Characterizing physical properties of ground surface based on Synthetic Aperture Radar (SAR) are crucial for geological target detection under Torrid Zone condition (Saepuloh et al., 2010). The backscattering intensity of SAR chiefly a function of two physical quantities of surface material, the surface roughness and dielectric parameters. The two parameters were proved effective to discriminate ground surface materials at volcanic field such as pyroclastic flows, lava, and lahars (Saepuloh et al., 2010). In this research, we focused to obtaining surface parameter related to the permeability based on Polarimetric SAR data. The surface permeability is a crucial parameter for estimating geothermal potential. Furthermore, analyzing spatio- temporal of surface permeability will be important to understand the dynamic of fluid extraction in geothermal power plant.
There were two could be modeled from PolSAR data, termed as the surface roughness and linear features. The two parameters were used as basis of surface permeability assessment according to the field measurement data. The surface roughness means the topographic expression of ground surface at horizontal line with gravel scale and controls the backscattering signals significantly (e.g. Duarte et al., 2008; Saepuloh et al., 2012; Saepuloh et al., 2015). The interaction between hydrothermal fluid and host rocks is aimed to be analyzed by surface roughness parameter. The resistance of the rocks presented by their roughness to the geological processes such as alteration due to geothermal process depends on rock type and/or thermal intensity. The surface roughness was identified for three geothermal manifestation zones, altered surfaces, mud pools, and hot springs in the selected study area of Mt. Wayang Windu, situated in the southern part from Bandung City, West Java, Indonesia (Fig. 1).
Figure 1: Study area located at Mt. Wayang Windu in West Java, Indonesia with sites of explored geothermal prospects in gray arc segments (Hochstein and Sudarman, 2008). Inset showing local topographic of study area presented by SRTM 30 m DEM (Farr et al., 2007).
2. FIELD MEASUREMENTS
Obtaining the appropriate model, we measured surface roughness at field in three directions: azimuth, range, and dominant topographic undulation. The azimuth and range directions follow the satellite movement and line of sight, respectively. A fixed pin meter with correlation length 30 cm similar to the L-band frequency was used for measurement. The polarimetric mode of the Phased Array Synthetic Aperture Radar (PALSAR) onboard Advanced Land Observing Satellite were used in this study. There are 269 points measured surface roughness at field with three different direction of measurement (Fig. 2 upper part). The field roughness measurements permit to establish three qualitative classes of geothermal surface manifestation: alteration zones, mud pools, and hot springs. These classes are depicted by photographs in Figure 3 with measurement methods using a pin meter. In this paper, the discussion is focused on the surface roughness characteristics at the three classes: Zone-1, 2, and 3 (Fig. 2 upper part). Zone-1 located between Bedil and Windu is composed by altered surfaces, Zone-2 between Puncak Besar and Gambung is composed by mud pools, and Zone-3 is composed by hot springs. The selected zones are presenting general geothermal surface manifestation at Wayang Windu geothermal field (Fig. 2 lower part). The ground surface of Zone-1 located at Wayang Crater is composed by vegetation, grasses, and fragmental rocks with size 5 – 30 cm. The rock surface generally has been altered to be soil with bright greyish to yellowish color. The vegetation is rare at alteration areas. Zone-2 located at Burung Crater is composed mainly by dense vegetation, grasses, and farms. The ground surface is composed chiefly by flat soils with mud pools at the crater area. Zone-3 located at Kertamanah is composed by grasses, farms, and ponds. The tea plantations are also existed at hilly terrain. The hot springs deliberate warm water about 55° C with pH 6.3 to the ponds. The ground surface is dominated by the almost flat soils and grasses.