EXPLORATIONEXPLORATIONGEOPHYSICSGEOPHYSICS

FORFOR

GEOLOGISTGEOLOGISTANDAND

ENGINEERENGINEERPRIHADI SA / 2002

COURSE INSTRUCTORCOURSE INSTRUCTOR

DR. PRIHADI SA.DR. PRIHADI SA.

PRIHADI SA / 2002

GEOELECTRICGEOELECTRIC

APPLICATIONS APPLICATIONS

1. GROUND WATER EXPLORATION2. MINERAL AND BASE METAL EXPLORATION3. GEOTHERMAL4. OIL AND GAS EXPLORATION, ESPECIALLY

WHEN SEISMIC REFLECTION IS TECHNICALLY AND ECONOMICALLY INEFECTIVE, SUCH AS :

• KARSTIVIED CARBONATE COVER• VOLCANIC COVER• ROUGH TOPOGRAPHY

PRIHADI SA / 2002

PRIHADI SA / 2002

STANDARD GEOELECTRICAL EQUIPMENT STANDARD GEOELECTRICAL EQUIPMENT

1. GEOPHYSICAL TRANSMITTER2. MULTI PURPOSE GEOPHYSICAL RECEIVER

DC RESISTIVITYDC RESISTIVITY

COMPLEX RESISTIVITYCOMPLEX RESISTIVITY

T E M T E M ( TRANSIENT ELECTROMAGNETIC )( TRANSIENT ELECTROMAGNETIC )

1. POWER GENERATOR2. TRANSMITTER CONTROLLER FOR CSAMT3. MAGNETIC SENSOR4. POROUSPOT ELECTRODES5. ELECTRODE AMPLIFIER

C S A M T C S A M T (CONTROLLED SOURCE AUDIO MAGNETO TELLURIC) (CONTROLLED SOURCE AUDIO MAGNETO TELLURIC)

PRIHADI SA / 2002

Ground wire Both wire and small coil Small coil ( ground) Small coil ( air )

Grounded wireGalvanic Resistivity Magnetometric resistivity ( MMR )

I P Magnetic IP ( MIP )Inductive C S A M T Some Time-domain EM (TEM) systems

( controlled-source audio magneto-telluric )

Small loopSlingram Airborne EMHorizontal-loop EM Time-domain towed-birdVertical-loop EM Helicopter rigid-boomTilt-angle methodGround conductivity meters (GCM)Some Time-domain EM (TEM) systemsCoincident loopBorehole systems

Large loop ( long wire )Large-loop systems -. Sundberg method -. TuramMany TEM systemsBorehole systems

Plane wareVertical antenna VLF-resistivity VLF VLFNatural -. Geomagnetic -. Field

Telluric -. Currents

RECEIVER TYPE

Classification based on Swift ( 1988 ), from John M. Reynolds, 1997, An Introduction to Applied and Environmental Geophysics

TRANSMITTER TYPE

PRIHADI SA / 2002

PRIHADI SA / 2002

TRANSMITTER RECEIVER DAN BOOSTER OYO MCOHM 21

PRIHADI SA / 2002RECEIVER GDP 16 AND GDP 32 ZONGE

PRIHADI SA / 2002TRANSMITTER ZONGE

PRIHADI SA / 2002ELECTRIC GENERATOR, TRANSMITTER ZONGE

PRIHADI SA / 2002

RESISTIVITYRESISTIVITY

Survey geolistrik dilakukan dengan menginjeksikan arus listrik ( I ) searah (DC) ke dalam tanah melalui dua elektroda dan mengukur responsnya berupa beda potensial (V) pada dua elektroda yang lain. Dengan susunan elektroda tertentu diperoleh parameter fisis tahanan - jenis semu (Apparent Resistivity).

Arus listrik sebesar I melalui titik O pada permukaan, dialirkan ke dalam tanah, yang dianggap sebagai media homogen dan isotropis. dan mempunyai tahanan jenis . Arus listrik tersebut akan menyebar dan membentuk medan listrik setengah bola ( Gambar ).

