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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