Discussions on Resolution of Different TDEM Survey Techniques for Detecting Water-Bearing Structures

Abstract

It is common when using TDEM to measure only inside the transmitter loop. In groundwater and environmental applications, this is almost ubiquitous. This situation arose because inversion applications were available only for central loop readings (Anderson in Improved digital filters for evaluating Fourier and Hankel transform integrals, 1975 [1]) and geoscientists thought of such readings as soundings similar to resistivity sounding applications. But, should we consider TDEM data as analogous to reflection data and measure proximate to the source or as analogous to refraction data and measure away from the source? In mining applications, three-dimensional modeling has long been available and the use of multiple measurements inside and outside loops has been common for three decades. In this paper, we examine several misunderstandings and problems associated with in loop approaches by comparing results from different TDEM survey techniques. We utilize both synthetic and field data for our studies. Both synthetic and field data indicates that the use of in loop data is potentially dangerous, as this location is poor for sensing resistive structure. In addition, single station inversion is limited without considerable knowledge of the geology within the study area, as 1D inversion is highly non-unique. The use of multiple data in a 1D inversion helps locate the correct model subspace and it appears that out-of-loop data has fewer possible models. Fixed loop surveys can provide more accurate deep inversion results if the ground is sufficiently one-dimensional. It also provide an array for faster surveys and more area coverage.