In April 10, 1997, a rice field in Nankan county, northwestern Taiwan was flooded by sixty to seventy kilolitres of fuel oil as a result of an accidental underground pipeline leakage. Immediate action to excavate contaminated soil and remove it to a landfill were done. However, it was believed that a significant quantity of light non-aqueous phase liquid (LNAPL) contaminant might have remained in the soil and infiltrated into the groundwater.This paper emphasizes the use of multiple geoelectric techniques to detect and map the LNAPL plume in this uncontrolled real-world site, and help monitor the effectiveness of the clean-up operation. A significant change in resistivity values was detected between polluted (> 140 ohm-m) and non-polluted areas (< 140 ohm-m). Repeated measurements were conducted at 1, 4 and 10 month intervals after the first measurements. These data were used to monitor variation and a possible spreading of the LNAPL plume over time. The total LNAPL masses were concentrated or diluted in the soil matching the variations of the water table. Two additional resistivity profiles were conducted to investigate the spatial distribution of the LNAPL contaminant plume within the study area.Electromagnetic induction and ground penetrating radar were also used to outline the resistivity zone defined by the plumes. The results of the survey serve to provide insight into the sensitivity of geoelectrical methods for detecting a shallow subsurface LNAPL plume, and help to provide valuable information related to monitoring the movement of an LNAPL plume over time in a study area.

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Geophysical surveying in water-covered and swampy areas is particularly challenging. This paper presents a new survey strategy for such surveying that integrates ground penetrating radar (GPR) and resistivity image profiling (RIP) methods at the water surface to investigate geologic structures beneath rivers, ponds, and swamps.Two test sites, a pond and a lake, have been selected to evaluate this new survey strategy. Experiments in both areas have been successful in delineating the structure of underlying gravel layers. The depth of water and shallow structures obtained from GPR data provided an effective constrain during processing of RIP data. Deeper structures were delineated using RIP data. The integration of GPR and RIP methods conducted at the water surface was successfully applied to map the Hsincheng fault crosscutting the Tourchyan River in Hsinchu County.This paper shows that the use of GPR and RIP at the water surface is efficient in mapping geological structures beneath water. The proposed approach suggests the potential for conducting geophysical surveys along rivers and drainage canals in urban areas, places where roads and buildings impede other methods designed to detect active fault.

Time-lapse methodology was applied to cross-hole electrical resistivity tomography (CHERT) to investigate two groundwater contamination sites. In the first case study, resistivity profiles were used to delineate the transport direction and spatial distribution of the contaminant, which can serve as a basis for adjusting the remediation treatment by the remediation team. In the second case study, changes in electrical conductivity were used to evaluate the remediation reagent's transport direction and area of effect, and this was used to indirectly verify the effectiveness of the remediation efforts. CHERT equipment was installed simultaneously at the monitoring wells, which enhanced the benefits of the boreholes, enabling them to be even more economical. In large-scale groundwater contamination sites or sites with complex hydrogeological environments, application of CHERT techniques can result in greater amounts of data, particularly in analyzing localized preferential flow paths. This data would be greatly beneficial to the remediation of groundwater contamination sites and long-term groundwater management.

Ground penetration radar imaging reveals a prominent reflector which is offset vertically by about 2.6 m at depth about 2.3 m across the scarp. Schlumberger depth sounding exhibits a disrupted marker that links the loci of maximum apparent resistivity at greater depths.Resistivity image profiling (RIP) surveys across the scarp yield either zones of low resistivity (two profiles) or disruption of high resistivity zones (one profile); those anomalies are suggestive of a low angle thrust fault.