Test-site calibration and validation of airborne and ground-based TEM systems

Validation and calibration of airborne and ground-based time-domain electromagnetic (TEM) systems are important to obtain high-quality data and thereby reliable and reproducible results. Validation and calibration become even more important when the TEM systems are to be used for low-anomaly groundwater and environmental applications. The recent extension of the Danish TEM test site has made it possible to perform detailed validation of airborne TEM systems and calibrate airborne TEM systems that cannot make hovering measurements. We evaluated the Danish TEM test site and used a test site calibration scheme recommended for ground-based as well as airborne TEM systems. Furthermore, we discovered an extended data set from the airborne TEM system, SkyTEM, from the test site, used for an extensive validation of the SkyTEM system. This validation included repeatability tests at different heights and comparisons with the reference sections obtained with ground-based measurements. The validation and comparison were performed directly on the inversion results and in data space, down to the single data gate values. The extensive validation of the SkyTEM system at the TEM test site revealed a very stable and reliable system. The data repeatability of the SkyTEM system at different heights and directions was, in general, well within the standard deviation (STD) of the data. The agreement between the ground-based reference model sections and the SkyTEM model section from different recording heights was very good. Likewise, the match between the ground-based reference data and the SkyTEM data was good and, in general, within 1.5 times the STD on the data. The positive outcome of the extensive validation also confirmed that data processing and modeling were performed at the highest standard.