Bulletin of the Marine Geology, Vol. 27, No. 1, June 2012, pp. 19 to 26

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Magnetic Anomaly Patterns Using Trend Surface Analysis Application (TSA) on Marine Geology Mapping in The Balikpapan Waters (Map Sheet 1813–1814), East Kalimantan

Pola Anomali Magnetik dengan Menggunakan Aplikasi “Trend Surface Analysis” (TSA) pada Pemetaan Geologi Kelautan, Lembar Peta 1813-1814, Perairan Balikpapan, Kalimantan Timur

Delyuzar Ilahude*

Marine Geological Institute, Jl. Dr. Junjunan No. 236 Bandung 40174 IndonesiaEmail: *ilahude_mgi@yahoo.com

(Received 3 January 2012; in revised form 19 April 2012; accepted 21 May 2012)

ABSTRACT: The application of Trend Surface Analysis (TSA) method an geological and geophysicalresearch in map sheets 1813-1814, Balikpapan Waters and its surrounding, shows the significant value ofresidual anomaly. The magnetic disseverance of regional and total anomaly value obtained the negativeanomaly between -50 nT and -350 nT and positive anomaly between +50 nT and +400 nT. The contour oftotal and regional anomaly shows the magnetic properties of rocks which characterizes the geologicalarrangements of the research areas. Residual anomaly yielded from the 2nd order value of regional anomalymight be correlated with the formation of basin structures in the central and northern parts of research area,which is interpreted as a part of Kutai Basin.

Key words : TSA method, magnetic anomaly, geology and geophisics, Balikpapan Waters.

ABSTRAK: Penerapan metode TSA dalam penelitian geologi dan geofisika di Lembar Peta 1813-1814, PerairanBalikpapan dan sekitarnya menunjukkan nilai anomali sisa yang cukup signifikan. Hasil pemisahan nilai anomalimagnet regional dan anomaly total diperoleh nilai anomali yaitu antara -50 nT dan –350 nT dan anomali positifantara +50 nT dan +400 nT. Kontur anomali total dan anomali regional memperlihatkan sifat kemagnitan batuanyang mencirikan tatanan geologi daerah penelitian. Anomali sisa dihasilkan dari nilai anomali regional orde ke 2,kemungkinan berkaitan dengan pembentukan struktur cekungan di bagian tengah dan utara daerah penelitian yangditafsirkan sebagai bagian dari Cekungan Kutai

Kata kunci : metode TSA, anomali magnet, geologi dan geofisika, Perairan Balikpapan.

INTRODUCTIONInformation on geophysical data, especially on

marine magnetics at Balikpapan Waters, and itssurroundings were relatively inadequate. Thus,magnetic methods utilized in geophysical researchs areexpected to contribute preliminary data to determine theform of magnetic anomalies in Balikpapan Waters andits surroundings.

Analysis of magnetic anomalies using TrendSurface Analysis (TSA) in Balikpapan Waters, EastKalimantan, is a part of activity of geological andgeophysical mapping that systematically conducted byMarine Geological Institute (MGI). The purpose of thisstudy is to determine both regional and residual total

magnetic anomalies pattern, which can well describegeological structure of the seabed surface.

By applying a method Trend Surface Analysis(TSA) from the calculation magnetic intensity total it isexpected that the application of this method can showtrend regional anomaly and residual. Furthermore, bothof these anomaly pattern will support the geologicalstructures interpretation of the study area.

This method is one of the oldest of mathematicaltechnique which is used by geologists to conduct aninterpretation of geological data and oil reserves thatwas applied in Moruga, Trinidad and Leduc Reefs,Alberta Basin, Canada ( Wharton, 1993 ). One of thechallenges that was faced by geologists in petroleumindustry by applying TSA is to achieve optimum result

20 Delyuzar Ilahude

in decision making. Analysis TSA can be used to definethe geometric 3D models. This modelling have beendone at several exploration activities and as a result themore oil reserve in complicated geological conditionwas found.

Polynomial equations and linear resultantproduced a graphical representation of a mathematicalequation ( Davis, 1986). Mathematical methods used toseparate these data are represented as maps and residualtrend maps. The exodus of the application of thismethod served in contour map and a three-dimensionalshape ( 3-D ).

