Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 Hybrid Gravimetry for Optimization Time Lapse Monitoring Data: A case study in Kamojang Geothermal Field Yayan Sofyan 1,4, Jun Nishijima 1, Yasuhiro Fujimitsu 1, Yoichi Fukuda 2, Makoto Taniguchi 3, Yunus Daud 4 1 Department of Earth Resource Engineering, Kyushu University, 744 Motooka Nishi-ku Fukuoka 819-0395 Japan 2 Department of Geophysics, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan 3 Research Institute for Humanity and Nature (RIHN) 4 Physics Department, University of Indonesia, Kampus baru UI Depok, Indonesia sofyan_hasan@yahoo.com Keywords: Hybrid gravimetry, Geothermal monitoring, Kamojang Geothermal Field ABSTRACT Time-lapse gravity monitoring is a way to assist in building up knowledge of the changes of subsurface condition. Gravity monitoring techniques have been applied to the investigation of dynamic processes in various types of geothermal and volcanic fields. Gravity changes enable characterization of subsurface processes: i.e., the mass of the intrusion or hydrothermal flow. A key assumption behind gravity monitoring is that changes in earth s gravity reflect mass-transport processes at depth (Battaglia et al., 2008). Combined absolute and relative gravimeter is an advance method that is newly used for monitoring. Use of an absolute gravimeter as a reference for other relative measurements, the technique called hybrid gravimetry (Okubo et al., 2002), was applied successfully to geothermal studies (Sugihara et al., 2008; Nishijima et al., 2010). We used relative gravimeters (LaCoste-Romberg or Scintrex) and were combined by absolute gravimeter (A-10 Micro-g) to monitor the subsurface condition. A10 absolute gravimeter is a new generation of portable absolute instrument and has accuracy 10 µgal. This method has many advantages for monitoring method. Some of the advantages are the ability to reduce uncertainties caused by regional gravity variations and reduce the drift correction factor. The repeated absolute gravity measurement was conducted in the Kamojang geothermal field in 2011. The repeated relative gravity measurement also was conducted in the same field. Some gravity benchmarks were measured using both absolute and relative gravimeter and was used as the reference benchmarks. The combined absolute and relative gravity measurement can reduce drift correction of relative measurement. In a long time period, the hybrid gravimetry method will improve the result of gravity change data for monitoring in Kamojang geothermal field. This present study introduces a time lapse monitoring method that will enhance reservoir monitoring using repeated precisely-gravity measurements. 1. INTRODUCTION Repeated gravity measurements in the Kamojang Geothermal Field (KGF) were carried out using both relative and absolute gravimeter since 2009. Comparison of gravity data is made between earlier and later measurement in the same benchmark. The purpose of repeated measurements during exploitation is to identify and describe variations in the gravity anomaly distribution in and around the reservoir area of the active geothermal field. This technique is relevant to the estimation of natural recharge into the reservoir and the response to production and injection rate (Sofyan et al., 2010). The precision gravity is also commonly used in the geothermal industry to constrain the change in water saturation in 3D reservoir simulation models of geothermal reservoir (Atkinson and Pedersen, 1998; Protacio et al., 2001). Combine gravity measurement using relative and absolute gravimeter at the same time provides solutions to reduce uncertainties caused by regional gravity variations and drift factor. Relative gravimeter has an advantage of a few times faster measurement that absolute gravimeter but gets many uncertainties and acquires relative value only. By tying the obtained relative gravity to absolute station, the drift correction of relative gravimeter is then characterized. A Hybrid gravity survey combines the advantage of the inherent long term stability of absolute gravity measurements with the simplicity and speed of relative gravity measurements to achieve the best of both survey methods (Micro g lacoste, 2010). The KGF is located in the Garut region of West Java, Indonesia (Figure 1). Reservoir area of KGF is about 21 km 2 and has a vapor dominated system. In late 1982, production of 30 MWe (Unit I) was started at KGF. Drilling development was continued and two 55 MWe units (Units II and III) were added in 1987. At the end of 2007, a 60 MWe (unit IV) was added to complete a 200 MWe installed capacity at KGF (Hochstein and Sudarman, 2008). More than 20 years since 1983, about 160 millions of tons of steam have been extracted from the reservoir and about 30 million tons of condensed and river water were injected into the reservoir system. The KGF is a steam dominated system which has only steam production with a small proportion water reinjected from the cooling tower condensate. KGF has enlarged rapidly the amount rate produced from 8 to 13 MT/ year, while injection and recharge rate to the reservoir has limited rate between 2 and 2.7 MT/year (Suryadarma et al., 2010). The KGF, the oldest geothermal field in Indonesia with large production in a more than a quarter century, requires precise monitoring using hybrid gravimetry. 1
