Improving the understanding of transient electromagnetic signals for near-surface applications by assessing the turn-off ramp

Abstract

The application of the transient electromagnetic (TEM) method to near surface problems depends mainly on the correct resolution of shallow subsurface layers. To resolve the near-surface layers using the TEM method, it is necessary to adjust the obtained signals using correct turn-off ramp times. Hence, the main aim of this work is the quantification of the turn-off ramp. Furthermore, the ramp times are applied to improve the inversion results for determining correct electrical resistivity logs. Additionally, two case studies with a focus on near surface problems are presented to evaluate the corrected TEM resistivity logs. Within the first case study a brick-clay deposit is investigated to quantify the clay content in the subsurface. The aim of the second case study is to develop a novel methodology to characterize a clayey landslide. The quantification of the turn-off ramp was done by using an oscilloscope to measure the signals within the transmitter loop. The brick clay deposit is investigated using electric and electromagnetic methods and the results are validated by comparison to the clay content measured from in-situ soil samples. The development of a novel methodology to characterize a clayey landslide is based upon the integral application of geophysical and geotechnical methods. By application of seismic refraction tomography, it was possible to delineate unconsolidated areas in the subsurface, while the electrical and electromagnetic methods were able to discriminate between different lithological units. The evaluation of the, in terms of the turn-off ramp, corrected TEM resistivity logs was successful, since the TEM method was able to obtain similar results as the well-established electrical resistivity method. The oscilloscope measurements were repeated at different sites and for multiple measurement configurations to provide a data base of turn-off ramp times, ranging between 0.1 and 10 s.