Titik yang terletak di dalam media mempunyai densitas (rapat arus) sebesar :

22 rIJ

Arus total yang menembus permukaan setengah bola adalah :

I jds r j 22

PRIHADI SA / 2002

Selisih potensial (dv) antara dua kulit yang berjarak dr adalah :

dvIr

dr2 2

Dengan mengintegrasikan persamaan di atas, diperoleh harga potensial titik P yang disebabkan oleh sumber arus O sebesar :

rIV

2

PRIHADI SA / 2002

PRIHADI SA / 2002

POTENSIAL OLEH SUMBER ARUS GANDA DI PERMUKAAN POTENSIAL OLEH SUMBER ARUS GANDA DI PERMUKAAN

r1 r2

POWER

MTotal potensial pada titik M oleh sumber arus C1 dan C2 :

21 VVVM

)11(2 21 rrIVM

PRIHADI SA / 2002

TEKNIK PENGUKURANTEKNIK PENGUKURAN

Pengukuran tahanan jenis pada umumnya menggunakan susunan empat elektroda terminal. Sepasang elektroda untuk menginjeksikan arus ke dalam tanah dan sepasang elektroda lain untuk mengukur beda potensial yang ditimbulkannya.

• Vertical Electrical Sounding atau DrillingUntuk mendapatkan variasi tahanan jenis listrik secara vertikal terhadap kedalaman, dibawah suatu titik dipermukaan.

• Electrical Mapping atau ProfillingUntuk mendapatkan distribusi tahanan jenis listrik secara lateral.

Dua teknik yang umum dipakai :

PRIHADI SA / 2002

CRITERIA WENNER SCHLUMBERGER DIPOLE-DIPOLE SQUARE

Vertical resolution Good Moderate Poor ModerateDepth penetration Poor Moderate Good ModerateSuitability to VES Moderate Good Poor UnsuitableSuitability to CST Good Unsuitable Good GoodSensitivity to orientation Yes Yes Moderate NoSensitivity to lateral inhomogenities High Moderate Moderate YesLabour intensives Yes Moderate Moderate Yes

(No *) (No*) (No*)Availability of interpretational aids Good Good Moderate Poor

COMPARASION OF DIPOLE-DIPOLE, SCHLUMBERGER, SQUARE, AND WENNER ELECTRODE ARRAYS

* When using a multicore cable and automated electrode array

from John M. Reynolds, 1997, An Introduction to Applied and Environmental GeophysicsPRIHADI SA / 2002

PRIHADI SA / 2002

WENNER RESISTIVITY METHODWENNER RESISTIVITY METHOD

Pengambilan data sounding dengan menyusun elektroda - elektroda arus dan elektroda potensial dalam satu garis lurus yang mempunyai jarak sama.

Potensial pada P1 :

V I1 2

1

1AM BM

V I1 2

1 12

a a

Potensial pada P2

V I 2 21 1

AN BN

V I2 2

1 12

( )a a

Beda potensial di P1 dan P2 :

V V V 1 2 V I

2a

Maka tahanan jenis media adalah :

VI

2a

K 2a

VI

K

dimana K : Faktor Geometri I : Arus ListrikV : Beda Potensial : Tahanan Jenis Semu

PRIHADI SA / 2002

PRIHADI SA / 2002

METODA INTERPRETASI METODA INTERPRETASI

Interpretasi data dilakukan di lapangan dan di laboratorium. Metoda yang dipakai :

1.1. PENCOCOKAN KURVAPENCOCOKAN KURVAMenggunakan kurva standar dan kurva bantu.

2.2. KUMULATIF MOOREKUMULATIF MOORETahanan jenis semu dibaca, diakumulasikan, dan diplot terhadap kedalaman. Perubahan harga tahanan jenis ditunjukkan oleh perubahan mencolok kemiringan grafik dan dapat diinterpretasikan sebagai batas lapisan.

PRIHADI SA / 2002

3.3. CARA BARNESCARA BARNES

Diperoleh tahanan jenis sebenarnya untuk suatu ketebalan tertentu. Susunan lapisan batuan di bawah permukaan dianggap merupakan tahanan listrik yang tersusun paralel.

1 1 1 21 1

1

1

R R Ra

R RL n n

L

n n

Misalkan perhitungan dilakukan terhadap tahanan jenis sebenarnya untuk tiap

ketebalan 1 m ( tiap bentangan elektroda @ = 1 m ), kemudian harga - harga ini

diplot terhadap kedalaman dengan memakai skala satuan.