Figure 1: Location and reservoir boundary of Kamojang Geothermal Field 2. ABSOLUTE GRAVITY MEASUREMENT AT KGF The A10 (#017) absolute gravimeter from Micro-g LaCoste has been used for precise monitoring at KGF. The A10 absolute gravimeter was a portable gravimeter that can be used at the reference station and at some selected sites close to the production and injection wells. The repeated gravity measurement using an absolute gravimeter was conducted at KGF with some advantages. This introduces a new technology of using a portable of absolute gravimeter for the monitoring geothermal reservoir. Absolute gravity measurement will enhance the method of reservoir monitoring using repeated precisely-gravity measurements. The gravity data monitoring using an absolute gravimeter was also independent of the possible temporal gravity change or regional variation at the reference base station and actually does not require the reference base station. The A10 used a laser, an interferometer, a long period inertial isolation device and an atomic clock to measure the position of the test mass very accurately (Nishijima et al., 2010). The setup measurement for one set data consisted of 100 drops. The precise absolute gravity data of one benchmark were estimated from 10 sets data and took about 50 minutes to one hour for measurement including preparations at each benchmark. The parameters also were characterized for drop interval about one second and set interval between two sets data about two minutes. The absolute gravity measurements at KGF were started in 2009 and then repeated in 2010 and 2011. The eleven absolute gravity benchmarks located inside of the reservoir boundary near to wells and one absolute gravity benchmark, which is PG48A, is located far from production and injection wells. The PG48A benchmark was used as the reference and part of a network with other eleven benchmarks inside of a reservoir boundary (Sofyan et al., 2014). Location of the absolute gravity benchmarks is adjacent to relative gravity benchmarks (Figure 2). 2
Figure 2: Absolute and relative gravity benchmarks at KGF There were 12 absolute gravity benchmarks in the field and a GIC benchmark located in the core sampling building. These absolute benchmarks were selected by available locations. Some criteria for the possible absolute benchmark are a flat surface, a wide area for the absolute equipment, small noise, good contact with the ground, and has accessed road by car to the benchmark. According to availability and limited access, we had a small number of absolute gravity benchmarks that fulfilled the criteria and our lack of measurement sites away from the reservoir. The absolute benchmarks were located inside the reservoir area except PG48A benchmark was located outside of reservoir boundary. The network of absolute gravity benchmarks in KGF used PG48A benchmark as a reference to others benchmark. We need the PG48A reference benchmark in this measurement as comparison study of gravity variation between inside and outside the reservoir boundary. Absolute gravity benchmarks were surrounded by relative benchmarks in KGF. The relative gravity measurement and the leveling surveys in the first period (1999 2005) consisted of a network of more than 51 benchmarks and covered an area of about 35 km 2. Relative measurement in 2008 using Scintrex relative gravimeter was only about 30 benchmarks due to some benchmarks were lost or broken. Detailed of relative gravity measurement at KGF was explained in previous research (Sofyan et al., 2011). We evaluated the absolute data, such as drops data, laser, and other parameters during measurement in the field. We prepared the measurement carefully to gain good results that were indicated by small, scattered data and uncertainty of the drops and laser data. We conducted measurement more than once at one benchmark that have large scattered and uncertainty of gravity data. The 3
average uncertainty of absolute gravity measurements at each benchmark in each year measurement was around 10 to 11 μgal. Uncertainty of each measurement was directly shown in display unit of absolute gravimeter. 