In the broadest sense, electrical, magnetic, and electromagnetic exploration and detection techniques are generally concerned with the determination of subsurface structure configuration or anomaly detection. The selection of a particular method is based on a priori expectations of the exploration volume and target. The differences have to do with the nature of the fields: “electric” and “magnetic” techniques generally imply static fields and passive exploration methods; “electromagnetic” techniques imply dynamic fields and active exploration methods. Electromagnetic methods also allow study and interpretation of material parameter changes with frequency.
Magnetic field phenomena may be broadly divided into potential and electromagnetic types. The potential magnetic fields are self-generated by materials that may be classed as “magnets”; primarily materials containing iron or nickel. The earth itself may be considered a potential field magnet, having well-known North and South poles. Electromagnetic fields are of a transient phenomena – having a time-dependent component. Lightning strikes are a common, “natural” cause of electromagnetic fields. These are simplified definitions; the earth’s field for example fluctuates over time, but may be considered a static potential field unless phenomena are being investigated over long periods of time.
The measurement instrument responds to both magnetic and electromagnetic fields with a frequency response of DC to some high frequency. The frequency limit depends on the particular geological configuration and instrument capability. The instrument used for this study has a magnetic field sensitivity of about 0.05 nT (500 nGs) with a bandwidth of approximately 100 kHz.
INTRODUCTION
To begin, equations describing frequency-domain electromagnetic fields in free space are presented. The field equations are then developed for conductive half-space regions, followed by the addition of a simple conductive anomaly. By assuming a linear transmission model for the half-space, a Laplace Transform is applied, allowing a simplified analysis of transient field effects to be discussed.
At high frequencies and early times of transient measurements, currents are concentrated near the source and the fields in the vicinity of a conductive anomaly are minimal. Within this time/frequency range, the influence of the anomaly is negligible – the higher frequency components contain little information of interest. For this reason, only the intermediate and low frequency parts of the spectrum will be considered to contain useful information. It is noted that “early”, “intermediate”, and “high” are relative terms dependent on spatial relationships as well as material properties. The following discussion will be based on the assumption that skin depth is much greater than the exploration dimensions or the conductor cross-section when a 2D or 3D model is considered.
By definition, the primary field does not contain information about the anomaly and may be considered as noise. The secondary field is related to the presence of an anomaly – currents are present within the anomalous conductor and these generate a secondary magnetic field which are functions of the conductivity of the anomaly and the surrounding media.
The overall process for this discussion may be summarized as follows:
A time-varying current in a coil on the surface generates a vertical magnetic dipole (). The dipole field creates a vortex electric field, causing charge movement – current – within the region of the dipole. The current creates a secondary magnetic field () generates an electric field – and additional currents – in the surrounding medium. These fields influence and interact with currents in the anomaly. This distribution of currents results in secondary magnetic (and electric) fields () which cause a distortion in the primary field. Interpretation of the anomaly characteristics is based on the analysis of this measured distortion.
This exploration method is best suited for the detection of conductive anomalies: the simple model to be discussed may be modified as the situation warrants. Other considerations such as alternative transmission sources, array methods, frequency signature analysis, and instrument capabilities are not part of this discussion.
That’s good for now.