0001-Metoda Gravitasi Kuliah

Gravimetric and magnetic exploration


• Role of Gravity and Magnetic Exploration– Potential Field Methods

+ natural source methods

+ non-invasive

+ inexpensive

+ fast

+ easy data collection, reduction, but... - non-straightforward interpretation

- low resolution

- ambiguous

- not always applicable


Method Advantages Disadvantages Cost Ratio

Magnetics Very fast, very cheap Poor resolution, not always applicable

1

Gravity Fast, cheap Poor resolution 10

Seismic Fine detail, good correlation to geology

$$$ 100

From a 1999 Edcon brochure advertising their aerogravity/magnetic surveys:"The cost of conducting an aerogravity/magnetic survey over a 5,000 square kilometer concession in South America is in the order of $200,000 to $300,000. The cost of a 3-D seismic survey over only 250 square kilometers can be ten times that amount."


• Pat Millegan, Marathon Oil, on use of G&M in industry:– Pat stresses the importance of diversifying your

skills: " seismic does NOT answer all the questions, all the time...there are MANY seismic failures (e.g., one current Marathon project). The main reason G&M does not see more use is true "ignorance". My job is 10-100 times harder when my "clients" (the exploration groups...I'm in a service group) know nothing about G&M. Please stress geophysical integration to your students. It is the smart way to explore, but you don't just throw G&M at everything...don't bother if the geology isn't conducive to geophysical results."


1972 Costs of Acquisition and Processing of Geophysical Data (Telford et al.)

x $106 %

Petroleum Exploration

seismic 802 89.7

surface grav/mag 17 1.9

airborne mag 6 0.7

Mineral Exploration

airborne Mag 19 2.1

ground mag 12 1.5

Other 34 3.8

Total 894 100

Gravity and Magnetics in a Nutshell

• Gravity is useful wherever the formations of interest have densities that are appreciably different from those of surrounding formations.– Some examples:– mapping sedimentary basins, where sedimentary rocks

consistently have lower density than basement rocks – salt bodies: low density of salt – groundwater studies (e.g., Cayman Islands)

• Magnetics is useful whenever object of investigation has a contrast in magnetic susceptibility or remanence– Some examples:– mapping structure on basement – mapping sedimentary basins – direct location of ores containing magnetite

Characteristics of gravity and magnetic data

• Gravity and magnetic anomalies can only be produced by horizontal changes in rock properties

• Multiple horizontal layers, with only vertical changes in density and magnetization, will not produce anomalies regardless of the magnitudes of these contrasts

Characteristics of gravity and magnetic data

• Anomaly amplitudes are generally proportional to density contrast, magnetization, acoustic impedance, electrical resistivity, etc..

• Large contrasts produce “high amplitudes” • Small contrasts produce “low amplitudes”• Anomaly wavelength is directly proportional

to the distance (or depth) from the source.• Long wavelengths associates with deep

sources while short wavelengths are produced by shallow sources

Gravity Method

In gravity prospecting, we measurevery small variations in the force ofgravity from rocks within the earth.Different types of rocks havedifferent densities, and the denserocks have the greater gravitationalattraction.

To the left is a “gravimeter”which measures the force ofgravity in the earth.

Figure courtesy of Lacoste-Romberg

Did you know … In oil exploration, we measure changes ingravity that may be only one-millionth or even one-ten millionthof the earth's total gravity field.

With a small kitchen scale, measure the weight of different rocks you find in your area. The heavier rockshave a greater gravitational pull than lightweight rocks.

Pyrite is a heavy rock Sandstone is a lighter rock

PENDAHULUAN• Gravitasi adalah gaya tarik menarik

yang terjadi antara semua partikel yang mempunyai massa

• Bentuk bumi yang tidak bulat sempurna dan relief bumi yang beragam disebabkan oleh distribusi massa jenis yang tidak merata, posisi titik di permukaan bumi, dan struktur geologi yang ada di bawah permukaan

Latar belakang Metode gravity

merupakan salah satu metode geofisika yang digunakan untuk mengetahui kondisi bawah permukaan bumi dengan cara mengamati variasi lateral dari sifat fisik batuan (densitas).