Apabila harga - harga tersebut dihubungkan, maka diperoleh kicks yang

akan memberikan gambaran korelasi batuan di bawah permukaan.

PRIHADI SA / 2002

4.4. FORWARD MODELINGFORWARD MODELING

Dilakukan dengan menggunakan perangkat lunak geolistrik.

Formula umum untuk perhitungan apparent resistivity : 1

22212111

11112

CPPCCPPCK

IVKa

K = faktor geometri

PRIHADI SA / 2002

PRIHADI SA / 2002

PRIHADI SA / 2002

Penampang tahanan-jenis semu konfigurasi Wenner sebelum (atas) dan sesudah (bawah) infiltrasi fluida konduktif.

PRIHADI SA / 2002

Model hasil inversi data konfigurasi Wenner alfa sebelum (atas) dan sesudah (bawah) infiltrasi fluida konduktif

PRIHADI SA / 2002

The Schlumberger electrical resistivity investigation consists of horizontal mapping and vertical sounding. The aim of the horizontal mapping investigation is to figure out the apparent resistivity values distributions, reflecting the lateral subsurface rock distributions. In the mapping survey the measured electrode distance i.e. are AB/2 = 500 m and AB/2 = 1,000 m, respectively.

SCHLUMBERGER SCHLUMBERGER

ELECTRICAL RESISTIVITY METHODELECTRICAL RESISTIVITY METHOD

PRIHADI SA / 2002

M

V

I

A BN

2b2a

MN < 0.2 AB

SCHLUMBERGERSCHLUMBERGER

The principles of measurement for sounding and mapping are the same.

The current electrode distances in sounding survey are logarithmically

increased. The spread distance ratio of MN to AB is

kept constant at 5 MN AB, whenever the potential electrode (MN) distance is

changed, the overlap measurement for the same AB/2 distance are carried out.

PRIHADI SA / 2002

The general formula to calculate the apparent resistivity is,

IVKa

π4 - 22

LK

where, L = AB and = MN

where, the geometric factor (K) for the Schlumberger electrode configuration is expressed as :

PRIHADI SA / 2002

a

n a

I

C1

C2

V

P1

P2

POLE - POLE POLE - POLE

PRIHADI SA / 2002

P1

a

n a

V

P2

a

I

C2C1

a

DIPOLE-DIPOLE DIPOLE-DIPOLE

PRIHADI SA / 2002

PRIHADI SA / 2002

MISE-Á-LA-MASSE METHODMISE-Á-LA-MASSE METHOD

Used two different current electrodes. The charged current electrode C-1 was located in the casing well, whereas C-2 was located about 5 km away from C1. The potential electrode P1 was moved surrounding C-1, whereas fixed potential electrode P2 was located 3 km away from C-1 in opposite direction from C-2. The general equation to calculate the apparent resistivity was expressed as a general formula of :

IVKa

(K is geometric factor for the Mise-á-la-masse electrodes configuration)

VERTICAL WELL CASEVERTICAL WELL CASE

The Mise-á-la-masse survey has been already proven as a quick method for mapping a geothermal prospective area (Kauahikaua, et.al., 1980, Tagomori et.al., 1984, Ushijima, 1989, Mizunaga, 1991). The electric potential due to a point source of current electrode on the ground surface of an isotropic and a homogeneous earth, if the potential at an infinite distance is assumed to be zero, is:

r1

2I V

PRIHADI SA / 2002

The formula of the potential electrode P1 can be derived by integrating the electric potential, because of the point source at the well head for the total depth along the vertical casing pipe.

The general formula for the potential calculation in the Mise-á-la-masse configuration is :

VP1P2 = VP1 - VP2 or can be written as :

P2C21

P1C21

P2C1P2C1

P1C1P2C1n12π

I V22

22

P1P2

PRIHADI SA / 2002

K is a geometric factor 1

22

22

P2C21

P1C21

P2C1P1C1

P1C1P2C1ln12πK

and,

P2C2

1P1C2

1

P2C1P1C1

P1C1P2C1ln1

2πIV

22

22

Y VI

if then a simple linear equation is expressed as Y X

where,

P2C2

1P1C2

1

P2C1P1C1

P1C1P2C1ln1

2π1X

22

22

PRIHADI SA / 2002

The theoretical apparent resistivity is derived by least square method.