The research area is located at Balikpapan Watersand its Surrounding, between 1°15Ê and 2°45Ê South,116°30Ê and 117°05Ê East, map sheet 1813-1814(Figure 1).

Water depths of study area ranges from 15 to 65meters, and more deeper toward the east and southeastof study area (Ilahude et al, 1999).

Application of magnetic methods in the study areawas conducted simultaneously with continuousmeasurement of sea water depth (sounding). Generally,the track lines had been applied to get a good data andhad been carried out by using MGI research vessel,Geomarin I. Balikpapan port was managed to be thestarting point of measurement, as well as the basestation of the research.

Principlly, magnetic method derived frommagnetic induction in the core of the earthÊs magnetic

field and permanent magnetization, which generallyhave different directions and intensities, and partiallymagnetized from the crystallization process (Telford etal, 1974). Therefore, the magnetic anomaly valueobtained was either a combination of both or the resultof pure induction. If the directions of permanentmagnetic field and magnetic induction were similar, thevalue of magnetic anomaly will amplify and vice versa.

Magnetic data processing was based on magneticintensity measurement to depict the pattern of magneticanomalies in the study area. Thus, the coordinateposition of longitude and latitude, and magnetic fieldintensity of the area will be acquired.

Measured magnetic field intensity is the sum valueof main earth magnetic field, variations in earthmagnetic field that related with variations in rockmagnetic susceptibility, remanent magnetic field, andvariations due to sun activity. Magnetic field intensitydata are strongly influenced by earthÊs magneticvariations, which related to the variations in rockÊsmagnetic susceptibility, generally known as localmagnetic anomaly.

TSA is one of the polynomial methods which candepict geographic model of magnetic anomaly patternin large scale. This method is a mathematical model oflinear regression in polynomial coordinates of X and Y.Based on the applying this model, the obtained patternof magnetic anomalies can describe the whole geologyorder in the area.

Magnetic anomalies analysis inthis paper will stress on the shape ofanomalies as a result of applied TSAmethod, which can describe thepattern of regional and residualanomalies. Regional magneticanomaly is a response from relativelydeep source anomalies, while residualmagnetic anomaly is a response fromshallow source anomalies. It isexpected that both residual andregional pattern of anomalies canmore be visible.

Regional and residual anomaliespattern is obtained by applying TSAmethod from magnetic intensitycalculation, may addresscomprehensive interpretation ofgeological structure in the researcharea.

Polynomial and linear resultantequations generated were graphicalrepresentations of mathematicalequations (Davis, 1986). Thismathematical method was applied toclassify represented data as trend and

108⁰00 116⁰00 W112⁰00

2⁰00

4⁰00 S

2⁰00

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4⁰00

6⁰00 N

Scale0 25 Km

Research area

N

Balikpapan

1813

1814

Brunai Darussalam

J A V A  S E A 

Figure 1. The research area in Balikpapan Waters and its surrounding, mapsheet 1813-1814 (Ilahude et al, 1999)

Magnetic Anomaly Patterns Using Trend Surface Analysis Application (TSA) on Marine Geology Mapping in The Balikpapan Waters 21

residual map. Its output will be presented in a contourmap of total magnetic anomalies of regional andresidual.

The main objective of this study is to map totalmagnetic anomalies, of regional and residual inBalikpapan and its surrounding waters, and to refine theform of anomalies along the trajectories, by applyingTSA method. This method is utilized to depict the orderof residual anomaly patterns from reduced total andregional anomalies.

Geologically, the study area is a part ofSampanahan Sheet, Balikpapan, East Kalimantan. Thisis a relatively stable geological structure formed by

nearly north-south direction of anticline and syncline(Heryanto et al, 1994). Thrust-faults and normal-faultswith almost northeast-southwest direction weregenerally easy to observe in this area.

