Cross‐borehole electromagnetic probing to locate high‐contrast anomalies

Abstract

Electromagnetic (EM) probing between boreholes is useful for locating high‐contrast geophysical anomalies such as a tunnel. Theoretical and experimental studies of EM field interaction with a tunnel show that minima in the received signal can be used for locating the tunnel. The theoretical studies show that as a transmitter and receiver are lowered in separate boreholes, the minima can be interpreted easily to yield both the lateral and vertical positions of the tunnel. The main mechanism of EM field interaction with the tunnel appears to be diffraction, and the spatial variation of the field strength is affected by the tunnel shape. Frequencies from 10 to 70 MHz were studied to assess the usable frequencies. The field in the receiver borehole was an effective diagnostic when a half‐wavelength in the surrounding medium was less than or equal to the diameter of the tunnel. EM probing at two test sites gave the locations of tunnels within 1 ft of the surveyed locations.

Characterization of Geotomographic Studies with the EMRE System

“…He calculated the attenuation constants of porphyry and sedimentary rocks at several frequencies [1]. The work was continued by Lager and Lytle [2], Lytle et al [3], and Somerstein et al [4]. Stolarczyk and Fry [5] used RIM to detect faults in the continuity of coal seams, and this can be considered as the starting point of RIM.…”

Radio Propagation in a Coal Seam and the Inverse Problem

“…However, the transmission measurements will still show that the seam is not uniform, and this information might be sufficient for practical mining applications. The same difficulties occur for other geophysical tomography applications when strong scattering is present in either electromagnetic or seismic cases [25].…”

“…Otherwise reflection and refraction occur, and they can be included, but only with significant increase in complexity [23]. When scattering becomes very important, even further changes in the processing are required [24], and in some cases it is useful to search for the scattering pattern of expected anomalies [25]. More experience with coal seam anomalies is necessary to determine the best method for processing data.…”

On image reconstruction algorithms for binary electromagnetic geotomography

“…The borehole tomographic technique in which electromagnetic waves are used is known in the literature [15,42] as Electromagnetic Geotomography (EG). The geometry of interest in EG is cross-borehole scanning.…”

“…This technique is also very helpful in localization and characterization of subsurface anomalies, such as cracks, cavities, tunnels or voids, which may result from lithological characteristics or human activities [22,42].…”