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Electro-Magnetic Methods in E&P
•Introduction
•EM: Diffusion or Propagation
•Electrical Methods
•Magneto-Telluric Methods
•Controlled Source EM methods
•Summary
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1953-1959: Primary school 's-Gravenzande1959-1964: Secondary school (HBS) 's-Gravenhage1964-1965: Lakeview High School, Battle Creek, USA1965-1968: University Leiden: Bachelors Geology1968-1972: University Utrecht: Masters Geophysics1972-1977: University Utrecht: Ph.D.
“Full wave theory and the structure of the lower mantle”1977-1982: Shell Research: Interpretation Research on lithology and fluid prediction.1982-1985: Shell Expro, Londen: Interpretation Central Northsea area
acquisition and interpretation of Vertical Seismic Profiles 1985-1988: Shell Research: Seismic Data processing,
evaluation of new processing methods for land and marine data.1988-1991: Shell Research: Interpretation methods,
development of interactive workstation methods1991- 1995: SIPM: Evaluation of Contractor Seismic data processing1995-2001: Shell Learning Centre Noordwijkerhout: Course Director Geophysics2001-2007: SIEP: Potential Field Methods2007- Geophysical Consultant (Breakaway, EPTS)
Courses on Geophysical Data Acquisition, Processing and Interpretation
Jaap C. Mondt
Electro-Magnetic Methods
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Introduction Electromagnetism
Q: Is electromagnenetics wave propagation or diffusion?
A: Wave propagation always involves attenuation & dispersion•Seismic waves
Diffusion = Wave propagation with (severe) attenuation•Perfume escaping from a bottle
A: EM can be considered to be wave propagation as well as diffusion. For high frequencies it has all the characteristics of wave propagation, For low frequencies it behaves more like diffusion
A: When it is time varying, namely a time varying electric field will generate a magnetic field, hence the name electro-magnetic.
Q: Source is a electrical dipole. When is it an electromagnetic source?
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4
:EquationPoisson
zJ)(Y
YμΨσμΨ
:EquationDiffusion
0Ψ
:Equation Wave
2
2h
002
122
T
ime
Der
ivat
ives
Resolution
Seismic waves
EM waves
Gravity
Resolution for Waves, Diffusion and Potential fields
4
:EquationPoisson
zJ)(Y
YμΨσμΨ
:EquationDiffusion
0Ψ
:Equation Wave
2
2h
002
122
5
Electromagnetics: Propagation or Diffusion ?
Q: What will be observed over time at A with the source at the origin O?
O
• early time• Intermediate• late time
A: Particle density will increase and then decrease again, this will givethe impression of a passing wave with an arrival time. 6
Diffusion: Skin depth / Wavelength
The skin depth, , is the distance over which the field strengthis reduced by the factor 1/e = 0.368 ~-8.686 dB
fkreal
503)(
1
The wavelength is
fkimag
3162)(
2
where is the resistivity in -m and f is the freq in Hz
(m)
(m)
7
Skin Depth/Wavelength for sea water, shales and reservoir
Sea water resistivity 0.3 Ohm-m 0.3 Ohm-mskin depth 300 m 600 m
wave length 1,886 m 3,771 m
Shale resistivity 1.0 Ohm-m 1.0 Ohm-mskin depth 900 m 1,800 m
wave length 5,657 m 11, 314 m
1 Hz 0.25 Hz
HC filled reservoir 50.0 Ohm-m 50.0 Ohm-m skin depth 3,500 m 1,800 m
wave length 22,000 m 44, 000 m
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Electrical methods
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Electrical Monopole
Current flow from a single surface electrode
Current density: i=I/(2πr²) Am-2 Potential gradient: δV/δr=-ρi=- ρi/(2πr²) Vm-1
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Rock resistivity
SI unit of resistivity : ohm-metre (Ωm)Reciprocal of resistivity is conductivity : Siemens/metre (S/m) 11
Fractional Current
The fraction of current penetrating below a depth Z for a current electrode separation L. Hence, 50% penetrates below L/Z=2 (Z=½L)
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The variation of apparent resistivity with electrode separationover a single horizontal interface between media with increasing resistivities with depth.
