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Electro-Magnetic Methods in E&P. 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

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Electro magnetic methods in e p

Electro-Magnetic Methods in E&P

Introduction

EM: Diffusion or Propagation

Electrical Methods

Magneto-Telluric Methods

Controlled Source EM methods

Summary


Jaap c mondt
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



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

  • Introduction Electromagnetism

    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
    Resolution for Waves, Diffusion and Potential fields

    Seismic waves

    EM waves

    Resolution

    Time Derivatives

    Gravity


    Electromagnetics propagation or diffusion

    Electromagnetics: Propagation or Diffusion ?

    • 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
    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
    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



    Electrical monopole

    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


    Rock resistivity
    Rock resistivity

    SI unit of resistivity : ohm-metre (Ωm)

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


    Fractional current
    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)


    Apparent resistivity

    Apparent resistivity

    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
    Summary separation for various resitivity sequences

    • 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.


    Magneto telluric mt
    Magneto-Telluric (MT) separation for various resitivity sequences


    Source solar flares
    Source: Solar separation for various resitivity sequences flares

    – 27 day cycle

    – main source of geomagnetic variations


    Source lightning main energy source at frequencies above 1hz
    Source: Lightning separation for various resitivity sequencesMain energy source at frequencies above 1Hz.

    EARTH

    Schumann resonances at 8, 14, and 21 Hz.


    Typical magnetic spectrum
    Typical magnetic spectrum separation for various resitivity sequences

    5pT

    pT= pico Tesla


    Time varying magnetic field
    Time varying magnetic field separation for various resitivity sequences

    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
    Depth of penetration separation for various resitivity sequences

    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
    MT versus CSEM separation for various resitivity sequences

    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
    Typical lay-out in the field separation for various resitivity sequences

    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
    E and H time series. separation for various resitivity sequences

    Time

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

    Total Segment duration=1024 secs.


    E and h components
    E and H components separation for various resitivity sequences

    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
    Impedances calculated from the measured components separation for various resitivity sequences

    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
    TE &TM separation for various resitivity sequences

    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
    Impedance matrix separation for various resitivity sequences

    The horizontal components can be written as a tensor

    These are decomposed into 2 apparent resistivities and phases

    The general relationship is


    Decomposition
    Decomposition separation for various resitivity sequences

    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
    Impedance Polarisation separation for various resitivity sequences

    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.


    1d sounding
    1D sounding separation for various resitivity sequences

    1D

    2D

    Libya NC171-5

    Inverted to give resistivity versus depth


    Example 2d sounding
    Example 2D sounding separation for various resitivity sequences

    1D

    2D

    1D

    2D

    INVERTED TO GIVE RESISTIVITY v. DEPTH X-SECTION


    Pseudo sections
    Pseudo sections separation for various resitivity sequences

    PERIOD

    APPARENT RESISTIVITY

    PERIOD

    PHASE

    DISTANCE ALONG PROFILE


    Pseudo sections1
    Pseudo sections separation for various resitivity sequences

    Res

    TE mode

    E parallel strike

    Phase

    Res

    TM mode

    E perp. strike

    Phase


    Summary magneto telluric
    Summary Magneto-Telluric separation for various resitivity sequences

    • 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


    Marine em
    Marine EM separation for various resitivity sequences

    CSEM: Controlled Source EM

    Or

    Sea Bed Logging


    Csem sea bed logging
    CSEM: Sea Bed Logging separation for various resitivity sequences

    Note: energy diffused through the air, seawater and subsurface


    Source and receivers
    Source and Receivers separation for various resitivity sequences

    EM receivers

    dropped at

    sea bottom

    EM Source towed above receivers


    What is recorded at different offsets
    What is recorded at different offsets? separation for various resitivity sequences

    Air waves

    DOMINATING WAVES

    Guided

    waves in

    the

    reservoir

    Air

    waves

    Direct

    waves

    HC

    Source-receiver distance


    In line galvanic broadside response induction
    In-Line (Galvanic) & Broadside response (Induction) separation for various resitivity sequences


    Troll off structure reference receiver
    Troll: Off structure reference receiver separation for various resitivity sequences

    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
    Troll: On structure versus Off structure receivers separation for various resitivity sequences

    Reservoir contour

    Towline

    Normalize by reference receiver

    Reference receiver

    Now the source is above the hydrocarns


    Troll depth estimate from phase plot
    Troll: Depth estimate from Phase plot separation for various resitivity sequences

    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
    Troll: Normalised magnitude at specific offset separation for various resitivity sequences

    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
    Troll (Gather Plot) Magnitude and seismic separation for various resitivity sequences

    Maximum Anomaly Positions


    Imaging
    Imaging separation for various resitivity sequences

    Well-1

    Well-2


    Imaging1

    Gather-plot (0.25Hz) separation for various resitivity sequences

    Median value at 5.5 km offset

    South-West

    North-East

    Water-depth (m)

    Normalized magnitude

    Offset relative to Rx01 (km)

    Imaging

    Depth Migration

    Resistivity : 15 ohm-m

    Thickness : 50 m

    NB: Seismic and SBL line is manually overlaid


    What is recorded at the different offsets
    What is recorded at the different offsets? separation for various resitivity sequences

    Air waves

    DOMINATING WAVES

    Guided

    waves in

    the

    reservoir

    Air

    waves

    Direct

    waves

    HC

    Source-receiver distance


    Brazil up down separation
    Brazil: Up-Down Separation separation for various resitivity sequences

    Raw Data

    Up-Down Separation

    Intow 0.125 Hz


    New electric gradiometer receivers mk iii
    New Electric Gradiometer receivers (MK III) separation for various resitivity sequences

    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
    Method I: “TM decomposition” separation for various resitivity sequences

    • 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
    Additional value of EM to seismic separation for various resitivity sequences

    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
    Conclusion separation for various resitivity sequences

    • 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)


    Summary1
    Summary separation for various resitivity sequences

    • 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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