Application of Apparent Resistivity Ratio on the Interpretation of Small Central-loop TEM Survey in Areas of Conductive Overburden

Abstract

Central-loop TEM sounding data are often interpretead with apparent resistivity as a function of depth. This can quickly give a qualitative understanding of the subsurface resistivity variation. However, when the resistivity contrast between anomalies and background is weak, especially when there is a conductive overburden, the apparent resistivity image is inadequate to quickly and visually identify local anomalies. As an alternative, the apparent resistivity ratios of each measured station to a reference station is proposed as a fast way to identify local conductive anomalies qualitatively. Modeling analyses are carried out on 1D layer model, 2D fault models, and a 3D prism model with conductive overburdens. The results show that the apparent resistivity ratio outperforms the apparent resistivity on intuitively reflecting the target anomaly. As an image enhancement technique, the apparent resistivity ratio performs data transformation on the original apparent resistivity data, normalizing the background response, and highlighting the anomaly. At last, the apparent resistivity ratio method is successfully applied on interpreting a small central-loop TEM survey on investigating karst structure beneath a freshwater river. It provides an alternative simple fast way for interpretation of central loop TEM measurements.