Near Surface Resolution and Turnoff Times in Airborne TEM Investigations

Aquifer vulnerability is a key issue in groundwater resource management. The airborne TEM method has demonstrated the potential to provide detailed information about the shallow subsurface resistivity distribution to constitute a valid background for vulnerability estimation. Combined with its ability to delineate aquifers in a variety of geological settings, the airborne TEM method is an efficient tool in large scale hydrogeological investigations. However, in sedimentary environments with moderate to high resistivities in the shallow subsurface it can be a challenging task to provide satisfactory nearsurface resolution applying airborne TEM measurements. It is well known that the near-surface resolution is highly dependent on the ability of a specific instrument to measure the early time part of the transient earth response, although the bandwidth of the receiver and the applied current waveform also have a significant impact on the resulting near-surface resolution. Focusing on the applied current waveform, most impulse response type TEM systems employ a linear current turn-off ramp. We show that the effect of a long linear ramp is to make the early time response similar to a step response, while a short linear ramp retains the impulse response character. The sensitivity of the step response to the shallow resistivity distribution is inferior to that of the impulse response, indicating that a short ramp is preferable for environmental investigations. In order to quantify the effects of the ramp length and of the unavoidable measurement dead-time on the near-surface resolution we have performed linearized inversion parameter analyses of 1D layered earth models for a generic airborne transient EM system. The analyses clearly demonstrate that specialized instrumentation, such as employed by the dual moment SkyTEM system, Sřrensen and Auken (2004), is necessary if aquifer vulnerability is to be estimated from airborne transient EM data in these types of sedimentary environments.