Analysis and Interpretation of Gravity data to modelling ‘PB’ Geothermal Field

The ‘PB’ field is located on the island of Sumatra which is related to the volcanic activity of Kaba hill which still retains residual heat from the magma. One of the geophysical methods used in this case is the gravity method where this method can be applied to determine the location of reservoirs, possible locations for reservoirs, and even to examine geological features and heat sources. In this paper, gravity data is used to interpret the structure and possibility of modelling the heat source in the ‘PB’ geothermal field. The gravity data was analyzed using integrated gradient interpretation techniques such as Horizontal Gradient (HG) and Euler Deconvolution (ED) methods. The CBA map produced shows two dominants high Bouguer anomalies underlying the ‘PB’ geothermal field associated with the presence of dacite and andesitic rocks. Analysis of fault structure based on geological data, HG and ED enhancement data on gravity anomalies was used as a reference for the initial model of subsurface conditions in the inversion model. The 2-D modelling results reflected high-density rocks associated with active ‘PB’ mountain volcanism. The conclusion obtained from the analysis and interpretation of the ‘PB’ geothermal system is that this area still has active volcanism, which is a heat source, with the presence of diatreme found through 2D gravity inversion modeling. The conceptual model results obtained by combining the integration of each reference and existing research show that the upflow zone is at the top of Kaba Hill and the outflow zone is at Babakan Bogor hot spring and Suban hot spring. Thus, these results can provide comprehensive information in analyzing and interpreting as well as the use of enhancement data gravity HG, ED and 2D slice inversion in the description of structures and modelling in the Geothermal ‘PB’ field.


Introduction
The gravity method is often used to analyse potential geothermal studies.The method can be used to determine potential areas, reservoir locations, possibly study geological structures and heat sources ( [1][2] [3][4]).In 2015, the Center for Geological Resources (PSDG) introduced geological and geochemical information for support the analysis of the potential of 'PB' geothermal field.The tectonic setting of the investigation area belongs to the Sumatra magmatic arc with a Quaternary volcanic environment.Geologically, the rocks in the 'PB' geothermal field are generally of andesite -basaltic type originating from Bukit Itam, Bukit 1960, Bukit Biring, Selojuang, Penyeluan and Kaba.The results of age determination (K-Ar) show that basalt lava product rocks were formed at the age of 500,000 years ago.The formation of the 'PB' geothermal field complex is influenced by tectonic activity in the direction of the Sumatra fault pattern and its antithetics which are oriented southwest northeast.The geothermal system is characterized by the appearance of manifestations in the form of solfatara, fumaroles, hot springs with the highest temperature of 94°C, neutral pH and rock alteration at the top of Kaba Hill (advanced argillic-argillic alteration) and at the Sempiang manifestation.The hot springs in the 'PB' geothermal system are bicarbonate and sulfate types in the immature water zone.The hot fluids in the 'PB' geothermal system are bicarbonate and sulfate types in the immature water zone.The reservoir temperature is taken through the calculation of CO2 gas geothermometer of Sempiang manifestation and the high SiO2 correlation in Babakan Bogor manifestation, so it is assumed to be around 250°C.Geothermal prospect areas based on CO2 anomaly data, Hg and structural patterns are divided into three areas with a total area of 38 km 2 spread around Sempiang manifestation, Babakan Bogor Manifestation and Suban hotsprings [5].
In 2018, PSDG conducted geophysical measurements consisting of gravity and MT in the southern direction of the 'PB' geothermal field [6].The conclusion is that the 'PB' geothermal field is a volcanic system associated with a quaternary volcanic system where the heat source comes from a new product of Kaba Hills, based on residual gravity anomalies there is structural control as evidenced by the presence of Sempiang hot springs, but babakan bogor hot springs are controlled by the Great Sumatran Fault (GSF) system.The existence of cap rock from gravity and MT modeling that has been done which is suspected as a young kaba alteration rock.Geothermal reservoirs in the PB field are under the cap rock with an indication of resistivity of 10 -60 ohms and a density of 2.4 gr/cm 3 [6].In 2019, PT PLN Indonesia conducted additional gravity geophysical data acquisition to the north of the 'PB' geothermal field in Figure 1.In this paper, gravity data is used to interpret the structure and possibility of modelling the heat source in the 'PB' geothermal field.The 'PB' geothermal field is in the Magmatic Arc region, which is accompanied by the appearance of a geothermal system.The 'PB' geothermal field geological condition can be seen in Figure 2. The history of Kaba Hill formation began in the early quaternary in the form of explosive eruptions that produced Old Kaba lava products I to III and pyroclastic products that flowed and fell.Along with the tectonic activity that developed in the Sumatra region at that time, the Kaba Mountain area experienced a violent eruption that formed the lip of the caldera, as seen on the map in the western and eastern parts of the Kaba Complex.This data is supported by the discovery of pumice and bombs up to lapilli sized scoria in the Babakan Bogor manifest and its surroundings.The large amount of scoria also supports the formation of a strong eruption with a large pyroclastic volume.After a major eruption that formed a caldera rim pattern, there was a vacancy and intensive fracturing.Northwest-southeast trending faults such as the Kaba fault and Sempiang fault facilitated subsequent magmatic eruptions from the Bukit Itam, Bukit 1960, Biring, Salojuang, and Kaba mount products, which were oriented almost west-east in accordance with the direction of the Itam fault.Each of these products is within the caldera rim of the Kaba Complex, while the Bandung fault, which develops on the southern Kaba body, facilitates the appearance of cinder cones such as those found in the Kepahiang Indah area.
The last eruption record obtained from the data of the Directorate of Volcanology states that in 1951 there was an eruption that produced the Vogelsang crater on the eastern side of the Kaba Complex.In 2002, there was an ash eruption and hydrothermal eruption in the Big Kaba Crater.To date, there has been no new lava formation that characterizes magmatic activity, but there are still small hydrothermal eruptions at the Kaba Besar Crater.This reflects the formation of a hydrothermal system in the area.Geological data of the geothermal area of Kepahiang shows the presence of rocks that have impermeable properties and contain clay minerals such as montmorillonite and kaolinite, which are quite high in the alteration area around the Sempiang manifestation.The alteration rocks formed are argillic to advanced argillic types.The alteration appears on pyroclastic flows and lava products of Kaba Hill.These host rocks are in the zone of the Sempiang fault structure, which runs almost north south.In addition to the alteration data, other possibilities that can be interpreted as host rocks are interpreted as a young lava product of Kaba that is massive and has not been strongly curdled.

