The subsurface three-dimensional modeling of ...

The subsurface three-dimensional modeling of volcano arc of Flores island based on gravity data analysis Yopiter Lukas Alexander Titi and Eko Minarto

Citation: 1788, 030106 (2017); doi: 10.1063/1.4968359 View online: http://dx.doi.org/10.1063/1.4968359 View Table of Contents: http://aip.scitation.org/toc/apc/1788/1 Published by the American Institute of Physics

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The Subsurface Three-Dimensional Modeling of Volcano Arc of Flores Island Based on Gravity Data Analysis Yopiter Lukas Alexander Titi1,a) and Eko Minarto1,b) 1

Department of Physics, Faculty of Mathematics and Natural Sciences Institute of Technology Sepuluh Nopember Sukolilo Surabaya 60111, East Java, Indonesia a)

Corresponding authors: [email protected] b) [email protected]

Abstract. The interpretation and three-dimensional modeling of the subsurface structure of the volcano arc of the Flores island based on the gravity data analysis have been done. This research is aimed for modeling subsurface structure utilized a secondary data of complete Bouguer anomaly gravity data obtained from Bureau Gravimetric International (BGI) using Grablox and Bloxer software. The modeling construction was performed by inversion technique applying the method of Singular Value Decomposition (SVD) and Occam inversion. The result indicates that Subsurface structure of the volcano area of the Flores island consists of sandstone, breccia and andesite have density value ranging from 2,42 g/cm3 to 2,62 g/cm3 and basaltic and lava have density values ranging from 2,65 g/cm3 to 3,24 g/cm3. The most dominating rocks in the study area are basaltic rocks have 2.73 g/cm3 point of average density. The existence of magma chamber in the volcanic arc of Flores island was estimated at a depth of 6 km.

INTRODUCTION The formation of the ranks volcanic arc in Flores island as well as being one of the regions have high levels of seismicity, as influenced by two main factors, namely the subduction zone in the South and back arc thrust activity in the North [1-2]. To understand the complex tectonic structure, further performed subsurface modeling. Geophysical modeling is the process of estimating the model and model parameters based on data observed on the surface of the earth [3]. The model reveals not only the representation of the geological conditions by physical quantities but also includes the mathematical or theoretical relationship between the model parameters and the response model [3]. Gravity method is one of the geophysical methods described the subsurface geology based on variations of the gravitational field on Earth caused by the difference of rocks density. In principle, this method is used because of its ability to differentiate between the density of the source of the anomaly and the density of the surrounding environment [4]. Inversion modeling is an analysis of measuring data by performing a curve fit between the theoretical or calculated data and field or measured data. The purpose of the inversion process is to estimate the physical parameters of rocks previously unknown [3]. The purpose of this study was modeling subsurface volcanic arc of Flores island and determining the structure of magma chamber.

EXPERIMENTAL METHOD The modeling was conducted on secondary data of complete Bouguer anomaly gravity that obtained from Bureau Gravimetric International (BGI) from http: //bgi.omp.obs-omp/fr with coordinates 119° BT - 124° BT and 7° LS – 9.5° LS spaced every 2 minutes or 3.7 km. This data is based on World Gravity Map (WGM) 2012 [5]. Complete Bouguer anomaly data was projected to horizontal plane using Dampney method on 26 km heights by Matlab program [6]. Separation of the regional and local anomaly with upward continuation method on 30 km heights. The processing of this data using the program Magpick Version 3.25 [7].

International Conference on Engineering, Science and Nanotechnology 2016 (ICESNANO 2016) AIP Conf. Proc. 1788, 030106-1–030106-6; doi: 10.1063/1.4968359 Published by AIP Publishing. 978-0-7354-1452-5/$30.00