Tectonic activity in this particular area, wereallegedly occurred since the Jurassic ultramafic,radiolarian chert and schist (Heryanto et al, 1994).During the Early Pliocene, granodiorite breakthroughoccurred, followed by uplifting, erosion, and leveling.This activity lasted until the Eocene, forming landdeposition of Tanjung Formation. Later in Oligocene,pool of sea occurred, yielding the limestones of BeraiFormation. This activity was occurred simultaneously

Figure 2. Geological Map of Kalimantan (Darman, 2007)

22 Delyuzar Ilahude

with classic sediment deposition of PamaluanFormation. During the Middle Miocene, sea shrinkagewere occurred and formed Warukin and Pulau BalingFormations. In Late Miocene, the deposition ceasedwith the uplifting and forming of Meratus, Barito,Kutai, and Pasir Basins. In the Pliocene – Plistocene,the leveling that deposited Dahor Formation occurred.

At the northern part of study area shows thatNeogen Basin is called the Kutai Mahakam Basin(Figure 2). This area as potential area for hydrocarbonexploration, because some field on Kutai MahakamBasin is produced the oil and gas.

METHODSMagnetic intensity data from the sea were

collected using a marine magnetometer system of G-877 geometrics model and proton procession model G-856. These magnetic sensor devices were pulled fromthe back of the ship, at a distance of three-times thelength of it. Devices were operated simultaneously with4 knots average of ship cruising speed. To determine theposition of field data collection, integrated satellitenavigation system was utilized by using C-Nav modelof DGPS Receiver satellite. The data was receivedevery two seconds and digitally processed usingHypack Software program.

To obtain the magnetic anomaly value, reductionof magnetic intensity effects due to EarthÊs magneticfield was performed. The value of measured magneticintensity (Tobs) was corrected to the datum of globalmagnetism (International Geomagnetic ReferenceField/IGRF) and to daily variation of magnetic intensity(vh), thus the total magnetic anomaly value wasobtained in nano Tesla (nT) units.

According to Telford, et al, (1990), the value oftotal magnetic intensity around magnetized rocksformulated as follows :

ΔT = Tobs – TIGRF ± TVH ......................................... (1)

ΔT = Total magnetic anomalyTobs = measured magnetic intensityTIGRF = theoretical magnetic intensity according to

IGRF at Tobs measurement stationTVH = magnetic intensity due to daily variations

Total magnetic anomaly data was processed intothe form of matrix calculation to obtain the value ofregional magnetic anomaly using the 2nd order ofTrend Surface Analysis (TSA), formulated byKrumbein (1963) with polynomial numbers as follows:

f (xi,yi) = bo + b1x + b2y + b3xn + b4xy + b5yn ......... (2)

f(xi,yi ) = regional anomalyb = polynomial constant

xi = the x-coordinates in each observation stationyi = the y-coordinates in each observation station n = order

Total magnetic anomaly is the sum total ofregional anomaly and residual anomaly, which can bemathematically written as follows :

T = f(xi,yi) + Rs ........................................................ (3)

T = total magnetic anomaly valueRs = residual anomaly f(xi,yi )= regional anomaly

Thus, residual anomaly (Rs) can be obtained fromthe difference between regional anomaly and totalmagnetic anomaly :

Rs = f(xi,yi) – T ...................................................... (4)

The value of regional and residual magneticanomaly was analyzed using Surfer-8 Software toacquire the magnetic anomaly contour map.

RESULTS

Magnetic Track Line

The application of TSA method in geological andgeophysical research at Balikpapan waters and itssurrounding, 1813-1814 map sheets, showingsignificant value of residual anomaly. The magneticdisseverance of regional and total anomaly valueobtained the anomaly between -50 nT and -350 nT andpositive anomaly between +50 nT and +400 nT. Theresults which is obtained from the field measurement isthe total magnetic field data which is still under theinfluence of external magnetic field components.

These influences were caused by magnetic fieldoriginating from the moon movements and the earthÊstemperature conditions, so the total anomaly data whichwere obtained needed to be corrected with thefluctuations of daily magnetic field variations. In thisstudy, the research trajectory was drawn north-southand east west directions, with crosscheck line in thesouthern part of research area with southwest-northeastdirections (Figure 3). Analysis of magnetic anomalieswas conducted using ΔT formulations to obtain the totalanomaly values, whereas the value of regional anomaly wasobtained using the TSA method. Meanwhile, the residualanomaly was obtained from the reduction of total anomalyvalues in the entire trajectory. From the results of total,regional, and residual magnetic anomaly, an anomalycontour was drawn to see the pattern (trend) of those threekinds of magnetic anomalies in Balikpapan and itssurrounding waters.

