Geophysical Characterization of a Subsurface Igneous Formation in Medina County, Texas

Prolific volcanism during the Late Cretaceous resulted in the emplacement of hundreds of igneous masses predominately within or atop the Austin Chalk in Central Texas. Often referred to as "serpentine plugs," the features are recognized to be submarine volcanic mounds trending northeast to southwest roughly parallel to the Balcones and Lulling fault systems.

In 2015 a driller attempted to install a well in the Edwards aquifer on a property in Medina County, Texas. The driller encountered what was described as a hard igneous lithology at approximately 91 m (300 ft) below surface. Initial attempts at drilling the well were unsuccessful due to the unexpectedly hard lithology. In addition, problems with wellbore deviation occurred. The driller suspected interaction with a fault plane was to blame for the deviation problems. After multiple dry holes the driller relocated by chance to an area that was absent of any subsurface igneous lithology.

The purpose of this project is to accurately determine the location, extent, and morphology of the suspected volcanic formation by utilization of complementary geophysical investigation techniques. It is hypothesized that the additional constraints and a high density of geophysical observation points will provide a more accurate and complete subsurface model of the igneous body than previously available data in the study area.