SOES6002: Modelling in Environmental and Earth System Science
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Transcript of SOES6002: Modelling in Environmental and Earth System Science
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SOES6002: Modelling in Environmental and Earth
System Science
CSEM Lecture 5Martin Sinha
School of Ocean & Earth Science
University of Southampton
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Recap and plan:
Yesterday: Layered models. Thin conductive layers. Frequency effects. Sea surface interaction and the ‘air wave’
Today: The importance of geometry – end-on vs. broadside
Thin resistive layers – an important class of models
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What controls signal propagation?
Signal propagation depends on: The earth (resistivity) structure Frequency Both the above affect skin depths But the transmitter is a dipole – So it also depends on DIRECTION
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Electromagnetic fields in the Earth
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Geometry
The transmitter is a horizontal dipole So signal propagation depends on
horizontal angle with respect to dipole axis
Refer to this angle as ‘azimuth’ Azimuth = 0o – ‘end-on’ Azimuth = 90o – ‘broadside’
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Plan view of source dipole axis and azimuth
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Polarization Ellipse
The field can be decomposed physically into two non-interacting ‘modes’
First corresponds to the radial component at the sea floor
Second corresponds to the azimuthal field at the sea floor
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These are orthogonal Each component has an
independent amplitude and phase So when combined, they sweep out
a ‘polarization ellipse’ Broadside – no radial field End-on – no azimuthal field
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Polarisation ellipse parameters
EE EE ee
EE EE ee
EE
EE
EE ee EE
EE ee EE
EE EE
EE EE
ii
ii
majmaj
ii
ii
11 11
22 22
22 11
22 11
11 22
1122
2222
11
22
2222
minmin
coscos sinsin
sinsin coscos
tantancoscos
2 1 2 1
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Azimuthal dependence of CSEM response
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Thin resistive layer models
Much of the ocean floor underlain by igneous (i.e. crystalline) oceanic crust – resistive
Continental margins – thick (many km) layers of sediments
High porosities, saturated with sea water – so much lower resistivities
But hydrocarbons and methane hydrates can dramatically increase resistivity – but generally only occur in isolated thin layers
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Seawater 0.3 Seawater 0.3 mm
Sediment 1 Sediment 1 mm
Sediment 1 Sediment 1 mm
Reservoir 100 Reservoir 100 mm
800 m800 m
1000 m1000 m
100 m100 m
halfspacehalfspace
HED source
Reservoir model – 1D
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Compare two models
1.5 km water depth 1 ohm-m sediments 50 m thick resistive layer, 180 ohm-
m, buried 950 m below sea floor Transmission frequency 0.25 Hz End-on and broadside calculations,
for both model with resistive layer and model without it
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Both geometries
Thin resistive layer
0.0001
0.001
0.01
0.1
1
0 2 4 6 8 10 12
Range (km)
Dim
en
sio
nle
ss
Am
p
Broadside
End-on
Broadside
End-on
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Result
For the end-on result, the thin layer has a huge effect on the amplitude
For the broadside result, the effect on amplitude is much smaller
Can demonstrate this more clearly by dividing the result for one model by the result for the other – ‘normalizing’
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Comparing models
Am plitude ratios
0
10
20
30
40
50
60
70
0 2 4 6 8 10 12
Range (km )
Rat
io Broadside
End-on
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Why use both?
