receiver distance, thus we expect to obtain a greater depth of investigation. Although the area of survey is limited with this type of method, it has advantages over conventional AEM such as a reduced sensitivity to flight level. To fully realise the method, we have worked to overcome two main problems: monitoring the motion of the receiver in the geomagnetic field, and cancelling the natural magnetic field variation and cultural noise without stacking in the time domain. To minimise motion noise, we utilised high-accuracy fibre optic gyros to monitor the pitch and roll of the magnetic sensor in the bird. We tested the method at a geothermal field in Beppu area in southwest Japan, and obtained a clear transient response of the vertical magnetic field and its time derivative over several kilometres distance from the source at around 300 m flight height. We expect that this method will be a successful approach for deep exploration targets.