SOES6002: Modelling in Environmental and Earth

System Science

CSEM Lecture 5Martin 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

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

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

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

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

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

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

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

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

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

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

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 6Martin 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

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

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

Resistive layer

Uniform+resistive layer

Uniform seafloor

Effect of a thin resistive layer – broadside.

Fields measured at the seafloor

Resistive layer.

Uniform seafloor

Uniform+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

1D background1D background

1D reservoir1D reservoir

2D reservoir2D reservoir

Edge effect in survey data

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.