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22 July 1994 Tunnel detection using the radio imaging method at the Otay Mesa site
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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.
© (1994) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Kenneth D. Mahrer and 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);


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