2.1 Corrections and calibration Hunt (2000) explained the correction factors of the gravity measurement were classified into the correction of variations with position, which varies with time and changes in position of mass in the earth. The correction and calculation were applied in the data processing of the observed gravity data in KGF. The drift, height and tide corrections are the standard correction factors that have to be calculated in the gravity measurement. The observed gravity data were firstly corrected for tide, height and drift corrections. Some of these corrections were directly provided by gravimeters and the rest were estimated manually. The seasonal changes in the shallow groundwater level can have a significant effect on the precise gravity data. Therefore the time of the absolute gravity measurements in 2009, 2010, and 2011 was scheduled during the dry season around July as the lower term of the rain falls rate in Indonesia. The same season of gravity measurement was expected to provide similar saturation and groundwater level in that area. The changes in the shallow groundwater level were calculated in the 16 shallow wells by the local people in KGF between 1999 and 2005 (Sofyan et al., 2011). The gravity correction that is caused by shallow groundwater level changes was very small and had an average value of about 0.845 μgal. The A10 absolute gravimeter has software that can correct directly the effects of earth tides, ocean load, barometric pressure, and polar motion in acquiring the gravity data. The different height of the absolute instrument while using a tripod in uneven surface of benchmark was measured in order to calculate the height correction factor in the software. Free air correction due to elevation changes was explained of about 0.3085 mgal/m. Figure 3: Calibration absolute gravimeter in 2009 and 2010 (left) and illustrated calibration in 2011 (right) In 2009 and 2010, we calibrated absolute gravity data at KGF with comparison study to the series of absolute gravity data using the same equipment at Kyushu University, Japan. We analyzed absolute gravity data in 2009 that were relatively stable while the tendency of linear decrease of about 0.12 µgal/ day was found in the series of absolute gravity data in 2010. We corrected absolute gravity data in 2010 by this calibration data. In 2011, the calibration process was carried out with a comparison with FG-5 absolute gravimeter. The FG-5 gravimeter has more accurate result but not portable equipment. The accuracy of absolute gravity data using FG-5 was about 2 µgal but its measurement took more than 5 hours nonstop for 10 set data. The calibration process was performed in two main equipment parts, which was laser and clock calibration. In the laser calibration, the effect of the frequency standard as well as the laser interferometer data on the quality of gravity data was investigated. Average frequency of blue and red lasers of measurements that have similar wavelength was compared to the nominal frequency of A10 absolute gravimeter. The gravity difference of laser calibration was about - 20 µgal. In the clock calibration, the rubidium clock of the absolute gravimeters was calibrated on the same day of laser calibration. According to oscilloscope analysis data, difference time of one wavelength between the A10 and FG5 was 4.6125 s. The gravity difference of clock calibration was about 23 µgal (Sofyan et al., 2014). 2.2 Absolute gravity data Gravity measurement using absolute gravimetry in 2009, 2010 and 2011 indicated mostly small scattered data and uncertainty factor around 10 Gal that was close to the accuracy of the A10 absolute gravimeter but 5 absolute data has uncertainty between 11 and 12 Gal. According to small uncertainty factor, we assumed the absolute gravity data was good data. The corrected gravity change between two measurements over same benchmarks were evaluated in terms of mass variations in subsurface. The trend of negative absolute gravity change ruled the distribution of gravity variation data between 2009 and 2010 in almost all absolute benchmarks except KMJ 11. Another comparison of absolute gravity data between 2010 and 2011 displayed a domination of negative gravity variation or associated with mass decrease in south and north area while west and east area showed positive gravity variation dominance. The absolute gravity change data are illustrated in Figure 4. 4
In the some benchmarks of the north and south area, the data of gravity changes between 2010 and 2011 more widely decreased in mass than that of changes in the first period between 2009 and 2010. The production rate between 2009 and 2011 has almost similar trend with 200 MWe installed capacity. Some make-up wells have been continuously added and some standby wells were activated according to a high production demand. According to the vapor dominated system of KGF, it had a very small amount of injected water. Figure 4: Absolute gravity data at KGF (Sofyan et al., 2012) 3. HYBRID MEASUREMENT AT KGF In the combined relative and absolute gravity measurement, we can optimize the relative gravity value by another calibration to absolute gravity value. The calibration factor from two absolute gravity benchmarks is: c i i 1 (1) g g oi g g o1 where c i, g 1, g i, g o1, g oi are calibration factor on benchmark i, absolute gravity at reference benchmark 1, absolute gravity at benchmark i, gravity observation relative at reference benchmark 1, gravity observation relative at benchmark i, respectively. We analyzed the combined 12 absolute gravity measurements and relative measurements in 2009 at KGF. The calibration factor between a difference of absolute and relative gravity data from 2 absolute benchmarks in one array of relative gravity measurements varied from 0.964 to 1.069. The calibration