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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

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

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:


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.


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


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


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


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


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).


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


,

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


22

0

2

0

22

0

10

000

,

,)()(14

p

kku

drJRRu

Ruj

IdsQ TMTETE

where


: 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,

//upload.wikimedia.org/wikipedia/commons/6/69/Complex_conjugate_picture.svg

http://www.answers.com/topic/mathematics

http://www.answers.com/topic/complex-number

http://www.answers.com/topic/real-number

http://www.answers.com/topic/imaginary-number

http://www.answers.com/topic/sign-mathematics

http://www.answers.com/topic/real-number

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.


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


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



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


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

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