Magnetic Anomaly Patterns Using Trend Surface Analysis Application (TSA) on Marine Geology Mapping in The Balikpapan Waters 23

Total Magnetic AnomalyTotal magnetic anomaly values are

obtained after correction to IGRF andvariation of daily magnetic intensity weredone. Distribution of total magnetic intensitygave the insight of its anomaly values between-650 nT and 100 nT. The contour of totalmagnetic anomaly was dominated by negativemagnetic anomaly located in the central andnorthern part of research areas. The pattern oftotal magnetic anomaly contour is obtained asa resultant of magnetic intensity componentswhich assumed to be associated with igneousrocks containing either high or lowsusceptibility.

The distribution of total magneticanomalies was determined into two separategroups: between +50 nT and +100 nT belongsto positive anomaly group, while from -50 nTand -650 nt belongs to negative anomalygroup. Through this distribution, positiveanomalies +100 nT were located in thesouthern part of research areas, marked withpink colored notation. These anomalies werepresumed to be related with geologicalstructures in the area, and also an influence ofmagnetic rocks derived from volcanic rocks ofMeratus. In the other hand, negative anomalies-650 nT formed a circle in the central ofresearch area, that was interpreted as a non-magnetic rocks with low susceptibility,suspected to be sedimentary rocks.

The drawn total magnetic anomalycontour shows that the pattern of negativeanomalies from -50 nT to -650 nT which arelocated in northern, central, and southern partof the research area, were presumed to be thepart of deep layer of Kutai Basin structure see(Figure 4).

Regional Magnetic Anomaly

Group of low magnetic anomalies werescattered along the northern to the southernpart of research area. To acquire the regionalmagnetic anomaly values, 2nd order of TSAformulation was applied by substituting thevalue of total magnetic anomaly at allmagnetic trajectories. Separation using TSA-order method yields two regional anomaliesdepicted below.

The values of regional magneticanomalies range from -150 nT to -410 nT.Regional magnetic anomalies were dominatedby negative anomaly groups. This conditionshowed that the lithological order of its

Balikpapan

116⁰30’ 116⁰45’ 117⁰00’  117⁰30’ E 

N

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Scale

Explanation :

Line of research

Research area

Index Map

Figure 3. Drawn trajectory of magnetic survey in Balikpapan Watersand its surrounding (Ilahude et al, 1999).

Balikpapan

Total M

agne

tic Ano

maly

116⁰30’ 116⁰45’ 117⁰00’  117⁰30’ E 

N

0 25 kmScale

nT

+100

-650

Figure 4. Total magnetic anomaly contour of Balikpapan waters andits surrounding

24 Delyuzar Ilahude

component rocks were dominated bysedimentary rocks with low magneticsusceptibility (Figure 5). This anomalyregional distribution magnetic delineatesglobally by layers of deeper rock, whileanomalous total and anomalous delineatesremaining layers of rock more shallow.

Residual Magnetic Anomaly

To acquire the residual anomalyvalues, reduction of total to regionalmagnetic anomaly values were carried out.From the results of the reduction,distribution of residual magnetic anomalycan be determined into another two groupsas follows: negative anomaly rangedbetween -50 nT and -350 nT, and positiveanomaly ranged between +50 nT and +400nT (Figure 6). Generally, the pattern of thisresidual anomaly in several locationsindicates some similarities to the totalmagnetic anomaly pattern (See Figure 4).

This form of similarity occurs sincethe residual anomaly is a reflection ofmagnetic basal rocks at a shallow seadepths. In the southern part of the researcharea, the occurence of positive anomalieswere relatively high (+400 nT). It issuspected that high value of positiveanomalies were related to polarizationeffect of magnetic rocks which wasdeformed by geological structure of thearea, while the negative anomaly valueswere suspected as the part of Kutai Basin(See Figure 2).

DISCUSSIONFrom the analysis of regional anomaly

by using the TSA method shows thatanomaly great in map residual not precludethe possibility anomaly caused by theinfluence of weathered rock magnetic thevolcanic rocks of Meratus. While the lowmagnetic anomaly allegedly sedimentaryrock is non magnetic in relatively shallowdepth.