Methodology
The Gravity survey was carried out by the PT PLN Persero in 2019.The measurements were conducted using a Scintrex CG-5, which has a repeatability accuracy of 0.005 mGal and a reading resolution of 0.001 mGal [8].The gravity survey measurements were focused on the northern part of the 'PB' geothermal field to the top of Kaba Hill.The CBA was also subjected to gradient analysis using Horizontal Gradient (HG) for source edge detection and in addition, a 3D Euler Deconvolution (ED) study was performed to obtain source locations and line patterns.

Horizontal Gradient (HG)
The Horizontal Gradient works in finding the boundaries of areas of contrasting density from gravity data.The method is based on the horizontal gradient of the gravity anomaly, resulting from a tabular body, tending to cover the edges of the body if the edges are vertical and separated from each other [9].The HG method in describing shallow and deep sources is very good.the following formula is used [9] : Where   and   are the derivatives of the gravitational field in the x-direction and y-direction, respectively.

Euler Deconvolution (ED)
Euler deconvolution is used to characterize the location and depth of the gravity source anomaly.The Euler Deconvolution equation in 3D is given by [10]: Where ( 0 ,  0 ,  0 ) is the position of the detected gravity anomaly source at (, , ),  is the regional value in the gravity data, and n is the structural index (SI).The SI has been chosen using prior knowledge of the source geometry.For example, SI = 2 for spheres, SI = 1 for horizontal cylinders, SI = 0 for faults, and SI = -1 for contacts [11].

Complete Bouguer Anomaly (CBA)
The complete Bouguer Anomaly Map for the 'PB' geothermal field is shown in Figure 3.The high anomaly shown in pink has a value range of 37-55 mGal and is interpreted as older volcanic rocks in the form of dacite and andesite rocks at the top of Kaba Hill to the east and south.Medium anomalies with a range of values of 7-36 mGal located in the central and southwestern parts are interpreted as the response of basaltic rocks dominated by pyroclastic rocks that have undergone weak-moderate weathering.Low anomalies with a value range of -8 to 6 mGal are scattered in the western, northwestern, and central parts of the study area, which are interpreted as the response of rocks that have undergone weathering from weak to strong levels due to increased hydro solution to strong due to increased hydrothermal solution.

