Electromagnetic Exploration Methods

The theory of electromagnetic fields and waves is described for electromagnetic geophysical methods for subsurface exploration and monitoring of subsurface processes. A distinction is made between potential, diffusive and wave fields. Point source-receiver response functions are derived in wavenumber-frequency, space-frequency and space-time domains.

In the potential field methods, we distinguish the magnetic field method from the geo-electric method. In the magnetic field method, receivers are used that measure the total magnetic field amplitude or its vector components. In these measurements, deviations from the Earth magnetic field can be understood as coming from a magnetised subsurface object. The measurements contain information about the object’s position, shape, and magnetisation that can be retrieved from the data. In the geo-electric method, two electrodes are used to generate a current in the ground and between two other electrodes the electric potential difference is measured. Depending on the electrode configuration the apparent resistivity is computed as the basis of inversion. The current distribution is optimised in a desired depth range by choice of electrode distance.

In the diffusive field electromagnetic methods, we discuss the MT method which uses naturally occurring noise as the source signal, and the so-called controlled source electromagnetic (CSEM) method which uses a horizontal electric or a magnetic dipole source. The horizontal electric field components are measured, possibly with the magnetic field vector. This configuration is used on land and offshore. The basic model is used to study the ability to detect a subsurface anomalous layer or object. Both time-domain and frequency domain methods are discussed.

Ground-penetrating radar (GPR) uses electric current antennas as source and receiver. Most applications acquire single-offset data. The model underlying data interpretation and imaging is scattering by a subsurface object. The expressions for the understanding of GPR data and creating a subsurface image are discussed in detail.

Theoretical and practical aspects of acquisition and processing for these methods are discussed and evaluated.