Seismic Detection and Analysis of Underground Laboratory Facilities

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2001

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An abandoned underground tunnel complex at Fort Hood, Texas, was made available for project study. The physical size and depth of this tunnel system were similar to the physical dimensions and burial depths of underground laboratory (UGL) targets that are the subject of this UGL Countermeasures program. The first seismic field test objective at the Fort Hood tunnel system was to determine if cultural activity within a tunnel can be detected with seismic sensors deployed on the surface near an underground facility. The second objective was to try to identify a tunnel target with these same surface-positioned sensors using reflected and refracted seismic wavefields generated by a weight-drop source stationed at locations around the tunnel target. Seismic test data were recorded with three-component (3-C) geophones to allow vector extrapolation of event arrivals to their subsurface points of origin. The recording system had only 24 active recording channels, which limited the number of 3-D geophones that could be used for target detection and analysis to eight. Test results verified that cultural activity within a tunnel can be detected by surface-positioned seismic sensors. Point-source disturbances, such as hammering at a fixed location within the tunnel, yielded more interpretable data than did disturbances from a distributed source, such as the movement of airwaves along the entire 1000-ft length of the study tunnel when the entry doors were opened and closed. These data show that passive seismic monitoring of a UGL site will provide valuable information about UGL structure and geometry. The weight-drop source produced sufficient seismic illumination of the chosen tunnel target and other near-surface discontinuities in the tunnel area. These data demonstrate that UGL targets can be illuminated appropriately for target analysis purposes with seismic wavefields if an adequate number of moderate-energy source stations are distributed across a target area. The results shown here are qualitative in nature. Quantitative analysis of the data will be done during the next funding period.

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