Mapping the Subsurface Structure of a Potential Shallow Geothermal Field in Northeastern Taiwan by Using the Seismic Reflection Method
Wednesday, 17 December 2014
The Ilan plane is situated in northeastern Taiwan, in a structure extension area at the southwestern end of the Okinawa Trough. The triangle shape Ilan plane is covered by thick sediment of several hundred meters, and is bounded by two mountain ranges at the northwest and the south. The outcrops collected from the northwestern and south mountain range are argillite and slate, respectively. The observed high geothermal gradient and the abundant hot spring indicate that the area has a high potential of geothermal energy at a shallow depth. This paper is to illustrate the subsurface structure of the area by using the seismic reflection method. The result will be used to build up a conceptual model for tracing the possible paths of hot water and search for a site for geothermal well. In 2013, two seismic lines were conducted to obtain a preliminary subsurface structure. We used two mini vibrators (EnviroVibe) as the seismic source and deployed 384 geophones in a split-spread to receive the data. The CMP profile in EW direction showed one clear interface separating the sediments and the bedrock and the other deeper event, dipping to the east. The profile in NS direction showed the same image of the bottom of the sediment; many indications of faulting were seen on that profile as well. A major fault was also interpreted in that profile; however, whether the rock types of the formations were slate or argillite was still to be identified. The result of the rock type will directly affect the location of the major fault, which could be a major path for the underground hot water. To detail the subsurface structure, we just completed 4 more seismic lines in the study area in this summer. The two EnviroVibe were used again, but the geophone was increased from 384 to 432 channels. The geophone spacing was chosen as 4m, and the shot interval was set at 16m. Currently, core boring in two test wells is still ongoing. Besides the geologic studies, the rock samples from both outcrops and borings will be used to measure the P and S wave velocities, respectively. The collected seismic data are now on processing. Integrating the seismic profiles with data from core borings, surface geology, and physical properties of the rock samples, we believe we are able to provide us a detailed structure for studying the first shallow geothermal field in Taiwan.