I. KONSEP DASARUNTUK DAPAT MEMAHAMI PENGARUH PERCEPATAN GAYA BERATPADA SUATU BENDA, PERLU DIPAHAMI DASAR DASAR ILMU FISIKAYANG DAPAT MEMBANTU DALAM MERUMUSKAN DAN MENGANALISISPERCEPATAN GAYA BERAT

PARAMETER DALAM GAYA BERAT ADALAH DENSITAS

2. PERCEPATAN GAYA BERATDASAR DARI METODE GAYA BERAT ADALAH HUKUM NEWTON MENGENAI GAYA TARIK MENARIK ANTARA 2 BUAH BENDA YANG DIRUMUSKAN SEBAGAI BERIKUT

DASAR DASAR PERCEPATAN GAYA BERAT

F = Gm1 x m2

r2

F F

m1 m2

r

Kaitan gaya gravitasi dengan geologi

• Kekuatan gravitasi di tentukan oleh kerapat-an massa-> massa per unit volume

• Kerapatan massa di gambarkan oleh “pusat massa” (point mass)

• Semakin tinggi kerapat an massa, semakin tinggi pula gaya tarik massa tersebut.

Perubahan percepatan gaya berat

Manfaat dan kegunaan metode gravity

• sensitive terhadap perubahan vertikal, oleh karena itu metode ini disukai untuk mempelajari kontak intrusi, batuan dasar, struktur geologi, endapan sungai purba, lubang di dalam masa batuan, kondisi terpendam dan lain-lain.

Anomali gravitasi

• Bentuk anomali di tentu-kan oleh bentuk massa dan kedalaman sumber penyebab anomali.

• Anomali gravitasi berasal dari batang silindris

• Anomali bersifat simetris • Harga maksimum diatas

pusat silinder sebesar 0.025 mgals

Hubungan gravitasi dengan pusat massa

• Percepatan gravitasi terbesar berada pada posisi “pusat massa”

• Bentuk anomali me-ngikuti bentuk massa

• Nilai anomali tergan-tung pada jarak, kerapatan massa dan volume.

Hubungan gravitasi dengan geologi

• Anomali gravitasi dinyatakan oleh perubahan “percepatan” yang ditimbulkan oleh perbedaan kerapatan massa.

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DETERMINATION OF BASIN

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DEVELOPMENT OF TIME -

STRATIGRAPHIC FRAMEWORK

DETECTION OF

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ENVIRONMENTAL - FACIES

ANALYSIS

RECONSTRUCTION OF

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GRAVITYGRAVITY

MAIN FIELD EQUIPMENT’SMAIN FIELD EQUIPMENT’S

Gravimeter : 2 unit La Coste and Romberg.

Positioning : 2 set GPS-Receivers

Elevation : 3 set Paulin Altimeter

Communication : 2 unit SSB radios ( 1 unit at field, 1 unit at head office), 4 unit Handy talky

Data Processing: Laptop PC, printer, software’s, diskettes, calculator

Camp Facilities

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PC PIV-1 G h

PC PIV-1 G h

DESIGN OF REMOTE DATA COMMUNICATION SYSTEMDESIGN OF REMOTE DATA COMMUNICATION SYSTEM

DATA PROCESSINGDATA PROCESSING

The data obtained from the sites are sent directly to the base camp and processed.

11. DATA REDUCTION. DATA REDUCTION

22. GRAVITY PROFILES. GRAVITY PROFILES

33. GRAVITY MAP. GRAVITY MAP

GRAVITY DATA PROCESSINGFLOW CHARTGRAVITY DATA PROCESSINGFLOW CHART

Dasar pengolahan data metode gravitasi adalah

= Mencari perbedaan harga gravitasi suatu titik ke titik lain di suatu tempat.

= Dimana massa tersebut hanya menyumbang sekiatar 0,05% dari harga gravitasi yang didapat.

= koreksi data.

Pengolahan Data Gravity

Pemrosesan data gayaberat yang sering disebut juga dengan reduksi data gayaberat, secara umum dapat dipisahkan menjadi dua macam, yaitu:

proses dasar proses lanjutan.

Proses dasar konversi pembacaan gravity meter ke nilai milligal, koreksi apungan (drift correction), koreksi pasang surut (tidal correction), koreksi lintang (latitude correction), koreksi udara bebas (free-air correction), koreksi Bouguer koreksi medan (terrain correction).

1. DATA REDUCTION1. DATA REDUCTION

The gravity data reduction consists of two types of correction which are internal and external correction.

The internal corrections are drift and tidal corrections.

The external corrections are ellipsoid gravity value, free air, bouguer, and terrain corrections.

DATA ACQUISITION PLANDATA ACQUISITION PLAN

1. CalibrationCalibration of the gravimeter is carried out several times : before and after a trip and every two weeks.

2. Base StationThe gravity base station in every location is established by tying the base station to the nearest standard base station to the location.

