Combined SBAS-InSAR and geostatistics to detect topographic change and fluid paths in geothermal areas

Abstract

Topographic changes or ground surface deformation provide critical information regarding the fluid ascent paths that form in permeable fracture zones. This study combines surface displacement data using the small baseline subset-interferometric synthetic aperture radar (SBAS-InSAR) technique, lineament density, topographic elevation, and water type in geothermally active areas to detect highly permeable zones to assess geothermal resource potential. The Bandung Basin (2000 km2) with several geothermal fields, including Kamojang (KGF), Wayang Windu (WWGF), and Tangkuban Parahu (TPGF), is selected as a case study. An ALOS PALSAR dataset composed of 20 descending and 24 ascending orbital modes acquired in 2007–2011 is used for SBAS-InSAR together with a digital elevation model dataset from the Shuttle Radar Topography Mission (SRTM) for lineament analysis. Although interferograms are available in only half of the study area owing to low coherence, ordinary kriging is successfully applied to clarify the vertical and horizontal E-W displacement over the entire area. Both displacements are in agreement with the GPS data within a 95% confidence level. The detected uplift and subsidence depend on the activity and location of production and injection wells in KGF and WWGF, whereas topographic change in TPGF is dominated by magmatic activity. An important finding is the geologic structural control over topographic change, and in particular, that displacement tends to abruptly change near faults, and the displacement pattern becomes complicated in closely distributed fault zones. A weakly positive correlation between vertical displacement and lineament density is observed in water types classified as sulfate, chloride, and bicarbonate. The largest vertical displacement occurs in the sulfate water type, which shows advanced hydrothermal alteration, weakened rocks, and large topographic changes owing to natural and anthropogenic factors.