1 Electric Methods
Cairo University
Instructor : Mahmoud Mekkawi
Professor of Applied Geophysics
* National Research Institute of Astronomy
and Geophysics (NRIAG-Helwan), Cairo.
Geomagnetism & Geoelectricity Dept.
https://nriag.academia.edu/mahmoudmekkawi
E-mail: [email protected]
Mobil: 01000 643 221
Electric methods Course -2017
- Yungul S., 1996. Electrical Method in Geophysical Exploration of Deep Sedimentary Basins.
Champman & Hall Press. - Reynolds M. John, 1997. An Introduction to Applied &
Environmental Geophysics. Geo-Sciences Ltd, UK. - Zhdanov S. Michael. 2009. Geophysical
Electromagnetic Theory & Methods. Elsevier. - Chave A. & Jones A. 2012. The Magnetotelluric Method, Theory & Practice. Cambridge Univ. Press.
I Electric methods:
- Units & Symbols
- Electrical Resistivity (Conductivity)
- Self Potential (SP)
- Induced Polarization (IP)
- Application
II EM Methods
- EM theory, Propagation & Spectrum
- Frequency Domain (FEM)
- Time Domain (TEM )
- EM Applications
III Magnetotelluric Method (MT)
- MT source field & Acquisition
- MT Types & systems
- MT processing & Interpretation
- MT Applications
IV Airborne &Marine EM
- Airborne (AEM) systems & Application
- Marine EM (Seaborne) & Application
Contents
Electrical Methods
(electric galvanic)
Soil contacting methods (EM-induction)
Non-contacting Electromagnetic
Electric
Resistivity
Induced
Polariztion
(IP)
Self Potential
(SP) Magnetotelluric (MT)
Electromagnetic (EM)
Variations of the electrical conductivity
of the subsurface
- Electrical conductivity
- Electrochemical polarization
Electromagnetic (inductive; a.c) methods
Electrical (galvanic; d.c) methods in the sub-surface investigations (DC, IP and SP)
Resistance : resistance to movement of charge Capacitance : ability to store charge Inductance : ability to generate current from changing magnetic field arising from moving charges in circuit
(Galvanic d.c. methods), electric direct current (I) is injected into the subsurface through the electrodes A & B. The potential, voltage difference (V), caused by the currents in the subsurface are measured with the potential electrodes M & N (voltage between the electrodes). * The mutual location of the electrodes A, B, M and N define the geometric (array) factor G. Resistivity:
Depth of investigation is controlled by the length of the array (roughly AB/3). Expanding the array provides deeper penetration.
I Electrical Methods Resistivity, Self Potential
& Induced Polarization
𝝆 = 𝑮𝚫𝐕
𝐈
Resistivity, IP, SP
Potential Electrodes
Electrical Electrodes
Conductive layer Resistive layer
Acquisition
Processing
Interpretation
The (EM) have the broadest range of different
instrumental systems:
* Time domain EM (TEM), measure with Time
* Frequency domain (FEM), measure one or more frequencies.
or
* Passive, utilizing natural ground signals (MT, AMT)
* Active, where an artificial transmitter is used
- Near field as in ground conductivity meters)
- Far field (using remote high powered military &
civil radio transmitters (VLF & RMT).
II EM Methods:
EM Induction
Methods
TX RX
Determination of resistivity as a function of distance Time (frequency) & depth.
Time (frequency) 1D -MT(m)
2D-MT (m)
Depth
Resistivity
Log Resistivity
3D-MT resistivity model at Kharga Reservoir Water Mekkawi et al., 2014
EM systems - Frequency Domain (FEM)
- Time Domain (TEM )
EM Applications: Groundwater-Mineral , Geothermal, Cavities, Faults, Landfill Survey, Geological and Permafrost mapping
III Magnetotellurics (MT) - MT source field & acquisition
- (AMT, CSAMT, MT & LMT)
- MT processing & Interpret.
- MT Applications
Deep structures, Geothermal
And volcanicity, active zones and Earthquakes, ground water, mineral and petroleum exploration.
Hx
Hy
Hz
Ex
Ey
MT Data Record vs. Time
MT Data Spectral (FFT)
Magnetotelluric (MT) :
MT method can be used to determine electrical properties of materials at relatively great depths.
Naturally occurring electrical currents, generated by magnetic induction of electrical currents in the ionosphere.
Recorded electrical (Ex, Ey) and magnetic signals (Hx,Hy, Hz) are used to estimate subsurface distribution of electrical resistivity.