3. Data Acquisition Methods

DRIFT CORRECTIONDRIFT CORRECTION is applied to eliminate the effect of spring fatigue of the La Coste instrument. This correction is derived by double check the starting base station at appropriate time interval.

TIDAL CORRECTIONTIDAL CORRECTION is applied to eliminate gravity of the sun and moon which are time function due to relative motion among earth, moon and sun. The tidal correction had been calculated in advance using computer by applying the Longman’s formula.

CONTOH METODA PENGUKURAN

D a y 1D a y 2

CONTOH METODA PENGUKURAN

ELLIPSOID EARTH GRAVITYELLIPSOID EARTH GRAVITY reference has to be applied to produce an earth

gravity value at the mean sea level as a function

of location latitude.

This reference implies an homogenous mass

distribution of the ellipsoid earth model.

The ellipsoid model in the IUGG 1979 formula is :

gg = 978.03185 (1 + 0.005278895 sin= 978.03185 (1 + 0.005278895 sin + +

0.000023462 sin 0.000023462 sin44 ) , mgal) , mgal

where g = theoretical gravity as function of

= latitude of the observation point.

Koreksi Spheroid dan Geoidspheroid referensi sebagai pendekatan

untuk muka laut rata-rata (geoid) dengan mengabaikan efek benda diatasnya.

- Koreksi Bougeur menganggap permukaan lempengan di atas bidang acuan rata, melainkan ada lembah dan bukit, sehingga tidak mewakili keadaan sebenarnya.

- - Adanya lembah dan bukit disekitar titik pengamatan akan menimbulkan efek-efek yang mengurangi percepatan gravitasi di titik amat.

- Koreksi medan yang dilakukan selalu berharga positif.

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FREE AIR and BOUGUER EFFECTFREE AIR and BOUGUER EFFECT

r r+h

FREE-AIR CORRECTIONFREE-AIR CORRECTION (FAC) is applied to estimate the earth gravity at certain altitude of an observation above mean sea level.

The free air correction formula is calculated for average earth radius at elevation h in meters.

FAC = - 0.3086 h, mgalFAC = - 0.3086 h, mgal

BOUGUER CORRECTIONBOUGUER CORRECTION (BCBC) is applied to estimate the earth gravity at elevation h above sea level with earth mass of density (gr./cm3) fill up the space of thickness h. This theoretical Bouguer correction can be written as:BC BC = = 2h 2h Gh = Gh = 0.04187 0.04187 h h, mgalwhere :G = 6.67 x 10-9 Cgs unit = the chosen density in gr./cm3

H = altitude of observation point in meters.

BOUGUER ANOMALYBOUGUER ANOMALY (BABA) is the difference between the observation gravity value (gobs) and the expected

earth normal gravity at an observation point.

BABA = gobs - (g - FAC + BC)

where the magnitude in the bracket is the expected earth normal gravity.

Pengukuran gaya berat sering dilakukan pada daerah dengan topografi yang cukup bervariasi. Koreksi terrain harus dihitung untuk menghilangkan efek relief permukaan bumi terhadap nilai anomali Bouguer yang dihitung.Koreksi ini dihitung sebagai efek gaya berat yang ditimbulkan oleh suatu badan massa tiga dimensional yaitu adanya bukit dan lembah di sekitar stasion pengukuran gaya berat.

TERRAIN CORRECTIONTERRAIN CORRECTION

INNER ZONE CORRECTIONINNER ZONE CORRECTION

To apply this correction, a simple topographic survey has to be performed at every gravity station along a radius of 35 and 68 meters which may be done before or after gravity reading.

Such survey should include the nature of local morphology and the distance to the gravity station which affects the observation.

The correction was directly calculated at the field by using a certain gravity terrain inner correction chart.

OUTER ZONE CORRECTIONOUTER ZONE CORRECTION

This correction was done by using the Hammer Chart, usually based on a topographic map of 1 : 250.000 scale. Applying the terrain correction, the Bouguer Anomaly (BA) can be refined to be a Complete Bouguer Anomaly (CBA) following this formula :

CBA = gCBA = gobs obs - (g- (g - FAC + BC - TC) - FAC + BC - TC)

or

CBA = BA + TC

696 698 700 702 704 706 708 710 712132

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LHD -4,8,9,10 LHD -6

LHD -7LHD -5LHD -1

LHD -2

LHD -3

GRAVITASIANOMALI BOUGUER

rapat massa = 2.67 gr/cm3

U

2 km

696 698 700 702 704 706 708 710 712132

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LH D -1

LH D -2

LH D -3

LH D -4,8,9,10

LH D -5

LH D -6

LH D -7LH D -5 LH D -7

GRAVITASIANOMALI

REGIONALPOLINOM FIT

ORDE - 2

U

2 km

696 698 700 702 704 706 708 710 712132

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LH D -1 LH D -5 LH D -7

LH D -4,8,9,10 LH D -6

LH D -3

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GRAVITASIANOMALI SISA

U

2 km

INTERPRETASI METODE GRAVITASI

Interpretasi kualitatif dilakukan dengan menfasirkan peta anomaliBougeur.

