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Universitas BrawijayaFebruary 21, 2014
FUNDAMENTAL OF ELECTROMAGNETIC METHODS IN EXPLORATION GEOPHYSICS
Djedi S. WidartoSr. Geoscientist / Chief New Energy & Green TechnologyUpstream Technology CenterPT PERTAMINA
Education : BS in geology (ITB, 1985) M.Eng. in mineral resources engineering (Waseda University, 1991) Dr.Sci. in science, geology and mineralogy (Kyoto University, 1994)
Working Experiences : March 1986 – June 2008, Research Ctr for Geotechnology, LIPI, Bandung
Last post: Principle Researcher in Applied Geophysics June 2008 – present, Upstream Technology Center, PT Pertamina (Upstream), Jakarta
Present Position: Senior geoscientist / Specialist in electromagnetic geophysicsChief of New Energy & Green Technology, Upstream Technology Center, PT Pertamina
Award : 2006, Peneliti Utama Terbaik Indonesia, Riset Unggulan Terpadu, Kementerian RISTEK 2006 2004-2005, National Science Council Scholarship Award, National Central Univ, Taiwan 1995 – 2008, Japan Society for the Promotion of Science, Research Scientist at Japanese universities (Kyushu,
Hokkaido, and Chiba Universities) 1997, TWAS/UNESCO Scholarship Award at the Flinders Univ of South Australia 1991-1992, SEG Scholarship Student (US) / ASIA 21 Scholarship Student (Japan)
Professional Membership : 1990 – present, Society of Exploration Geophysicists (SEG) 1989 – present, European Society of Geoscientists and Geoengineers (EAGE) 1989 – present, Society of Exploration Geophysicists Japan (SEGJ) 1986 – present, Indonesian Association of Geophysicists (HAGI) 1986 – present, Indonesian Association of Geologists (IAGI) 2007 – present, Inter-association Working Group EMSEV (Electromagnetic Studies on Earthquakes and
Volcanoes)
Djedi S. Widarto
GEOPHYSICS:The study of the earth by quantitative physical methods, especially by seismic reflection and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods (Sheriff, 1999).
EXPLORATION GEOPHYSICS / GEOPHYSICAL PROSPECTING / APPLIED GEOPHYSICS:Making and interpreting measurements of physical properties of the earth to determine subsurface conditions, usually with an economic objective, e.g., discovery of fuel or mineral deposits. Properties measured include seismic, gravity, magnetic, electric, and temperature (Sheriff, 1999).
PETROLEUM/GEOTHERMAL GEOPHYSICS:Making and interpreting measurements of physical properties of the earth to determine subsurface conditions related to hydrocarbon/geothermal.
Geophysical Methods
Surface Methods
Seismic Methods : Seismic reflection methods Surface wave (refraction) methods Micro-earthquake
Potential Field Methods : Gravity & magnetic
Electrical Methods Resistivity methods Self-potential Mise a-la masse methods Induced polarization
Electromagnetic Methods Magnetotelluric (natural + controlled-source) methods Time-domain electromagnetic methods Ground penetrating radar Very-low frequency methods Seismo-electric method
Nuclear Methods Nuclear magnetic resonance (NMR) method
Borehole Methods
In-Hole Procedures
Cross-Hole Procedures
Surface to Borehole Procedures: Velocity surveys
Vertical seismic profiling
Logging Techniques: Electrical methods Acoustic logging Nuclear logging
Flow logging Other methods of logging
Techniques applying physical laws (or theory) to the study of the solid Earth,
Estimation of subsurface physical property distribution by measuring relevant parameters:
Geophysical Methods
Method Measured Rock Property
SEISMICS Travel time & amplitude Elastic moduli (density & velocity)
GRAVITY Variation in gravitational field Density
MAGNETICS Variation in magnetic field Magnetic susceptibility
ELECTRICAL / ELECTROMAGNETICS
Specific resistivity Electrical conductivity
GPR Travel time Dielectric constant
NUCLEAR Variation in natural radioactivity Nuclear decay
Method and Survey Procedure
Expected Anomaly
Interpretation
Aero- or ground magnetic (covers a large area)
low anomaly Ql : can be associated with thermally altered zones
Qt : geometry (?)
