CAN YOU USE GEOPHYSICAL TOOLS TO INVESTIGATE KARST? MAYBE

Any geotechnical investigation in karst terrane must rely heavily upon a geologic understanding of the problems related to the particular karst characteristics of the locale. Is the subsurface metamorphosed, old (Cambrian), recent (Eocene/Miocene), flat or “bent”? Once the local geological model is understood, including the numerous unidentified vagaries inherent in karst terrane, conventional test borings (using rotary-wash techniques, SPT sampling and split-barrel coring), test pits and percussion drilling (less powerful equipment preferred), can be used to fill-out and firm up the geologic model developed from readily available sources (geologic mapping and literature, aerial photography and site reconnaissance).

Geophysical tools have been used in many instances to attempt to characterize the subsurface in karst areas. Even when reported by engineering geophysicists, the results have not always lived up to expectations. Many “successful” geophysical investigations have no hard data to backup the results. Geophysical tools used in karst include micro-gravity, seismic reflection/refraction, cross-hole tomography, ground penetrating radar, Spectral Analysis of Surface Waves (SASW) and electrical resistivity/conductivity.

The authors will present some examples of data and the proffered interpretations. In general, we believe that some techniques are applicable in the younger, flat-lying carbonates (e.g., Florida coral), while others may be more appropriate in the mid-continent, flat-lying carbonates. However, virtually any geophysical technique used to characterize the fractured and folded (bent) carbonates of the Appalachians are fraught with problems. The reasons for potential success and failure will be discussed in relation to the geologic regime with emphasis on the problems presented by “bent” karst interpretations.

In no instance do we believe that any geophysical techniques can be used as a stand-alone tool. They must be used in conjunction with direct, invasive test data from borings, probes and test pits. Use only with care and trepidation.