Abstract
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.
Highlights
The concept of resistivity surveying is based on potential field measurement; TAO, Vol 17, No 2, June 2006 resistivity data from water-covered areas is influenced by the water layer (Loke 2000)
Geoelectric signals are often distorted in an underwater environment, this preliminary study shows that Ground penetrating radar (GPR) and resistivity image profiling (RIP) surveys can be integrated to successfully provide details on geological structures beneath water layers
The estimated water depth gleaned from RIP is usually less accurate than that taken from GPR
Summary
The concept of resistivity surveying is based on potential field measurement; ,. Ground penetrating radar (GPR) has been successfully employed to map water depth and underwater stratigraphy in shallow lakes and rivers filled with fresh water (Annan and Davis 1977). Image profiling (RIP) is a newly developed multi-electrode method that has been successfully used in land surveys It is suitable for areas where lateral variations of geological structure have rendered vertical electrical sounding (VES) unsuitable (Griffiths and Barker 1993). RIP can provide relatively more lateral and vertical information on sub-water strata than conventional DC sounding (Loke 2000) It improves survey efficiency by performing measurements at the surface of the water; and topographic effect can be eliminated when conducting such measurements. The crosscutting of the Hsincheng fault through the Tourchyan River of Hsinchu County was mapped
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