∑

∑n

1i

2i

n

1iii

t

X

YX

n

1i

2i

n

1iai

2i

n

1i

2i

n

1i i

i2i

t

X

X

X

XY

X

Finally, the theoretical potential at a certain point of j location can be calculated as

iXV tti

and the potential and the resistivity difference are :

itiii XYVYΔV ti

taii PRIHADI SA / 2002

DIRECTIONAL WELL CASEDIRECTIONAL WELL CASE

The directional well problem derived by using the similar procedure with the vertical well casing case. The Geometric factor for the Mise-á-la-masse survey at a directional well casing pipe as is defined by the following equation :

1

21

2cossincos1

cossin22cos21222

21

2cossin2cos1

cos22

11212K

rr

rrrn

r

rn

PRIHADI SA / 2002

P (x ,y)

y

z

x

r

K ick o ff P o in t

2

1

ELECTRIC POTENTIALELECTRIC POTENTIALFOR FOR

A DIRECTIONAL WELLA DIRECTIONAL WELL PRIHADI SA / 2002

PRIHADI SA / 2002

CONTROLLED SOURCE AUDIO FREQUENCY CONTROLLED SOURCE AUDIO FREQUENCY MAGNETO-TELLURIC METHOD MAGNETO-TELLURIC METHOD

(CSAMT) (CSAMT)

The CSAMT measurement area is similar to the Magneto-Telluric method.

In the MT measurement the source of magnetotelluric wave is natural, whereas

for the CSAMT measurement the source is dipole discharged current.

In the CSAMT, the distance of the dipole source and receiver is kept between 3 – 5 , in order to get a plane wave source assumption as a natural magnetotelluric wave. The close distance between transmitter to receiver will produce near field effects, as indicated by increasing apparent resistivity and decreasing phase if the frequency is decreased.

PRIHADI SA / 2002

ELECTRIC FIELD

INPUT :- Pre Amp

MAGNETIC FIELDINPUT :

- Pre Amp

- Power SupFILTER ANDAMPLIFIER

A/D CONV.

6 - 24 CHAN

DIGITALTAPE RECORDER

MONITOR

ATENUATORCHANNEL H

ATENUATORCHANNEL E

GENERATORSIGNAL

COIL

PRIHADI SA / 2002

During the CSAMT survey, the subsurface electrical formation response data was measured by changing the frequency during the field survey. The sub-surface skin depth () relation is

ffaa 503

μπδ

PRIHADI SA / 2002

Therefore, the deeper penetration can be recorded by using the lower frequency. The apparent resistivity calculation derived from the measurement of electric field and magnetic intensity at each point is

where,ρa = apparent resistivity (-m)

= electric field (mV/km)

= magnetic field ()

xE→

yH→

2

→

→

2.0

Y

Xa

H

Ef

PRIHADI SA / 2002

The Bostick depth transform as described by Jones (1983) was applied to confirm the 1-D maximum depth penetration of each data point

( )( )MM

B -1+1

×ρ=ρ a

B = computed Bostick resistivity,ρa = apparent resistivity, M value is the slope of the apparent resistivity curve on a log-log plot which is approximated using numerical differentiation method, where,

0

aa

μf π2ρ

=h ;)f( log ∂)ρ( log ∂

=M

mH /10π4μ 7- h = depth (m) PRIHADI SA / 2002

PRIHADI SA / 2002

MODELING OF CSAMTMODELING OF CSAMT

Apparent resistivity is considered as a measured resistivity value of the assumed layered earth model.

The apparent resistivity value is relied on the true resistivity and the electro-magnetic frequency.

where,

= electric field intensity,= electric capacity density,= magnetic field intensity,

μ = magnetic permeability (μo= 4π x 10-7 H/m),= magnetic induction (in isotropic medium = μ ),

ε = permittivity (εo= x 10-9 F/m),= electric displacement (in isotropic medium = ε ),

σ = conductivity (σo = 0 S/m),

→E

φ

→H

→B

→H

π361

→D

→E

The Maxwell equation is as follows :

tBE

→

∇

J ∇→

tDH 0∇

→B

→∇ D; ; ;

= electric current density (in isotropic medium =σ ),→J

→E

zk

xi

yj∇ (Cartesian)

PRIHADI SA / 2002

In the isotropic medium, the relation between impedance, electric and magnetic fields within the layer boundary is given by

=HE

Z

A layered model for the CSAMT interpretation, where the subsurface has difference true resistivity values.

