Cross-borehole electromagnetic probing to locate high-constrast 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 transimitter 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 feld 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. 20 figures.
- Research Organization:
- Univ. of California, Livermore
- OSTI ID:
- 5413164
- Journal Information:
- Geophysics; (United States), Vol. 44:10
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
42 ENGINEERING
01 COAL, LIGNITE, AND PEAT
BOREHOLES
ELECTROMAGNETIC SURVEYS
CAVITIES
COORDINATES
ACCURACY
IN-SITU GASIFICATION
TUNNELS
COAL GASIFICATION
DIFFRACTION
ELECTROMAGNETIC FIELDS
MHZ RANGE 01-100
MONITORING
SPATIAL DISTRIBUTION
UNDERGROUND
COHERENT SCATTERING
DISTRIBUTION
ELECTRICAL SURVEYS
FREQUENCY RANGE
GASIFICATION
GEOPHYSICAL SURVEYS
IN-SITU PROCESSING
LEVELS
MHZ RANGE
PROCESSING
SCATTERING
SURVEYS
THERMOCHEMICAL PROCESSES
580203* - Geophysics- Geophysical Survey Methods- (1980-1989)
422000 - Engineering- Mining & Underground Engineering- (1980-)
010404 - Coal
Lignite
& Peat- Gasification