Page 73 - Demo

A Subsoil Inspection in a Mangrove Habitat Using Electrical Resistivity Tomography61Marine Research of the National Academy of Marine ResearchDOI: 10.29677/MR.202406_4(1).0005 A field resistivity inspection can find the resistivity variation zones and identify the sliding zones in aroadside high sliding-prone natural slope in Taiwan before and after storm events (Luo, 2007). The field ERT method has been used to explore the soil profiles of two newly-constructed dams, including silty clay and sandy silt (Wei, 2013). The investigation results showed inspection images that presented the same characteristics as those of back-filled, rolled, and compacted soil layers. Engineers face challenges in interpreting two-dimensional ERT results due to varying field geological conditions caused by either a fault or reservoir leakage. The spatial resolution, reliability of inverted resistivity section, boundary condition, and three-dimensional effect may mutually affect the interpretation of testing images. By matching the changes in the two-dimensional images for long-term monitoring, engineers can identify the spatial changes in the resistivity images and give reasonable explanations (Hung, 2013). An elapsed-time ERT technique was developed to monitor the potential seepage in the damaged dam slopes nearby a concrete spillway in Taiwan (Wang et al., 2023). Tracing the downhole brine seepage could effectively identify the possible direction and path of the seepage flowing through the rockfill dam mass in a few hours. The spatial resistivity variations can provide information about the potential sliding surface and the accumulation zones of the seepage in the downstream side slope of the dam. An on-site investigation was required to inspect the possible groundwater leakage through an earth dam during either its full-level storage period or empty storage period (Wang et al., 2018). Significant variations on the resistivity space distribution were feasibly measured for tracing aberration leakage position on the dam mass. The pervious silty sand was reasonably identified as the potential leakage section, which was located at the old river channel. A potential seepage belt could gradually develop along such a residual permeable layer. Combined with the salt watertracing technique, ERT could be used as a rapid and non-destructive method to estimate the hydraulic conductivity coefficient values in soils (Chen, 2020). Both the downhole and cross-hole ERT approaches at different locations could effectively detect the difference in resistivity distribution caused by tracer transmission. Combined with in-door groundwater simulation analysis, engineers could directly estimate the groundwater flow rate, and then estimate the on-site hydraulic conductivity coefficient values. The four field investigation cases in Taiwan indicate that the ERT can efficiently identify the inclination of strata up to 5-6 meters deep (Yang, 2005). Deeper positions show less sensitivity in resistivity images. The resulting resistivity images are strongly affected by the difference between survey layout direction and strike of strata and the resistivity variations in geomaterials. The simulation results showed: (1) the angle between the survey line and the fault strike is less than 60 degrees and the three-dimensional effect is obvious; (2) when the angle is greater than 60 degrees, the three-dimensional effect gradually disappears (Hung, 2013). The experiments to identify the correlation between soil moisture and resistivity were conducted in sandboxes under the wetting and drying periods (Yao, 2007). When the soil conditions change from a wet state to a drystate, or from a dry state to a wet state, the regression results of electrical properties are different; this is calledthe hysteresis effect. The conductivity or resistivity in soil is also heavily affected by temperature under ahigh-water content condition. Slow wetting rates could effectively reduce hysteresis in resistivity measurements (Lin, 2009). In addition, when measuring the groundwater level with the resistivity inspectionmethod, the groundwater level is affected by the capillary phenomenon in soils (Wu, 2018). The capillaryphenomenon leads to partially saturated soil around the water table, which decreases the resistivity values insoils. The impact effect is positively related to the capillary phenomenon. A higher degree of capillarityincreases a deeper measurement depth and higher inspection error percentage in soils.