To scatter anomalous value of residualvalue were obtained a picture that anomalyoccupy the southern part of the study areawith yellow color, notation brown to red,reflecting sensorial magnetic of bedrockhas high susceptibility. The estimatetion ofpositive catter anomalous will associatedwith volcanic rocks that derived from

Balikpapan

Regio

nal M

agne

tic Ano

maly

116⁰30’ 116⁰45’ 117⁰00’  117⁰30’ E 

N

0 25 kmScale

nT

Balikpapan

Residu

al  M

agne

tic Ano

maly

116⁰30’ 116⁰45’ 117⁰00’  117⁰30’ E 

N

0 25 kmScale

nT

+400

‐350

‐350

Figure 5. Regional magnetic anomaly contour of Balikpapan Waters andits surrounding

Figure 6. Residual magnetic anomaly contour of Balikpapan Waters andits surrounding

Magnetic Anomaly Patterns Using Trend Surface Analysis Application (TSA) on Marine Geology Mapping in The Balikpapan Waters 25

Volcano Meratus. While anomalous negative innorthern and central regions will represent thesedimentary rock, even though the sedimentary rocksome time will represent by low magnetic nature.Another possibility the relative low magnetic anomalycan also be caused by igneous rock or rock acidicmetamorphic. From the application of the TSA method,the patterns of residual sensorial anomalous magneticreflecting magnetic bedrock susceptibility that have ahigh anomaly, whereas at some places the negativeanomalies is also found. To reduce this negativeanomaly of the relation lithologi in the sedimentaryrock of study area is not magnetic interpretation ofKutai Basin.

Based on the result of TSA method applicationshow that the sedimentary deposits is thick and wideenough. This condition is indicated by regional negativeanomaly map (Figure 5 ).

The difference in positive and negative anomalyvalue, on each map is presented by a colour in order tomake easy interpretation. The interpretation of TSAmethod will show us the lithology and order bedrock asapart of sediment of the Kutai Basin.

CONCLUSIONSApplication of TSA method yields a significant

magnetic anomalies pattern that is considered to behighly related to geological structure under the seabed.Residual anomalies separate the root cause ofanomalies in the deeper layer, and high value ofmagnetic anomaly with large dimensions were depictedin the regional and residual magnetic anomalies.Positive anomalies found in the southern part of theresearch area were assumed due to the magnetic rocksderived from Meratus peaks. These anomalies were thesource of relatively shallow anomalies which related tohigh magnetic susceptibility in the southern part ofresearch area. In the northern and central part of it,negative residual magnetic anomalies were found andassumed to be low susceptibility rocks whichinterpreted as the part of Kutai Basin.

ACKNOWLEDGMENTSThe authors would like to thank Mr. DR.Ir.

Susilohadi the Head of MGI for his great support andhelp in finishing this paper.

REFERENCES[1] Davis, G.V., 1986, Numerical Methods in

Engineering and Science, London, Proc. Conf.p. 213.

[2] Heryanto, R., Supriatna, S., Rustandi, E.,Baharudin, 1994, Peta Geologi LembarSampanahan, Kalimantan, PPPG.

[3] http://en.wikibooks.org/wiki/Darman, 2007,in The  Geology  of  Indonesia/Kalimantan/2011.

[4] Ilahude, D., Usman, E., Ibrahim, A., Setyanto,A., Salahudin, M., Setyadi, D., Rachmat, B.,1999, Penyelidikan Geologi dan GeofisikaKelautan Daerah Perairan Kalimantan Timur,Lembar Peta 1813-1814, Laporan Intern PusatPenelitian dan Pengembangan GeologiKelautan, Bandung.

[5] Krumbein, W.C., 1963, Confidence intervals onlow-order polynomial trend surfaces, JournalGeophysical Research, 68, 5869-5878.

[6] Telford, W. M., Geldart, L. P., Sheriff, R.E.Keys, D.A., 1974, Applied Geophysics,Cambridge University Press, London-NewYork-Melbourne.

[7] Telford, W. M., Geldart, L. P., Sheriff, R.E.,1990, Applied Geophysics, CambridgeUniversity Press, Cambridge, Second Edition.

[8] Wharton, S.R., 1993, An Integrated Approachto the Study of Pliocene Gros Morne ReservoirSands For Developing Play Concepts, MorugaEast Field, Trinidad.

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