Schlumberger resistivity mapping and sounding (concentrated in the area between broad magnetic low and high)
low anomaly Ql : can be associated with thermal fluids upflow and outflow zones
Qt : shallow resistivity structure
self-potential (across high and low resistivity areas)
high or low anomaly
Ql : ascending thermal fluid (and / or descending cold water)
Qt : fluid flow (?)
gravity (covers low and high magnetic areas)
high or low anomaly
Ql : existence of deep structure, i.e. intrusive body or caldera structures
Qt : geometry of those above (the upper structure must be closely defined)
Probable Sequence of Geophysical Exploration Methods Used to Investigate Young Volcanic Geothermal Prospect (revised from Sudarman, 1983)
Method and Survey Procedure
Expected Anomaly
Interpretation
Thermal gradient and anomalous temperature (to figure out the cause of low resistivity layer)
high anomaly
Ql : uprising or horisontal thermal fluid movement, if depth to resevoiris relatively shallow
Qt : defined the upper structure
Magnetotelluric sounding low anomaly
Ql : can be associated with thermal fluids upflow and outflow zones
Qt : deeper resistivity structure
micro-seismics (M< 3) high anomaly
Ql : permeable zones, hydrothermal activity zone
Qt : velocity distribution (?)
Probable Sequence of Geophysical Exploration Methods Used to Investigate Young Volcanic Geothermal Prospect (revised from Sudarman, 1983)
Natural Source Magnetotelluric
MT is a geophysical method to estimate subsurface electrical property (resistivity or conductivity) distribution by measuring naturally time-varying EM fields,
Dependence of electric and magnetic phenomena on the conductivity of the medium can be exploited to study the structure of solid Earth,
Source of MT signals comes from interaction of the Earth’s permanent magnetic field with particles from solar wind and with atmospheric lightning which induce electric currents in the subsurface, thus
no need for transmitter, simplifies the logistics
random signals, low S/N (dead band ~1 Hz)
What is Magnetotellurics (MT) ?
Methods to estimate subsurface electrical property (resistivity) distribution by measuring (naturally time-varying) EM fields over a range in frequencies :
Magnetotellurics (MT, f < 10 Hz), Audio-frequency MT (AMT, f > 10 Hz), Controlled-Source Audio-Frequency MT (CSAMT), …
Transient EM / Time-Domain EM, Very Low Frequency EM (VLF-EM), LOTEM, …., etc.
Ground Probing Radar (GPR)
Airborne EM, Marine CSEM,…etc.
What is Magnetotellurics (MT) ?
transmitter generates
time varying EM field
induces Eddy currents
in the conductor (Earth)
generate secondary
magnetic field
electric and magnetic
field are sensed at the
receiver
Electromagnetic Induction
Electrical Resistivities of Rocks
Resistivity
Res
isti
vity
[O
hm
-m]
Hyd
rate
s
OIL
SA
ND
S
0.3 Wm
Interpreting subsurface resistivity: Impact of pore fluids and geologic processes on resistivity
Saline brine
Clay alteration
Dissolution
Temperature
Pressure
Faulting
Shearing
Hydrocarbons
Carbonate cementation
Silicification
Metamorphism
DecreaseIncrease
f > 1 Hz
f < 1 Hz
natural electromagnetic field10−4 – 10+4 Hz
Natural Electromagnetic Fields
z
x y
Hx
Hz
Ex
Hy
Ey
NaturalSignal
ShortPeriod
Longperiod
For a homogeneous or layered (1-D) medium
Ex = Z Hy Z = scalar impedance
For a medium with 2-D symmetry
Ex = Zxy Hy
Ey = Zyx Hx Zxy ≠ Zyx
Z = vector impedance
For a general 3-D medium
Ex = Zxx Hx + Zxy Hy E = Z H
Ey = Zyx Hx + Zyy Hy Z = tensor impedance
Electric (E) and magnetic (H) fields relationship
Infinite distance of source – sounding site
plane wave assumption, time invariance of the source
simplifies analysis of the governing equations
Frequency domain and wide frequency bands
intermediate to deep investigation depth
Wide range of applications
regional scale geological studies/tectonics
mineral, geothermal and oil exploration
Characteristics of MT Method
Characteristics of MT Method
Resistivity contrast
There must be a significant resistivity contrast within the depth of investigation for the method to be useful
Contrast of 5:1 or greater
Resolution depends on thickness and depth of unit being mapped:
About 5~10% of depth, e.g. the top of a horizon at 10000 m can be mapped to +- 500 m
MT Advantages:
Great depth of penetration
Provides information in non-seismic or poor seismic areas
No transmitter required
Light-weight equipment - very portable
Good production rate
Can access almost anywhere
Little impact on environment
Better resolution than grav / mag
