Signal extraction from 4D transient electromagnetic surveys using the equivalent source method

Abstract

Emerging applications of transient electromagnetic (TEM) surveys, such as monitoring of groundwater aquifers or tracking fluid movement within petroleum reservoirs, require the acquisition of high resolution time-lapsed (4D) data sets, using three-component receivers to maximize information. However, separating signal from noise in TEM surveys can be difficult, and the magnitude of the time-lapse difference is often on the order of the noise. A method is developed for extracting signal from noisy three-component TEM data sets collected in fixed-loop, multiple-receiver surveys using a novel application of the equivalent source technique. This method takes advantage of the relationship between the three components of the observed decaying magnetic field to improve data contaminated by uncorrelated random noise, such as that due to receiver coil misalignment and location errors. Both synthetic and field three-component TEM data are used to demonstrate the viability of the technique. The results suggest it is feasible to use the method in processing time-lapsed TEM data, allowing for an improved signal to noise ratio in 4D survey data.