Deep Ground Truthing Complex Geology

Westward was recently tasked with creating a preliminary mine plan as well as locating a suitable place to drill a deep industrial water well(s) for a limestone quarry site in southwest Texas.  The project focused on evaluating the very complex geologic structure in the area, which is a result of numerous surface and subsurface intrusive igneous bodies. These igneous bodies influence and compartmentalize the groundwater in the area, alter the groundwater gradient, and impact the amount of minable limestone. Most of the igneous bodies have no surface expression and were only detected by a previously flown aeromagnetic survey and some very limited drilling. 

Based on detailed Westward surface geologic field mapping, very preliminary and limited drilling, as well as the aforementioned aeromagnetic data, Westward’s geologic team designed and performed an electrical resistivity program, to approximate depths of 1,000 feet. A total of six separate resistivity transects were completed on orientations parallel and perpendicular to the main structural trend at the project site. The resistivity method was chosen to better define the geologic structure, better identify specific formations and possibly locate groundwater bearing strata, both at depth and over a large area. Each profile was approximately 2,780 feet in length, and the results verified numerous field mapped faults and a couple of potentially large buried igneous bodies, which generally (although not completely) coincide with portions of the aeromagnetic data. A core hole was previously drilled in the immediate vicinity of one of the igneous bodies. The core showed the presence of a volcanic tuff from 30 feet – 90 feet (total depth) below ground surface. The resistivity survey in this area also identified a very conductive clay layer in the subsurface, which is above the mineable unit, functionally creating an interburden that is considered waste. The results from the resistivity study, coupled with the field mapping, are useful in refining and quantifying the vertical and horizontal limits of the minable material. Better defining and understanding the complex structure at the site plays an essential and critical role in the development of a detailed mine plan as well as in siting the location(s) for water well(s).