Well-developed 2-D / 3-D interpretation procedures
MT Disadvantages:
Coupling with lateral conductors (e.g. sea)
Irregular natural signal and industrial noise
Resolution less than seismic
Complex data processing
Static shift of apparent resistivity curves sometimes significant
Inversion techniques rely on smooth models, tougher to interpret in complex areas
Skin effect and penetration depth
Skin effect = exponential EM wave attenuation with depth
Skin depth () = depth in a homogeneous medium at which the amplitude becomes 1/e or 63% of the original field strength:
A () = A exp (- ) = A exp (-1) = (2 / µ0) 1/2 500 (.T)1/2
in meter, in Ohm.m, T in seconds
Skin depth is associated to penetration depth of EM
Skin effect and penetration depth
Skin-depth 500 (T) ½
Effective depth d /√2 330 (T) ½
Lower frequency(or higher period) andhigher resistivity
~ slower attenuation~ deeper penetration
Principles of MT sounding i.e. wide frequency band measurement probes different parts (depths)of the subsurface
Z
Depth
MT Data Acquisition (field set-up)
MT Field Set-up
Magnetic SensorInduction Coil
Electric Sensor Pb-PbCl2
Receiver SystemMTU-5A Phoenix
Receiver SystemMTU-5A Phoenix
Audio-Frequency MT (AMT)
and
MT field set-up
Satellite-Synchronized Magnetotellurics
Phoenix MT System 2000
MT time series
Time (sec)
Mag
net
ic a
nd
Ele
ctri
c Fi
eld
s In
ten
sity
Data processing sequence
To extract impedance tensor Zfrom observed EM fields (time series of E and H),
Spectral analysis and transfer function estimation
Analysis of subsurface properties contained in Z
Acquisition, Data Processing and Results
Measurement of orthogonal EM fields (time series)
Ex , Ey , Hx , Hy
Data processing to extract impedance tensor
Ex = Zxx Hx + Zxy Hy
Ey = Zyx Hx + Zyy Hy E = Z H
Apparent resistivity and phase
ij
ij
ijij
o
ijaZ
ZZ
.Re
.Imtan
1 1)(
2
)(
Apparent resistivity and phase sounding curvesa (Ohm.m) and (degree) vs frequency (Hz)
Medium quality data(Class-B)
320 Hz ~ 0.5 Hz: good
0.5 ~ 0.00055 Hz: med - poor
good
Apparent resistivity and phase sounding curvesa (Ohm.m) and (degree) vs frequency (Hz)
Pseudo-section
2-D plot of apparent resistivity and phase data from MT sounding on a profile
horizontal axis is distance or station position
vertical axis is frequency or period (increasing periods downward ~ increasing depth)
Color contoured:
low resistivity (or high impedance phase) ~ red
high resistivity (or low impedance phase) ~ blue
Qualitative 2-D resistivity distribution for preliminary interpretation
Data Presentation
Resistivity pseudo-section-1
Freq
uen
cy (
Her
z)
Distance (km)
0 2000 4000 6000 8000 10000 12000 14000 16000 18000
DISTANCE (m)
6000
5000
4000
3000
2000
1000
0
PS
EU
DO
-DE
PT
H (
m)
A B C D
4 6 8 10 12 14 16 18 20 22 24 26
APP. RESISTIVITY (Ohm.m)
Resistivity pseudo-section-2
MT Data Modeling
1
10
100
1000
AP
P.
RE
SIS
TIV
ITY
(O
hm
.m)
obs. data
calc. data
0.001 0.01 0.1 1 10 100 1000
PERIOD (sec.)
0
45
90
PH
AS
E (
de
g.)
1 10 100 1000
RESISTIVITY (Ohm.m)
10000
1000
100
DE
PT
H (
m)
?
1
10
100
1000
AP
P.
RE
SIS
TIV
ITY
(O
hm
.m)
obs. data
calc. data
0.001 0.01 0.1 1 10 100 1000
PERIOD (sec.)
0
45
90
PH
AS
E (
de
g.)
1 10 100 1000
RESISTIVITY (Ohm.m)
10000
1000
100
DE
PT
H (
m)
MT 1-D smooth modeling
OCCAM inversion (Constable et al., 1987), ABIC (Mitsuhata, 1991)
Markov Chain Monte Carlo (MCMC) algorithm (Grandis et al., 1999)
Assuming skin depth () = investigation depth (D) in a
layered medium, then transform apparent resistivity –
period curve (α vs. T) to resistivity – depth curve ( vs. D)
data model
αi , Ti αi , Di
B , Di or
Bostick Transform
2/1500 iaii TD
i
ai
i
i
iaiB
Td
dTs
Ts
Ts
log
log)(
)(1
)(1
900
,45
1
1
1
M
M
MaiB
B is Bostick resistivity (Ohm-m)Di is Bostick depth (meter)
Bostick Transform
1-D Bostick Model
First approximation of (z) at each MT sounding site
Used to construct 2-D pseudo-section with vertical axis in depth or pseudo-depth
Pseudo-depth from Bostick transform
too deep, usually down-scaled
(z) from Bostick transform as initial guest to 1-D model
resistivity and depth iterative adjustments using 1-D
forward modelling
Bostick Transform
2-D resistivity section from 1-D models on a profile
Correlation of resistivity units from station to station
Correlation of resistivity units with geology and lithology
Bostick Transform
Source: USGS Pubs Web-site
Bostick Transform
Source: USGS Pubs Web-site
First used for academic and geothermal
Map plate boundaries, structural-tectonic studies, volcanoes, alteration, etc.