In depth variation ( )

of the wave propagation direction, the magnetic field consisting of transmission and reflection is

∑1

n

iii hz

)( 0kz

y eHzH

PRIHADI SA / 2002

For layer i=1 to n can be expressed as iiii

i

ziki

ziki

i

zyeBeAH -

and the electric field consisting of transmission and reflection is

i

i

zy

x z

HE i

-

For layer i=1 to n can be expressed as izx i

E

iiii

i

zikii

zikii

izx eBikeAikE --

where, k is a wave number, n

nik

PRIHADI SA / 2002

If the frequency is very low, an impedance general formula for the n-1 layer is

1-nz1-n1-n

1-

n

1-

Zarctanhhi-tanhωμ k

kk

Zn

zn

and for the half space of the nth layer ,

( )n

z kZ

n

ωμ=

to compute the surface impedance Z(0).

PRIHADI SA / 2002

The calculation is starts with the impedance computation at the nth-layer or

nkZ

ωμ=1

In other words, Z1 is the measured impedance in the surface of nth-layer, Z2 is the measured impedance in the n-1 layer,…etc. Zn-1 is the measured impedance in the 2nd-layer and Zn is the measured impedance in the 1st-layer or the field observed data.

ωμ

Zarctanhhi-tanhωμ 1z

111

1-nk

kk

Z n

PRIHADI SA / 2002

Therefore, the measured apparent resistivity for nth-layer is

2

1z112

1

2 2

ωμZ

arctanhhi-tanhμω2.0 1-n

k

kkfa

A ir

h

h

h

o

1

2

n-1

n

o

1

2

n-1

n

o

1

2

n-1

n

....

...

...

h

1

2

n-1

n

SUBSURFACE GEOMETRY OF THE LAYERED EARTH MODEL PRIHADI SA / 2002

PRIHADI SA / 2002

Pemancar untuk navigasi kapal selam yang memiliki daya besar dengan medan elektro-magnetik frekuensi rendah (15 – 30 kHz) dimanfaatkan dalam survei VLF. Medan primer yang dipancarkan oleh antena menginduksi benda-benda konduktor di bawah permukaan. Benda-benda konduktor tersebut kemudian menghasilkan medan sekunder yang ditangkap oleh alat penerima.

METODA VLFMETODA VLF

KUMPARAN 1 KUMPARAN 2 KUMPARAN 3

KONDUKTORPEMANCAR PENERIMA

PRIMER SEKUNDER

PRIMER

PRIHADI SA / 2002

2

11

1"'

Q

LsKMK

HpHs

dimanaM: Induktansi antara pemancar dan konduktorHp: Medan PrimerHs: Medan Sekunder Q : Rapat massa medan elektromagnet

Untuk konduktor yang sangat baik Q >> komponen Hs mempunyai fasa 1800 terhadap komponen Hp.

Komponen ini disebut komponen real atau in-phase.

Sebaliknya untuk konduktor yang buruk Q 0 komponen Hs mempunyai fasa 900 terhadap komponen Hp.

Komponen ini di sebut komponen imajiner atau out-phase / quadrature.

PRIHADI SA / 2002

Kedua komponen ini yang dideteksi dalam pengukuran yang menggunakan metoda VLF. Interpretasi data VLF dibuat berdasarkan teknik filter linier :

I(0) = K (-0.25 hI(0) = K (-0.25 h-2-2 + 0.323 h + 0.323 h-1-1 – 1.446 h – 1.446 h00 + 1.446 + 1.446 hh+1+1 – 0.323 h – 0.323 h+2+2 + 0.205 h + 0.205 h+3+3))

dimana K : konstanta yang bergantung pada jarak

antar titik pengukuranh : harga pengukuran pada titik sebelumnya

(h-n) atau titik sesudahnya (h+n)PRIHADI SA / 2002

Electrokinetic(elektrofiltation)(electromechanical)(streaming)

Diffusion potentialLiquid-junction

Nernst potential ( Shale )