Apparent resistivity
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Variation of apparent resistivity as a function of electrode separation for various resitivity sequences
a: At large enough electrode separation the apparent resistivity will equal the true resistivity.b: The intermediate higher/lower resistivity will appear at intermediate electrode separation.c: The deeper the higher/lower resistivity the larger the electrode separation (a) needed to observe its value.
a cb
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Summary
• Currents flow through the whole subsurface between electrodes.• 50% of the current flows in the subsurface above/below half the electrode spacing.• Commonly used field layouts: Wenner and Schlumberger configuration.• Wenner configuration: simpler (same spacing current and potential electrodes).•T here is “some” depth discrimination in the observed apparent resistivity.• True Inversion is needed to obtain better depth / spatial discrimination.
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Magneto-Telluric (MT)
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Source: Solar flares
– 27 day cycle– main source of geomagnetic variations
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EARTH
Source: LightningMain energy source at frequencies above 1Hz.
Schumann resonances at 8, 14, and 21 Hz.
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Typical magnetic spectrum
5pT
pT= pico Tesla
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Wave-front of time-varying magnetic fields
Time-varying magnetic fields induce electric fields in the earth. The amplitudes of these are proportional to the resistivity.
Time varying magnetic field
Induced electric field
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Skin depth: depth at which incident magneticfield is attenuated to 1/e of its orginal value
Skin depth in metres = 500 SQRT(ρ/f)
With ρ is resistivity of earth f is measurement frequency.
Hence, by varying frequency, we vary the depth of penetration.
Depth of penetration
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MT versus CSEM
In MT the subsurface is derived from the relationship between the measured electric and magnetic data. This relationship is given by the (complex) transfer function called impedance tensor (Z) with elements: Zxy= Ex/Hy. The MT transfer function Z relates the horizontal electric field components Ex and Ey to the
magnetic field components Hx and Hy .
The vertical magnetic component Hz is related to the horizontal
magnetic components via the Tipper vector: Hz = (A)Tx Hx +
(B)Ty Hy and is only present in case of 3D structure (hence only
3D structures lifts the magnetic vector out of the horizontal plane, tips the vector up or down.
MT is an inductive method and senses conductivity in the subsurface. 24
•H=magnetic field component•E=electric field component
Battery
Acquisition &processing unit
Computer
Magnetic sensors
electrode
Commonelectrode
electrode
electrode
electrodeEy
Ex
Hx
Hz
Hy
Typical lay-out in the field
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E and H time series.
Time
Channels (top to bottom) are Ex,Ey, Hx, Hy, and Hz.Total Segment duration=1024 secs. 26
Time series are processed to give spectral estimates of the measured parameters, i.e. 2 electric and 3 magneticfields at each site.
These are denominated ExEyHxHyHz
E= electric and H=magnetic; x,y,z refer to the measurement axes.
E and H components
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Spectra are combined to give impedances (Zij), thus
Zxy=Ex/Hy and so on.
Since Ex etc are complex numbers, it follows that the impedances are also complex. In other words, they have an amplitude and a phase.
The full MT site therefore has 4 horizontal impedance elements (Zxy, Zyx, Zxx, and Zyy), and also two vertical magnetic ones (Tzx and Tzy).
Impedances calculated from the measured components
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TEEx
Hy
Hz
Strike
TM
Strike Hx
Ey
Ez
StrikeTM
TE
TE &TM
Traditionally the 2D sections were chosen in the dip direction.Hence, the TE has an E vector parallel to strike, whereasTM has an E vector in the dip direction, which crosses the structure and is more sensitive to its resistivity. Namely, the currents can’t go around the resistivity, whereas in TE they could.Hence, TM mode will show hydrocarbons in a traditional 2D acquisition.