Use for petroleum starting ~1980
1980’s: many in-house groups
Shell, Amoco, Arco, CGG
1990’s: most work outsourced to contractors and consultants:
Geosystems (Italy, UK, US), Zonge (USA)
Phoenix (Canada), Metronix (Germany), Russia, Japan, etc.
CASE STUDIES: Historical Perspectives
Historical Perspectives
MT method in Indonesia
introduction and application since mid 70s
geothermal exploration
foreign contractors (BEICIP, CGG, NGS, …)
MT expertise gained in early 90s
start of economic crises in mid – late 90s
MT equipment acquired by institutions (LIPI, Elnusa,
PSG, PSDG, ANTAM …) in ~2004
introduction to HC & geothermal explorations
Central Java Transect (AMT & MT, 1995-1996)
Bengkulu Transect (AMT & MT, 1997)
Flores Transect (AMT & MT, 1998)
Cimandiri Fault Zone West Java (AMT & MT, 1999-2000)
Crustal scale studies
Seram, Mollucca (1998)
Tanjungkerta, West Java (1999, 2000)
Kawengan and West Banyuasin, East Java (2004)
Brebes and Losari, Central Java (2005)
Banjar, West Java (2009)
Petroleum exploration
Geothermal and Volcanology
Hululais, Jambi (MT, 1995)
Seulawah, Aceh (MT, 1995)
Sumurup, Jambi (MT & TDEM, 1997)
Lahendong, North Sulawesi (CSAMT)
Kamojang, West Java (CSAMT)
Cimanggu Hot Spring, West Java (AMT, 1998)
Guntur-Galunggung, West Java (AMT, 2000-2001)
Ungaran, Central Java (AMT, 2002-2003)
Guntur volcano (AMT & MT, 2003-2004)
Papandayan volcano (AMT & MT, 2007)
2-D Modeling by FEM
2-D Inverted Resistivity Model Result
AMT Survey on Guntur volcano
1.0
1.5
0.5
-0.5
0.0
0.0 0.5 1.0 1.5 2.0
10000
3000
300
30
3
1000
100
10
1 [ohm-m]
REFERENC E
G. GUNTUR
3.5
~ 8
.5 k
m
(se
ism
icdata)
Sta.
Cit
iis [
+15
23
m]
SOUTH-SOUTHWESTING [km]EASTING [km]
0.0 0.5 1.0 1.5 2.0
1.0
1.5
0.5
-0.5
0.0
Magmachamber
GN_01
GN_02
GN_03
CMK
GN_05
GN_06
PTR
_01
[+1
44
6.3
m]
PTR_02
PTR_06
PTR_05
PTR_03
PTR_04
PTR_07
PTR_08
PTR_09
PTR_10
Tiltmeter/WatertubeStation
INVERTED 2-D RESISTIVITY MODEL BENEATH GUNTUR VOLCANO AS VIEWED FROM NORTH TO THE SOUTH OF THE VOLCANO
AMT Survey, Cimanggu Hot Spring, West Java
App. Resisitivity (Ohm.m)
0 5 10 15 20 25 30 35 40
1
2
3
4
LO
G F
RE
QU
EN
CY
(H
z)
10
30
70
200
400
1000
3000
1 2 3 4 5 6 7 8 9 10
11
12
13
14
15
16
17
18
19
0 5 10 15 20 25 30 35 40
DISTANCE (x 100m)
0
5
10
15
EL
EV
AT
ION
(x
10
0m
)
Resisitivity (Ohm.m)
5
20
50
200
500
3000
10000
pseudosection(data)
2-Dsmooth model
hot-spring hot-spring
MT on Papandayan Volcano
MT on Papandayan Volcano
Geologic Interpretation on Geothermal Structure
Controlled-Source Electromagnetic (CSEM)
Controlled-Source Audio-frequency Magnetotellurics (CSAMT/CSMT), and
Time-Domain / Transient Electromagnetics(TDEM/TEM).
Answers:
A. Stronger signal compared to the natural one enhance S/N ratio;
B. Electrical conductivity is closely linked to fluid properties.
Why are Controlled-Source EM methods worth such attention ?
