Magnetotelluric Method Applied for Exploration of Geothermal Resources in Sumatra

Pertamina has conducted geothermal exploration in Sumatra showing that the depths to the top of most of the geothermal reservoirs are about 1000-2000 meters. Previous geophysical exploration work by Pertamina emphasized the use of DC-resistivity methods such as the Schlumberger method. However, these techmques can typically only penetrate to a depth of less than 1000 meters and are too shallow to map the geothermal system in most areas. With the magnetotelluric (MT) method, investigation to depths of greater than 1000 meters are routine, whch gives the explorationist a better picture of the deep resistivity structure. Pertamina now applies MT methods to explore for geothermal resources in most area. In conjunction with the MT data. time-domain electromagnetic (TDEM) data are usually measured to correct for static effects in the MT data. The first step in interpreting MT data, it is common to use maps of apparent resistivity at a certain period typically 1 second to delineate an anomaly of low resistivity area. Resistivity cross-sections are then constructed through the most interesting areas starting with one-dimensional (l-D) models, which are spliced together along the cross-section. Twodmensional (2-D) resistivity modeling could also be applied if the data showed to warrant a 2-D interpretation. In general it is found that the resistivity layers can not be clearly correlated with 1itholoD of the altered rocks and hydrology of the system. An important part of the interpretation process is to integrate the evaluations with other available data. This is necessary due to low resistivity anomalies are not always prospective as geothermal targets. For ths reason, integration with other geologic and drilling data, if available, can help to eliminate low resistivity anomalies from consideration that are not prospective. The MT method that has been Conducted in the areq of Sibayak appear to detect the reservoir rocw coincide with resistivity substratum of about 45 S2m The very resistive (greater than 100 S2m) su stratums show formations with high temperature bq low permeability. In the Ulubelu geothermal projea area, a 1200 meters drills hole in the conductive ar correlated well with the top of loss circulation a dnlling fluids and water level. To date there is n dnlling information for deeper resistivity structure., The top of the resistive sub-stratum was corresponq to the top of the intrusive body met by t emperam gradient well in Sumurup, Jambi-West Sumatra. confirmed that the top of the resistive substra 1 ,"