THG Geophysics was asked to investigate an open hole that was exposed during excavation activities. The opening measured approximately 7 feet by 3 feet and was 8 feet deep. The objective of the study was to characterize the extent and identify a potential source of the opening.
To map the extent of the surface void , THG designed a high-resolution, shallow geophysical survey. We deployed electrical imaging, ground penetrating radar (GPR), and microgravity. A GF Instruments ARES electrical resistivity imaging (EI) system was deployed to map the subsurface to investigate for potential voids by analyzing the resistivity contrast of subsurface materials. A Sensors and Software ground penetrating radar system configured with a 250 MHz antenna was used to analyze to a depth of approximately 15 feet. Finally, a microgravity survey was conducted to further characterize the subsurface by assessing the Earth’s pull of gravity at measurement points.
An irregularly shaped resistive feature was identified in the electrical imaging data extending over a large portion of the site, west of the open void. The GPR survey located several structures indicative of voids near the open hole. Additionally, a high amplitude GPR response was mapped to the west of the open hole that correlated with the shape and location of the resistive feature identified in the electrical imaging data. The gravity data also indicated extension of the open void to the west, but was limited due to restrictions on accessible survey area.
The resistive feature that was mapped with both GPR and EI could be indicative of a more competent rock structure than that containing the nearby open void, or a highly weathered, porous (air-filled) structure.