Electrical conductivity and fluids:
Rock conductivity is a direct function of porosity,
Rock conductivity is a direct function of permeability,
Rock conductivity is a direct function of fluid conductivity (clearly – need other information or assumptions to separate effects).
Only a few physical properties are available for remote sensing:
Density, acoustic velocity, magnetization, conductivity,
Only seismics and EM can use an active (man-made) source.
Conductivity of Earth materials
Electrical resistivity has units of Wm.
Current I
A 1 m3 cube of 2 Wm rock would have a series resistance of 2 W across the faces,
Conductivity is just the reciprocal of resistivity: = 1 / (S/m)
mL
RA
A
LR W
Conductivity is proportionality constant in Ohm’s Law for continuous media:
JEorEJ
Where J is current density (A/m2) and E is electric field (V/m)
Faraday’s Law says that a moving (or time-varying) magnetic field will induce electric fields in a conductor.
It is also expressed in terms of the change in the magnetic induction B with time t:
Electromagnetic Induction
C
dt
ddlE
a length element of the loop
t
BE
magnetic flux [weber = 108 maxwell]
‘Minus’ sign in Faraday’s Law shows that conductors attenuate EM fields and so EM fields propagate in resistive materials.
Rate of cutting of lines of magnetic flux in Maxwell/sec
Magnetic induction [1 Tesla = 1 weber/m2 = 104 gauss = 109 gamma]
Voltage
Electric field strength [V/m]
Ohm’s Law says that a current will be generated from the electric field in a good conductor.
Ampere’s Law says that the current I will generate a secondary magnetic field.
Electromagnetic Induction
C
IdlB
EJ
HB 0
H is magnetizing force or magnetic field strength
[1 ampere turn/m = 4p10-3 oersted]
o = permeability of free space = 4p10-7 Henry/meter
= permeability of the medium
Electromagnetic Induction
CSEM and MT lack the resolution of seismic methods, but can constraindepths very much better than potential field methods through the skin depth:
For a plane wave incident on a uniformconductor,
is called the skin depth. It is the distance that field amplitudes are reduced by 1/e, or 37%. In practical units,
where circular frequency f=/2p=1/T
2s
deptheffectivemTd
mTf
s
s
3502
5001500
Skin depth is not a measure of resolution, but is a guide to the
maximum distance EM energy can propagate.
A terminology grouping all electromagnetic techniques which use their own transmitter. Examples are time-domain/transient electromagnetics (TDEM/TEM) and controlled-source audio MT (CSAMT),
CSEM is an active geophysical method to estimate subsurface electrical property (resistivity or conductivity) distribution by measuring generated secondary EM fields,
Source of EM signals is transmitted from, in general, a grounded-dipole wire & loop-wire (horizontal & vertical) sources,
The receivers are, in general, fluxgate magnetometer (3 comp.), induction magnetic coil + electric sensors.
What is Controlled-Source Electro-
Magnetics (CSEM) ?
CSAMT (freq: 10 Hz ~ 20 kHz) / CSMT (freq: <1 Hz ~ 20 kHz) frequency domain
• Manufacturers: Phoenix Geophysics Ltd (Canada), Zonge Ltd (US), Metronix BmgH (Germany), Neoscience Ltd (Japan, local), etc.
• Services: Phoenix Geophysics Ltd (Canada), Geosystem Srl. (Italy), Zonge Ltd (US), Indonesia (PT Elnusa Geosains?, PT Aneka Tambang: 2 systems in end 2008?),
• Targets: mainly geothermal, several in oil and gas, groundwater and civil engineering
What is Controlled-Source Electro-
Magnetics (CSEM) ?
Time-Domain EM / Transient EM / MULTI-TEM
• Manufacturers: Phoenix Geophysics Ltd (Canada), Lamontagne Geophysics Ltd. (UTEM, Canada), Zonge Ltd (US), Metronix BmgH (Germany), Geonics Ltd (Canada), Neoscience Ltd (Japan, local), etc.
• Services: Phoenix Geophysics Ltd (Canada), Geosystem Srl. (Italy), Zonge Ltd (US), Lamontagne Geophysics Ltd. (UTEM, Canada), Geonics Ltd (Canada), Metronix BmgH (Germany), SIROTEM (Aussie), Neoscience Ltd (Japan, local), Indonesia (PT Elnusa Geosains), PGS, WesternGeco,
• Targets: mainly geothermal, several in oil and gas, groundwater and civil engineering, sulfide (conductive) minerals
What is Controlled-Source Electro-
Magnetics (CSEM) ?
Long-Offset TEM (LOTEM) Tx – Re offset is
approximately equal to or greater than the exploration depth
• Manufacturers & Services: K-M Strack (Western Atlas Internatl. Ltd)
• Targets: oil and gas, geothermal, and structural tectonic studies
What is Controlled-Source Electro-
Magnetics (CSEM) ?
