An example of 3D conductivity mapping using the TEMPEST airborne electromagnetic system

Abstract

The TEMPEST airborne electromagnetic system is designed to measure the information that is required to derive accurate, high-resolution three-dimensional conductivity estimates of the subsurface. TEMPEST is configured with a transmitter loop located on a fixed-wing aircraft and receiver coils located in a towed bird. The system measures the EM response of the ground over a wide-bandwidth (25 – 37.5 kHz). The transmitter waveform is a square wave with 50% duty cycle, i.e. equal on and off times, and variable switching ramp. Very low noise levels are achieved by recording the received signal at a high sampling rate, 75 kHz, then applying sophisticated signal processing techniques. The signal processing operates in the frequency domain to perform a full deconvolution of the measured response, removing the system transfer function characteristics and dynamically compensating for variations in the transmitted waveform. The broad bandwidth allows the variable primary field effects that result from changes in coupling between the receiver coils in the towed bird and the transmitter loop to be more accurately removed by reducing the uncertainty in the ground response. To assist interpretation, the deconvolved ground response signal is converted to a 100% duty cycle square-wave B-field response, allowing a single transient decay to be presented for the full 20 ms half cycle length available for a base frequency of 25 Hz. The system geometry can be accurately monitored. This involves measuring the orientation of both the transmitter loop located on the aircraft and the receiver coils located inside the towed bird. These orientation measurements are used to compensate the ground response data for the effects that result from variations in the system geometry as the aircraft flies along the survey line. A 3D conductivity grid is produced by combining a series of 1D inversions. In addition to standard conductivity sections, the contents of these data volumes can be displayed as conductivity slices and iso-conductivity surfaces. Data from Walford Creek, Queensland, define conductivity distributions in close agreement with ground EM and drilling information.