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The anomaly target that wants to interpret relatively localized namely to model the structure of shallow nearsurface zone of the volcano of Flores island. Modeling conducted on the residual gravity anomaly using Grablox (Version 1.6e) and Bloxer (Version 1.6e) computer programs based Graphical User Interface (GUI). The 3D density contrast model for the study area is estimated using the Grablox software. Grablox is a gravity interpretation and modeling software based on a 3D block model. The main objective is to optimize the density contrast and or the shape and dimension of the density variations [8]. In this study, the total gravity field data of 5100 data points are used to create an initial model of 5000 minor blocks in X, Y, and Z directions. The initial model for modeling the residual anomaly created by the X direction of 558, 20 km as many as 20 minor blocks. The length of the Y direction of 284.50 km minor as many as 25 minor blocks, thus forming a minor block 500 for each layer. Blocks in the vertical direction or Z direction is divided into 10 layers and a depth of 15 km. The results of the initial model display for residual anomalies can be seen in Fig. 1, density parameters used for the first layer to the fourth layer is made equal to the density value of 2.67 g/cm3 in the zone of the volcanic arc and a density value of 2.35 g/cm3 in the area surrounding zones that experienced volcanic arc sedimentation. The fifth to eighth layers given input density values greater than the upper layer density value of 2.72 g/cm3 with the assumption that getting into the rock formations would be more massive, while the density value of 2.99 g/cm3 at volcanic zones with assuming the magma intrusion. Ninth and tenth layers given input density value of 2.90 g/cm3 as more massive rock layers. The initial models of this rock density will be conducted inversion process. The modeling construction was performed by inversion technique applying the method of Singular Value Decomposition (SVD) and Occam optimization [9-10]. The optimization process is done in stages starting from the base optimization, density optimization, Occam density optimization, height optimization and Occam height optimization. Optimizations are done for the purpose of reducing the error rate or error between the data model and the measurement data to obtain the right model. The results of the 3D modeling are presented in Bloxer software. The final stage is modeling and interpretation data.

FIGURE 1. The initial model data anomaly residual on bloxer program based GUI.

RESULTS AND DISCUSSION Based on Occam Height Optimization Residual anomaly data in Fig. 2, it is a final optimization of the whole process optimization on the inversion method to obtain the same gravity anomaly pattern between measured and calculated model used for interpretation. The results Occam height optimization have minimum RMS error (0.324353E-01) between measured and calculated data as shown in Fig. 4(a) and 5(a). The value of error (0.324353E-01) indicates the difference in data mismatch between block height and density gravity anomaly measured and density and block height gravity anomaly calculated each data point is very small. The results of the optimization Occam height contours show the similarity between the measured model and calculated model.

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(a)

(b)

FIGURE 2. Occam height optimization residual anomaly data (a) Computed (b) Measured. Based on residual anomaly Fig. 3, represented by positive and negative anomalies with anomaly value ranging from -95 to 55 mGal. This shows that the rocks shallow structures have the value of density contrast vary from positive to negative. Residual anomaly contour obtained is used to model the structure of shallow volcanic zone of Flores island. The results are obtained made profile cross section of the Y-axis in the direction North-South at position Latitude X = 314.58 km of which cross mount Ambulombo area and the X-axis in the direction East-West trending at position longitude Y = 9.025,57 km are cross some volcanic area include Mount Egon at X = 439 km, Mount Kelibara at X = 370 km, Mount Kelimutu at X = 369 km, Mount Ranakah at X = 301 km and Mount Inelike at X = 387 km.

FIGURE 3. Map of residual gravity anomaly contours volcano arc of Flores Island.

The modeling results have 20 cross section X-direction and 25 cross section Y-direction. For the interpretation chosen one cross section by each X and Y directions. The results of the cross-sectional profile will be used for the interpretation of the subsurface. In this case, the cross section profile are made to represent the subsurface interpretation that passes the area an active volcano on Flores island. The misfit between measured and computed gravity fields, cross section profile and density contrast model resulted from 3D inversion can be seen in Fig. 4 and Fig. 5.

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(a)

(b)

(c) FIGURE 4. 3D density contrast model (a) The misfit between measured and computed gravity fields (b) Cross section (c) Density contrast model resulted from 3D inversion for the profile X=314,58 km .

