3D inversion modelling of gravity data for identification of subsurface structures in the mud volcano area of Sedati, Sidoarjo

Many mud volcanoes are found in Central Java and East Java in the north, but research on mud volcanoes is still limited. This research focuses on the Sedati mud volcano area, Sidoarjo with a research area of 500x400 meters. Measurement of gravity method data with 88 points spaced 50 meters to determine the value of rock density and subsurface structure of Sedati mud volcano area. From the data that has been obtained, data processing is carried out to obtain a map of the distribution of regional and residual gravity anomaly values. From the anomaly data, 3D inversion was carried out to obtain a map of the subsurface structure. The 3D gravity inversion modelling results show that there is high density rock block which is interpreted as a basin that holds the reservoir from the mud volcano. Near the surface and around the mudflow in the study area, the distribution of low density is caused by the presence of mud content. There is a possibility that there is a fault zone with a west-east direction from the study area. The results of the 3D gravity model incision show that the weak zone in the study area is a zone of accumulation of mud which has a density value of 1.8 g/cm3 at a depth of 20 meters to 500 meters.


Introduction
Mud volcano is a general term used to describe structures that emit water, mud, or other hydrocarbonderived materials.The cause of the formation of mud volcanoes is confirmed to be the result of the endogenous activity (from within the earth).According to Satyana and Asnidar [1], mud volcanoes also identified with the intrusion of relatively easy-moving material due to buoyancy on a plastic structure that has not been fully compacted with a high-pressure difference and reaches the surface.The existence of mud volcanoes is caused by seismic activity which is quite intense and triggers the release of high-pressure fluid derived from hydrocarbon derivatives of illitization of clay at subsurface depths [2].There are 20 mud volcanoes on the island of Java, and 14 of them are in East Java [3].One of the mud volcanoes in East Java is a mud volcano in Buncitan Village, Sedati District, Sidoarjo Regency.Sedati mud volcano has 5 KM from the beach and around the mud volcano, there are residential areas.When viewed from the geological map sheets of Surabaya and Sapulu [4], this Sedati mud volcano is in the Lidah Formation.The Lidah Formation is known as the clay facies of the Pucangan Formation.Information from geological maps is not sufficient to obtain a model of local subsurface structures in the study area, so a survey using geophysical methods is needed.
The gravity method is one of the geophysical methods that can be used in modeling subsurface structure.The gravity method can provide a model of the subsurface structure of the earth based on the distribution of the density values of the constituent rocks [5].The gravity method can be used because it can delineate the subsurface structure [6] in the form of an indication of the presence of mud which is characterized by low-density contrast with the surrounding rock [7].Research on mud volcanoes using gravity data has been carried out in Golestan Province, Iran [8] which succeeded in obtaining a gravity data model with a density variation of 2.55 to 2.239 gr/cm 3 .A gravity is also obtained model in Lokbatan, Azerbaijan with gravity values of -3 mgal to 5 mgal [9].In the research area, research has been carried out using the microtremor method which states that the Sedati mud volcano has a bedrock layer thickness of up to 70 m [10].However, this research is not sufficient to obtain a 3-dimensional model of the subsurface structures.Therefore, this study uses gravity data in modeling the structure of the Sedati mud volcano.In this modeling, an inversion process will be carried out to obtain a 3-dimensional model of the subsurface structure, to produce better information.The results of this study expected to provide information for the identification of faults in the Sedati mud volcano area.
The research location of the Sedati mud volcano is administratively located in Buncitan Village, Sedati District, Sidoarjo Regency, East Java Province.Geographically, the research area of this final project located at the coordinates of 7°23'59.1"S and 112°47'21.3"E.This area is 22 km from Surabaya City and 17 km from Sidoarjo City.The area of this mud volcano reaches 2.25 ha, while the research area has a size of 500 m (to the east) x 400 m (to the north).

