An engineering site investigation using non-invasive geophysical approach

Subsurface geological formation is essential to validate design assumptions for the construction of deep engineering structures, especially in the weathered terrains. The geological formation can be delineated using resistivity values through an electrical survey. However, the subsurface resistivity alone is ambiguous to interpret the subsurface geological units. As part of an ongoing investigation to select the key methods towards this end, an integrated geophysical survey through a combination of electrical resistivity tomography (ERT), induced polarization (IP), magnetic method and joint profile method (JPM) was carried out in a weathered terrain of South Huizhou, China. Resistivity, IP, and magnetic data were obtained using a variety of survey parameters. Subsurface resistivity was calibrated with upfront boreholes lithology to constrain geological formation into four discrete layers such as topsoil cover with resistivity 2-3257 omega m, highly weathered layer having resistivity 2-636 omega m, partly weathered layer with resistivity range 448-1204 omega m, and unweathered bedrock having resistivity 791-116,497 omega m. The integration of ERT with IP, magnetic and JPM delineated four faults namely F1, F2, F3 and F4, and several localized fractures. The weathered layer, fractures and faults were marked as the weakest zones for engineering projects, whereas the unweathered fresh bedrocks were identified as the most appropriate locations for the construction of deep structures in the study area. The weakest zones unsuitable for engineering structures were delineated as the most appropriate places of groundwater occurrence in the studied area revealed by low resistivity ranging from 2 to 1204 omega m and overlapped by low chargeability less than 14.8 ms. This non-invasive geophysical approach suggests the most suitable locations highly significant not only for the future construction of engineering structures but also the exploitation of groundwater resources in the investigated area.