Pseudo 3d Gravity Modeling to Determine Intrusion Contact of Parang Area, Karangsambung, Central java, Jndonesia

Dwipaningtyas+, Bestman W.Simamora, Dian Enggelia

Geophysical Engineering, Institut Teknologi Bandung, Indonesia

Abstract. 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-Australian 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/cm3 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 Area The acquisition of this gravity method was held on Karangsambung Formation, Kebumen, Cental Java,

Fig. 1: (A) 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

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2012 International Conference on Geological and Environmental SciencesIPCBEE vol.3 6(2012) © (2012)IACSIT Press, Singapoore

Indonesia, located at 7.5260-7.560 S and 109.6600-109.6820 E, where Parang intrusion as the main survey in this area. The survey area encompasses approximately 2 x 3 km2 with 135 stations. Each station has interval space of 100 m. The base is located precisely 7.54640 S and 109.67240 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 (Anshori, 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.

Fig.2: Geological map and Cross Section of Karangsambung area (Asikin, 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 Anomaly (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 Air Correction, we can acquire the estimation of average density from linear gradient (Telford, 1990).

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∆ 0.3086 0.0419 ∆ (2)

CBA 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. Anomaly regional is obtained by using moving average toward CBA. Later, the residual will be produced through CBA’s reduction by regional.

After 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 And Analysis The CBA, 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.

Fig.3: (A) Complete Bouguer Anomaly (CBA) map, (B) Regional anomaly map and (C) Residual anomaly map. Residual map is obtained from CBA 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 1350 that cut the high anomaly structure and one slice, the azimuth of 450, 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/cm3. Some rocks have varying density values, they are diabase intrusion 2.5 to 3.2 gr/cm3, lava basalt 2.8 to3.3 gr/cm3, claystone 1.63 to 2.6 gr/cm3, 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/cm3. 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/cm3, and the obsolete rock has about -0.1 to -0.3 gr/cm3 .

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Fig.4:. Anomaly 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.

After 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.

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.

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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. Acknowledgemets The authors would like to thank Dr. Susanti Alawiyah and Dr. Setyaningsih for their guidance during

this work. To the Chairman of Geophysical Engineering Study Program, Dr.Agus 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. And to our beloved friends, Waskito, Habibie, Drana, and Rifan for sharing their knowledge.

8. References [1] Anshori, Chusni. 2007. Petrogenesa Basalt Sungai Medana Karangsambung, Berdasarkan Analisis Geokimia.

Jurnal Riset Geologi & Pertambangan Jilid 17 No.1 (2007) 37-50.

[2] Asikin, S.,Handoyo, A., 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 Applications. 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. Applied Geophysics : Second Edition. Cambridge Univerity Press : Cambridge

Fig. 6: Pseudo 3D gravity modeling. The model is based on the lines of azimuth 1350

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