Identification of Rock Characteristics Using the Microtremor Inversion Method at Air Putih Geothermal Field, Lebong Regency

Research has been carried out to identify rock characteristics based on Vs values using the microtremor method in the Air Putih geothermal field, Lebong Regency, which aims to determine the geothermal prospect area on rock characteristics using Vs values based on the microtremor method. The data obtained in this study were 20 measurement points obtained from data analysis by obtaining the H/V curve through Geopsy software and HVSR inversion to obtain the Vs value of the inverse. The results of this calculation indicate that at a depth of 0-4 meters the homogeneous Vs value 200 m/s, at a depth of 4 meters the Vs value 200-440 m/s, at a depth of 20 meters the value of Vs 440-880 m/s and a depth of 20 meters homogeneous Vs value of 880 m/s and for the distribution of Vp values, at a depth of 0-5 meters the Vp values vary between 2000-2900 m/s, at depths of 6-20 meters in general the value of the Vp wave velocity is the same in the range of 3100-3800 m/s and at depth 21-100 meters the Vp value ranges from 3900-5100 m/s. This shows that the greater the depth, the denser the rock.


Introduction
Geothermal energy is a renewable energy source in the form of thermal energy generated and stored in the earth's core [1].The increasing need for energy and the scarcity of fossil energy sources have spurred other countries.To reduce dependence on on fossil energy by utilizing geothermal energy to produce electrical energy.Geothermal energy is a renewable energy that is environmentally friendly (clean energy) compared to fossil energy sources, but the utilization of geothermal energy in Indonesia is generally not optimal.
Based on Law Number 21 of 2014, geothermal utilization is classified into two types, namely direct and indirect utilization.One of the geothermal energy potentials found in Bengkulu Province is the geothermal energy source located in the Air Putih area, North Lebong sub-district, Lebong district, which has a potential of around 173 MWe, located in the position of 3°LS and 102°BT.
Based on field observations, the geothermal area is located at a topographic height of 446 m above MSL and there are also sources of high temperature hot water (100°C -115°C) located in several places, the source is a surface manifestation of a geothermal system that has not yet been explored by researchers [2].According to Toth and Bobok (2017), Geothermal Energy is heat energy that comes from the crust, mantle, and core of the earth with high temperatures, when the earth's components are in hotter conditions than conditions on the surface, heat energy will continue to move from hotter subsurface conditions to the surface through impermeable rock [3].The existence of a geothermal energy system is usually characterized by surface manifestations.Some surface manifestations show symptoms such as fumaroles and solfatara, hot springs, hot mud, soil vapor, geysers, craters, and alteration rocks.The important components of a geothermal system according to Suharno [4]  Various exploration methods can be used to identify potential geothermal energy sources in the subsurface, geophysical methods are methods that are generally often used for the exploration of geothermal energy sources [5].The geophysical method used in this research is the microtremor method and processed by the HVSR (Horizontal Vertical Spectre Ratio) method.This method is a method of comparing the spectrum of the horizontal component to the vertical component of the microtremor wave [6].Microtremor consists of a basic variety of Rayleigh waves, it is thought that the peak period of the microtremor H/V ratio provides the basis of the S wave period.The H/V ratio in microtremor is the ratio of the two components that theoretically produces a value.The dominant period of a location can basically be estimated from the peak period of the microtremor H/V ratio [7].
The dominant frequency value can provide information about the type and characteristics of rocks found in the study area.Amplification is the magnification of seismic waves that occurs due to significant differences between layers, in other words, seismic waves will experience magnification, if they propagate in another medium that is softer than the medium that was passed before.The greater the value of the difference produced, the greater the magnification experienced by the wave will be [8].Previous research conducted by Arintalofa et al. conducted research on geothermal manifestation areas by processing microtremor data that had been measured using the Horizontal Vertical Spectral Ratio (HVSR) method, the conclusion was that the Diwak and Derekan geothermal manifestation areas were in a geothermal system located in the zone of outflow, the appearance of manifestations in the form of hot springs in the Diwak and Derekan research areas is due to the presence of faults in the form of normal faults which are considered as weak zones and cause the release of geothermal fluids to the surface [9].
Microtremor measurement is a passive seismic measurement to record vibrations generated by earth activity or human activity, this method is used to determine the condition of the subsurface structure based on its dominant frequency and amplification factor.The HVSR method was first introduced by Nogoshi and Iragashi who stated the relationship between the ratio of horizontal and vertical components to the ellipticity curve of Rayleigh waves which was later refined by Nakamura [10].
The HVSR analysis method is the vibration of shear waves stored in sedimentary materials or materials located on top of bedrock.The HVSR Equation formula is: The subsurface Vs value obtained from the HVSR curve can be used to determine the Vs value based on soil classification [11].Based on the above background, this study aims to determine the geothermal The microtremor data analysis process is obtained from the field in the form of vibration waves by the PASI Mod Gemini 2 Sn-1405 Seismometer in the form of SAF files, processed using Geopsy Software with the HVSR method to produce H / V curves to obtain the dominant frequency parameters (0) and amplification (0).HVSR inversion using Dinver Software aims to model the structure under the earth's surface using the principle of Monte Carlo (MC) by reducing misfit.Parameter calculations are performed iteratively until the H/V curve matches the measured H/V curve and obtains the shear wave velocity (Vs).After the data processing is completed, the final result is a 2.5 dimensional surface model.

Results and Discussion
The velocity value of shear waves (Vs) to identify the characteristics of subsurface rocks in the Air Putih geothermal field area of Lebong Regency has been classified based on.Figure 1 is a 1 dimensional profile of the shear wave velocity (Vs) value against depth in meters from microtremor data starting from the H/V curve.The structural analysis of the shear wave velocity value Vs is guided by the table of soil site groups according to SNI 1726 (BSN, 2019).regarding the relationship of the Vs value with the site group and the assumption of the layer structure in the rock, especially the hardness level of the rock and its characteristics.

Conclusion
The value of shear wave velocity (Vs) in Air Putih, Lebong Regency shows that at several points of the data collection location the wave velocity Vs at a depth of 1 meter to 100 meters does not change much, meaning that it is assumed that the type of material in the layer at that point is homogeneous.And most of the research locations show that at a depth of 100 meters the Vs value varies until the largest value is 1000 m/s, this indicates that at a depth of 100 meters it is assumed that the rock type is hard rock (hard rock) because given the nature of seismic wave propagation in the subsurface medium, the greater the Vs value, the denser the rock and vice versa.

Figure 2 .
Figure 2. Profile 1 D microtremor data in the form of primary wave velocity (VP) and shear wave (Vs).

Figure 3 .
Figure 3. Distribution of Vp values at a depth of 5 meters.

Figure 4 .
Figure 4. Distribution of Vp values at a depth of 10 meters.

Figure 5 .
Figure 5. Distribution of Vp values at a depth of 30 meters.

Figure 6 .
Figure 6.Distribution of Vp values at a depth of 50 meters.

Figure 7 .
Figure 7. Distribution of Vp values at a depth of 100 meters.

Figure 9 .
Figure 9. Distribution of Vs values at a depth of 5 meters.

Figure 10 .
Figure 10.Distribution of Vs values at a depth of 10 meters.

Figure 11 .
Figure 11.Distribution of Vs values at a depth of 30 meters.

Figure 12 .
Figure 12.Distribution of Vs values at a depth of 50 meters.

Figure 13 .
Figure 13.Distribution of Vs values at a depth of 100 meters.

Table 1 .
Group of soil or rock sites according to SNI 1726.

Table 2 .
The value of seismic wave velocity in volcanic rocks according to Gardner and House (1987).