Em Geophysics Unibraw 21.02.2014

FUNDAMENTAL OF ELECTROMAGNETIC METHODS IN EXPLORATION GEOPHYSICS Djedi S. Widarto Sr. Geoscientist / Chief New Energy & Green Technology Upstream Technology Center PT PERTAMINA

Universitas Brawijaya February 21, 2014

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Education :   

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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 geophysics Chief of New Energy & Green Technology, Upstream Technology Center, PT Pertamina

Award : 

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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 : 



Djedi S. Widarto

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)

 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

Borehole Methods

Seismic Methods :   

Seismic reflection methods Surface wave (refraction) methods Micro-earthquake

Potential Field Methods : 

Gravity & magnetic

In-Hole Procedures Cross-Hole Procedures Surface to Borehole Procedures:

   

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Resistivity methods Self-potential Mise a-la masse methods Induced polarization

Logging Techniques:

   

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

Electrical methods  Acoustic logging  Nuclear logging  Flow logging Other methods of logging 

Electromagnetic Methods 

Velocity surveys Vertical seismic profiling 

Electrical Methods



Geophysical Methods  Techniques applying physical laws (or theory) to the study of the solid

Earth,  Estimation of subsurface physical property distribution by measuring relevant parameters: 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

Probable Sequence of Geophysical Exploration Methods Used to Investigate Young Volcanic Geothermal Prospect (revised from Sudarman, 1983) Method and Survey Procedure

Expected Anomaly

Interpretation

Aero- or ground magnetic (covers low anomaly a large area)

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

Thermal gradient and anomalous temperature (to figure out the cause of low resistivity layer)

high anomaly

Magnetotelluric sounding

low anomaly

Interpretation Ql : uprising or horisontal thermal fluid movement, if depth to resevoir is relatively shallow Qt : defined the upper structure

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 (?)

Natural Source Magnetotelluric

What is Magnetotellurics (MT) ?  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.

Electromagnetic Induction 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

Electrical Resistivities of Rocks

OIL SANDS

Resistivity [Ohm-m]

Hydrates

Resistivity

0.3 Wm

Interpreting subsurface resistivity: Impact of pore fluids and geologic processes on resistivity Saline brine

Hydrocarbons

Clay alteration

Carbonate cementation

Dissolution

Silicification

Temperature

Metamorphism Pressure

Faulting Increase Shearing

Decrease

natural electromagnetic field 10−4 – 10+4 Hz

f > 1 Hz

f < 1 Hz

Natural Electromagnetic Fields

Natural Signal

Ey

x

Ex Hy

Hx Hz

Short Period Long period

z

y

Electric (E) and magnetic (H) fields relationship 

For a homogeneous or layered (1-D) medium Ex = Z Hy



For a medium with 2-D symmetry Ex = Zxy Hy Ey = Zyx Hx



 Z = scalar impedance

 Zxy ≠ Zyx  Z = vector impedance

For a general 3-D medium

Ex = Zxx Hx + Zxy Hy Ey = Zyx Hx + Zyy Hy

 E=ZH  Z = tensor impedance

Characteristics of MT Method 

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  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:

MT Disadvantages:

 Great depth of penetration

 Coupling with lateral

 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

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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

~ slower attenuation ~ deeper penetration

Depth

 Lower frequency (or higher period) and higher resistivity

 Principles of MT sounding i.e. wide frequency band measurement probes different parts (depths) of the subsurface Z

MT Data Acquisition (field set-up)

Receiver System MTU-5A Phoenix

Magnetic Sensor Induction Coil

MT Field Set-up Receiver System MTU-5A Phoenix

Electric Sensor Pb-PbCl2

Audio-Frequency MT (AMT) and MT field set-up

Satellite-Synchronized Magnetotellurics

Phoenix MT System 2000

Time (sec)

MT time series

Magnetic and Electric Fields Intensity

Data processing sequence  To extract impedance tensor Z

from 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)

E x , E y , Hx , Hy  Data processing to extract impedance tensor

Ex = Zxx Hx + Zxy Hy Ey = Zyx Hx + Zyy Hy

E=ZH

 Apparent resistivity and phase

 a(ij ) 

1

 o

Zij

2

(ij )

 Im.Zij   tan    Re .Zij  1

 Apparent resistivity and phase sounding curves

a (Ohm.m) and  (degree) vs frequency (Hz)

 Apparent resistivity and phase sounding curves

a (Ohm.m) and  (degree) vs frequency (Hz) good

Medium quality data (Class-B) 320 Hz ~ 0.5 Hz: good 0.5 ~ 0.00055 Hz: med - poor

Data Presentation  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

Frequency (Herz)

Resistivity pseudo-section-1

Distance (km)

Resistivity pseudo-section-2 A

C

B

D

0

PSEUDO-DEPTH (m)

1000 2000 3000 4000 5000 6000

0

2000

4000

6000

8000

10000

12000

14000

DISTANCE (m)

4

6

8

10

12

14

16

18

20

22

APP. RESISTIVITY (Ohm.m)

24

26

16000

18000

MT Data Modeling RESISTIVITY (Ohm.m) 1

100

100

10

obs. data calc. data 1 90

PHASE (deg.)

10

100

DEPTH (m)

APP. RESISTIVITY (Ohm.m)

1000

1000

45

0 0.001

0.01

0.1

1

PERIOD (sec.)

10

100

1000

10000

?

