5
Electromagnetic 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
EM Course 2015-2017
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 & Case Studies
IV Airborne &Marine EM
- Airborne (AEM) systems & Application
- Marine EM (Seaborne) & Application
Contents
III Magnetotelluric Method (MT) - MT source field & Acquisition
- MT Types & systems
- MT processing & Interpretation
- MT Applications & cases studies
Magnetic Coils
Hx Hy Hz
Amplifiers, digitizer, etc. Electrodes
E-Lines
Ex Ey
Computer GPS antenna
E
W
N
S
* MT Books
* Christopherson K., Jones A. &Mackie R., 2002. Magnetotellurics
for Natural Resources: From Acquisition though Interpretation SEG Course. http:// www.mtnet.info
Magnetotelluric (MT)
Hx
Hy
Hz
Ex
Ey
TM
TE
I-Acquisition
MT Data II-Processing
Modeling
III-Interpretation
Resistivity
Phase shift
- MT Symbole & unit
E the electrical field (V/m)
H the magnetic field intensity (A/m).
f the frequency (Hz) =[s-1] = 1 / T
k Wave number [1/m];
00
i
cossin
sincosR
TR
cossin
sincosθR
z,y,x
yyyxxyxx ZZZZ ,,,
T
Cartesian coordinates, z positive downwards
Z MT impedance tensor with complex components
(transfer functions)
Rotation matrix for clockwise
rotation ( < 0) with:
Rotation matrix for anti-clockwise
rotation ( > 0) with:
Regional strike coordinates
Angular frequency ; [1/s]
R
X, y, z
Resistivity & phase Curves
ρxy=TM
ρyx=TE
Hx
Hy
Hz
Ex
Ey
MT Data Record vs. Time
Magnetotelluric (MT) Basic Concept is an EM method which uses:
* Passive surface measurement of the earth’s natural electrical (Ex & Ey) and magnetic (Hx, Hy, Hz) fields
* Measure changes in E and H with time
* Transformation into frequency domain by using FFT.
* Frequency range 104 Hz to 10-4 Hz (Broadband MT)
* Used to derive the electrical resistivity (conductivity)
structure of the subsurface.
Phyx Phxy
xy
xyyx
HEyx
HEf
/tan
/5
1
1
2
Vertical component of the geomagnetic field (Hz) = A*Hx + B*Hy A and B are complex functions of the frequency.
Real part of A & B represent the induction arrows which used to define the resistive and conductive structure of the earth
(Geomagnetic deep sounding method).
Hx
Hy
Hz
*
*
*
yx
yxxy
HEyx
HEf
/tan
/5
1
1
2
Induction Arrows (Tipper)
MT Station SW- Cairo Mekkawi 2007
MT history & Theory:
According to Maxwell (1861):
1. Electric current and changing magnetic field induce
magnetic field (Örstedt, Ampere and Maxwell)
2. Changing (time varying) magnetic field induce
electric field (Faraday)
- Theory proposed by French (Cignard) & Russian (Tikanov) 1950’s
- MT system developed 1960’s
- First used for academic ~1970
(Map plate boundaries, geothermal, …etc.)
MT impedance (Z)
Plan wave
Ex (z,ω) = E0 e−kz
Hy (z,ω) = E0 (k/μω) e−kz
Zxy (ω) = Ex (z,ω) / Hy (z,ω)
= √(μωρ) e i𝝅/4
ρa(ω)=(1/μω) ∥Z (ω)∥2
After Fourier transforming the E(t) and H(t) data into the frequency domain the MT
surface impedance is calculated as:
Z(ω)= Ex (ω) / Hy (ω)
Apparent resistivity is: ρa,xy = (1 / ωμ0) ∣ Z ∣2
= (1 / ωμ0) ∣ Ex / Hy ∣
2
Phase Shift is: φxy(ω)=tan-1 [Z (ω)]
2
/5
1xyyx HE
f
yx HExy /tan 1
MT Source Fields
* Low Frequency: Micropulsations
* Interaction of solar wind with Earth’s magnetic field
(magnetosphere)
* Sunspot Cycle : Important for low frequencies < 8 Hz
SEG Course-2002
* High Frequencies: World-wide thunderstorm activity
* Energy travels around Earth in waveguide
* Bounded by Earth surface and ionosphere
* Lightning: Source field almost always present
SEG Course-2002
Penetration Depth
* Short Period (High Frequency)= Shallower Penetration
* Long Period (Low frequency) = Deeper penetration
increase
Skin Depth: δ ≈ 0.5 √ (ρ *T) km ρ a subsurface resistivity T a period of variations.