Methodology
In this study, the data from the measurement of gravity in the mud volcano area were used.The data obtained from the measurement results are as many as 88 gravity measurement stations with a space of 50 m using the LaCoste & Romberg G-118 gravimeter.The distribution of gravity measurement stations is more focused on the area around the center of the mud volcano iin the center of the study area.The data that have been obtained are then carried out with general corrections such as drift, tidal, latitude, free air, and Bouguer corrections, to obtain complete Bouguer correction data.The data used in the 3-dimensional modeling process is residual anomaly data (local), so the residual anomaly is separated from the regional anomaly by using an upward continuation filter to obtain a residual anomaly map (figure 1).
This study uses an upward continuation filter which is carried out by transforming the potential field measured on a certain surface into a potential field on another surface further from the source [11].The result of this filter is a regional anomaly map, so to get a residual anomaly map, it is necessary to reduce the value of the complete Bouguer anomaly with the regional anomaly that has been obtained.Then this residual anomaly will be used as input data for the 3-dimensional (3D) modeling process which produces the distribution of rock density values.In this study, subsurface structure modeling using 3D inversion will be carried out.This 3D inversion process uses software made by Pirttijarvi ( 2004), namely the Grablox 1.6 software [12] which will produce a density distribution model in 3D.Pirttijarvi (2004) in his software, grablox 1.6, uses two inversion methods namely Singular valued Decomposition (SVD) inversion and Occam inversion [13] to get a good model with low error.
Zhao (2011) explains that Singular Value Decomposition inversion is a technique of parsing a mtrix into two matrices.Zhao (2011) also gives an example of SVD inversion, which is to decompose matrix A into matrix U and matrix V, if written mathematically [7], we get an equation like this: A = U S V T (1) where U is a left-hand orthogonal matrix, S is a diagonal matrix, and V is a right-hand orthogonal matrix.
Grablox software also uses the Occam inversion process, Constable (1987) explains that the Occam inversion is an inversion method that utilizes the roughness level of the model which can be written mathematically as follows: (2) where, ||ðm||2 is roughness, μ-1 is Lagrange multiplier, ||Wd-WGm||2 is misfit, and X2 is error.
Grablox 1.6 software integrates the two inversions in the creation of a 3D cross-sectional model to produce a smaller error value (difference between the observed values and the calculated data).This integration is carried out because the inversion modeling is usually not unique or singular [14], which means that the inverted model can produce several subsurface models.The model of the SVD inversion process and the Occam inversion will produce a more accurate density model.

Results and Discussion
The research area to be modeled on the Grablox 1.6 software has a size of 500 meters to the east (X-axis) and 400 meters to the north (Y-axis), and the target depth of the model is 600 meters to the vertical (Zaxis).For the initial model, the X-axis divided into 33 blocks (nx), the Y-axis divided into 27 blocks (ny), and the Z-axis divided into 10 blocks (nz) to produce 8910 minor blocks that make up the major blocks as shown in Figure 2. Then the anomaly data residuals are used as input data for the 3D inversion process to the existing initial model.The inversion process is carried out in stages such as optimalization of the base (base0, density (density), Occam density (Occam d), beam height (Height), and Occam beam height (Occam h).After all the steps are carried out, a 3-dimensional (3D) density cross-sectional model will be obtained.The results of the 3D density cross-sectional model will be display in a 2D density cross-sectional model.The 2D density cross-sectional model will display the vertical distribution of rock density from the results of the 3D density cross-sectional model.In this study, there are 6 2D cross sections (Figure 3), 4 incisions have a west-east direction, and 2 more incisions have a north-south direction.The cross-section of the 2D model of the A-A' path found at Y = 9181.55(Figure 3) is shown in Figure 4.This section is the southernmost section of the study area, and this section has a maximum anomaly of about 5 mgal.This is due to the presence of a layer of hard rock with high density (red color) with a value of 2.6 -3.00 g/cm 3 in the west with coordinates 697.2 to 697.3 which has a depth ranging from 0 meters and at coordinates 697.3 to 697.8 which has a depth ranging from 350 meters.And there is a minimum anomaly of about -15 mgal on the surface.This is due to the very low rock mass (dark blue) with a density value of 2.2 -1.8 g/cm 3 at coordinates 697.3 -697.7 and starting from a depth of Z > 50 meters.The possibility of this rock thought to be a weak zone and an area where mud volcano reservoirs exist.While at coordinates 397.6 there is a continuity form the weak zone east to west which is indicated by the arrow, this possibility is due to a fault that becomes the path of the mud to move to a larger reservoir.
The cross-section of the 2D model of the B-B' path found at Y = 9181.61(Figure 3) is shown in Figure 5.This incision is in the middle of the study area, and this section has a maximum anomaly value of 5 mgal.This is due to the presence of a medium-density layer of sediment on the surface (green color) with a value of 2.4 g/cm 3 spread over the surface.And there is a minimum anomaly of about -50 mgal on the surface.This is due to the presence of a very low rock mass (blue -purple), which has a density value of 2 -1.8 g/cm 3 which is spread throughout the incision and has a depth from 0 meters to 500 meters.This lowdensity area is an area that has visible manifestations on the ground surface, such as the presence of very small mudflows scattered at coordinates 697.3 to 697.5.The low anomalous value in this area caused by the very high salt content contained in the mud.This low anomaly also usually appears in the salt dome area, the closer the salt dome to the surface, the lower the anomaly that will be measured, and vice versa.The cross-section of the 2D model of the C-C' path found at Y = 9181.6(Figure 3) is shown in Figure 6.This incision is in the middle of the study area, and this section has a maximum anomaly value of 10 mgal.This anomaly spread at coordinates 697.45 to 697.7.This is due to the presence of areas with a layer of sediment (burial area) that has a medium to high density (green to red color) with a value of 2.4 -3 g/cm 3 spread over at coordinates 697.45 to 697.7 at a depth of 0 to 300 meters.And there are also at a depth of 400 meters and below.And there is a minimum anomaly of about -50 mgal and below.And there is a minimum anomaly of about -50 mgal on the surface.This weak zone area is a continuation of the weak zone in the B-B' incision, and is an area where mud is present.In this incision, it can be clearly seen that there may be a fault which is the path of the mud entering the reservoir of the Sedati mud volcano.The cross-section of the 2D model of the D-D' path found at Y = 9181.81(Figure 3) is shown in Figure 7.This incision is in the north direction of the study area, which is dominated by a high anomaly with a maximum value of 35 mgal.This is caused by a layer of rock that has a hogh density (red color) with a value of 2.8 to 3 g/cm 3 spread at coordinates 697.2 to 697.7 and has a minimum anomaly value of 5 mgal on the surface.In this section, there is a low density with a value of 1.8 g/cm 3 at coordinates 697.3 with a depth of 200 meters to 350 meters which is the limit of the mud volcano reservoir Sedati.The cross-section of the 2D model of the E-E' trajectory at Y = 697.60(Figure 3) is shown in Figure 8.This section is taken in a north-south direction and has a maximum anomaly value of 30 mgal.This is caused by the mass of rock layers that have a high density (red color) with a value of 2.8 to 3 g/cm 3 spread at coordinates 9181.5 to 9181.9 with a depth of 0 to 600 meters.And there is an anomaly of at least -10 mgal on the surface.This is due to the presence of a weak zone with low density (blue to purple) with a value of 2 to 1.8 g/cm 3 at a depth of 20 meters to 500 meters.This weak zone area is thought to be an area of accumulated mud in the Sedati mud volcano.The same thing was seen in the F-F' incision (Figure 9). Figure 9 shows the weak zone at coordinates 9181.55 to 9181.8 at depths ranging from 0 meter to a depth of 450 meters.On the surface of this incision, there is the appearance of a fairly large mudflow which spewed out the mud with a high salt content so that it affected the anomalous value on the surface with a minimum vlaue of -50.In the mudflow at Sedati there is also a bubble that comes out and emits gas.The gas content in the mud volcano reservoir also affects the density in the weak zone, the more gas contained, the lower the density.The results of the 3D density cross-sectional modeling are also analyzed from the distribution of the density values that have been obtained (Figure 10).The model image shows low-density rock spread from the surface to a depth of 500 meters.This low density is a weak zone under the surface with a value of 1.9 to 1.8 g/cm 3 .While this high-density value surrounds a weak zone which can be said to be mud-holding basin, this high-density has a value from 2.5 to 3.00 g/cm 3 .In addition, there are also studies conducted on volcanoes using gravity [16].Their gravity modeling results show that the LM Seamount is a low density (2.1 g/cm3) seamount (Fig. 13), with a slightly higher density.compared to unconsolidated abyssal sediments (< 1.7 g/cm3).Coupled with serpentinite mud volcanoes with low bulk density and low seismic velocities, Longmen Seamount is most likely a serpentinite mud volcano in the slowly expanding central oceanic region.

