2012 International Conference on Geological and Environmental Sciences IPCBEE vol.3 6(2012) (2012)ICSIT Press, Singapoore Pseudo 3d Gravity Modeling to Determine Intrusion Contact of Parang rea, Karangsambung, Central java, Jndonesia Dwipaningtyas +, Bestman W.Simamora, Dian Enggelia Geophysical Engineering, Institut Teknologi Bandung, Indonesia bstract. Karangsambung, Central Java is an area in Indonesia with high geological complexity, located in Melange Complex. There is a rock-unification between two plates, i.e. Indo-ustralian and Eurasian which creates the appearance of various rock, including igneous rock, sediment and metasediment. This observation is exactly centered at Parang area because it still becomes discussion among the experts whether it is an intrusion or just a igneous boulder. Gravity method is applied in this research by using residual data for shallow depth, with the final purpose result is pseudo-3d modeling of Parang clearly, where in this model it can be determined the contact between the diabase intrusion and obsolete rock. In addition, range of contrast density on diabase intrusion is about 0.3-0.4 gr/cm 3 toward background density. Keywords: Gravity method, Karangsambung, Parang intrusion. 1. Introduction Gravity method in geophysical exploration is based on its principle to determine the variation of gravitational field on earth. Generally the gravity method is applied in the first step of geophysical exploration in order to estimate subsurface structure, such as salt domes, intrusion and fault..the reason why Parang intrusion become interesting because this area had been crucial to be a discussion matter among the experts since previous years, because of its appearance whether it can be classified as an intrusion or just an igneous boulder. 2. Location of Study rea The acquisition of this gravity method was held on Karangsambung Formation, Kebumen, Cental Java, B Fig. 1: () The acquisition map with remote sensing image. The station is symbolized by the yellow dot and red dot shows the base station. (B) Topography Map. + Corresponding author. Tel.: + 6281572422251; fax: +6222-2534137 E-mail address: dwipaningtyas@gmail.com 89
Indonesia, located at 7.526 0-7.56 0 S and 109.660 0-109.682 0 E, where Parang intrusion as the main survey in this area. The survey area encompasses approximately 2 x 3 km 2 with 135 stations. Each station has interval space of 100 m. The base is located precisely 7.5464 0 S and 109.6724 0 E. Gravimeter La-Coste Romberg G-502 is used for this measurement and GPS for knowing the position and high of the surface level (topography) accurately. The acquisition was taken place in June 2011. 3. Regional Geology Karangsambung, Central Java, is one of the most enticing geological view areas in the world because of its various structure and formation where the melange complex is situated in. It is an area in South Serayu Mountain zone (van Bemmelen, 1949 op.cit. Hadiyansyah, 2005). Melange complex in Karangsambung have a wide volcanic rocks distribution. Those rocks are identified as the rocks which formed in the ocean floor (nshori, 2007). They had undergone both folding and lifting process and finally exposed on the surface. Karangsambung formation is deposited on Lok Ulo Melange Complex. The contact between these two units in some places is an unconformity combined with tectonic borders. This formation comprises scaly clay with blocks of limestone, conglomerates, sandstone, claystone and basalts. B Fig.2: Geological map and Cross Section of Karangsambung area (sikin, et all 1992). The red rectangle is the acquisition area of Karangsambung Formation. Parang Intrusion is symbolized by the orange-colored area, it is designated there that the intrusion shoves into Karangsambung Formation (Teok). 4. Methodology Basic equation method in this method is based on Complete Bouguer nomaly (Telford,1990). (1) Where is the station reading, is the theoretical gravity, is the latitude correction, is the free-air correction, is the Bouguer correction, and is the terrain correction. Parasnis method is applied to estimate background density from field result. By arranging Equation (1) and Free ir Correction, we can acquire the estimation of average density from linear gradient (Telford, 1990). 90
