Protecting Secure Facilities From Underground Intrusion Using Seismic/Acoustic Sensor Arrays

The geologic setting of an area determines the suitability of a given technique for locating visually ob-scured features such as tunnels in the shallow subsurface.In the area surrounding the test tunnel, the upper 6 meters of sediments were deposited as part of a delta during a time of higher sea level, when low-gradient rivers carried fine-grained sediments into a shallow sea. The resulting sediments vary in thickness, density, moisture content, and color—both horizontally and vertically. These sediments are now overlain by fine-grained, windblown material. Subsequent natural and man-caused processes have changed much of the original sedimentary layering. The upper sediments can be cemented with either calcium carbonate (calcite) or calcium sulfate (gypsum) to a depth of about 30 centimeters and are difficult to dig through. In the upper layer, the gypsum forms veinlets some 5 millimeters in diameter and spaced quite closely throughout the layer. Crystals of gypsum up to 3 centimeters in length are present in the upper layers. The lower sediment layers are typically devoid of visible gypsum crystals. These veinlets are hard, making digging difficult and producing definite signatures that can be picked up with the sensors (see Figure 1). At some locations in the study area near the surface, there are substantial areas of white cemented sand that is locally called “gatch” (see Figure 2, page 73). Gatch forms when carbonate or sulfate minerals (calcite, gypsum, or both) are deposited by movement and evaporation of water in the pore space of previously deposited sand.