028 Jgee 030 Irham Subsurface

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International Journal of Geology, Earth & Environmental Sciences ISSN: 2277-20! "#nline$

%n #en %ccess, #nline International Journal %vaila'le at h tt : (( ))) * ci ' t e c h * o r g ( + g e e * h t m

20! ol* ".$ Setem'er-/ecem'er, * 2-21(Irham et al*

Research Article

3oyright 20! 4 3entre for Info 5io 6echnology "3I56ech$ 1

SUBSURFACE MODELING OF YOGYAKARTA BASIN USING

INVERSION METHOD OF GRAVITY DATA

*Irham Nurwidya!" M#$% Kir&ai S#B#

'% Si(ma!"

'ad )auy"

'

! /eartment of hysics, /ionegoro 8niversity Semarang, In9onesia,

2 /eartment of hysics, Ga9+ah a9a 8niversity, ;ogya 3 km and are mostly located in !antul"raben Yogyakarta. This study aims to determine the depth or thickness of the top basementsedimentary basins and modeling conditions of Yogyakarta !asin based on the constituent rock densitycontrast value.#e used of a $a%coste & 'omberg gravity meter type "%( M)' and "*+ Trimble ,avigations 4-00

$+ to measure the height measurement point The distribution of the point measurements was

performed by a semi grid pattern with the measurement path passing through the e/isting road or

footpath to the distance about % km and the distance between the point of measurement varies around

0.1 km % km. The relative gravity data is measured then doing the process among others calibration

readings into the unit of mgal drift correction high correction tools tidal correction normal gravity

correction free air correction !ouguer correction and terrain correction. The gravitational field of data

processi ng had been done with completed called The 2omplete !ouguer nomaly 2!5 in

topography at the point of measurement5. The 2! data residing in topography then pro6ected on a

horiontal plane at a height of 100 m with the e8uivalent point source approach to mass at a depth of 1000 m using 9amneymethod.

The 2! data which had been on a flat surface and then separated used upward continuation

method. Therefore the local component removed and lived regional component then do further modelingand interpretation. :n this study used the inversion method using "rav39 software in order to determine

the condition of layering of sedimentary rocks in the subsurface in this study area. !ased on the

inversion modeling results it can be concluded that the rocks making up the Yogyakarta !asin has

a value of density contrast varies between %0.- g ; cc to 0.< g ; cc with bedding pattern in the form

of syncline structure. The syncline structure has a depth of sedimentary rock in the middle between - km

to7 km and on the edges have a depth of km to 3 km.

Keywords: Inverse etho9, Gravity etho9, /ensity 3ontrast, Su'surface, ;ogya


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International Journal of Geology, Earth & Environmental Sciences ISSN: 2277-20! "#nline$

%n #en %ccess, #nline International Journal %vaila'le at h tt : (( ))) * ci ' t e c h * o r g ( + g e e * h t m

20! ol* ".$ Setem'er-/ecem'er, * 2-21(Irham et al*

Research Article

3oyright 20! 4 3entre for Info 5io 6echnology "3I56ech$ 2

0015. :ndian%ustralian oceanic plate relative moves to the speed of about -%7. cm ;year is approachingthe @urasian continental plates are relatively stable. The "lobal plate tectonics of :ndonesia is presented in >igure 3 !MA" 035.!ecause the subduction events Yogyakarta !asin is in the fore arch basin as tectonically very

active forming ancient volcanism which is active today is Mount of Merapi which is the most active

volcano in the world. +ince Yogyakarta !asin position near the subduction one earth8uakes tendsto occur naturally.

http://www.cibtech.org/jgee.htmhttp://www.cibtech.org/jgee.htmhttp://www.cibtech.org/jgee.htmhttp://www.cibtech.org/jgee.htm


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Fi+ur, $- Iu(!ra!i" ". !h, r,(,ar/h ar,a Fi+ur, '- D,!ai !"0"+ra0hy i 'D ad 1D 2i,w

". !h, r,(,ar/h ar,a

The earth8uake on May -%th 00- occurred due to the activation of a fault in the Yogyakarta !asin@lnashai et al*, 00(5. s most e/perts in earth sciences +ulaiman et al*, 00(B +arah et al*, 00(BMulyaningsih et al*, 00ault. !ecause of the position of Yogyakarta !asin at the fore arch basin which is estimated age is still young andrelatively shallow depth so that predicted less prospects for the hydro carbon Aoesumadinata


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Merapi. The second is strong undulation unit hills that is stretched along the east side of the =pak 'iver.

