Very Early Times In Aem - Resolution Of The Near-surface For Hydrogeological Applications

Abstract

Since the 2000’s helicopter transient electromagnetic (HTEM) systems have widespread in hydrogeological/environmental investigations. They are usually more limited in terms of near-surface resolution compared to helicopter frequency EM systems (HFEM). A new version of the SkyTEM system, called Mini-SkyTEM or SkyTEM 101, is now capable of measuring at very early times, only few μs after the turn-off of the current in the transmitter loop. The system payload is very light (350 kg) with a transmitter loop of only 130 m² (Figure 1a). The particularity of all SkyTEM systems is the measurement of both a low (LM) and a high (HM) moments (Figure 1b). This dual-moment configuration allows getting information both from near surface and deep layers. The HM gives a depth of investigation of ~100 m for an average resistivity of 50 Ωm, but the more interesting feature of the system is its earliest time in the LM which is only 2-3 μs from end of ramp (~ 3μs). The very early times or the high frequency content can be interpreted only if two key points are handled. The first one is the calibration of the EM system. The AEM system has been calibrated following the procedure described by Foged et al. (2013) where a time and amplitude factor shifts are estimated above a well-known reference site to match the measurements and the modeled reference response. The accuracy of this calibration is critical in the present case, since a bad time shift estimation of less than 1 μs can cause non-negligible differences in the top 30 m. The second point to consider is the primary field residual, or coil response (CR), which affects the first gates right after the turn-off of the current. This CR is induced by small residual current in the transmitter wire and needs to be modeled especially when ground resistivity and/or flight altitude increase, reducing the strength of the ground EM response. This CR is measured at high altitude. Since measurements have confirmed its stable shape, only its amplitude, which varies due to small bending of the frame during the acquisition, has to be determined during the inversion as an amplitude correction factor. With the consideration of the CR, it is possible to interpret the very early times right after the turn-off of current (Schamper et al., 2012).