An Improved Early‐Channel VTEM Helicopter System for Near‐Surface Applications

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The helicopter-borne Versatile Time Domain Electromagnetic System (VTEM) is a geophysical instrument which has been in continuous development, utilizing the most recent advances in digital electronics and signal processing for deeper penetration; higher spatial resolution; better resistivity discrimination; and increased detection of a broad variety of conductive targets. Since its inception in 2002, more than 1 300 000 line kilometers have been flown around the world, in a wide variety of geological environments, primarily involving mineral exploration, but also including near-surface groundwater applications. However, although the VTEM system has progressively achieved marked improvements in its deep penetration characteristics, at the same time its near-surface imaging capability has been limited by its early time data. This is significant because early time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas. Yet many time-domain AEM system have problems obtaining quantitative early-time data due to a variety of issues, namely system bandwidth. Recently, workers and researchers in shallow electromagnetic sounding have confirmed the calibration accuracy of VTEM mid to late delay time data (>100μsec to 10msec) while at the same time highlighting the quantitative incompatibility issues with its early channel data and near-surface conductivity layering. Processing strategies, such as deconvolution, aimed at correcting system imperfections, such as bandwidth, have been proposed with promising results but are still not routinely implemented. in an effort to address this issue, Geotech has embarked on a system design strategy aimed at improving the early-channel VTEM data and achieving fully calibrated, quantitative measurements closer to the transmitter current turn-off. This development has led to new model of VTEM system designed specifically for near-surface, high resolution applications, while maintaining reasonably optimal deep penetration characteristics. Results have shown a significant improvement in quantitative VTEM data at earlier times than previously achieved, approaching 5μseconds after the current turn-off.