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ZyyZyx
ZxyZxxZ
The horizontal components can be written as a tensor
These are decomposed into 2 apparent resistivities and phases
The general relationship is
2
5
1xyxy Z
f
Impedance matrix
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The most usual decomposition technique is to compute the parameters in the directions in which they are at their maximum and minimum for each relevant frequency. (Principal Axis Rotation)
Increasing period increasing depth
apparent resistivity
phase
xy
yx
xyyx
Decomposition
xy =TE in case of 2D geology yx = TM in case of 2D geology
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The same data can be plotted as impedance polarization ellipses for each frequency:
Zxy
Zxx
These show the azimuthal variation of Z (hence resistivity).Here, the minimum apparent resistivity is N-S (parallel to strike) and the maximum is E-W.
N
Impedance Polarisation
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Libya NC171-5
1D
2D
RESISTIVITY, OHM.M
1 10 100D
EP
TH
, M
0
1000
2000
3000
4000
5000
6000
7000
8000
Inverted to give resistivity versus depth
1D sounding
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2D
INVERTED TO GIVE RESISTIVITY v. DEPTH X-SECTION
1D
1D
2D
Example 2D sounding
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APPARENT RESISTIVITY
PHASE
PERIOD
DISTANCE ALONG PROFILE
Pseudo sections
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PERIOD
TE modeE parallel strike
TM mode E perp. strike
Res
Phase
Res
Phase
Pseudo sections
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Summary Magneto-Telluric
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• Passive method: using a natural source (solar activity, lighting)• Given the low “propagation”velocity in the subsurface the EM source-waves travel vertical downwards.• The frequency is low and hence the skin-depth very large.• In the field only receiver equipment is needed.• Is used as an early exploration tool (basin detection)• As it detects resistivity/conductivity it is used for mapping basement
CSEM: Controlled Source EM
Or
Sea Bed Logging
Marine EM
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CSEM: Sea Bed Logging
Note: energy diffused through the air, seawater and subsurface
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EM receivers dropped at sea bottom
EM Source towed above receivers
Source and Receivers
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HC
Source-receiver distance
DOMINATING WAVES
Directwaves
Air waves
Guided
waves in
the
reservoir
Airwaves
What is recorded at different offsets?
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In-Line (Galvanic) & Broadside response (Induction)
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Towline
Reservoir contour
Reference receiver
1E-15
1E-14
1E-13
1E-12
1E-11
1E-10
1E-09
1E-08
-10000-9000-8000-7000-6000-5000-4000-3000-2000-10000
Source - Receiver Offset [m]
Mag
nit
ud
e [V
/Am
²]
SW Offset NE
0
0 Offset NE
Troll: Off structure reference receiver
Note: the receiver and source are both not above the the hydrocarbons
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-12000-10000-8000-6000-4000-20000
Offset [m]
Towline
Reservoir contour
Reference receiver
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-10000-9000-8000-7000-6000-5000-4000-3000-2000-1000
Source - Receiver Offset [m]
No
rma
lise
d E
lec
tric
Ma
gn
itu
de
Normalize by reference receiver
Troll: On structure versus Off structure receivers
Now the source is above the hydrocarns
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Towline
Reservoir contour
Reference receiver
½ offset at split = depth BML of anomaly
SW Offset NE
0
Troll: Depth estimate from Phase plot
Note the source is SW (not above the hydrocarbons) and NE of the receiver
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0,0
0,5
1,0
1,5
2,0
2,5
-12000-10000-8000-6000-4000-20000
Offset (m)
No
rmal
ised
Mag
nit
ud
e
Towline
Reservoir contour
Troll: Normalised magnitude at specific offset
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Troll (Gather Plot) Magnitude and seismic
Maximum Anomaly Positions
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Gather-plot Ubah-Crest
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0 -1000
-500
0
500
1000
1500
Well-1 Well-2
Imaging
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-1
-0,5
0
0,5
1
1,5
2
2,5
3
-6000 4000 14000 24000 34000 44000
-1000
-500
0
500
1000
1500
Gather-plot (0.25Hz)
Median value at 5.5 km offset
Offset relative to Rx01 (km)
No
rma
lize
d
ma
gn
itu
de
Wa
ter-
de
pth
(m
)
South-West North-East
Depth Migration
Resistivity : 15 ohm-mThickness : 50 m
NB: Seismic and SBL line is manually overlaid
Imaging
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HC
Source-receiver distance
DOMINATING WAVES
Directwaves
Air waves
Guided
waves in
the
reservoir
Airwaves
What is recorded at the different offsets?