Conclusion
The results of the 3D gravity inversion model show that there are high-density rock blocks (3 -3.3 g/cm 3 ) which are interpreted as basins that hold the reservoir from the mud volcano.Also found on the surface and around the mudflow in the study area, there are low-density rock blocks (1.8 -2 g/cm 3 ) which are

Figure 4 .
Figure 4. Cross-section of the 2D model of the A-A' trajectory

Figure 5 .
Figure 5. Cross-section of the 2D model of the B-B' trajectory

Figure 6 .
Figure 6.Cross-section of the 2D model of the C-C' trajectory

Figure 7 .
Figure 7. Cross-section of the 2D model of the D-D' trajectory

Figure 8 .
Figure 8. Cross-section of the 2D model of the E-E' trajectory

Figure 9 .
Figure 9. Cross-section of the 2D model of the F-F' trajectory

Figure 11 .
Figure 11.Distribution of low density values Based on research conducted by Arafin [15], two cross-sections along mutually perpendicular directions (SW-NE and NW-SE) of the low-density geological body at the Tabaquite mud volcano are shown in Figures 12.The shape of the low-density body, elongated in the SW-NE direction, coincides with the strike direction of the Naparima Fold belt.The absence of gravity anomalies associated with the Piparo mud volcano indicates a large and dynamic density contrast that evolved over ~25 years of mud volcanic regeneration.The size and density contrast properties are advantageous for monitoring Trinidad's mud volcanoes using the gravitational method.

Figure 12 .
Figure 12.A SW-NE cross-section of the low density (left) and A NW-SE cross-section of the low density (right)

5th
International Conference on Research and Learning of Physics (ICRLP 2022) Journal of Physics: Conference Series 2582 (2023) 012009 IOP Publishing doi:10.1088/1742-6596/2582/1/01200910interpreted as weak zones influenced by the presence of mud.This study also obtained an indication of a fault with a west-east direction from the study area.