So the end-on result is more sensitive than the broadside result
So why bother to use both? Answer is – distinguishing between
classes of models Another important case – when resistivity
at depth is greater for some other reason e.g. porosity, salt …
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Sediment over salt
layer over thick resistor
0.0001
0.001
0.01
0.1
1
0 2 4 6 8 10 12
Range (km)
Dim
en
sio
nle
ss
Am
p
Broadside
End-on
Broadside
End-on
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Sediment over salt
Am plitude Ratios
0
10
20
30
40
50
60
0 2 4 6 8 10 12
Range (km )
Am
p R
atio
Broadside
End-on
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Summary
Inline and broadside responses can be sensitive to different aspects of the structure
“Broadside” corresponds to azimuth 90 and Sazim in modelling code
“Inline” corresponds to azimuth 0 and Srad in modelling code
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SOES6002: Modelling in Environmental and Earth
System Science
CSEM Lecture 6Martin Sinha
School of Ocean & Earth Science
University of Southampton
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Comparing polarizations
So thin resistive layers are a class of model that leads to ‘splitting’ of amplitudes between modes
Whereas thicker resistive layers are a class of model that do not
But why should this be happening? Need to use some physics to
understand our models
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Direction of currents
We can think of the source dipole as generating two polarizations of current loops
Loops in the horizontal plane – inductively coupled between layers
Loops in the vertical plane – carrying electric current across the boundaries between layers
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The field lines of a dipole
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Horizontal electric dipole in layered earth
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Seawater 0.3 Seawater 0.3 mm
Sediment 1 Sediment 1 mm
Sediment 1 Sediment 1 mm
Reservoir 100 Reservoir 100 mm
800 m800 m
1000 m1000 m
100 m100 m
halfspacehalfspace
HED source
Reservoir model – 1D
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Horizontal current loops : ‘PM Mode’
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Vertical current loops : ‘TM Mode’
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Direction of currents
Think of the source dipole as generating two polarizations of current loops
Loops in the horizontal plane – ‘PM Mode’ – inductively coupled between layers, main contribution to broadside
Loops in the vertical plane – ‘TM Mode’ – carrying electric current across the boundaries between layers, main contribution to inline
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Effect of a thin resistive layer – in-line
Fields measured at the seafloor
Resistive layer
Uniform+resistive layer
Uniform seafloor
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Effect of a thin resistive layer – broadside.
Fields measured at the seafloor
Resistive layer.
Uniform seafloor
Uniform+resistive layer
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Case study 1 – Does it work in practice?
• The first trial survey was carried out in 2000.
• It was a collaborative research project between SOC, STATOIL, and Scripps Institution of Oceanography, and
• The target was a known hydrocarbon bearing reservoir.
• Results were presented at the 64th Conference of the EAGE in May 2002.
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DASI deployment (North Atlantic, November 2001)
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Data processing
• Initial processing involves extracting the component of the recorded electric field which corresponds to the known source signal and combining this with acoustically derived navigation data on source and receiver locations.
• The resulting amplitude and phase of the received electric field as a function of source-receiver separation and geometry form the basis for analysis and interpretation.
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Field data from a known reservoir
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West Africa 2000:
0.25Hz data from Line 1
Electric field strength Normalised field strength
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2-D effects
In this course, we are going to limit ourselves to 1-D – i.e. uniform layers
In practice, we can also run models and analyse data in 2-D and 3-D. Models are more complex, but the principles are just the same
For example, detecting the ‘edge’ of a reservoir -
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Detecting the edge of a reservoir
CSEM sounding for hydrocarbon exploration:
Effect of reservoir edge.
2.5 D model
Transmitting dipole aligned across the
structure
In-line source-receiver geometry
Radial field amplitude and phase
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1D background1D background
1D reservoir1D reservoir
2D reservoir2D reservoir
Edge effect in survey data
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CSEM sounding for hydrocarbon exploration:
Effect of the edge of the reservoir
2.5-D model, invariant direction up the page, variable direction across the
page
Transmitting dipole aligned up the page
Component shown – amplitude of Pemax
Colour contours: normalised by amplitudes from a 1-D structure with
no hydrocarbon
White contours: absolute field values
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Dipole aligned parallel to edge:
Transmitter and receiver both over reservoir: response extremely similar to the 1-D case
If either transmitter OR receiver moves off the edge of the reservoir, the signal very rapidly reverts to looking like the case for no hydrocarbon
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CSEM sounding for hydrocarbon exploration: Field trials offshore West Africa,
October/November 2000
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West Africa 2000:
2D ‘view’ of the reservoir
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Case Study 2
Using both geometric modes is important not only for the ‘thin resistive layer’ classes of models
Having both data types has been crucial in studies of mid-ocean ridges
Example – Lau Basin study, the Valu Fa Ridge
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Anatomy of an active hydrothermal system:
The Valu Fa Ridge, Lau Basin, SW Pacific.