0.3086 0.0419 (2) CB map is intended to analyze the anomaly pattern in observed area generally. Furthermore, separation between regional and residual is carried out because the information about the source (residual or anomaly plane) is an important thing as a part of entire anomaly in earth. If separation works correctly, we are about to get the reflection of density variation in crust, and the interpretation result must be geologically adjusted. nomaly regional is obtained by using moving average toward CB. Later, the residual will be produced through CB s reduction by regional. fter getting the residual anomaly, a forward method is built to ascertain the 2D design anomaly. However, this method requires repeated calculation of gravity measure that denoted in Cartesian coordinate with Green s function (Blakely, 1995).,,,,,, (3) 5. Result nd nalysis The CB, Regional and Residual contour map are made by Surfer 9.0. We also execute Model Vision 9.0 to make 2D- anomaly gravity geology structural design and pseudo-3d design. 93.5 9167500 9167000 2.6 2.2 1.8 91 9166500-1.2 1.4 1 0.6 88.5 9166000 0.2-0.6 93.5 9165500-1 -1.4 C 9165000 B C -1.8-2.2 9164500 352500 353000 353500 354000 0 500 1000 Fig.3: () Complete Bouguer nomaly (CB) map, (B) Regional anomaly map and (C) Residual anomaly map. Residual map is obtained from CB map, reduced by regional map as stated before 2D and pseudo-3d design are made in residual map anomaly by creating six lines with the azimuth of 135 0 that cut the high anomaly structure and one slice, the azimuth of 45 0, crosses the other six slices. Each slice has 100 m long. By making 2D modeling, some parameter properties are needed, such as background density, density of rock information, and geological map. Based on Parasnis method, the result of background density is about 2.8 gr/cm 3. Some rocks have varying density values, they are diabase intrusion 2.5 to 3.2 gr/cm 3, lava basalt 2.8 to3.3 gr/cm 3, claystone 1.63 to 2.6 gr/cm 3, and sandstone 1.61 to 2.76 gr/cm3 (Telford, 1990). In the 2D modeling figure, line c is picked as representation of the other five slices, and one slice, line x, as modeling comparison. Parang Diabase intrusion is assumed as the red one, the pink one as pillow lava basalt, and the brown one as more obsolete rock such as claystone and sandstone. The diabase intrusion modeling that we made has a contrast density 0.3 to 0.4 gr/cm 3. Based on cross section in geological map, Parang intrusion has branches to 500 m deep, but our modeling have a grid to 300m deep, so only two intrusions are made in the modeling, and the branches cannot be seen. The lava basalt has contrast density is about 0.1 to 0.2 gr/cm 3, and the obsolete rock has about -0.1 to -0.3 gr/cm 3. 91
Fig.4:. nomaly Residual Map. The map is overlapped by topography map and the track of acquisition. Topography map has a function as a media to detect geomorphology commonly. The existence of this map will predict how the subsurface section might be detected. Six slices from the top(north-west) is line a, line b, line c, line d, line e, line f and one cross section line is line x. fter six slices are made in the 2D modeling, the pseudo 3-D modeling will be created. It can be seen that each line have continually smooth modeling with the other slices, so the modeling is in accordance with the surface fact, which will be similar with the the subsurface. a b Fig. 5: The 2D modeling. (a) The 2D modeling of line c. (b) The 2D modeling of line x. The red one is diabase intrusion, the pink one is lava pillow, and the brown one is the obsolete rock. 92
6. Conclusion In summary, after finishing Pseudo 3D gravity modeling completely, it can be seen that Parang area is exactly an intrusion not an igneous boulder which vertically infiltrated in Karangsambung Formation. So we can name Parang area as a dike intrusion. 7. cknowledgemets The authors would like to thank Dr. Susanti lawiyah and Dr. Setyaningsih for their guidance during this work. To the Chairman of Geophysical Engineering Study Program, Dr.gus Laesanpura M.Sc, as the one who is responsible person in field camp lecture, Karangsambung, Central Java. To our adviser, Dadi, who had already taught us to operate Model Vision 9.0. nd to our beloved friends, Waskito, Habibie, Drana, and Rifan for sharing their knowledge. 8. References Fig. 6: Pseudo 3D gravity modeling. The model is based on the lines of azimuth 135 0 [1] nshori, Chusni. 2007. Petrogenesa Basalt Sungai Medana Karangsambung, Berdasarkan nalisis Geokimia. Jurnal Riset Geologi & Pertambangan Jilid 17 No.1 (2007) 37-50. [2] sikin, S.,Handoyo,., Busono, H., dan Gafoer, S.1992. Geologic Map of Kebumen Quadrangle, Java, scale 1:100000.Geological Research and Development Centre, Bandung. [3] Blakely, R.J. 1995. Potential Theory in Gravity and Magnetic pplications. Cambridge University Press.Cambridge [4] Hadiyansyah, D. 2005.Karakteristik Struktur Formasi Karangsambung, Daerah Karangsambung dan Sekitarnya.Kecamatan Karangsambung karangayam, Kabupaten Kebumen, Jawa Tengah. Undergraduate thesis, Geology Engineering Study Program ITB, not published. [5] Telford, W. M., Geldart, L. P. Sheriff, R. E.. 1990. pplied Geophysics : Second Edition. Cambridge Univerity Press : Cambridge 93