6. Time Domain Electromagnetic Prospecting Methods

Abstract

INTRODUCTIONIn the traditional frequency-domain electromagnetic (FEM) methods of exploration the ground is energized by passing an alternating current (ac) through an ungrounded loop situated usually on or above the surface of the earth. The primary field of the loop will induce eddy currents in all conductors present in the earth. The secondary electromagnetic (EM) fields due to these induced currents, together with the primary EM field, are recorded with a suitable receiver at various points in space. In general, the secondary EM field at the receiver, which contains all the information regarding the underground conductors, may be several orders of magnitude smaller than the primary field. Under these conditions the separation of the measured total EM field into its primary and secondary parts is difficult.This fact led to the idea of using time-domain electromagnetic measurements (TEM), often referred to as transient EM techniques. In TEM measurements a strong direct current (dc) is usually passed through an ungrounded loop (Figure 1). At time t = 0 this current is abruptly interrupted. The secondary fields due to the induced eddy currents in the ground can now be measured with a suitable receiver in the

Geophysical imaging of alpine rock glaciers

“…From the general shape of this curve, we can infer the presence of a relatively resistive layer sandwiched between two more conductive layers, one of which approaches the surface. The solid line in Figure 3b represents the best-fitting model obtained from the inversion program, and the gray shaded area represents 'equivalent' models that explain the data nearly as well as the best-fitting model (Nabighian and Macnae, 1987). The significant decrease of resistivity at 54 AE 15 m depth coincides with the rock-glacier-bedrock interface.…”

An Electric Resistivity Study of the Chelungpu Fault in the Taichung Area, Taiwan

“…It can be correlated to the gravel layer of the alluvium that appears on the banks of the Tali Stream. Based on the geological setting and the resistivity spectra of rocks including the resistivity measured on the outcrops of the formations around the study area (Keller and Frischknecht 1966;Nabighian 1988;Cheng 1990;Ho and Chen 2000;Cheng et al 2002), the middle layer is interpreted to be a layer of sandy alluvium as it has a resistivity of 30 -150 Ω-m, and the bottom layer is interpreted to be the late Pleistocene gravel beds because of its high resistivity at 300 -2000 Ω-m.…”

Response Analysis of Data Acquired by Marine Loop Electromagnetic System Using Three-Dimensional Modeling Based on Integral Equation

“…특히 소형 루프형 전자탐사를 해양 환경에서 적용하는 연구들 은 국내에서 뿐만 아니라 국제적으로도 시간영역(time domain) (Nakayama and Saito, 2011;Swidinsky et al, 2012) 및 주파수 영역에서 (Ko et al, 2012;Müller et al, 2012;Müller et al, 2011 (Nabighian, 1988;Zhdanov et al, 2006) Ko et al (2012)). Note that the anomaly appears as a face-to-face ellipses shape and is marked in a circle.…”