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### Electro-Magnetic Methods in E&P

### Electromagnetics: Propagation or Diffusion ?

### Electrical Monopole

### Apparent resistivity

Introduction

EM: Diffusion or Propagation

Electrical Methods

Magneto-Telluric Methods

Controlled Source EM methods

Summary

Jaap C. Mondt

1953-1959: Primary school 's-Gravenzande

1959-1964: Secondary school (HBS) 's-Gravenhage

1964-1965: Lakeview High School, Battle Creek, USA

1965-1968: University Leiden: Bachelors Geology

1968-1972: University Utrecht: Masters Geophysics

1972-1977: 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 methods

1991- 1995: SIPM: Evaluation of Contractor Seismic data processing

1995-2001: Shell Learning Centre Noordwijkerhout: Course Director Geophysics

2001-2007: SIEP: Potential Field Methods

2007- Geophysical Consultant (Breakaway, EPTS)

Courses on Geophysical Data Acquisition, Processing and Interpretation

Q: Is electromagnenetics wave propagation or diffusion?### Introduction Electromagnetism

- 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

Q: Source is a electrical dipole. When is it an electromagnetic source?

A: When it is time varying, namely a time varying electric field will generate a magnetic field, hence the name electro-magnetic.

Resolution for Waves, Diffusion and Potential fields

Seismic waves

EM waves

Resolution

Time Derivatives

Gravity

- early time
- Intermediate
- late time

O

Q: What will be observed over time at A with the source at the origin O?

A: Particle density will increase and then decrease again, this will give

the impression of a passing wave with an arrival time.

Diffusion: Skin depth / Wavelength

The skin depth, d, is the distance over which the field strength

is reduced by the factor 1/e = 0.368 ~-8.686 dB

(m)

The wavelength is

(m)

where r is the resistivity in W-m and f is the freq in Hz

Skin Depth/Wavelength for sea water, shales and reservoir

0.25 Hz

1 Hz

Sea water resistivity 0.3 Ohm-m 0.3 Ohm-m

skin depth 300 m 600 m

wave length 1,886 m 3,771 m

Shale resistivity 1.0 Ohm-m 1.0 Ohm-m

skin depth 900 m 1,800 m

wave length 5,657 m 11, 314 m

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

Current flow from a single surface electrode

Current density: i=I/(2πr²) Am-2

Potential gradient: δV/δr=-ρi=- ρi/(2πr²) Vm-1

Rock resistivity

SI unit of resistivity : ohm-metre (Ωm)

Reciprocal of resistivity is conductivity : Siemens/metre (S/m)

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)

The variation of apparent resistivity with electrode separation

over a single horizontal interface between media with

increasing resistivities with depth.

Variation of apparent resistivity as a function of electrode separation for various resitivity sequences

a

b

c

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.

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.

Source: LightningMain energy source at frequencies above 1Hz.

EARTH

Schumann resonances at 8, 14, and 21 Hz.

Time varying magnetic field

Wave-front of time-varying magnetic fields

Induced electric field

Time-varying magnetic fields induce electric fields in the earth.

The amplitudes of these are proportional to the resistivity.

Depth of penetration

Skin depth: depth at which incident magnetic

field 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.

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.

Typical lay-out in the field

electrode

Acquisition &

processing unit

Ey

Battery

Ex

electrode

Hx

Common

electrode

electrode

Hy

Computer

Hz

electrode

Magnetic sensors

- H=magnetic field component
- E=electric field component

E and H time series.

Time

Channels (top to bottom) are Ex,Ey, Hx, Hy, and Hz.

Total Segment duration=1024 secs.

E and H components

Time series are processed to give spectral estimates of the

measured parameters, i.e. 2 electric and 3 magnetic

fields at each site.

These are denominated

Ex

Ey

Hx

Hy

Hz

E= electric and H=magnetic; x,y,z refer to the measurement axes.

Impedances calculated from the measured components

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).

TE &TM

Strike

Ex

TE

Hy

TE

Hz

Strike

TM

Strike

Hx

TM

Ey

Ez

Traditionally the 2D sections were chosen in the dip direction.

Hence, the TE has an E vector parallel to strike, whereas

TM 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.

Impedance matrix

The horizontal components can be written as a tensor

These are decomposed into 2 apparent resistivities and phases

The general relationship is

Decomposition

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)

=TE in case of 2D geology

= TM in case of 2D geology

apparent resistivity

phase

Increasing period increasing depth

Impedance Polarisation

The same data can be plotted as impedance polarization ellipses

for each frequency:

N

Zxx

Zxy

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.

Summary Magneto-Telluric

- 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: Sea Bed Logging

Note: energy diffused through the air, seawater and subsurface

What is recorded at different offsets?

Air waves

DOMINATING WAVES

Guided

waves in

the

reservoir

Air

waves

Direct

waves

HC

Source-receiver distance

Troll: Off structure reference receiver

Reservoir contour

Towline

0 Offset NE

Reference receiver

Note: the receiver and source are both

not above the the hydrocarbons

0

SW Offset NE

Troll: On structure versus Off structure receivers

Reservoir contour

Towline

Normalize by reference receiver

Reference receiver

Now the source is above the hydrocarns

Troll: Depth estimate from Phase plot

Reservoir contour

Towline

0

Reference receiver

SW Offset NE

½ offset at split = depth BML of anomaly

Note the source is SW (not above the hydrocarbons) and NE of the receiver

Troll: Normalised magnitude at specific offset

2,5

Towline

2,0

Reservoir contour

1,5

Normalised Magnitude

1,0

0,5

0,0

0

-2000

-4000

-6000

-8000

-10000

-12000

Offset (m)

Troll (Gather Plot) Magnitude and seismic

Maximum Anomaly Positions

Gather-plot (0.25Hz)

Median value at 5.5 km offset

South-West

North-East

Water-depth (m)

Normalized magnitude

Offset relative to Rx01 (km)

ImagingDepth Migration

Resistivity : 15 ohm-m

Thickness : 50 m

NB: Seismic and SBL line is manually overlaid

What is recorded at the different offsets?

Air waves

DOMINATING WAVES

Guided

waves in

the

reservoir

Air

waves

Direct

waves

HC

Source-receiver distance

New Electric Gradiometer receivers (MK III)

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

Traditional receiver

Method I: “TM decomposition”

- TM decomposition refers to removing the TE mode

At the receivers there are no E source:

Measure and calculate Ez from :

Additional value of EM to seismic

Oil

Oil and

Gas

Gas

LSG

Brine and

LSG

Brine

Brine

Oil

Oil Sw = 0.2

LSG Sw = 0.95

Gas Sw = 0.2

LSG

Gas

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)

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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