Magnetic susceptibility and VLF-R investigations for determining geothermal blowout contaminated area: a case study from Alaşehir (Manisa/Turkey)

Abstract

Western Turkey is one of the most spectacular regions of widespread, active continental extension in the world. The most prominent structures of this region are E–W trending Gediz and Buyuk Menderes Grabens. Geothermal activity around city of Manisa in Gediz Graben has been investigated by many researchers and many geothermal boreholes were drilled in order to produce electricity. In the middle of May 2012, a geothermal blowout occurred 150 m away from the deep drilling (depth 1,100 m), in Alkan village of Alasehir resort in city of Manisa. After that, four big blowouts took place at the same area and thermal water had been diverted to Alkan stream for almost 4 months. The boron (B) and Fe levels of thermal water are 87 and 11.2 ppm where the background B and Fe values of topsoil vary between 0.2–1.5 ppm and 3.08–5.63 ppm, respectively. The objective of this study was to utilize joint magnetic susceptibility and VLF-R investigations to determine possibly contaminated area with B, related to high Fe level on the topsoil and thermal water in the subterranean layers. It is observed that magnetic susceptibility values vary between 500 and 790 cgs (×10−6) in the vicinity of blowouts and 150–340 cgs (×10−6) at a distance of 50 m around, where the background average value is 20 cgs (×10−6), by using the data collected at 346 stations. Besides, the VLF-R studies were carried out along 18 profiles at three different frequencies. Laterally constrained two-layer inversion was applied to each station; in addition to the inversion of all profiles for each frequency, all lines were stacked, 2-D resistivity maps were obtained, and the contaminated area was determined approximately as 35,000 m2. The results show not only satisfactory agreement between magnetic susceptibility and VLF-R data, but also provide fast and valued interpretation of geochemical and geophysical properties of the geothermal contaminated area.