Assessment of near-surface mapping capabilities by airborne transient electromagnetic data — An extensive comparison to conventional borehole data

Abstract

A newly developed helicopter transient electromagnetic (TEM) system has the ability to measure very early times within just a few μs after the turn off of the primary current. For such a system, careful calibration and accurate modeling of the electromagnetic (EM) response is critical to get true resistivities of the very shallow geologic layers. We discovered that this leads to resolution of the same level or in some cases even better than what can be obtained from airborne frequency EM systems. This allowed a range of important applications where high and accurate resolution is mandatory, e.g., geotechnical applications such as urban planning, railroad and road investigations, landslides or distribution of raw materials, and assessing aquifer vulnerability. We evaluated the results of a pilot survey covering the Norsminde catchment south of Aarhus, Denmark, where we found that near-surface layers (top 30 m) can be mapped with an accuracy of a few meters in a complicated glacial sedimentary environment. The mapping of the geologic layers was assessed by a detailed analysis in which we developed a general methodology for crosschecking the EM and borehole data. This methodology is general and can easily be adapted to other data types and surveys. After rating the quality of the boreholes based on a list of predefined criteria, we concluded that the EM data matched with about three-quarters of the boreholes located within less than 15 m from the closest EM soundings. The remaining quarter of the boreholes fell into two groups in which half of the boreholes were of very poor quality or had inaccurate coordinates. Only eight of all the boreholes could not be reproduced by the data, and we attributed this to be caused by very strong lateral or vertical geologic variations not resolvable by the TEM technique.

Three-dimensional geological modelling of AEM resistivity data — A comparison of three methods

“…As a consequence of a very short turn-off time of the SLM, it is possible to use data at very early times (5-6 μs from begin of ramp). According to Schamper et al (2014), the best lateral resolution of the top 30 m is estimated to be in the order of 30-40 m, whereas the depth of investigation is around 100 m. The SkyTEM survey covers 2000 line km with an in-line sounding spacing of about 15 m, resulting in more than 100,000 soundings. The survey has been flown with three different line spacings: in the eastern part the nominal line spacing is 100 m; in the western part it is 50 m; and in a smaller area west of the town Odder, cross-lines have been flown with a nominal line spacing of 50 m (Fig.…”

“…1) acquired in the framework of the NiCA research project in 2011 , using a high resolution version of the SkyTEM system (Sørensen and Auken, 2004) called the SkyTEM101 system ). The SkyTEM101 system was developed with the objective to increase the resolution of the top 30 m. Schamper et al (2014) evaluated the SkyTEM101 results against boreholes and found a good fit for more than 75% of the boreholes. The SkyTEM101 system has a small transmitter loop from which it operates with two moments: a super-low moment (SLM) of 910 A m 2 and a high moment (HM) of 7150 A m 2 .…”

“…Processing and inversion of the survey are described in detail in Schamper et al (2014). One of the important steps in the processing is manual inspection and removal of data that are affected by couplings to man-made installations.…”

Large-scale 3-D modeling by integration of resistivity models and borehole data through inversion

“…The 100 m horizontal discretization of the CF model and cluster model was selected to match the computational grid setup of a subsequent groundwater model. A detailed overview of resolution capabilities of the Norsminde SkyTEM survey is given by Schamper et al (2014b), including an extensive comparison to borehole data.…”

“…The SkyTEM data were collected with the newly developed SkyTEM 101 system (Schamper et al, 2014b). The SkyTEM 101 system has the ability to measure very early times, which improves the resolution of the near-surface geological layers when careful system calibration and advanced processing and inversion methodologies are applied (Schamper et al, 2014a).…”