Surface and Subsurface Fracture Zones Modeling Using Automatic Lineament Analysis and Geostatistical Method, with Case Study of Wayang Windu Geothermal Field, West Java, Indonesia

Abstract

Geological structures such as faults and fractures in a geothermal prospect area might be an indication of a permeable zone for geothermal fluids through fractures that appear inside the rocks. Such geological structures can be identified by lineaments in satellite images and aerial photographs. In this study, we determine the effect of pixel size on the results of automatic lineament extraction and examine the relationship between lineaments at the Wayang Windu geothermal field (WWGF) and nearby geothermal areas, automated lineament results which were extracted from digital elevation models (DEMs) with pixel sizes of 0.5 m, 1 m, 2 m, 3 m, 4 m, 5 m, 10 m, 15 m, and 20 m based on aerial photographs, DEM Nasional (DEMNAS) images, and Shuttle Radar Topography Mission (SRTM) images. Under similar parameters, larger pixel sizes revealed fewer and longer lineaments than smaller pixel sizes. The type of image (i.e., satellite images or aerial photographs) used for lineament extraction did not affect the results. Based on lineament density maps, the lineaments from images with smaller pixels had a wider distribution area than those with larger pixels. Surface geothermal manifestations, such as hot springs, showed good correlation with lineaments found in images with smaller pixel sizes (≤ 5m), which were found in moderate to high lineament density zones. Based on the lineaments extracted from SRTM images, each geothermal field in southern Bandung (Patuha, WWGF, Darajat, Kamojang) had a local geological structure that greatly correlated with the lineament extraction results. All of the lineaments in the geothermal fields were primarily oriented in the NE–SW direction (i.e., N65°E for Patuha, N51°E for WWGF, N57°E for Darajat, and N24°E for Kamojang). Subsurface fracture zone modeling was conducted at WWGF to estimate the permeability zones based on the fracture density from production well data using the Ordinary Kriging (geostatistical) method. The results revealed the direction of subsurface fracture dips and strikes as well as the relationship between subsurface fracture geometries and surface lineament structures. In general, the subsurface fracture zones featured strikes in a NE–SW direction, with dip angles ranging from 74° to 85° and fracture density ranging from 36 to 43 fractures per 10 m. Additionally, the fracture density linearly corresponded to the depth of wellpads. Compared to the surface lineament extraction map, the majority of lineaments were in the NE–SW direction, similar to the major strike of the subsurface fracture zones.