Airborne TDEM/TEM:
• Japan, US, Australia, China, Canada
• Targets: minerals, volcano studies & geothermal
Marine CSEM/TDEM/TEM/MT:
• Services: EMGS, PGS, CGG, OHM, WesternGeco, etc.,
• Targets: oil and gas, methane hydrate,
• Cases: Gulf of Mexico, North sea, Nigeria, Brazil, Colombia, Canada, and offshore West Africa,
• ExxonMobil Resistivity Mapping Surveys: West Africa (23 cases), South America (9 cases), North America (10 cases), Nile delta (2007, 2008).
What is Controlled-Source Electro-
Magnetics (CSEM) ?
The polarization of the fields can be selected by the orientation of the transmitting source and the signal strengths do not depend upon the time of day and season,
Signals are stronger (greater S/N ratio) and hence higher accuracy, therefor the receiving equipment does not need to be as sensitive as that for natural MT/AMT,
Because of the coherent signal, the usual signal processing and enhancement techniques are far more effective,
It is less affected by lateral resistivity variations when providing sounding information (Ward, 1983), and
The surveys can be much faster than of using natural source.
Why Controlled-Source Electro-
Magnetics (CSEM) ?
BUT,
It is more expensive & logistically inefficient (esp. tensor CSMT) for very long cross-country traverses over those natural fields,
One (technical) disadvantage: if the survey area is closer to the transmitter, the resistivity of the deep layers cannot be accurately determined (Goldstein and Strangway, 1975; Yamashita, 1984) the plane-wave assumption is no longer true. To avoid such effect, the receiver must be placed at some distant where the transmitted EM field becomes satisfy a plane wave assumption or as the so-called far-field response Lf 4 x skin-depth = 2000 /f.
Why Controlled-Source Electro-Magnetics (CSEM) ?
Multi-layered half space model, with a current dipole of sufficiently small length.
I : current intensity, Amp
ds : dipole length, m
r : distant of dipole source center to P(x,y,0), m
: conductivity, S/m or resistivity, Ohm-m
x, y, z : Cartesian axes
Characteristics of EM Fields
Horizontal components of the EM fields Ex, Ey, Hx and Hy at P(x,y,0) due to the dipole are described as follows (e.g. Wait, 1966, 1970; Daniels, 1974; Murakami, 1986):
,14
0
0 0
0
Qr
x
xdrJR
u
IdsjE TEx
p
Characteristics of EM Fields
,
Q
r
y
xEy
pdrJRR
ur
y
x
IdsH TMTEx 1
0 0
)()(4
,)()(4
14
1
0
0
0
p
p
drJRRr
x
x
Ids
drJRIds
H
TMTE
TEy
Characteristics of EM Fields
22
0
2
0
22
0
10
000
,
,)()(14
p
kku
drJRRu
Ruj
IdsQ TMTETE
where
Characteristics of EM Fields
: dielectric constant
: magnetic permeability
: angular frequency
RTE , RTM : reflection coefficients of TE and TM mode
EM wave at the Earth surface
Jo, J1 : Bessel functions
Ex, Ey, Hx and Hy : (usually) complex numbers
Acquired parameters:
E-field : Magnitude + Phase
H-field : Magnitude + Phase
Calculated parameters (these are calculated automatically in the field): Cagniard resistivity:
E : mV/km
H : nano-Tesla (nT) or gamma
Phase difference (degree): E-phase minus H-phase,
= E - H
Survey Design & Acquisition
2
5
1
y
xa
H
E
f
are measured as a function of frequency
Based on their configuration:
Scalar Tx: 1 system; Re: Ex/Hy or Ey/Hx
Vector Tx: 1 system; Re: Ex, Ey, Hx and Hy
Tensor Tx: 2 systems (Tx-1 and Tx-2); Re: Ex1, Ex2, Ey1, Ey2, Hx1, Hx2, Hy1, and Hy2
Survey Design & Acquisition
Scalar CSAMT/CSMT
Scalar CSAMT/CSMT
Scalar impedance is calculated from a pair of orthogonal horizontal E- and H-fields : Zxy = Ex/Hy or Zyx = Ey/Hx,
In the frequency domain, the relationship between E and H is shown by the transfer function:
Ex() = Z()·Hy()
In an homogeneous area or 1-D structure, scalar impedance provides complete information the resistivity structure,
For complex structure as in mining and geothermal areas, the resistivity models analyzed from scalar impedances can be ambiguous and confusing due to the heterogeneity.
