Towed transient electromagnetic survey for groundwater investigation - challenges and solutions.

Abstract

SUMMARY Towed transient electromagnetic survey (TEM) is conducted by towing loops of wire behind a vehicle, one transmitting and several receiving. The loop sizes must be large enough to transmit/receive moments well in excess of EM noise levels while loop inductances must be kept low in order to keep turn-off time and system response low enough to resolve shallow detail. Long systems with the towing vehicle well in front of two separate nonmetallic loop support structures are practical. As such a system must fit through farm gates, between trees, and along road margins without undue traffic disruption, it must be capable of folding to legal road vehicle width. Processing involves most of the normal complications of airborne EM survey but with the additional complication that the system cannot be elevated high above the ground to isolate system response. Additional challenges are identification and removal/minimization of noise and metallic source anomalies: 1. created by movement of coils through the magnetic field of the earth, 2. created by buried telephony cables, and 3. from other sources such as power lines, buried metal pipes and fences. Much of Australia is reasonably navigable and not densely covered with infrastructure and it is in these parts of Australia where towed TEM systems, which can be deployed and manoeuvred flexibly, have a market for groundwater and other exploration.