Soes6002 modelling in environmental and earth system science
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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:. 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

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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:

  • 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


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


Electromagnetic fields in the Earth


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’


Plan view of source dipole axis and azimuth


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


  • 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


Azimuthal dependence of CSEM response


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


Reservoir model – 1D

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

  • 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


Both geometries


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’


Comparing models


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 …


Sediment over salt


Sediment over salt


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


SOES6002: Modelling in Environmental and Earth System Science

CSEM Lecture 6

Martin Sinha

School of Ocean & Earth Science

University of Southampton


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


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


The field lines of a dipole


Horizontal electric dipole in layered earth


Reservoir model – 1D

Seawater 0.3 Wm

800 m

HED source

1000 m

Sediment 1 Wm

Reservoir 100 Wm

100 m

Sediment 1 Wm

halfspace


Horizontal current loops : ‘PM Mode’


Vertical current loops : ‘TM Mode’


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


Effect of a thin resistive layer – in-line

Fields measured at the seafloor

Uniform+resistive layer

Uniform seafloor

Resistive layer


Effect of a thin resistive layer – broadside.

Fields measured at the seafloor

Uniform+resistive layer

Uniform seafloor

Resistive layer.


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.


DASI deployment (North Atlantic, November 2001)


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.


Field data from a known reservoir


West Africa 2000:

0.25Hz data from Line 1

Electric field strength

Normalised field strength


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 -


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


Edge effect in survey data

1D reservoir

2D reservoir

1D background


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


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


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


West Africa 2000:

2D ‘view’ of the reservoir


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


Anatomy of an active hydrothermal system:

The Valu Fa Ridge, Lau Basin, SW Pacific.


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