Vector CSAMT/CSMT
Vector CSAMT/CSMT
The impedance is calculated from a set of orthogonal horizontal E- and H-fields (Vozoff, 1986) :
Ex = ZxxHx + ZxyHy
Ey = ZyxHx + ZyyHy,
The tensor impedance Z is a complex number, then apparent resistivity and phase are calculated by:
HZEH
H
ZZ
ZZ
E
E
y
x
yyyx
xyxx
y
x
,
2
0
.
1ijija Z
ij
ij
ijZ
Z
Re
Imtan 1
xyxyxxxxxx EHZEHZEE ***
Vector CSAMT/CSMT
yyxyyxxxyx EHZEHZEE ***
Defining the impedance tensor:
xyxyxxxxxx HHZHHZHE ***
yyxyyxxxyx HHZHHZHE ***
xyyyxxyxxy EHZEHZEE ***
yyyyyxyxyy EHZEHZEE ***
xyyyxxyxxy HHZHHZHE ***
yyyyyxyxyy HHZHHZHE ***
*ii XX
*iiYY
*jiYX
Power-spectra:
Cross-spectra:
X and Y are magnetic or electric field at i- or j-direction
Tensor CSAMT/CSMT
Tensor CSAMT/CSMT
Best technique to be used in a very complex with strong regional anisotropy (i.e. volcano and geothermal studies, mineral exploration)
Full tensor solution to the impedance may be preferred,
The determinant of the impedance tensor is invariant under rotation and hence is not influenced by the orientation of the measuring coordinates and source orientation (Eggers, 1992; Geophysics),
The tensor is more representative of most situations since it does permit to obtain intrinsic apparent resistivity under 2- or 3-D structures (Cantwell, 1960; Swift, 1967; Sims, Bostick and Smith, 1971; Eggers, 1982),
It improves resolution of complex geologic structure compared to single source CSAMT (Sanberg and Hohmann, 1982; Otten and Musmann, 1985; Uchida et al., 1989)
Tensor CSAMT/CSMT
Defining the ‘full’ impedance tensor, two source polarizations are required. The MT tensor data set become:
Ex1 = ZxxHx1 + ZxyHy1
Ey1 = ZyxHx1 + ZyyHy1, (1)
Source-1 :
Ex2 = ZxxHx2 + ZxyHy2
Ey2 = ZyxHx2 + ZyyHy2, (2)
Source-2 :
Source-1 and -2 are transmitted independently !
Tensor CSAMT/CSMT
Multiply (1) and (2) to reference signal and (* denotes complex conjugate), we obtain:
*1yH *
2xH
*22
*22
*22
*22
*22
*22
*11
*11
*11
*11
*11
*11
xyyyxxyxxy
xyxyxxxxxx
yyyyyxyxyy
yyxyyxxxyx
HHZHHZHE
HHZHHZHE
HHZHHZHE
HHZHHZHE
Complex Conjugate
In mathematics, complex conjugates are a pair of complex numbers, both having the same real part, but with imaginary parts of equal magnitude and opposite signs.
For example, 3 + 4i and 3 − 4i are complex conjugates. The conjugate of the complex number z
where a and b are real numbers, is
For example,
Tensor CSAMT/CSMT
The ‘full’ impedance tensor elements can be obtained as follows:
*11
*22
*22
*11
*11
*22
*22
*11
*11
*22
*22
*11
*11
*22
*22
*11
*11
*22
*22
*11
*11
*22
*22
*11
*11
*22
*22
*11
*11
*22
*22
*11
yyxxxyyx
yyxxxyyx
xx
yyxxxyyx
yyxyxyyy
yx
yyxxxyyx
yxxxxxyx
xy
yyxxxyyx
yyxxxyyx
xx
HHHHHHHH
HEHHHEHHZ
HHHHHHHH
HHHEHHHEZ
HHHHHHHH
HEHHHEHHZ
HHHHHHHH
HHHEHHHEZ
2
0
1ijij Z
ij
ij
ijZ
Z
Re
Imtan 1
Tensor CSAMT/CSMT
In 1-D Earth case, we obtain:
)()(
0)()(
yxyxxyxy
yyyyxxxx
orZorZ
orZorZ
In 2-D Earth case, Zxy and Zyx have a maximum or minimum, parallel or perpendicular to the strike.
If the x- or y-axis is along strike : Zxx = Zyy = 0If neither axis is along strike : Zxx + Zyy = 0
In 3-D Earth case : Zxx ≠ Zyy ≠ 0
Processing & Modeling : General Procedures
Objective: To extract the observed signals a set of smooth, repeatable functions representing the Earth’s response to be used in interpreting the Earth’s conductivity structures
The response functions consist of:Impedance tensor,Apparent resistivities and phases,Principle directions,Skew, andEllipticity
The processing steps:1. Pre-processing step (in the field, real time), and2. Advanced processing step (at labo)
Processing & Modeling : General Procedures Pre-Processing Steps (in the field, real time): Compute Fourier coefficient by convoluting signal wave data to obtain
real (cosine wave) and imaginary (sine wave) parts for E and H, and Gain phase correction to convert the measured E and H fields to
common units (mV/km and nT).
