| Summary: | 碩士 === 國立臺灣海洋大學 === 應用地球科學研究所 === 101 === In this study, we used the electrical resistivity method for imaging subsurface structures of the lake bottom sediments in the Da-hu Lake in Ilan, Taiwan. Floating passive electrodes were utilized for the surveys in the study area. We deployed 24 survey lines across the lake, and executed the survey with Schlumberger and Wenner array. There are two geological cores, DH-7A and DH-7B, located at different locations in the Da-hu Lake. The cores were carefully collected for the purpose of the sediment and dating analysis, as well as the geochemical testing for the studies of climate change in the past 10000 years. The inverted images provide good understanding for the spatial relationships of the sediment and basement structures. The sediments above the resistive rock basement are consisted of the sand sediments (from 45 to 60 ohm-m) and mud sediments (20 to 25 ohm-m). The results show that a sandy layer with resistivity of 45 to 60 ohm-m is located at 3 to 8m under the water surface in the east part of the lake, and the outcomes confirm the findings from the DH-7A core data. The resistivity spatial distribution suggests that this sand layer is possibly brought in the lake from the East. We found that a high resistivity layer (>200 ohm-m) at about 5m under the water surface in the south part of the lake, according to outcrops of southern station, that might be the detrital sediments from lakeside to the lake. A sharp linear resistivity structure was found in the west part of the lake. Its strike is consistent to the observations from the surface outcrops of normal fault near the west part of the lake. Consequently, we infer that this structure line might be a stretch of the normal fault and the two cores are located at the hanging wall as well. In addition, the results show we could use the electrical resistivity imaging method with floating passive electrodes to investigate the lake-bottom in details.
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