Urban Geophysics: Geophysical Signature of the Mount Bonnell Fault and its Karstic Features in Austin, Texas
Environmental Geophysics Associates, 2000 Cullen Ave., Ste. 7, Austin, Texas 78757
Although most karstic regions are characterized by caves, collapsed features, and sinkholes, such features often do not have surface expressions, and their presence may go unrecorded. Central Texas and the Greater Austin metropolitan area have been built on the karstic limestone (Lower Cretaceous of Glen Rose Formation and Edwards Aquifer) in the Balcones Fault Zone, and the growth of Austin area is expanding. Nearsurface karst features in the Austin area have a profound effect upon geotechnical engineering studies, such as structural foundations (residential buildings, shopping malls), utility excavations, tunnels, pavements, and cut slopes. Thus, the practice of geotechnical engineering is and has been a challenging proposition in the Austin area.
Geophysical methods are sporadically used to estimate the locations and parameters of these karst features prior to any of the above-mentioned geotechnical studies. Opinions concerning the effectiveness of these geophysical surveys are mixed, and geophysical techniques are not generally recognized as primary tools in engineering-scale studies.
However, remarkable advances in the manufacturing of geophysical instruments over the last ten years have made geophysics a viable tool for geotechnical studies of karst features. Data quality have improved with the advent of continuous data collection. The data are better processed and interpreted by new and improved software packages that produce improved subsurface imaging and mapping.
Thus, integrated geophysical surveys can provide new insights into the near-surface karstic features in the Glen Rose Formation and Edwards Aquifer. The author conducted geophysical surveys (ground penetrating radar [GPR], resistivity imaging, magnetic [G–858], conductivity [EM–31] and natural potential [NP]) at two locations where the Mount Bonnell Fault is present, along the northern limiting boundary of the Balcones Fault Zone. Results indicate that all methods successfully imaged significant karst anomalies across the known fault locations. Integration of all these anomalies provides a much better understanding of near surface geology defined by the caves, voids, collapsed materials, sinkholes, and the fault itself.
AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012