Constructing a Quaternary stratigraphic framework of Choushui river alluvial fan delta by seismic reflection and magnetotelluric surveys

Land subsidence is a severe problem in Changhua and Yunlin Counties because of over-exploitation of groundwater. As a result, the government promotes mitigation strategies, including building the Hushan Reservoir and artificial lakes, to replace the usage of groundwater with surface water. According to the recent data, although the amount of subsidence in the Choushui River alluvial fan in Changhua and Yunlin Counties is much less than 20 years ago, the rate of subsidence maintains several centimeters per year in some area. After considering the land subsidence in the shallower strata caused by groundwater pumping, we still got a compressed status in the deep. This observation shows that there could be other reasons leading to the subsidence in the deeper strata. The sedimentary deposits and structural features in the deep might directly cause deformation of the strata and the surface. On top of groundwater pumping, such deformation could make the land subsidence more serious and cause problems in the operation and management of water resources. Moreover, the current hydrogeological models for groundwater management are mainly based on the shallow wells (mostly no more than 300 meters), which are sparsely distributed. Hence, there is very little understanding of the deep geometric characteristics of the Choushui River alluvial fan. In order to clarify the reasons for the deep strata subsidence and to understand the three-dimensional morphology of the Choushui River Basin, we will deploy long-distance seismic reflection and Magnetotelluric (MT) surveys (40-60 km) to construct high-resolution interpretation of Choushui River alluvial fan. This two-year project would provide not only a more comprehensive and detailed hydrogeological model but also important information about water resources management in Yunlin County.