R. J. Lytle

Cross-borehole electromagnetic probing to locate high-contrast anomalies

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.

COMPUTER MODELING OF ANTENNAS NEAR THE GROUND

ABSTRACT An accurate and efficient numerical method based on the rigorous Sommer-feld theory is described for modeling antennas near an interface such as the ground. The Sommerfeld integrals are evaluated by numerical integration along contours in the complex plane and two-dimensional interpolation is used subsequently to obtain the many Sommerfeld integral values needed for the moment-method solution of an integral equation. These methods permit modeling an antenna within 10-6 wavelengths of the ground for about two to four times the computation time for the same antenna in free space. Results showing currents and radiation patterns are included.

Electrode configuration influence on resistivity measurements about a spherical anomaly

The trade-offs involved with subsurface resistivity probing are studied for the case of a spherical anomaly. In particular, two-, three-, and four-electrode configurations are investigated. The three- and four-electrode configurations have similar results and have significantly better remote detection capabilities than does a two-electrode configuration. Cross-borehole probing is capable of detecting anomalous conditions at a greater range than is single-borehole probing.