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# SOES6002: Modelling in Environmental and Earth System Science - PowerPoint PPT Presentation

SOES6002: Modelling in Environmental and Earth System Science. CSEM Lecture 5 Martin Sinha School of Ocean & Earth Science University of Southampton. Recap and plan:. Yesterday: Layered models. Thin conductive layers. Frequency effects. Sea surface interaction and the ‘air wave’

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### SOES6002: Modelling in Environmental and Earth System Science

CSEM Lecture 5

Martin Sinha

School of Ocean & Earth Science

University of Southampton

Recap and plan: Science

• 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

• 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

Geometry Science

• 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’

Polarization Ellipse Science

• 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

• These are orthogonal Science

• Each component has an independent amplitude and phase

• So when combined, they sweep out a ‘polarization ellipse’

• End-on – no azimuthal field

i Science

f

E

=

E

e

1

1

1

i

f

E

=

E

e

2

2

2

i

D

f

E

e

cos

a

-

E

sin

a

E

2

1

min

e

=

=

i

D

f

E

e

sin

a

+

E

cos

a

E

2

1

maj

2

E

E

cos

D

f

1

2

tan

2

a

=

2

2

E

-

E

1

2

Polarisation ellipse parameters

Thin resistive layer models Science

• 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

Reservoir model – 1D Science

Seawater 0.3 Wm

800 m

HED source

1000 m

Sediment 1 Wm

Reservoir 100 Wm

100 m

Sediment 1 Wm

halfspace

Compare two models Science

• 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

Result Science

• 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’

Why use both? Science

• 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 …

Summary Science

• 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

### SOES6002: Modelling in Environmental and Earth System Science

CSEM Lecture 6

Martin Sinha

School of Ocean & Earth Science

University of Southampton

Comparing polarizations Science

• 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

Direction of currents Science

• 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

Reservoir model – 1D Science

Seawater 0.3 Wm

800 m

HED source

1000 m

Sediment 1 Wm

Reservoir 100 Wm

100 m

Sediment 1 Wm

halfspace

Direction of currents Science

• 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

Fields measured at the seafloor

Uniform+resistive layer

Uniform seafloor

Resistive layer

Fields measured at the seafloor

Uniform+resistive layer

Uniform seafloor

Resistive layer.

• 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.

Data processing Science

• 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.

West Africa 2000: Science

0.25Hz data from Line 1

Electric field strength

Normalised field strength

2-D effects Science

• 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 -

CSEM sounding for hydrocarbon exploration:

Effect of reservoir edge.

2.5 D model

Transmitting dipole aligned across the structure

Radial field amplitude and phase

Edge effect in survey data Science

1D reservoir

2D reservoir

1D background

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

• 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

CSEM sounding for hydrocarbon exploration: Field trials offshore West Africa, October/November 2000

West Africa 2000: offshore West Africa, October/November 2000

2D ‘view’ of the reservoir

Case Study 2 offshore West Africa, October/November 2000

• 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

Anatomy of an active hydrothermal system: offshore West Africa, October/November 2000

The Valu Fa Ridge, Lau Basin, SW Pacific.