Theoretical treatment of amplitude calibration problems in HEM data

Abstract

A recent increase in the environmental usage of Airborne EM has shown the need to accurately provide values of depth and conductivity. Calibration problems in helicopter EM data produce imprecise conductivity depth images (CDIs), maps and sections, which are essential requirements in order to target smaller, near?surface objectives, such as salinity outbreaks. To ensure agreement between ground?truth, such as conductivity logs and CDIs, data recalibration is often applied before processing. Ground?based methods, if available, have spatial limitations. An alternative statistical average method has been developed to provide theoretical consistency. In conductivity?independent aßdomain, the median response of a variety of expected synthetic models based on expected geology is calculated and compared with the median of the larger amplitude field data. The data are then rescaled in the data domain so that the recalibrated median response lies exactly on the theoretical curve. The amplitude rescaling was applied to an HEM dataset, collected in the Riverland area in South Australia. The results were compared using maps and CDI images of the raw and recalibrated data. The original delivered data produced CDI images that were generally inconsistent with borehole conductivity data. However, rescaling to ensure `thin?sheet' consistency has produced remarkable agreement between ground truth and the CDI sections.