Electromagnetic Borehole Geophysics
Electromagnetic borehole geophysics exploits the law of induction, whereby a voltage potential is induced across an electrical conductor within a time-varying magnetic field. Electromagnetic borehole-geophysical tools induct electromagnetic energy in the formations surrounding the borehole and measure the response produced by the earth materials. Thus, by considering how different electrical properties affect electromagnetic wave behavior, electromagnetic borehole methods can depict variations in such properties with depth.
Electromagnetic borehole tools induce a time-varying alternating current (AC) within an internal transmitter coil, which ultimately generates a magnetic field having the same frequency and phase. This primary magnetic field penetrates the earth materials adjacent to the borehole to an investigation radius dependent on tool specifications and subsurface environmental conditions. An electromotive force (i.e., voltage potential) forms across conductive bodies or materials within the measurement zone, and a secondary electromagnetic wave is emitted.
Electrical current flows within the conductors while influenced by the primary magnetic field and generates a secondary magnetic field, which is detected by the tool. The secondary field induces current flow within a receiver coil, and the borehole instrument records the returning signal and compares it to the transmitted signal. These data can then be processed and interpreted in attempts to characterize the subsurface electric properties and their variation throughout the length of the borehole.
Electromagnetic energy propagation through the subsurface is related to the three electromagnetic properties of matter. Electrical conductivity and magnetic susceptibility govern the magnitude and phase of the received signal, which can be digitally analyzed to determine their variations in the measurement zone. Electromagnetic energy reflection is dependent on dielectric permittivity contrasts and is exploited to produce high resolution images of the borehole-intersected formations. The electromagnetic borehole methods most relevant to environmental geophysics are listed below.