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Tunnel and cavity detection are of paramount importance for security and construction requirements. The development and implementation of tools for detection is being sponsored by the Department of the Army to fulfill many of these needs. Borehole sonar, using equipment by SONEX, was used at two sites, Idaho Springs, Colorado and Manatee Springs, Florida, to detect and locate a manmade tunnel, and a water-filled cave. Crosshole sonar techniques were used with the receiver and transmitter in separate holes separated horizontally by 20 to 89 feet and vertically by 0 to 20 feet. The results demonstrate that even under poor conditions the SX-7 borehole sonar can be used to reliably detect, locate, and define tunnels between holes spaced up to 65 feet (30 meters) apart. Under normal or optimal conditions, detection up to 100 feet or more should be possible. Water-filled caves are detectable, and in the one example seen, a distinctive signature was noted. Further experimentation is desirable to develop characteristic signal signatures for differing rock conditions and tunnel or void-fill materials. Additional development for tuneable sonic sources is warranted to achieve optimum penetration without losing target resolution. Additional resolution in cavity and void mapping can be gained by holographic processing of the phase-shifted crosshole signals. Research into implementation of this should be considered. In summary, the SX-7 borehole sonar is well-suited now for field use in detecting and mapping tunnels, caves and similar targets, and could be immediately deployed in the field. Additional development could increase both the detection and the resolution. (Author)