Geophysical records of anthropogenic sinkhole formation in the Delaware Basin region, Southeast New Mexico and West Texas, USA

A significant minority of sinkholes that formed in gypsum bedrock in the Delaware Basin region are of human origin. These anthropogenic sinkholes are often associated with improperly cased abandoned oil wells or with solution mining of salt beds in the shallow subsurface. In July 2008, a sinkhole formed abruptly at the site of a brine well in northern Eddy Co., New Mexico. The well operator had been injecting fresh water into underlying salt beds and pumping out the resulting brine for use as oil field drilling fluid. Borehole problems had prevented the operator from conducting required downhole sonar surveys to assess the dimensions of subsurface void space. The resulting sinkhole formed in just a few hours by catastrophic collapse of overlying mudstone and gypsum and in less than 1 month had reached a diameter of 111 m and a depth of 64 m. Fortuitously, a seismometer had been deployed 13 km southeast of the brine well a few months earlier and precursor events were captured on the seismograph record a few hours before the subsurface cavity breached the surface. Four months later, another sinkhole collapse occurred in northern Eddy Co., again associated with a brine well operation. These events prompted the New Mexico Oil Conservation Division to review its regulations regarding brine well operations in the southeastern New Mexico oil fields. A third brine well within the city limits of Carlsbad, N. Mex., has been shut down to forestall possible sinkhole development in this more densely populated area. Electrical resistivity surveys have been conducted adjacent to the Eddy Co. sinkholes to assess the potential for additional subsidence or collapse events in the future.