Advanced Processing Steps (at Labo): Estimate auto-power and cross-power spectra (Vozoff, 1991), Testing statistical hypothesis/random data analyses (Bendat and
Piersol, 1971): S/N ratio, polarization parameters and their azimuth (Fowler et al., 1967), and coherencies (Reddy and Rankin, 1974),
Impedance tensor analyses (Cantwell, 1960; Vozoff, 1972; also Goubau et al., 1978 & Kao and Rankin, 1977),
Tensor apparent resistivities and phases (Vozoff, 1972, 1991; Eggers, 1981),
Invariant apparent resistivities and phases (Eggers, 1982; Ranganayaki, 1984) reduce 4 resistivities and phases into single
resistivity and phase.
Processing & Modeling : General Procedures
Auto-power & cross-power spectra:
Testing statistical hypothesis/random data analyses :S/N ratio 2Polarization parameters 0.9 (90%), Coherencies 0.9 (90%)
Impedance tensor analyses & Tensor apparent resistivities and phases in the previous chapter
**** ,,, yyxxyyxx EEEEHHHH
****** ,,,,, yxyyxyyxxxyx EEEHEHEHEHHH
Processing & Modeling : General Procedures
Invariant apparent resistivities and phases (Eggers, 1982; Ranganayaki, 1984) To obtain representative 1-D model from any Z;
reduce 4 resistivities and phases into single resistivity and phase.
Arithmetic average :
Geometric average :
Determinant :
yxxyyyxxD
yxxyG
yxxyA
ZZZZZ
ZZZ
ZZZ
2/1
5.0
reiminv
inv
ZZ
Z
1
2
tan
1
Impedance tensor Z obtained from measurement in a coordinate
system can be rotated mathematically to obtain Z’ in other
coordinate system (axes rotated + clockwise)
Z’ = R Z RT
Impedance tensor rotation
y
y’
x x’
cossin
sincosR
Processing & Modeling : General Procedures
Processing & Modeling : General Procedures
For 1-D resistivity variations:
xyinv
xyxyinv Z
2
1 2
yxxyinv
yxxyyxxyinv ZZ
1
For 2-D resistivity variations:
TE- and TM-modes Apparent Resistivities and Phases:
2
)//(
2
)//(
1
1
yxyxstrikeHTM
xyxystrikeETE
Z
Z
reyximyxyx
rexyimxyxy
ZZ
ZZ
..1
..1
tan
tan
Processing & Modeling : General Procedures
1-D Modeling, Forward (ex. Kaufman and Keller, 1981, pp. 68-74):
2
11
11211
231
22121
111
...cothcothcoth...
...cothcothcothcoth
nnnnnn
a
hk
hkhk
2/1
)(
)( 12
p
a
aD
D
1-D Modeling, Inversion (ex. Bostick, 1977):
n and hn are resistivity and thickness of layer n-th, and k=(-/)1/2
CSAMT/CSMT CASE STUDIES
CSAMT Field Setup
2-D and 3-D interpretation of CSMT data in the Bajawa geothermal field, Flores (Uchida et al., 2002)
2-D and 3-D interpretation of MT data in the Bajawa geothermal field Flores, Indonesia (Uchida et al., 2002)
Graben CSAMT Survey Lines, Borealis Project, Walker Lane gold belt, western Nevada, USA
The dark purple color represents low resistivity/high conductivity.Drill holes in the centre of the dark purple correlate to highquartz-pyrite and higher grade ore intercepts
The dark purple color representslow resistivity/high conductivity.Drill holes in the centre of the darkpurple correlate to high quartz-pyrite and higher grade oreintercepts
2-D Smooth-Model Resistivity Inversion, Scalar CSAMT Data
The Los Olivos Project is located 5 km east of Valle de Olivos near Hidalgo de Parral in the State of Chihuahua, Mexico
Sur de Guerrero, Taxco, Mexico CSAMT Survey
Line 2- Comparison of 1-D and 2-D inversion models
1D modeled resistivities at draped depths of 100 m and 200 m
2D modeled resistivities at draped depths of 100 m and 200 m
2D modeled resistivities along Taxco Line 4 and geologic cross-section through drillsite C (TA443 ore discovery).
Wadi Almarsad ProjectThe Kingdom of Jordan Line 10
Thank You