Integrated Geophysical Surveys Applied to Karstic Studies Over Transmission Lines in San Antonio, Texas

CPS Energy^TM of San Antonio constructed three electric transmission lines (Fair Oaks Ranch Area, Stone Gate-Hill Country and Stone Gate to Green Mountain) in the last three years across the boundaries of the Edwards Limestone Aquifer in San Antonio, Texas. The purpose of geophysical studies (2-D resistivity imaging, ground penetrating radar and natural potential) was to determine voids, caves or any other karstic features beneath the power pole locations. Results of 2-D resistivity imaging, ground penetrating radar (GPR) and natural potential (NP) surveys helped to determine locations and depth of voids in the near surface as deep as 40 feet. In this way, we relocated the pole locations, if necessary, to reduce the potential for the construction to impact threatened or endangered species and to locate more solid limestone to seat the pole footings. One hundred fifty-four (154) pole locations were studied along these three transmission lines. During the drilling of these pole locations, which were based on the geophysical data, three significant (>5 feet in diameter) and two insignificant (<5 feet in diameter) voids were encountered. If a cave or void was encountered, an engineering evaluation was conducted to determine if the pole installation can proceed as planned, or if the pole installation design needs to be changed, or if the hole should be abandoned and a new hole drilled. Thus, the purpose of this paper is to discuss the geophysical data obtained over some of the drilled pole locations with significant voids and why the geophysical surveys did not detect them. In addition, we discuss the geophysical signature of a known karstic feature such as Tick'n Delight Cave in the vicinity of these pole locations.