Geophysical Identification of Evaporite Dissolution Structures Beneath a Highway Alignment

Vast deposits of evaporites are present in the vicinity of Carlsbad, New Mexico. While evaluating a section of State Highway 128, sinkholes were discovered near and within the highway right-of-way. Several sinkholes, with dimensions of roughly 0.3 meter, were found in the vicinity of a dip section in the highway. Surface seismic refraction geophysics was used to both identify the presence of and delineate seismic anomalies in the shallow subsurface. A twelve channel seismograph with 36 meter geophone array and sledge hammer energy source was used. Typical seismic refraction results provided subsurface profiles where harder sedimentary rock was lying in a near horizontal orientation, where it dipped significantly, and where it was missing. In addition, anomalous loss of signal between geophones across the seismic lines indicated the presence of subsurface anomalies, including significant fractures. Refraction microtremor surface wave seismic was also performed. Resulting s-wave profiles indicated the presence of lower velocity rock horizons underlying the harder rock, provided a cross-check with seismic refraction interpretations and provided a deeper depth of investigation than the seismic refraction results. Test pits located based on seismic profiles confirmed the presence of shallow gypsiferous solution structures.