Short Period high frequency
Shallower Penetration
Long Period Low frequency
Deeper Penetration
For example:
* Sedimentary environment
ρ ~ 100 ohm.m &
frequency of 100 Hz
the penetration depth
is about 0.5 km
* Igneous rocks
ρ ~ 1000 ohm.m &
frequency of 10 Hz
the penetration depth
is about 5.0 km
Layered (1-D) Earth
ρ1 = 1000 ohm.m
ρ2 = 30 ohm.m
ρ3 = 500 ohm.m
104
103
102
101 Apparent resistivity
Ohm.m
Impedance Phase
(deg.)
10-3 10-1 101 103 Period (s)
80
60
40
20 Longer period à deeper
Using a range of periods a
depth sounding can be
obtained
0
Typical Resistivity Values
Source field almost always present (BUT
weak in AMT deadband during
daytime)
LMT MT
AMT
CSAMT
RMT
broad-band MT
6 5 4 3 2 1 0 -1 -2 -3 -4 -5
MT = magnetotellurics
AMT = Audio MT
LMT = Long MT
CSAMT = Control Source AMT
RMT = Radio MT
Log ( f ) Hz
Types of MT
MT (broadband) Generally refers to recording from 10-4 Hz to 104 Hz
AMT – Audio MT Refers to “Audio” frequencies recording > 10 Hz to 104 Hz
LMT – Long period MT recording from (1,000 s to 10,000 s ) (0.001 Hz to 0.00001 Hz) • CSAMT– Control Source Audio MT recording from 10-3 Hz to 10-5 Hz • RMT- Radio MT recording from 10-3 Hz to 10-6 Hz CSAMT = Control Source AMT RMT = Radio MT
Types of MT
Smirnov. MT Course
MT Acquisition
* One station set-up: 1-2 hours
* 2-4 stations per day
Smirnov. MT Course
Site Requirements
Magnetic sensors
Electric field sensors
Data acquisition systems
1 - Instrument case
2 - Pb-PbCl –electrodes
3 - Three induction coils
4- Fluxgate magnetometer (3-comp)
5 – data logger (24 bit, 6 cha)
6 - 12V battery (50 – 120 Ah)
7 - GPS-antenna and cable
8 - GSM-antenna and modem
9 - Laptop
2
3
4
Digital Acquisition Unit
Batteries
E-Lines
Computer
Coils layout in Epinal –France
Mekkawi & Schnegg, 2003
N
E
Hz
S
W
Hx
Hy
H-Coils
Electrode
Coils layout in Aswan, Egypt
Mekkawi & Schnegg, 2002
- Detailed prospect (spacing = 0.5 -1 km on profiles)
- Regional: areal coverage (spacing = 2-10 km on grids)
Remote Area Acquisition
Advanced MT Acquisition
Remote Area Acquisition
Advanced MT Acquisition
Broadband Magnetotelluric Metronix –Germany www.metronix.de
High frequencies
(>1 Hz) to 10k Hz
Low frequency
< 1 to 0.001 Hz
ADU –7e
Broadband MT in SW Saudia, 2011
Sub-basalt imaging with broadband magnetotellurics inNW Saudi Arabia, SEG 2011.
Daniele Colombo, Tim Keho, Emad Janoubi, Saudi Aramco Wolfgang Soyer, WesternGeco
Resistivity & Phase Data
ρxy =TE-Mode
ρyx =TM-Mode
2D-MT inversion
ρxy =TE-Mode
ρyx =TM-Mode
Resistivity Phase
ρyx =TM-Mode ρxy =TE-Mode
2D-MT modeling
Integrate with other Tool
MT- Interpretation , Processing & Interpretation