1000

MT 1-D smooth modeling  OCCAM inversion (Constable et al., 1987), ABIC (Mitsuhata, 1991)  Markov Chain Monte Carlo (MCMC) algorithm (Grandis et al., 1999) RESISTIVITY (Ohm.m) 1 100

100

10

obs. data calc. data DEPTH (m)

APP. RESISTIVITY (Ohm.m)

1000

1

PHASE (deg.)

90

1000

45

0 0.001

0.01

0.1

1

PERIOD (sec.)

10

100

1000

10000

10

100

1000

Bostick Transform  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  αi , Ti

model αi , Di

Di  500  aiTi 

1/2

 B   ai 

B , D i

s(Ti )  B is Bostick resistivity (Ohm-m) Di is Bostick depth (meter)

1  s(Ti ) 1  s(Ti )

d log  ai or d log Ti

1 M  B   ai 1M    M 1 ,  45  0    90

Bostick Transform

1-D Bostick Model

Bostick Transform  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

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

CASE STUDIES: Historical Perspectives  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.

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

Crustal scale studies  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)

Petroleum exploration  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)

Geothermal and Volcanology 

Hululais, Jambi (MT, 1995)

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Seulawah, Aceh (MT, 1995)

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Sumurup, Jambi (MT & TDEM, 1997)

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Lahendong, North Sulawesi (CSAMT)



Kamojang, West Java (CSAMT)



Cimanggu Hot Spring, West Java (AMT, 1998)

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Guntur-Galunggung, West Java (AMT, 2000-2001)

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Ungaran, Central Java (AMT, 2002-2003)

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Guntur volcano (AMT & MT, 2003-2004)

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Papandayan volcano (AMT & MT, 2007)

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2-D Modeling by FEM



2-D Inverted Resistivity Model Result

AMT Survey on Guntur volcano

0.5

PTR_10

PTR_09

PTR_08

PTR_07

PTR_05 PTR_06

PTR_04

PTR_03

PTR_01 [+1446.3 m]

PTR_02

REF ERE NC E 1 [ohm-m] 3 10 30 100 300 1000 3000 10000

Tiltmeter/Watertube Station

(seismic data)

Sta. Citiis [+1523 m]

GN_06

GN_05

GN_03

GN_02

G. GUNTUR

3.5 ~ 8.5 km

1.0

GN_01

1.5

CMK

INVERTED 2-D RESISTIVITY MODEL BENEATH GUNTUR VOLCANO AS VIEWED FROM NORTH TO THE SOUTH OF THE VOLCANO

1.5 1.0 0.5

0.0

0.0

- 0.5 0.0

0.5

1.0 EASTING [km]

1.5

Magma 2.0 chamber

0.0

0.5

1.0

SOUTH-SOUTHWESTING [km]

1.5

2.0

- 0.5

AMT Survey, Cimanggu Hot Spring, West Java

3000

pseudosection (data)

LOG FREQUENCY (Hz)

4

1000

3

400

200

2 70

30

1 0

5

10

15

20

25

30

35

10

40

App. Resisitivity (Ohm.m)

19

18

17

16

12 13 14 15

11

10

8

7

6

5

4

3

2

1

9

hot-spring

hot-spring

10000

15

2-D smooth model

ELEVATION (x 100m)

3000

500

10 200

50

5

20

0

5

0

5

10

15

20

25

DISTANCE (x 100m)

30

35

40 Resisitivity (Ohm.m)

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).

Why are Controlled-Source EM methods worth such attention ? Answers: A. Stronger signal compared to the natural one  enhance S/N ratio; B. Electrical conductivity is closely linked to fluid properties.

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. Conductivity is just the reciprocal of resistivity:  = 1 /  (S/m) A 1 m3 cube of 2 Wm rock would have a series resistance of 2 W across the faces,

R Current I

L

A 

RA  L

Wm

Conductivity is proportionality constant in Ohm’s Law for continuous media:

J  E

or

E  J

Where J is current density (A/m2) and E is electric field (V/m)

Electromagnetic Induction Faraday’s Law says that a moving (or time-varying) magnetic field will induce electric fields in a conductor.

 E  dl  

C

d dt

magnetic flux [weber = 108 maxwell] Rate of cutting of lines of magnetic flux in Maxwell/sec

a length element of the loop

It is also expressed in terms of the change in the magnetic induction B with time t:

B E   t

Magnetic induction [1 Tesla = 1 weber/m2 = 104 gauss = 109 gamma]

Voltage Electric field strength [V/m]

‘Minus’ sign in Faraday’s Law shows that conductors attenuate EM fields and so EM fields propagate in resistive materials.

Electromagnetic Induction Ohm’s Law says that a current will be generated from the electric field in a good conductor.

J  E Ampere’s Law says that the current I will generate a secondary magnetic field.

 B  dl

 I

C

B



0

H

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 constrain depths very much better than potential field methods through the skin depth: For a plane wave incident on a uniform conductor,

 s  2  

is called the skin depth. It is the distance that field amplitudes are reduced by 1/e, or 37%. In practical units,  s  500 1 f   500 T m d  s

2  350 T m effective depth 

where circular frequency f=/2p=1/T Skin depth is not a measure of resolution, but is a guide to the maximum distance EM energy can propagate.

What is Controlled-Source ElectroMagnetics (CSEM) ?  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 groundeddipole wire & loop-wire (horizontal & vertical) sources,  The receivers are, in general, fluxgate magnetometer (3 comp.), induction magnetic coil + electric sensors.

What is Controlled-Source ElectroMagnetics (CSEM) ?  CSAMT (freq: 10 Hz ~ 20 kHz) / CSMT (freq: