Processing and inversion of commercial helicopter time-domain electromagnetic data for environmental assessments and geologic and hydrologic mapping

Abstract

Helicopter time-domain electromagnetic (HTEM) surveying has historically been used for mineral exploration, but over the past decade it has started to be used in environmental assessments and geologic and hydrologic mapping. Such surveying is a cost-effective means of rapidly acquiring densely spaced data over large regions. At the same time, the quality of HTEM data can suffer from various inaccuracies. We developed an effective strategy for processing and inverting a commercial HTEM data set affected by uncertainties and systematic errors. The delivered data included early time gates contaminated by transmitter currents, noise in late time gates, and amplitude shifts between adjacent flights that appeared as artificial lineations in maps of the data and horizontal slices extracted from inversion models. Multiple processing steps were required to address these issues. Contaminated early time gates and noisy late time gates were semiautomatically identified and eliminated on a record-by-record basis. Timing errors between the transmitter and receiver electronics and inaccuracies in absolute amplitudes were corrected after calibrating selected HTEM data against data simulated from accurate ground-based TEM measurements. After editing and calibration, application of a quasi-3D spatially constrained inversion scheme significantly reduced the artificial lineations. Residual lineations were effectively eliminated after incorporating the transmitter and receiver altitudes and line-to-line amplitude factors in the inversion process. The final inverted model was very different from that generated from the original data provided by the contractor. For example, the average resistivity of the thick surface layer decreased from ∼1800 to ∼30 Ωm, the depths to the layer boundaries were reduced by 15%-23%, and the artificial lineations were practically eliminated. Our processing and inversion strategy is entirely general, such that with minor system-specific modifications it could be applied to any HTEM data set, including those recorded many years ago.

Paleo-megalake and paleo-megafan in southern Africa

“…We corrected, calibrated, processed, and inverted the data using new procedures that allow commercial data designed for mineral exploration to be used for hydrogeological investigations (Viezzoli et al, 2008;Podgorski et al, 2013). To highlight the most important features, the resultant three-dimensional (3-D) resistivity model has been spatially smoothed and displayed using a simplifi ed three-color scheme in Figures 2 and 3.…”

Airborne and ground‐based transient electromagnetic mapping of groundwater salinity in the Machile–Zambezi Basin, southwestern Zambia

“…Thus the uncertainties about how the digital filtering was conducted by Geotech () in addition to transmitter–receiver timing errors and amplitude shifts necessitated calibration of the airborne TEM data set with accurate ground‐based TEM measurements (Podgorski et al . ) in order to obtain a more reliable inversion and interpretation. Interpretation of resistivity data obtained from TEM measurements is described in more detail in the Discussion section.…”

“…The procedure and considerations used for processing and calibration of the airborne data set are similar to those of Podgorski et al . () who report a final time shift of and a final shift factor of 1.44. For this study, the time shift was determined as —44.929 ± 10.05 μ s, and the shift factor was determined as 1.071 ± 0.112 based on averaging of the calibration results at 14 sites.…”

“…Most importantly, accurate ground‐based TEM data were used to calibrate the VTEM data in order to eliminate absolute amplitude inaccuracies and timing errors between receiver and transmitter instrumentation (Podgorski et al . ). Contaminated early time and noisy late time gates were semi‐automatically eliminated for each record.…”

“…Podgorski et al . () described the Okavango Delta as an alluvial mega‐fan and used airborne TEM data constrained by ground‐based TEM seismic data and borehole records to provide additional evidence for an ancient large‐scale alluvial fan complex underlying the Makgadikgadi sediments. Their airborne TEM data were a reprocessing of a commercially obtained versatile TEM (VTEM) data set from the Okavango Delta affected by systematic errors and with large uncertainties regarding key system parameters.…”