22 July 1994 Tunnel detection using the radio imaging method at the Otay Mesa site
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Abstract
We demonstrated that radio imaging method (RIM) surface-to- surface, borehole-to-surface, and borehole-to-borehole sensing technologies at the Otay Mesa test site east of San Diego, CA could detect and delineate a horizontal 4 X 6-foot (cross- section) tunnel buried at a depth of approximately 45 feet. Utilizing monochromatic, continuous wave electromagnetic signals from a magnetic dipole source operating in the range between 22 kHz and 15 MHz, we confirmed the effectiveness of two general approaches: (1) mapping the electrical conductivity contrast between the country rock (sandstone) and the tunnel (i.e. the void and surrounding desiccation fractures) and (2) locating a cable (i.e. conductor) within and running the length of the tunnel from its induced, secondary radiation. Surface-to-surface RIM, utilizing a gradiometer receiver, mapped the 2D plan view location of the tunnel. Borehole-to-surface delineated both the depth and plan view location of the tunnel. Borehole-to-borehole RIM delineated the depth of the tunnel.
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Kenneth D. Mahrer, Kenneth D. Mahrer, William A. Mondt, William A. Mondt, } "Tunnel detection using the radio imaging method at the Otay Mesa site", Proc. SPIE 2217, Aerial Surveillance Sensing Including Obscured and Underground Object Detection, (22 July 1994); doi: 10.1117/12.179932; https://doi.org/10.1117/12.179932
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