Interpretasi kuantitatif dilakukan dengan cara pemodelan .

1.pemodelan maju digunakan untuk melihat respon gravitasi yang ditimbulkan dari model geologi yangdibuat. Sedangan

2.pemodelan mundur digunakan untuk membuatmodel geologi dari pengaruh medan gravitasi daerah penelitian.

Bouger Gravity Contour Image

Pre-cretaceous high-density basement

(derived from Bouguer gravity)

Regional gravity map

132 136 140 144 148

-5.0

0.0

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LH D -4 LH D -5 LH D -2 LH D -3

SELATAN U TA R A

andesit basaltik terubah (2.5 gr/cc)

tu ffa , ign im brite (2 .0 gr/cc)

andesit (2 .6 gr/cc)

sedim en (2.2 gr/cc)


in trusi d iorit (2 .9 gr/cc)

data

perh itungan

GRAVITASIPROFIL

ANOMALI SISADAN

MODEL 2-DIMENSI

696 700 704 708 712

-5.0

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BAR AT TIM U R

LH D -1 LH D -5 LH D -7

data

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andesit basaltik terubah (2.5 gr/cc)

tu ffa , ign im brite (2 .0 gr/cc)



in trusi d iorit (2 .9 gr/cc)

GRAVITASIPROFIL

ANOMALI SISADAN

MODEL 2-DIMENSI

What is a reasonable geologic interpretation for the NE-SW blue anomaly (A-B) on this magnetic map of northwesternmost Texas? Contour interval = 100 nT, blues are lows and yellows and reds are highs, and the anomaly is about 70 km (45 miles) long. Click in circle of your choice.

A. A pile of volcanic rocksB. A reversely polarized dikeC. A narrow horst of basement rock uplifted to shallow depth

Magnetic Method

Magnetic PoleBatuan yang mengandung mineral magnetik dapat terinduksi oleh medan magnet bumi sehingga pengukuran variasi spasial medan magnet dapat digunakan untuk memperkirakan keberadaan mineral tersebut

Magnet bersifat dipol (dwi-kutub) sehingga interpretasi hasil pengukuran anomali magnetik relatif lebih sulit jika dibandingkan dengan anomali gravitasi yang bersifat monopol.

Batuan yang mengandung mineral magnetik dapat terinduksi oleh medan magnet bumi sehingga pengukuran variasi spasial medan magnet dapat digunakan untuk memperkirakan keberadaan mineral tersebut.

Magnet bersifat dipol (dwi-kutub) sehingga interpretasi hasil pengukuran anomali magnetik relatif lebih sulit jika dibandingkan dengan anomali gravitasi yang bersifat monopol.

Reduksi ke kutub dan reduksi ke ekuator adalah proses simulasi kondisi medan magnet di kutub atau di ekuator dimana arah medan magnet bumi masing - masing vertikal (inklinasi = 90o) atau horisontal (inklinasi = 0o). Dengan demikian diperoleh anomali yang bersifat monopol.

Prinsip reduksi ke kutub dan ke ekuator adalah perkalian anomali magnetik dengan fungsi transfer filter pada domain frekuensi.

Magnetic Anomaly and Geology

• Magnetic survey measures variations along the earth’s surface.

• It records both main and induced fields.

• The magnetic susceptibility of the rocks exert greater influence to the induced field

• Variations in the strength of magnetic field allow us to locate rocks body having high magnetic susceptibility.

Geophysical Instruments magnetometer, airborne survey

http://www.geoinstruments.com.au/_vti_bin/shtml.dll/DEFAULT.HTM/map2

Comparisons of interpretationMagnetic versus gravity

• Magnetic data allows better estimate of depth than the gravity data because : large magnetization contrasts between crystalline basement rocks and volcanic rocks and the overlying sedimentary rocks.

• Density, on the other hand, does not vary so dramatically.

• So magnetic anomalies are generally thought to be produced by distinct source bodies, while gravity anomalies may be produced by the cumulative effect of several density variations.

Documents

0001-Metoda Gravitasi Kuliah