Based on the results of a cross-sectional, density values for cross section profile mount Ambulombo area direction North-South range between 2.37 g/cm3 to 3.05 g/cm3 as shown in Fig. 4(c). While the cross section profile that cuts some volcanoes are trending East-West have density values ranging from 2.25 g/cm3 to 3.24 g/cm3 as shown in Fig. 5(c). From the results of the both cross-section profile are visible masses with high density right under the mountain areas. For the first cross-sectional profile with latitude UTM 9.015 km to 9.040 km, at depths 6 km has a rock density values ranging from 2.67 g/cm3 to 3.05 g/cm3 which is located just below the mount Ambulombo areas are interpreted as lava rock and basalt rock. The mass was alleged as the magma chamber of the mount Ambulombo. As for the depth of 0 km to 5 km is dominated by a rock that has density values ranging from 2.42 g/cm3 to 2.62 g/cm3. According to the geological information and based on the density of rock, Mount Ambulombo’s category state is young volcano [4, 11-12]. It can be interpreted that this area is dominated by sandstone, breccia and andesite. Similarly for the second cross section, starting at a depth of 6 km of rock density values ranging from 2.65 g/cm3 to 3.24 g/cm3 which are interpreted as lava rock and basalt rocks just below the area of an active volcano. Mass with high-density values indicated as magma chamber that supply magma to active volcanoes located on the Flores Island.

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(a)

(b)

(c) FIGURE 5. 3D density contrast model (a) The misfit between measured and computed gravity fields (b) Cross section (c) Density contrast model resulted from 3D inversion for the profile Y=9025,57 km.

CONCLUSIONS Subsurface structure of volcano area of the Flores island consists of sandstone, breccia and andesite that have density value ranging from 2,42 g/cm3 to 2,62 g/cm3 and basaltic and lava from 2,65 g/cm3 to 3,24 g/cm3 point of density. The most dominating rocks in the study area are basaltic rocks have the average density 2.73 g/cm 3. The existence of magma chamber in the volcanic arc of Flores island was estimated at a depth of 6 km.

ACKNOWLEDGMENT The author thank Dr. rer. nat. Eko Minarto for valuable guidance encouragement for this research. The author also thanks to Bureau Gravimetric International (BGI) as provider of gravity data and thank to Indonesia Endowment Fund for Education (LPDP) as the sponsorship during the research.

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REFERENCES 1. R. McCaffrey, J. Geophys. Res. 93, 163 (1988). 2. E.A. Silver, N.A. Breen, and H. Prasetyo, J. Geophys. Res. 91, 3489 (1986). 3. H. Grandhis, Introduction in Geophysical Inversion (Geophysics Association of Indonesia (HAGI), Jakarta, 2009), pp. 2-6. 4. W.M. Telford, L.P. Geldart, and R.E. Sheriff, Gravity Methods in Applied Geophysics (Cambridge University Press, New York, 1990), pp. 6-16. 5. S. Bonvalot, G. Balbino, A. Briais, M. Kuhn, A. Peyrefitte, N. Vales, R. Biancale, G. Gabalda, G. Moreaux, F. Reinquin, and M. Sarrailh, World Gravity Map (Bureau Gravimetrique International, Toulouse, 2012). 6. C.N.G. Dampney, Geophysics 34, 39 (1969). 7. R.J. Blakely, Transformations in Potential Theory in Gravity and Magnetic Applications (Cambridge University Press, New York, 1995), pp. 313-324. 8. M.A. Khalil, F.M. Santos, and M. Farzamian, J. Geophys. Res. 103, 104 (2014). 9. S.S. Ganguli, and V.P. Dimri, J. Appl. Geophys. 95, 23 (2013). 10. Y. Li, and Y. Yang, Phys. Earth Planet. Inter. 189, 9 (2011). 11. S. Koesoemadinata, Y. Noya, and D. Kadarisman, Geological Map of the Ruteng Sheet East Nusa Tenggara (Geological Research and Development Centre Ministry of Energy and Mineral Resources, Bandung, 1994). 12. N. Suwarna, S. Santosa, and S. Koesoemadinata, Geological Map of the Ende Sheet East Nusa Tenggara (Geological Research and Development Centre Ministry of Energy and Mineral Resources, Bandung, 1989).

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