1. Electro-Magnetic Methods in E&P Introduction EM: Diffusion or Propagation Electrical Methods Magneto-Telluric Methods Controlled Source EM methods Summary 1

2. 2 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 Jaap C. Mondt

3. Electro-Magnetic Methods 3

4. 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? 4

5. Time Derivatives Resolution Seismic waves EM waves Gravity Resolution for Waves, Diffusion and Potential fields 5

6. 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 give the impression of a passing wave with an arrival time. 6

7. Diffusion: Skin depth / Wavelength The skin depth, , is the distance over which the field strength is reduced by the factor 1/e = 0.368 ~-8.686 dB The wavelength is where  is the resistivity in  -m and f is the freq in Hz (m) 7

8. Skin Depth/Wavelength for sea water, shales and reservoir Sea water resistivity0.3 Ohm-m0.3 Ohm-m skin depth300 m600 m wave length1,886 m3,771 m Shale resistivity1.0 Ohm-m1.0 Ohm-m skin depth900 m1,800 m wave length5,657 m11, 314 m 1 Hz 0.25 Hz HC filled reservoir50.0 Ohm-m50.0 Ohm-m skin depth 3,500 m1,800 m wave length22,000 m44, 000 m 8

9. Electrical methods 9

10. 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 10

11. Rock resistivity SI unit of resistivity : ohm-metre (Ωm) Reciprocal of resistivity is conductivity : Siemens/metre (S/m) 11

12. 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) 12

13. Electrode Configuration The generalised form of the electrode configuration used in resistivity measurements. 13

14. Wenner and Schlumberger configurations Wenner: ρ=2πaΔV/I Schlumberger: ρ= πL²/2l. ΔV/I 14

15. The variation of apparent resistivity with electrode separation over a single horizontal interface between media with increasing resistivities with depth. Apparent resistivity 15

16. 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. acb 16

17. 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. 17

18. Magneto-Telluric (MT) 18

19. Source: Solar flares – 27 day cycle – main source of geomagnetic variations 19

20. EARTH Source: Lightning Main energy source at frequencies above 1Hz. Schumann resonances at 8, 14, and 21 Hz. 20

21. Typical magnetic spectrum 5pT pT= pico Tesla 21

22. 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 22

23. 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. Depth of penetration 23

24. 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 E x and E y to the magnetic field components H x and H y. The vertical magnetic component H z is related to the horizontal magnetic components via the Tipper vector: H z = (A)T x H x + (B)T y H y 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

25. H=magnetic field component E=electric field component Battery Acquisition & processing unit Computer Magnetic sensors electrode Common electrode Ey Ex Hx Hz Hy Typical lay-out in the field 25

26. E and H time series. Time Channels (top to bottom) are Ex,Ey, Hx, Hy, and Hz. Total Segment duration=1024 secs. 26

27. 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. E and H components 27

28. Spectra are combined to give impedances (Z ij ), thus Z xy =E x /H y and so on. Since E x 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 28

29. TE Ex Hy Hz Strike TM Strike Hx Ey Ez Strike TM TE TE &TM 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. 29

30. The horizontal components can be written as a tensor These are decomposed into 2 apparent resistivities and phases The general relationship is Impedance matrix 30

31. 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 Decomposition =TE in case of 2D geology= TM in case of 2D geology 31

32. 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 32

33. Libya NC171 -5 1D 2D Inverted to give resistivity versus depth 1D sounding 33

34. 2D INVERTED TO GIVE RESISTIVITY v. DEPTH X- SECTION 1D 2D Example 2D sounding 34

35. APPARENT RESISTIVITY PHASE PERIOD DISTANCE ALONG PROFILE Pseudo sections 35 PERIOD

36. TE mode E parallel strike TM mode E perp. strike Res Phase Res Phase Pseudo sections 36

37. Summary Magneto-Telluric 37 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

38. CSEM: Controlled Source EM Or Sea Bed Logging Marine EM 38

39. CSEM: Sea Bed Logging Note: energy diffused through the air, seawater and subsurface 39

40. EM receivers dropped at sea bottom EM Source towed above receivers Source and Receivers 40

41. HC Source-receiver distance DOMINATING WAVES Direct waves Air waves Guided waves in the reservoir Air waves What is recorded at different offsets? 41

42. In-Line (Galvanic) & Broadside response (Induction) 42

43. Towline Reservoir contour Reference receiver SW Offset NE 0 0 Offset NE Troll: Off structure reference receiver Note: the receiver and source are both not above the the hydrocarbons 43

44. Towline Reservoir contour Reference receiver Normalize by reference receiver Troll: On structure versus Off structure receivers Now the source is above the hydrocarns 44

45. 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 45

46. 0,0 0,5 1,0 1,5 2,0 2,5 -12000-10000-8000-6000-4000-20000 Offset (m) Normalised Magnitude Towline Reservoi r contour Troll: Normalised magnitude at specific offset 46

47. Troll (Gather Plot) Magnitude and seismic Maximum Anomaly Positions 47

48. Well-1Well-2 Imaging 48

49. Gather-plot (0.25Hz) Median value at 5.5 km offset Offset relative to Rx01 (km) Normalized magnitude Water-depth (m) South-WestNorth-East Depth Migration Resistivity : 15 ohm-m Thickness : 50 m NB: Seismic and SBL line is manually overlaid Imaging 49

50. HC Source-receiver distance DOMINATING WAVES Direct waves Air waves Guided waves in the reservoir Air waves What is recorded at the different offsets? 50

51. Brazil: Up-Down Separation Intow 0.125 Hz Raw Data Up-Down Separation 51

52. 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 receivers 52

53. 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 : 53

54. Brine Brine and LSG Oil Oil and Gas LSG Oil  Sw = 0.2 LSG  Sw = 0.95 Gas  Sw = 0.2 Additional value of EM to seismic 54

55. 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) 55

56. 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 56