Bedrock Study Using Resistivity Imaging

THG geophysics was contracted to perform a geophysical investigation to characterize bedrock and identify potential anomalous bedrock features as part of a proposed expansion project for a cement plant in South Carolina. During development projects, like this one, using geophysical survey techniques to analyze and characterize bedrock can save a great deal of time and money for engineers, developers and construction contractors when compared to time consuming invasive techniques like geotechnical drilling.

THG designed a geophysical survey to investigate the bedrock using 2-D electrical resistivity imaging, or electrical imaging (EI).  Considering the site geology (weathered sandstone and limestone) and surface conditions, EI was selected as the most appropriate method for imaging the subsurface.  The technique offers efficient data collection, easy interpretation, and robust processing methodologies.  EI profiles were collected in perpendicular orientations across the 125-acre property, spaced appropriately to achieve the necessary degree of site coverage.  The EI profiles imaged to a depth of 97 feet below grade.

Generally, geologic units have a common apparent resistivity value.  Low apparent resistivity values are typically associated with soils, saturated materials, and highly weathered bedrock, whereas high apparent resistivity values are associated with rock (increasing with rock competence).  Based on the observed resistivity values, bedrock was characterized and mapped across the entire site.  Additionally, a geostatistical grid using a kriging routine was generated and used to visualize how bedrock may undulate between survey profiles.