The use of distributed sensor arrays in electrical and electromagnetic imaging

Abstract

Electrical methods for exploring the earth, such as direct current resistivity, induced polarization and electromagnetism are used for numerous exploration, engineering and environmental applications. Common to all these applications is the desire to obtain the clearest possible image of the target. This thesis analyses and develops methods for improving signal to noise ratio for electrical methodsThe ability to recover subsurface information from electrical exploration methods is dependent on the limits of signal detection which is strongly influenced by instrumentation and the conductivity structure of the Earth. Multiple sensors can be used to collect data efficiently over a survey area. Such multi-receiver arrays can improve the signal-to-noise ratio. However, the use of multiple sensors can also be exploited to improve the signal fidelity from each sensor, which may then translate to more accurate geological models and/or greater depth of investigation. In this thesis a two step algorithm for the removal of harmonic noise and atmospheric transients is presented. The first step is the removal of harmonic noise from each sensor using a non-linear single value decomposition (SVD) inversion technique to model a modulated sinusoid to narrow band noise sources. The second step is spherics attenuation using an iterative technique of signal stripping then removing residual coherent noise across the array combined with robust statistical measures in the tacking process. I show that this approach can recover signals otherwise buried in noise and that under certain conditions, signal to noise ratio can be improved by more than 46 dB. The algorithms designed here are applicable to any type of electrical or time domain electromagnetic survey conducted with a multi-receiver array.