Examining lake-bottom structures with the resistivity imaging method in Ilan's Da-Hu Lake in Northeastern Taiwan

In this study, we used the electrical resistivity method for imaging subsurface structures of lake-bottom sediment in Da-Hu Lake in Ilan, Taiwan. Floating passive electrodes were used for the surveys in the study area. We deployed eighteen survey lines across the lake, and executed the survey with the Schlumberger and Wenner array. There are two geological cores, DH-7A and DH-7B, located at different locations in Da-Hu Lake. The cores were carefully collected for the purpose of the sediment and dating analyses, as well as for the geochemical testing, which can be useful to climate-change studies covering the past 10,000 years. The inverted images clarify the spatial relationships between the sediment and basement structures. The sediments above the resistive rock basement consisted of the sand sediments (from 45 to 60 Ω m) and mud sediments (less than 30 Ω m). The results of our study show that a sandy layer comprising slate debris with a resistivity of 45 to 60 Ω m is located 3 to 8 m under the water's surface in the eastern part of the lake, and the outcomes confirm the findings from the DH-7A core data. The resistivity spatial distribution suggests that the lake's sand layer originally came from a region east of the lake. We also found a sharp linear resistivity structure in the western part of the lake. The structure's strike is consistent with observations from the normal fault's surface outcrops near the western part of the lake. Consequently, we infer that this structure line might be a stretch of the normal fault and that the two cores are located at the hanging wall as well. In addition, the results show that we can use the electrical resistivity imaging method with floating passive electrodes to investigate the lake-bottom in detail.