Residual Anomalies
Residual anomaly obtained from the subtraction between complete Bouguer anomaly and regional anomaly.The residual anomalies in the 'PB' geothermal field are shown in Figure 4.In the figure, the residual anomalies range from -4 to 2 mGal where the Suban hotsprings manifestation and Sempiang manifestation are between the 2 mGal and low anomaly -4 mGal.This indicates that both manifestations are controlled by structures.The residual anomaly map was overlaid with structural map.There are many indications of geological structures in the form of faults interpreted from local anomalies.A strange pattern of density and contour lines can be seen in the core area of the study.This line indicates the caldera structure that covers the Kaba Hill area.The southwest area of the local anomaly map shows a density of anomalies that are thought to be segments of the active NW-SE trending Sumatran Fault structure that controls manifestation at the Babakan Bogor manifestation.

Horizontal Gradient (HG)
HG was performed on the complete Bouguer anomaly map to clarify the presence of fault structures and the results can be seen in Figure 5. High HG values shown in red indicate the presence of geological structures in the form of faults.Geological data in the form of fault structures are shown with yellow lines on the HG anomaly contour map which has similarities with the interpretation of geological structures.

Euler Deconvolution (ED)
The ED results shown in Figure 6 provide depth estimates from 50 meters to above 1500 meters.The Euler solution in this area is mostly dominated by depths of 50-600 meters.The structural interpretation of the HG does not always give the same results as the ED.One possible indication is due to the dip of the interpreted fault.This will be seen in the gravity modeling data.The ED with a straight-line pattern may indicate the presence of faults in the study area.

Inversion model
The results of the inversion of the research area were carried out by inversion of the local anomaly map with a depth of ± 4000 m by ignoring topography.Data processing using zondGM2D includes slicing 2 passes SW-NE, NW-SE, and interpreting density boundaries and possible heat source models.Iteration using Levenberg-Marquardt for 20 times with a 2% error.High anomalies are shown in pink with a value of 9.35 x 10 -2 g/cm 3 with low anomalous values in blue with a value range of 8.7 x 10 -2 g/cm 3 .The 2D model can be seen in Figure 7 (SW-NE line) and Figure 8 (NW-SE line), where the Bukit Kaba area indicates a high density that spreads to form a geological structure in the form of a diatreme.
In figure 8, there is a high density anomaly that is suspected to be impermeable dacite and andesite and possibly caprock.This is in accordance with the lithological depiction [7] and future modeling and indicates that volcanism is still active in Bukit Kaba [12].For anomalous values that have a range of 1 x 10 -2 g/cm 3 to 2.9 x 10 -2 g/cm 3 and are yellow in color, it indicates a fault structure and the presence of a caldera [13] .In figure 8, the Suban hot spring is indicated to have occurred due to a fault.Low-density rocks with values ranging from 4.8 x 10 -2 g/cm 3 to 8.7 x 10 -2 g/cm 3 are assumed to have undergone weathering or alteration from weak to strong levels due to the rise of hydrothermal solutions and the emergence of existing structures and scattered rocks from rock lithologies in the form of andesite lava rocks and pyroclastic flows that have undergone alteration [14].It is known from MT measurements based on [15] that the Base of Conductor (BOC) is located at a depth of 1800 m with host rocks in the form of argillic and advanced argillic type alteration rocks [5].

Conceptual model
From the integration of geological, geochemical, and geophysical data shown in Figure 9. Hydrothermal flow in the PB geothermal system is shown using arrows flowing from the south [15] and north.The upflow covers the rim location of the Kaba Hill caldera and to the south at Sempiang hot spring, while the outflow goes southwest to Babakan Bogor manifestation and northwest to Suban hotspring.It is known from MT measurements based on [15] that the acid zone is inside the caldera rim and is 6 developing due to the active volcanism in the new Kaba Hill.The BOC is located at a depth of 1800 m [15], with host rocks in the form of argillic and advanced argillic-type alteration rocks [5].

Conclusion
The conclusion obtained from the analysis and interpretation of the 'PB' geothermal system is that this area still has active volcanism, which is a heat source, with the presence of diatreme found through 2D gravity inversion modeling.The high-density anomaly located on the NW-SE trajectory is thought to be impermeable dacite and andesite rocks and possibly cap rocks.In the southwest area, the Babakan Bogor manifestation is controlled by the GSF, as depicted by the 2D gravity inversion modeling.In the northwest area, the Suban hot spring manifestation is controlled by faults.The caldera in the 'PB' geothermal system is thought to be an impermeable fault, so that the fluid cannot flow up to the surface.The conceptual model results obtained by combining the integration of each reference and existing research show that the upflow zone is at the top of Kaba Hill and the outflow zone is at Babakan Bogor hot spring and Suban hot spring.It is known from MT measurements based on [15].that the BOC is located at a depth of 1800 m with host rocks in the form of argillic and advanced argillic-type alteration rocks [5].