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
EM: Diffusion or Propagation
Controlled Source EM methods
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
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.
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.
The skin depth, d, is the distance over which the field strength
is reduced by the factor 1/e = 0.368 ~-8.686 dB
The wavelength is
where r is the resistivity in W-m and f is the freq in 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
SI unit of resistivity : ohm-metre (Ωm)
Reciprocal of resistivity is conductivity : Siemens/metre (S/m)
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.
a: At large enough electrode separation the apparent resistivity will equal
the true resistivity.
b: The intermediate higher/lower resistivity will appear at intermediate
c: The deeper the higher/lower resistivity the larger the electrode separation (a)
needed to observe its value.
– 27 day cycle
– main source of geomagnetic variations
Schumann resonances at 8, 14, and 21 Hz.
pT= pico Tesla
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.
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.
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.
Channels (top to bottom) are Ex,Ey, Hx, Hy, and Hz.
Total Segment duration=1024 secs.
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
E= electric and H=magnetic; x,y,z refer to the measurement axes.
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).
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.
The horizontal components can be written as a tensor
These are decomposed into 2 apparent resistivities and phases
The general relationship is
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
Increasing period increasing depth
The same data can be plotted as impedance polarization ellipses
for each frequency:
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.
Inverted to give resistivity versus depth
INVERTED TO GIVE RESISTIVITY v. DEPTH X-SECTION
DISTANCE ALONG PROFILE
E parallel strike
E perp. strike
CSEM: Controlled Source EM
Sea Bed Logging
Note: energy diffused through the air, seawater and subsurface
EM Source towed above receivers
0 Offset NE
Note: the receiver and source are both
not above the the hydrocarbons
SW Offset NE
Normalize by reference receiver
Now the source is above the hydrocarns
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
Maximum Anomaly Positions
Gather-plot (0.25Hz) separation for various resitivity sequences
Median value at 5.5 km offset
Offset relative to Rx01 (km)Imaging
Resistivity : 15 ohm-m
Thickness : 50 m
NB: Seismic and SBL line is manually overlaid
Intow 0.125 Hz
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
At the receivers there are no E source:
Measure and calculate Ez from :
Oil Sw = 0.2
LSG Sw = 0.95
Gas Sw = 0.2