factor of relative gravity data before drift correction had larger difference than that of relative gravity data after drift corrected. The relative gravity data was quite good because of the value of the calibration factor was close to 1, which means the best fit between relative and absolute gravity data. We assumed that PG48A as a reference benchmark for the calculation of the calibration factor because of its location outside of the reservoir. The PG48A gravity data from relative and absolute gravity measurement should be the most stable value. The absolute gravity value of relative gravity measurement using Scintrex in this benchmark was expected same with the true absolute value from A10 gravimeter. We made a comparison of graphs between absolute data from the relative measurement (raw relative data, drift corrected relative data, calibrated relative data) and the absolute data from A10 absolute gravimeter. The graph explained by the calibrated relative data had the closest value to the true absolute gravity data. There were 6 benchmarks that had difference values of less than 10 µgal between the absolute gravity value of relative and absolute measurement. The other 6 benchmarks of comparison between the relative and absolute measurement had a deviated value of more than 100 µgal. The comparison graph can be seen in Figure 5. 5
Figure 5: The comparison absolute gravity data at KGF between relative and absolute measurement 4. DISCUSSION Gravity monitoring using only relative measurement always assumed the reference or base station is constant. In a long period, this assumption will be not true because the regional gravity has possible change during repeated measurement due to many factors. In the previous research (Sofyan et al., 2014), we discovered the possibility of changes at the base station of gravity monitoring in KGF has increased about 3 μgal/year. This variation comes from regional gravity change and other factors. Absolute gravity measurement in geothermal field is the best way to account for a regional effect, and to correct for changes at the base and the reference station over time. Some needs to be considered in the absolute gravity measurement using A10 gravimeter is the accessibility of the equipment and time consumption of the measurement. The absolute benchmark needs wide access to let people bring A10 gravimeter by car. Scintrex relative gravimeter has advantages of small body, not heavy, easy to carry in the field and also short time to measure. The hybrid gravimetry is combined between the precise absolute measurement and fast relative measurement. Geothermal monitoring using the portable absolute gravimeter at KGF is very valuable to evaluate the accuracy of gravity data using relative gravimeter. The calibration of relative gravity data with absolute data in 2009 can obtain more precise data of relative data. The largest deviation of KMJ28, the calibrated gravity data in this research, was possibly caused by the large uncertainty of absolute gravity measurement at this benchmark more than 11 µgal and large scattered data. The comparison graph described the calibrated relative data has the closest value to the true absolute gravity data. 5. CONCLUSIONS We carried out repeated absolute gravity measurement at twelve benchmarks in KGF. Some corrections and calibration for relative and absolute gravity data were calculated to obtain more accurate data. The uncertainties of the absolute gravity values were around 10 µgal that was close to the accuracy of absolute instrument and some of them are more than 11 µgal. The relative gravity data was also quite good because the value of the calibration factor was close to 1 as the best fit between relative and absolute gravity data. The comparison of graphs between the absolute data from relative measurement (raw relative data, drift corrected relative data, calibrated relative data) and the absolute data from A10 absolute gravimeter was made to analyze the relative data. The graph explained the calibrated relative data (using the calibration factor) had the closest value to the true absolute gravity data. In the data in 2009, there were 6 benchmarks has a difference value less than 10 µgal between the absolute gravity value of relative and absolute measurement. The other 6 benchmarks of comparison between relative and absolute measurement had a large deviated value. The analyzed method of hybrid gravimetry data in 2009 was also applied to data in 2010 and 2011. The use of the absolute gravimeter is the best way to account for a regional effect, and to correct for changes at the base and reference station over time. The hybrid gravimetry, a combination of absolute and relative gravity measurements, provides a direct solution for precise value in the larger gravity station network in the geothermal area. The hybrid gravimetry will improve the result of gravity change data for monitoring. ACKNOWLEDGMENT The first author acknowledges this research activity was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 25.03068. 6
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