Integrated geophysical study of near-surface faults in the Wilcox Group, Texas, with application to lignite mining

Abstract

Texas is the sixth-largest coal producer in the nation, a significant fraction of which is in the form of lignite, a soft coal found close to the surface and mined by opencast operations. To open up or expand a mine costs tens of millions of dollars. It is therefore important both economically and environmentally to investigate subsurface geological structures before expanding mine operations. A study has been carried out in two sites, located in an area where the richest lignite bearing formations in Texas are exposed. Several geophysical tools, namely magnetics, frequency and time domain electromagnetics (FEM and TEM), ground penetrating radar (GPR) as well as seismic refraction were tested in an attempt to qualitatively determine the most efficient and reliable tools for locating faults. We have found that fault location at the relatively noise-free Walnut Creek mine site is best performed by magnetics, owing to a distinct total field magnetic anomaly. Fault location at the Leholski Road site, a frequently-used farm access road surrounded by gates, fences and other metallic objects, is most clearly determined using a large-offset (~40m) FEM loop-loop configuration: while the other methods were not as efficient as fault-locators, each method offers certain advantages. For example, GPR shows great promise as a high resolution tool for the very shallow subsurface. TEM inversions require prior knowledge of the geology for formulation of the initial model. Refraction is found to be more suitable for velocity determination of formations with considerable impedance contrast, as opposed to fault location, at least in the type of sedimentary rocks found in our site.