Applications of TEM in Taiwan

Abstract

Based on the results of a large set of field data from Sirotem transient electromagnetic (TEM) soundings carried out in Taiwan, three case studies demonstrate the applications of TEM in Taiwan. The transmitter-receiver array used was the in-loop configuration with the dimensions of the square transmitter loops varying from 20 m to 400 m on a side for effective exploration depths of 20 m to 500 m. The Occam's smooth model inversion was employed to reduce the temptation to over interpret the data and to eliminate arbitrary discontinuities in simple layered models. The inverted results were filtered and then organised into both depth slice and cross sections to study the 3D geoelectric structures of the survey area. The first case study is located on the southwestern coast of Taiwan. The 3D resistivity images show fresh-water-saturated sand and/or gravel sediments in the northern part of the survey area, while a widespread sea-water intrusion is evident in the South. These findings are supported by more than 140 wells. The second case study is located in eastern Taiwan. Based on the TEM sounding results, one predominant electrical discontinuity can be recognised in the Longitudinal Valley that agrees well with the known suture trace on the surface between the Philippine Sea and the Eurasian plates. The third case study is in the urbanised Taipei Basin. The 3D resistivity images display basement topography and trace the probable faults of the Taipei Basin. These findings are supported by more than 180 wells. Therefore, TEM surveys can work around urbanised cities, where ambient electrical noise is high, and open spaces limited.