Roughly 20% of the United States is underlain by limestone or dolomite. Shallow karst, riddled with cavities and fractures enlarged by dissolution, poses a costly problem for engineers and developers. Nearly every conceivable geophysical method—microgravity, ground-penetrating radar, electromagnetics, seismic reflection, refraction tomography, seismic surface waves, resistivity, spontaneous potential, and induced polarization—has been conscripted in the war against damage caused by subsidence. In this paper, we will discuss two unusual methods that we have tested for mapping the extent of shallow karst: smoke tests and mise-à-la-masse resistivity. Both methods presuppose at least one borehole intersecting a cavity. Given the highly irregular nature of karst networks, the discovery of a cavity though drilling is sufficient to alert engineers that the site poses a potential problem, but little else can be determined using just the driller's log from a single borehole. The two methods we discuss provided a simple, inexpensive way to map the strike and extent of the karst network at a site in eastern Pennsylvania.

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