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SC02A_SC031M raw (in)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-10000-9000-8000-7000-6000-5000-4000-3000-2000-1000
Source - Receiver Offset [m]
No
rmal
ised
Ele
ctri
c M
agn
itu
de
SC02_SC031M raw (in)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-10000-9000-8000-7000-6000-5000-4000-3000-2000-1000
Source - Receiver Offset [m]
No
rmal
ised
Ele
ctri
c M
agn
itu
de
SC03_SC031M raw (in)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-10000-9000-8000-7000-6000-5000-4000-3000-2000-1000
Source - Receiver Offset [m]
No
rmal
ised
Ele
ctri
c M
agn
itu
de
SC02A_SC031M updown separated (in)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-10000-9000-8000-7000-6000-5000-4000-3000-2000-1000
Source - Receiver Offset [m]
No
rmal
ised
Ele
ctri
c M
agn
itu
de
SC02_SC031M updown separated (in)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-10000-9000-8000-7000-6000-5000-4000-3000-2000-1000
Source - Receiver Offset [m]
No
rmal
ised
Ele
ctri
c M
agn
itu
de
SC03_SC031M updown separated (in)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-10000-9000-8000-7000-6000-5000-4000-3000-2000-1000
Source - Receiver Offset [m]
No
rmal
ised
Ele
ctri
c M
agn
itu
de
Brazil: Up-Down Separation
Intow 0.125 Hz
Raw Data Up-Down Separation
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The new receiver consists of (1or 3 m length) dipoles at the end of 4 long perpendicular arms. This will provide us with the horizontal derivatives of the horizontal E components. In this set-up there is no longer a need for a vertical dipole, nor for the measured orientation of the receivers, nor for magnetic measurements to suppress the airwave
New Electric Gradiometer receivers (MK III)
Traditional receiver New Electric Gradiometer receivers52
Method I: “TM decomposition”
•TM decomposition refers to removing the TE mode
z
Ez
y
Ey
x
ExEdiv
0
XXXXE
VVX
12
12)()(
XXXEXEE XXX
X12
12)()(
At the receivers there are no E source:
Measure and calculate Ez from :x
Ex
y
Ey
z
Ez
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Brine
BrineBrine andLSG
Oil
Oil
Oil andGas
Gas
Gas
LSG
LSG
Oil Sw = 0.2LSG Sw = 0.95Gas Sw = 0.2
Additional value of EM to seismic
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Conclusion
• CSEM uses a active source.• Electric currents in the subsurface using an electric dipole• In-line (vertical currents) for thin horizontal layers• Broadside (horizontal currents) for background resistivity• In-line will detect thin hydrocarbon bearing layers• Interpretation using magnitude and phase of recorded signal• Inversion for detailed imaging
• Additional value of EM to seismic data: resistivity (hydrocarbons)
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Summary
• Electromagnetism (EM) is generated by a time varying electric or magnetic source
•On land by using a positive and negative pole in the ground
•In a marine survey the EM field is generated by a tiime varying dipole
•The response is measured by electric and magnetic dipole receiver on the surface
•The measurement contains of the subsurface and above surface response
•The subsurface response should be separated from the above surface response•Land: time separation•Marine: vertical dipole source and / or processing
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