A revised inversion model parameter formulation for fixed wing transmitter loop – towed bird receiver coil time-domain airborne electromagnetic data

Abstract

The conversion of data to conductivity for fixed wing transmitter loop ? towed bird receiver coil time-domain airborne electromagnetic (AEM) systems, such as TEMPEST, would ideally utilise complete knowledge of the system geometry and measurements for all three mutually perpendicular components of the received signal. In practice, not all of this information is available. We use a layered inversion that integrates TEMPEST survey data with a priori conductivity information from the survey area. Total (primary plus secondary) field data from both the X (horizontal in-line) and Z (vertical) components are used. Receiver coil pitch angle and transmitter loop to receiver coil horizontal and vertical separation parameters are included as unknowns in the inversion. Borehole conductivity data are used to build a reference conductivity model that acts as a constraint to stabilise the partitioning of the measured signal into primary field and ground response contributions. Smoothness constraints are applied to the conductivity values in the 1D model. The quality of the inversion output was assessed through comparison of the conductivity predictions with borehole conductivity values and shallow single-frequency ground EM measurements. This showed that the new formulation more accurately predicted conductivity than two previous sets of conductivity predictions.