Geophysical methods provide a non-destructive non-intrusive method of subsurface investigation. Oftentimes, however, the target is difficult to “see” due to lack of contrast between the target and background. This lack of contrast can be rectified through the use of a sentinel device. A series of sentinel devices were used in downtown Pittsburgh, Pennsylvania to determine potential subsidence beneath a building sidewalk. This sentinel strategy was performed to predict future deformation of the sidewalk and to prevent foot-traffic hazards.
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