Mineral potential Constant

from John M. Reynolds, 1997, An Introduction to Applied and Environmental Geophysics

TYPES OF ELECTRICAL POTENTIALS

Electrochemical potential

Variable with time

PRIHADI SA / 2002

Mineral potentials1. Sulphide ore bodies

2. ( pyrite, chalcopyrite, pyrrhotite,sphalerite, galena )

3. Graphite ore bodies4. Magnetite + other electronically

conducting minerals5. Coal

6. Manganese

7. Quartz veins Positive ~ ten of mV

Background potentials9. Fluid streaming, geochemical reactions, etc Positive + / - negative <= 100 mV10. Bioelectric ( lants, trees ) Negative <= 300 mV or so

11. Groundwater movement Positive or negative, up to hundreds of mV

12. Topography Negative, up to 2 V

NO.

TYPES OF SP AND THEIR GEOLOGICAL SOURCES

Negative ~ hundreds of mV

SOURCE TYPE OF ANOMALY

PRIHADI SA / 2002

No. MATERIAL NOMINAL RESISTIVITY ( m )

SULPHIDES1. Chalcopyrite 1.2 x 10-5 - 3 x 10-1

2. Pyrite 2.9 x 10-5 - 1.53. Pyrrhotite 7.5 x 10-6 - 5 x 10-2

4. Galena 3 x 10-5 - 3 x 102

5. Sphalerite 1.5 x 107

OXIDES6. Hematite 3.5 x 10-3 - 10-7

7. Limonite 103 - 107

8. Magnetite 5 x 10-5 - 5.7 x 10-1

9. Ilmenite 10-3 - 5 x 10

10. Quartz 3 x 102 - 106

11. Rock Salt 3 x 10 - 1013

12. Anthracite 10-3 - 2 x 105

13. Lignite 9 - 2 x 102

14. Granite 3 x 102 - 106

15. Granite (weathered ) 3 x 10 - 5 x 102

16. Syenite 102 - 106

17. Diorite 104 - 105

18. Gabbro 103 - 106

19. Basalt 10 - 1.3 x 107

20. Schists ( calcareous and mica ) 20 - 104

21. Schist ( graphite ) 10 - 102

22. Slates 6 x 102 - 4 x 107

23. Marble 102 - 2.5 x 108

24. Consolidated shales 20 - 2 x 103

25. Conglomerates 2 x 103 - 104

26. Sandstones 1 - 7.4 x 108

27. Limestones 5 - 107

28. Dolomite 3.5 x 102 - 5 x 103

29. Marls 3 - 7 x 1030. Clays 1 - 102

31. Alluvium and Sand 10 - 8 x 102

32. Moraine 10 - 5 x 103

RESISTIVITIES OF COMMON GEOLOGIC MATERIALS

PRIHADI SA / 2002

No. MATERIAL NOMINAL RESISTIVITY ( m )

33. Sherwood Sandstone 100 - 40034. Soil ( 40% of clay ) 835. Soil ( 20% of clay ) 3336. Top Soil 250 - 170036. London Clay 4 - 2037. Lias Clay 10 - 1538. Boulder Clay 15 - 3539. Clay ( very dry ) 50 - 15040. Mercia mudstone 20 - 6041. Coal measures clay 5042. Middle coal measures > 10043. Chalk 50 - 15044. Coke 0.2 - 845. Gravel ( dry ) 140046. Gravel ( saturated ) 10047. Quartenery / Recent sands 50 - 10048. Ash 449. Colliery spoil 10 - 2050. Pulverised fuel ash 50 - 10051. Laterite 800 - 150052. Lateritic soil 120 - 75053. Dry sandy soil 80 - 105054. Sand clay / clayey sand 30 - 21555. Sand and gravel 30 - 22556. Unsaturated landfill 30 - 10057. Saturated landfill 15 - 3058. Acid peat waters 10060. Acid mine waters 2061. Rainfall runoff 20 - 10062. Landfill runoff < 20 - 100

63. Glacier ice ( temperate ) 2 x 106 - 1.2 x 108

64. Glacier ice ( polar ) 5 x 104 - 3 x 10-5*

65. Permafrost 103 - > 104

RESISTIVITIES OF COMMON GEOLOGIC MATERIALS

PRIHADI SA / 2002

PRIHADI SA / 2002