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

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:EquationPoisson

zJ)(Y

YμΨσμΨ

:EquationDiffusion

0Ψ

:Equation Wave

2

2h

002

122

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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)

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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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