8/16/2019 019-ICGES2012-S10007

1/5

 

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 inMelange Complex. There is a rock-unification between two plates, i.e. Indo-Australian and Eurasian whichcreates 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 datafor shallow depth, with the final purpose result is pseudo-3D modeling of Parang clearly, where in thismodel it can be determined the contact between the diabase intrusion and obsolete rock . In addition, rangeof 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 anigneous 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

A B

89

2012 International Conference on Geological and Environmental Sciences

 IPCBEE vol.36(2012) © (2012)IACSIT Press, Singapoore

8/16/2019 019-ICGES2012-S10007

2/5

Indonesia, located at 7.5260-7.56

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 km2  with 135 stations. Each station has

interval space of 100 m. The base is located precisely 7.54640  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 GeologyKarangsambung, 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 thefree-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).

A

B

90

8/16/2019 019-ICGES2012-S10007

3/5

  ∆ 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 geologicallyadjusted. 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 Vision9.0 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 45

0, crosses the other six slices. Each

slice has 100 m long. By making 2D modeling, some parameter properties are needed, such as backgrounddensity, 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/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/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 .

  -  1.   2

-0.2

 -  0.  2

352500 353000 353500 354000

9164500

9165000

9165500

9166000

9166500

9167000

9167500

-2.2

-1.8

-1.4

-1

-0.6

-0.2

0.2

0.6

1

1.4

1.8

2.2

2.6

0 500 1000

C

  8  8.  5

9   1   

   9   3.   5

  9  3.  5

A

A   B   C

91

8/16/2019 019-ICGES2012-S10007

4/5

 

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.

a

b

92

8/16/2019 019-ICGES2012-S10007

5/5

 

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 

93