The U.S. Army Engineer Research and Development Center (ERDC) has worked with U.S. Law<br>Enforcement Agencies (LEAs) since 1995 to address the problem of clandestine tunnels beneath the<br>U.S./Mexico border. ERDC has performed tunnel-related research, equipment development, or tunneldetection<br>missions at the request of the LEAs, coordinated by Joint Task Force 6 (JTF-6, Fort Bliss, TX,<br>now known as JTF-N for Northern Command, US Army). This support to LEAs has revealed the<br>importance of understanding the geologic context of a suspected tunnel site as a basis for selecting the<br>appropriate geophysical tools and interpreting anomalies indicated by geophysical data. Tunnel detection<br>missions always involve multiple tools and techniques. A combination of geophysical instruments is used to<br>record data based on different physical principals. When interpreted in a regional geologic context, the<br>combined geophysical methods improve the likelihood of success for tunnel detection.<br>A variable-frequency electromagnetic survey tool was developed in the 1990s as part of tunneldetection<br>research, and proven at a tunnel test bed near Otay Mesa, CA. Also at the Otay Mesa site, an<br>ERDC-led team installed and tested a prototype passive-seismic fence, a system that can detect machine and<br>impact noise during the tunnel excavation process. This seismic fence concept has strong potential for<br>deterring tunneling in geographic areas where tunnels have been found most frequently and where cultural<br>clutter limits the usefulness of surface geophysical techniques and tunnel detection.<br>Current ERDC tunnel detection efforts (March 2005) are coordinated with the National Geo-<br>Intelligence Agency (NGA, formerly NIMA) to combine electromagnetic and radar methods with emerging<br>technology in microgravimetry.


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