2
,
,1 ))(
)((tan)(
xy
yx
H
E
2
,
,
5
1)(
H xya
E
f
yx
2D-MT resistivity for hydrocarbon reservoir exploration
MT Marine acquisition
3D-MT resistivity model at Kharga Reservoir Water. Mekkawi et al., 2014
2D-MT resistivity model Mekkawi et al., 2011.
Electrical Methods (RES, IP & SP)
Slide
1
Basic Concepts
Data Acquistion
Pseudosection
Inversion (1D,2D, 3D)
Case History
Electrical (Conductivity) Resistivity:
is a property of rock or material which determine how easily
electric current to flow when voltage is applied to the rock.
In the environment of the earth’s surface, most rock forming are: conductors (metals & water), semiconductors & insulators.
Metals
• Charge carriers are electrons that are not firmly attached to atoms in the lattice.
• Both the number of charge carriers and mobility are high.
• This gives a very low resistivity (e.g. copper ρ < 10-8 ohm-m).
Water
• Close to the surface, the fluid in the pore space is often water. If the water contains dissolved ions, then the resistivity of the water will be low because the ions can easily move.
• As the salinity of the brine increases, the resistivity decreases as more charge carriers become available and the resistivity decreases.
Semiconductors
• Semi conduction occurs in minerals such as sulphides and the charge carriers are electrons or ions.
• Compared to metals, the mobility number of charge carriers are lower, and thus the resistivity is higher (typically 10-3 to 10-5 ohm-m).
• This type of conduction occurs in igneous rocks and usually shows a temperature dependence of the form (thermally activated). When a mineral is molten, ions can freely move and the resistivity decreases.
Insulators
• In minerals such as diamond, there are very few charge carriers. To produce a charge carrier, a carbon atom would need to be removed from the crystal lattice. This requires a lot of effort and thus the mobility would be very low. As a consequence, the resistivity of pure diamond is very high resistive (ρ > 1010 Ωm)
• Carbon occurs in two forms, graphite and diamond.
While diamond has a high resistivity (no charge carriers), graphite has a structure that allows electrons to easily move parallel to sheets of carbon atoms. This gives a
very low resistivity (ρ = 8 x 10-6 Ωm)
Depends on directly on:
- Porosity,
- Permeability,
- Pore fluid saturation,
- Temperature and
- The presence of the conducting materials.
other causes of electrical conductivity: - Add clay minerals
- Graphite films,
- Iron oxides and metallic sulphides
- Partial melting
- Increase the salinity of the pore fluid
- Fracture rock to create extra pathways for current flow
- improve interconnection between pores
Electrical Resistivity (Conductivity):
Influencing factors
Influencing degree Geological
conditions of
rock mass Low resistivity ------
High resistivity
Porosity
Saturated
condition Large ----------- Small Weathered
and fault
fractured zones Unsaturated condi
tion Small ----------- Large
Pore fluid resistivity
(Groundwater) Low -------------- High
Components of
groundwater
Water saturation Large ----------- Small Groundwater
table
Water content by volume
(Porosity and water
saturation)
Large ----------- Small
Weathered
and fault fractured z
ones
Clay content Much ----------- Little Weathered and altere
d zones
porosity
R w w
nF S
F aR
w
m ( )
R
w
a
1) by Archie (1942)
F : factor
: res. Rock (ohm-m)
: res. water(ohm-m)
S : saturation
(0.5-2.5), m (1.3-2.5), n (2)
+ - V
RF I
+ +
+ +
- -
- -
Pore Fluid
1 1 1
R w sF
2) by Patnode and Wyllie (1950)
: resis soil (ohm-m) s
+ -
V
Rm I
Rf
Rock Matrix
Pore Fluid
+ + + +
- - - -
Clay Content (after Patnode & Wyllie,1950)
Cation Exchange Capacity (CEC) (after Waxman & Smith, 1968)
Surface Conduction (after Katube & Fume, 1987)
: Clay Content (after Patnode & Wyllie,1950)
Cation Exchange Capacity (CEC) (after Waxman & Smith, 1968)
Surface Conduction (after Katube & Fume, 1987)
