| Title | Resistivity Structures of Lahendong and Kamojang Geothermal Systems Revealed from 3-D Magnetotelluric Inversions, A Comparative Study |
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| Authors | Imam B. Raharjo, Virginia Maris, Philip E. Wannamaker, David S. Chapman |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Lahendong, Kamojang, magnetotellurics, MT, geothermal |
| Abstract | Three-dimensional magnetotelluric (MT) inversions of Lahendong and Kamojang datasets have been performed to obtain their resistivity structures. Gauss-Newton finite diference method of Sasaki was used, with several modifications including changing the parameter step solution to modified Cholesky and invoking a different system of parameter smoothing weights. The 2005 and 2008 MT datasets were binned to sets of frequencies for efficient inversions and provided a depth range of sensitivity from ~100 m to ~5 km, where the final misfits were three and two respectively. Lahendong field has a liquid dominated reservoir in the north part and a two-phase region in the south part. The current electricity production is 40MW. The inversion reveals a subsurface dome with the resistivity of 20-60 ohm.m covered by a conductor, e.g. <10 ohm.m, that depicts a propylytic reservoir encapsulated by geothermal clay caps. The peak of the dome is situated under Linau acidic lake. The conductor rapidly thickens and deepens to the north and west, and gradually thickens to the south indicating a deeper prospect. The biggest well LHD-23, >35MW, penetrates the center block of the dome, where the propylitic region at 1.5 km deep is about 3x4 km2. Kamojang on the other hand, is a vapor system hosted in a caldera surrounded by volcanic ranges. The fumaroles are situated in the north-east corner of the caldera at a higher elevation. The inversion reveals a spectacular finding. At a shallow depth, a conductor from the fumarolic area extends down to the caldera and merges with the conductive formations. The conductor becomes pervasive with depth down to about one km where the propylitic zone starts taking places. Within the production depth, e.g. 1.5 km, a subsurface u-shape proplytic ridge facing west is observed. The ridge represents the vapor zone that feeds the 200MW power plants. The shape of the ridge opens and vanishes to the west where the region becomes more conductive and associates with two-phase characteristics. An intriguing vertical resistive column, e.g. ~ 100 ohm.m feeding the Kamojang fumarolic area is also observed. The inversion suggests that the Kamojang caldera structure dictates the subsurface u-shape propylitic ridge in a steam dominated region. |