Assessing shallow rock conditions over a brine cavern using seismic methods

The 16 July 2008 collapse of a brine well southeast of Artesia, New Mexico, has led to concern about the stability of a brine well facility located in Carlsbad, New Mexico. Characterization, monitoring, and mitigation studies were initiated in 2009, and emergency response procedures are in place should it collapse into a large sinkhole that could threaten nearby structures, two highways and a major irrigation canal. Results of precision survey indicate that differential subsidence is occurring across the facility and is greatest in the vicinity of the brine well. It is anticipated that ground movement-induced fracturing or crushing of competent, shallow caliche horizons could be a precursor to collapse of the material over the brine well cavern into a large sinkhole. A 24-channel seismograph was used to obtain compression wave (p-wave) seismic refraction and refraction microtremor surface wave (s-wave) data at selected locations in the vicinity of the brine well to characterize the condition of the shallow caliche horizons. When intact and unfractured, these caliche horizons are anticipated to be high strength with high seismic p-wave velocities of at least 3,000 m/s. Fracturing or crushing of these materials dramatically reduces the material mass strength and seismic velocities. Shallow seismic profiles totaling about 730 m were completed around the facility. High seismic velocities were interpreted in the caliche adjacent to the highways abutting the facility, while low seismic velocities were interpreted adjacent to a major irrigation canal where measured subsidence was greatest.