Towed transient electromagnetic survey using various loop configurations

Abstract

Towed transient electromagnetic (TEM) survey, coupled with resistivity modelling software is an effective method of detailing small scale groundwater conceptual models and assisting with near surface geological investigations. Practical investigation depth ranges from 1m to 100m or more given the restrictions of today's electronics and practical trailer dimensions. Towed TEM survey using loops on trailers behind land vehicles or boats may be conducted using various loop configurations. Due to the loop area and separation requirements of loops from each other and from towing vehicles, design of trailers and/or sleds must be tightly integrated with design of loop configurations. Although separated loops (slingram configuration) are good for avoiding mutual inductance problems and may permit exploration to maximum possible depth, they are difficult to tow, especially around corners. Alternative arrangements with overlapping loops or bucking coils, all on a single platform, permit design of more practical platforms. On such platforms, not only must mutual inductance of coils be minimized but practical means of minimization are limited by achievable dimensional accuracy and stability of towed platform designs. Design is further restricted by the need to avoid use of metallic materials in most places and the need to separate and/or de-couple the metal survey vehicle from the loops. Case studies showing results of survey conducted with various platforms will be presented.