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.
Cross‐borehole seismic velocity and high‐frequency electromagnetic (EM) attenuation data were obtained to construct tomographic images of heavy oil sands in a steam‐flood environment. First‐arrival seismic data were used to construct a tomographic color image of a 10 m by 8 m vertical plane between the two boreholes. Two high‐frequency (17 and 15 MHz) EM transmission tomographs were constructed of a 20 m by 8 m vertical plane. The velocity tomograph clearly shows a shale layer with oil sands above it and below it. The EM tomographs show a more complex geology of oil sands with shale inclusions. The deepest EM tomograph shows the upper part of an active steam zone and suggests steam chanelling just below the shale layer. These results show the detailed structure of the entire plane between boreholes and may provide a better means to understand the process for in situ heavy oil recovery in a steam‐flood environment.
Experiments were conducted to determine the in situ relative dielectric constant ϵr and skin depth δ of hard rock in a permafrost region. In the Brooks Range in northern Alaska, measurements of transmission attenuation and transmission phase shift were done between the surface and the hole as well as from hole to hole over the frequency range of 5 to 50 MHz. Relative dielectric‐constant and skin‐depth results were in the ranges 5 to 30 and 20 to 140 m, respectively.
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