Validating land-based FDEM data and derived conductivity maps: Assessment of signal calibration, signal attenuation and the impact of heterogeneity

Abstract

Frequency domain electromagnetic (FDEM) responses can be related to electrical conductivity (EC) and magnetic susceptibility of the subsurface. Strides in FDEM instrumentation and EM modelling drive the rapid growth of land-based applications. However, the ease of use of FDEM instruments is starkly contrasted by data complexity and laborious calibration. When facing high conductivities, linear approximations between the quadrature phase and EC are no longer valid and the depth of investigation (DOI) is uncertain, complicating an accurate delineation and characterization of the subsurface. To assess the signal attenuation, a simple method for the calculation of the DOI was established and to explore the FDEM calibration and its potential for accurate conductivity mapping in a high conductivity setting, a land-based survey was conducted using a custom approach. The tailored approach relied on the collection of multi-configuration FDEM data as well as cone-penetration testing (CPT) data (with a mounted EC module). All data were collected in a two day time span to ensure maximum correspondence of all (temperature and moisture dependent) data. The CPT-EC data resulted in discrete profiles with a high vertical resolution and the FDEM data allowed good area coverage. The CPT conductivity logs were then used to calculate theoretical instrument responses (to evaluate FDEM data uncertainty or offsets) and obtain optimal inversion parameters. An excellent correlation between the forward modelled CPT-EC and measured FDEM data was uncovered, advocating the use of the applied forward modelling routine and the combination of CPT-EC and FDEM. Unfortunately, the presence of large discrepancies in the comparison data impeded the use for levelling purposes. This was likely the result of the erroneous assumption of lateral homogeneity made during the 1D forward modelling, demonstrating the large impact discrete, contrasting subsurface bodies can have. Following, the FDEM data were inverted to assess the accuracy potential.