This area is composed of sedimentary rocks. The last is the plateau which is part of the Eighlands

#onosari. This


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area is composed of limestone and reefs. The +tratigraphy of the study area generally composed

of gravity sedimentation results in the Miocene epoch Toha et al*, ormation. This is characteried by calcarenite with inserts sandstone andmudstone. This formation deposits at the ?pper Middle @ocene. #ungkal "amping formation is anintegral and inseparable formation +urono ormation and the !utak >ormation. Therefore it is called Aebo%!utak >ormation. The fourth is the +emilir >ormation Tmse5. :t is composed of interbedded of tuff pumice breccia tuff dacite and andecite tuff tuffaceous mudstone and shale.

This formation deposits at the end of the $ower Miocene and the oldest rocks which e/posed in the

study area. The fifth is the ,glanggran >ormation Tmn5. This formation is composed of volcanic

breccia with fragments of andecite flow breccias agglomerates lava and tuff. The Middle Miocene

formations bottom and inter%fingering with +emilir >ormation. The si/th is the +ambipitu >ormation

Tms5. :t deposits at the Middle Miocene. This formation is composed of sandstones and successively of

shale siltstone tuff and conglomerate. This formations is deposits over the ,glanggran >ormation. Theseventh is the =yo >ormation. This formation is composed by succesively of bioclastic limestone

marl with inserts calcarenit and fragmental limestone conglomerate. :t deposits in shallow

e/posure conditions with agitation calm waves in the Middle Miocene epoch. The eight is the#onosari >ormation Tmpw5. :t is composed of limestone reefs calcarenite and tuffaceous

calcarenite. This formation is deposited at Middle Miocene to ?pper Miocene and is aligned above

the +ambipitu >ormation. The last and the youngest formation is the Aepek >ormation. :t is composed

successively of layered limestone and marl. This formation deposits at ?pper Miocene with a thickness


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of 00 m and is stratigraphically related inter% fingering with the #onosari >ormation. The top layer

is the alluvial deposits of the Mount Merapi product.


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2.2. MethodsThe study was conducted by measuring the relative gravitational field in t he area that served asthe

research ob6ect namely !antul regency area which is the largest research area all of the cityof Yogyakarta area a small part area in +leman regency "unung Aidul regency and Aulon *rogoregency as presented in >igure . Those measurements were taken at each point and at each measuring point predetermined path to completion. 9istribution locations measuring point is semi%grid andrandom located on the roads side are there in the area of research. :n making measurements uselooping and binding system to drift correction.The main e8uipment used to measure the gravitational field at the points of measurement is a unit of $a

% coste & 'omberg gravity meter type "%( M)' which has a precision in the order of micro gal galG g ; cm

5. The ne/t e8uipment are two units one set5 of "*+ Trimble ,avigations 4-00 $+ type

to

measure the coordinates of the point of measurement position and height5 and a "armin ::: plus "*+

for field orientation.

The supporting e8uipment used in this study include a geological compass to measure the direction of strike and dip of outcrops a topographic map study area to determine the points of measurement a

geological map to see rocks distribution and a ,ote book which includes "olden +urfer +oftware

Microsoft @/el "rav39 and Matlab for data processing and modeling.

9ata measurement such as time coordinates and altitude measurement point and the data areobtained from gravity meter. The data readings obtained from the gravity meter then they are calibratedin units of mgal then the correction of tidal and drift correction of the obtained value of gravityobservations gobs5. The value of gobs then corrected to normal gravity gravity latitude5 free air

correction !ouguer correction and terrain correction Telford et al*, rom the measurements results of the gravity field survey using appropriate processing methoddescribed

in the previous chapter obtained the regional component of 2! as presented in >igure

-.:t can be seen in >igure - the value of 2! in the central part of the study area in the southwest toward

the northeast were indicated by the dark blue color.


