Non-invasive 2D and 3D TL-ERT monitoring of moisture content fluctuation and associated geotechnical implications in an engineering site: a Niger Delta case study
摘要
Time-lapse electrical resistivity tomography (TL-ERT) has evolved as a powerful investigation tool that can be used in sequence with common point-based procedures to model seasonal moisture content (SMC) dynamics within the subsurface. This serves as a means of monitoring abnormal accumulation of water beneath structures in engineering and mine sites. These applications are crucial to mitigating the potential risk of geotechnical instabilities. TL-ERT data sets from Delta State, southern Nigeria, were employed in monitoring seasonal moisture content (SMC) dynamics at an engineering site. Twelve (12) 2D ERT profiles acquired in a 100 by 80 m2 grid during two time seasons were used to perform 2D and 3D ERT simultaneous inversion for monitoring of SMC using a TL-ERT inversion code. The entire 2D ERT data acquired in the x and y directions at the engineering site were merged to create 3D ERT data sets in the rainy and dry seasons. The 2D ERT data characterized the subsurface to a depth of 17.1 m, with high and low water content (with a resistivity range of 200–356 Ωm) observed in both seasons, attributed to the presence of soil layers with differences in their water-retention capacity. Borehole data obtained from the engineering site shows that the subsurface geology consists of clayey sand, fine sand, coarse sand, and lateritic sand. The fine/clayey sand layers with fine particles that retain more water can initiate geotechnical instabilities such as landslides, sinkholes, or mass movement, making the overlying materials susceptible to downslope movement and failure. The 3D ERT slices of inverted resistivity in the x and y directions imaged the subsurface to a depth of 19.8 m and revealed high and low water content in both seasons. The 3D ERT dynamic slice shows the precise anomalous water accretion zones, and the anomaly detected was at a depth of 6.6 m within the subsurface. These results serve as a proxy to track anomalous accretion of water underneath structures and intrusion or seepage into structures. These processes often create a low electrical resistivity (ER) anomaly that can be used to predict degradation of buildings, road structures, and railway embankments. The percentage differences in electrical resistivity (ER) between the monitoring periods showed little or no significant change in the ER of the sandy layer. Based on these findings, TL-ERT (a non-invasive geophysical technique), along with in-situ geotechnical data, can be applied for long-term geotechnical stability monitoring and mining site performance to mitigate the environmental impacts of structural instabilities of buildings, road structures, and railway embankments.