A proposed gas fired power generating plant is being constructed over an area of historic subsurface coal mining. In order to reinforce the subsurface from potential mine subsidence, extensive grout injection operations were implemented. During these operations, small sink holes appeared at the surface and anomalous features were encountered during drilling. THG was contracted to image the subsurface of the site to characterize the anomalous features and investigate the nature of the sink holes.
THG designed and deployed a 2-D electrical resistivity imaging survey to image the site. Over 10,000 feet of high resolution electrical imaging data were collected and imaged to a depth of over 125 feet below grade. THG was able to correlate the electrical imaging data with boring logs for accurate lithologic control during interpretation. The resulting data sets exhibited numerous features of concern that indicated mine room collapse in several areas of the site and provided explanations for the sink holes.
Site conditions were not conducive for quality data collection; however, THG implemented a unique, site-specific QAQC program that helped to mitigate field obstacles (mud, inclement weather, active construction, etc). Data quality is a primary concern for THG so that we may maximize the value of our services for our clients. The application of geophysics in this case, though challenging, was a valuable and cost-effective means to provide our client the information they desired.