Conductive Pore Fluid
Insulating Rock Matrix
Conductive
Ion Double Layer
(Surface Conduction)
Fd cs
d
c
1 1 1
R w sF
s
Dispersed Sphere Particles Model
w
c
r
c
w
m
r
1
1
11
(after Bussian, 1983)
Porous
Media
Archie’s
Formula
Parallel Resistance
Model
Dispersed Sphere
Particle Model
Insulating matrix Conductive matrix Pore Fluid
Experiment
Specimen
PC PC
Tap water
Spindle Spindle
Lid
Filter papers
Specimen
PC PC
Tap water
Spindle Spindle
Lid
Filter papersA
L
I
VR ( )( )
V A AR
I L L
),( : R m)(ohm:
)(: mL
)(: 2mA
Data processor
Specimen holder
Function
generator Signal conditioner
Data processor
Specimen holder
Function
generator Signal conditioner
Result
10-1
100
101
102
103
Pore fluid resistivity (ohm-m)
102
103
104
105
Res
isti
vit
y (
oh
m-m
)
Measured values
Archie's fomula
Parallel resistances model
100
101
102
103
Pore fluid resistivity (ohm-m)
100
101
102
103
104
Resi
stiv
ity
(o
hm
-m)
Glass beads
Archie'formula
Clayey sands
Parallel resistances model
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
1x10 0
1x10 1
1x10 2
1x10 3
1x10 4
Granite
Rhyolite
Tuff
Andesite
Basalt
Sandstone
Shale
Chert
Limestone
Crystalline schist
Mudstone
Decomposed granite soils
Clay
Landslide soils
Weathered formations
Resistivity (ohm-m)
1 Electrical (DC Res., IP, SP)
2 MT (AMT, CSAMT, MT, LMT)
3 EM (induction)
Time Domain (TEM)
Frequency Domain (FEM)
DC resistivity
Pole-pole array
Pole-dipole array
Dipole-dipole array
Schlumberger array
Wenner array
Modify pole-pole array
Resistivity
1) Ohm’s Law
V IR
RL
A
Resistivity is measured ohm-m and is reciprocal of conductivity
L= length (m) A= area (m)
2
(1)
(2)
A
LR
A
L
V
I
(1)and (2)
JE
(J ): current density (E ) : electrical field
Electric field )( : Rm)(ohm: m)(ohm:
2) Point Current Source
r
P I
C
Ohm’s law
22 r
IJ
22 r
I
r
VE
Potential difference
r
IEdrV
r
2
Current density
3) Two current sources
C1(+)
C2(-)
P1
P2
r11
r12 r21
r22
1211
1
11
2 rrr
IV
2221
2
11
2 rrr
IV
(C1)
(C2)
22211211
21
1111
2 rrrrr
IVVV
I
VG
1
22211211
11112
rrrrG
P1, P2 are potential difference
Resistivity
G (geometric factor)
Res.Apparent a
I
VG
I
VGa
1 Sounding-Profiling
2) Wenner 1) Schlumberger
ana na na nana
M N A B M N A B
Sounding survey
VES
Profiling survey
methods
3. Dipole-dipole
na
5. Wenner
1. Pole - pole
2. Pole - dipole na a
aa na
4. Schlumberger ana na
na nana
2 different Array
Resistivity methods
3. Dipole-dipole
5. Wenner
1. Pole-pole
2. Pole - dipole
4. Schlumberger
2 na
2 1n n a( )
n n n a( )( ) 1 2
n n a( ) 1
2 na
ΔV2πna
IΔV
2πn(n+1)aI
ΔVπn(n+1)(n+2)a
I
n n a( ) 1
ΔV2πna
I
method Resistivity Resistance
0 2 4 6
Depth
0
20
40
60
80
100
Resis
tivity
Pole-Pole
Pole-dipole
Dipole-dipole
Schlumberger
Wenner
(Depth of investigation)
Pole-pole > Pole-dipole >
Dipole-dipole > Schlumberger >
Wenner
Pole-pole array Pole-dipole array
Dipole-dipole array Schlumberger array
Wenner array
Pole-pole > pole-dipole > dipole-dipole
>> Schlumberger >> Wenner
Model Study
After, KIGAM, 2008 (S. Korea)
pole-pole, pole-dipole, dipole-dipole : n = 2.
Schlumberger, Wenner :
model 1 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
0
20
40
60
80
Dep
th
100 ohm-m
1,000 ohm-m 1,000 ohm-m10 ohm-m
4321 20 21 22
100
Case History After, KIGAM 2008
Potential Electrodes
Electrical Electrodes
Conductive layer Resistive layer
Acquisition
Processing
Interpretation
Inversion
2D-Model acquisition
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0 20 40 60 80 100 120 140
Distance (m)
Borehole information
DH-06
DH-05
BH-78
DH-07
BH-93
BH-91
BH-92
BH-76
BH-75
BH-77
DH-09
DH-08
N
0 50 100m
Scale
Afetr, KIGAM, 2008
0
70
98
139
198
279
392
562
791
1113
1595
2246
3160
4529
Resistivity(ohm-m)
152300 152400 152500 152600 152700
167500
167600
167700
167800
167900
168000
168100
° °
°
° °
°
pumping well
borehole
BH-93
BH-92
BH-75
DH-05
DH-06
(10m) borehole
10m
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