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This 8ualitatively reflected the condition of the subsurface rocks which had small density contrast

value or the relative thickness of the sediment rocks. +outh east north and northwest area have a

relatively greater value 2! indicated with a yellow brown to redB it reflected the conditions in the

subsurface rocks. These conditions had a high density contrast value or the thickness of the sediment

rocks that is relatively shallow.


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Fi+ur,


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Fi+ur, $$- Su&(ur.a/, /r"(( (,/!i"d,(i!y /"!ra(! .r"m !h, u00,r 2i,w a!a d,0!h ". $444 m &,"w (,a ,2,

Fi+ur, $1- Su&(ur.a/, /r"(( (,/!i" d,(i!y/"!ra(! .r"m !h, u00,r 2i,w a! a d,0!h ". 7444m &,"w (,a ,2,

Fi+ur, $'- Su&(ur.a/, /r"(( (,/!i" d,(i!y/"!ra(! .r"m !h, u00,r 2i,w a! a d,0!h ". '444m &,"w (,a ,2,

Fi+ur, $7- Th, d,0!h ". !h, &,dr"/= 3r,d /""r6

.r"m !h, u00,r 2i,w ". !h, Y"+ya=ar!a Ba(i

>rom modeling result as shown in the figure above it can be concluded that the basin of Yogyakarta form

+yncline +tructure with a depth of sedimentary rock in the middle between - km %7 km and on the edgesat a depth of %3 km. !ased on the rocks density contrast value of the Yogyakarta !asin varied betweenminimum at % 0.- g ; cc to ma/imum at 0.< g ; cc which form the layering of the +yncline +tructure. Therock with a density contrast I 0.0- g ; cc to 0.0 g ; cc was presented in the dark blue color which wasinterpreted as alluvial.The rock with a density contrast between 0.0 g ; cc to 0. g ; cc was presented in blue color which was

interpreted as the =ya >ormation the Aepek >ormation and the #onosari >ormation. The rockswith density contrast of 0. g ; cc to 0. g ; cc was presented in light blue color which was interpreted as

the ,glanggran >ormation and the +ambipitu >ormation. The rock with a density contrast 0. to 0.33 g ;

cc was presented in light blue color interpreted as the +emilir >ormation. The rock with a density

contrast0.33 % 0.4ormation.

Therock with a density contrast 0.4


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depth of sedimentary rocks in the middle reaches - km to 7 km and at the edge varies between %3

km. The rocks in the Yogyakarta !asin form%bedding row from bottom to top are the igneous rocks the

metamorphic rocks and the sedimentary rocks. The sedimentary rocks are composed by the

#ungkal% "amping >ormation the Aebo%!utak >ormation the +emilir >ormation the ,glanggran

>ormation the


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+ambipitu >ormation the =ya >ormation the #onosari >ormation the Aepek >ormation and the top

is the alluvial deposits.

Suggestions!ased on the research results it is suggest to e/pand the area of research for uncover the structure of the

folds in the Yogyakarta !asin syncline pair anticline. :t has not been conducted yet because of thelimited funds and time. Moreover it is suggested to review the "eochemistry method to know the possibility of Eydro%2arbon prospect.

ACKNO)LEDGEMENT

#e deepest gratitude goes to *rof. 9r 96oko +antoso M.+c. through 9r. M +arkowi M.+i. #ethank them for their kindness and willingness so that we are able to use the "rav39 software as alicensee. #e

appreciation goes to many others whose name are not mention here for helping in writing and

completing this article.

REFERENCES

Ba=,y R> 3$8896# otential 6heory in Gravity an9 agnetic %lications 2ambridge ?niversity *ress5 ,ew York 44.

Ea(hai US% Kim S>% Yu G> ad Sidar!a D 3'4456# The Yogyakarta earth8uake of May 7 00-

M@2 report no. 07%0. 17.

Gra! FS ad ),(! GF 3$8orward Modeling and :nversion of "ravity 9ataover 39 +tructures. ?!2%"eophysical :nversion >acility 9epartment of @arth and =cean +ciences?niversity of !ritish 2olumbia )ancouver !ritish 2olumbia 4-.

K",(",madia!a R? 3$8516# Geology inya< /an Gas 5umi "etroleum Geology$ :T! *ress5 !andung


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028 Jgee 030 Irham Subsurface