Reservoir Geophysics


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Reservoir Geophysics

  1. 1. 4/26/2014 1 Reservoir Geophysics and Geology Arthur Godfrey, Dr. BATTE 1 2 • The technical journals, especially The Leading Edge, are the best sources of material. • Kearey, P., Brooks, M., and Hill, I., 2002, An introduction to geophysical exploration. 3rd edition, Blackwell Publishing, 262p. • Gluyas, J., and Swarbrick, R, 2004, Petroleum geoscience, Blackwell, 359p. • Sheriff, R E, and Geldart, L P, 1995, Exploration seismology, 2nd edition, Cambridge University Press. • Brown, A R, 1996, Interpretation of three-dimensional seismic data. • American Association of Petroleum Geologists, Memoir 42, 4th edn. • Sheriff, R E, (ed), 1992, Reservoir geophysics, Society of Exploration Geophysicists, Tulsa. References
  2. 2. 4/26/2014 2 3 LECTURE 1 Introduction to Geophysics What Is Problem Number One? In case you had not noticed, the basic problem is that most rocks are opaque! As a result, we have several alternatives to finding out what is lurking below the surface. We can use guesswork. 4
  3. 3. 4/26/2014 3 We do that with most domestic applications, such as house foundations or installing a swimming pool. We can dig or drill a hole. BUT HOW DO WE KNOW WE ARE DIGGING OR DRILLING IN THE RIGHT PLACE? We can get the Big Picture first with GEOPHYSICS. 5 What Is Geophysics? Geophysics uses the methods of classical physics to obtain a “geophysical image” of the subsurface. For every standard physical property, there is a corresponding geophysical technique. For example: • Density ↔ Gravity method • Magnetic susceptibility ↔ Magnetic method • Electrical conductivity ↔ Resistivity or EM methods • Velocity & density ↔ Seismic method 6
  4. 4. 4/26/2014 4 The geophysical image of the subsurface is not always the same as the optical or geological image. Recall however that a standard suite of geophysical logs generates an “image” different to core photographs. 7 Natural and Induced Fields Gravity and Magnetic methods measure the spatial variations in the naturally occurring fields. Radiometric methods can also be included within this group. These fields can vary slightly with time, but field and processing techniques usually seek to remove this aspect. 8
  5. 5. 4/26/2014 5 The major advantage of using natural fields is that there is no cost associated with establishing or maintaining them. There are also relatively few disturbances which prevent their measurement. 9 Gravity and Magnetic methods have traditionally been used to provide regional perspectives of the geology by taking measurements at relatively large station separations. However, station spacing has been considerably reduced in recent times, resulting in dramatic improvements in the resolution of geological features. 10
  6. 6. 4/26/2014 6 11 The figure shows a gravity survey for detecting salt structures. Survey results are contoured on a map so that patterns of gravity variation indicative of these features can be recognized. In the above example, a pattern showing a small decrease in gravitational attraction suggests the presence of a low-density salt dome. Gravity surveys for salt domes 12 A Magnetic Image of Geology
  7. 7. 4/26/2014 7 Seismic and Electrical methods on the other hand inject energy into the ground and measure parameters related to the source and energy propagation through the earth. Induced fields normally produce a more detailed image of the subsurface. However, this is usually achieved at substantially greater cost. 13 RESERVOIR ROCK Source Receiver Seismic Structure
  8. 8. 4/26/2014 8 Application of Potential Field Methods Potential field methods, namely Magnetics, Radiometrics, and Gravity are used extensively in the mapping of fold belts for mineral exploration. Costs are very low, such as $10/km for Airborne Magnetics and Radiometrics, and about $100/km for Airborne Gravity. Potential field methods essentially map lateral changes in rock properties. 15 16 Note Although depth-related, information can usually be recovered, with its accuracy considerably less than that achieved with boreholes or Seismic methods, which map vertical variations.
  9. 9. 4/26/2014 9 In petroleum exploration, potential field methods can be extremely valuable in determining the regional geological and structural setting. Furthermore, potential field methods can readily detect transform faulting, where the movement is predominantly in the horizontal direction. 17 18 Figure shows marine seismic recording. Ship-borne recording instruments gathering seismic data. The process is much faster than its land-bound equivalent, but accurate navigation is vital.
  10. 10. 4/26/2014 10 19 Frequently, there can be a close correlation between faulting detected in the sediments with seismic reflection methods, with that in the basement detected with magnetic surveys. One often drives the other. Magnetic data are also used to detect igneous intrusions in sedimentary basins. Benefits of Geophysical Methods •Rapid coverage which is usually not restricted by access. • Uniformity of sampling. • Substantial depths of investigation below the surface. • Data acquisition parameters can be varied to suit the target parameters. 20
  11. 11. 4/26/2014 11 • Geophysics can have a minimal environmental impact. • Geophysics provides quantitative, bulk, in- situ measurements. • Insensitive to vegetation. • Can be recycled many times. 21 Implications of 3D Seismic Reflection Methods The biggest cost in petroleum exploration is the dry hole. Offshore, a borehole can cost ~$25M+. A typical 3D seismic survey can cost ~$1M. One large US based petroleum E&P company estimated that in the 1990s, their cost of finding oil fell from ~$US9/barrel to ~$US1/barrel, due largely to the reduction in dry holes with 3D seismic reflection methods. 22
  12. 12. 4/26/2014 12 Basic Seismic Methods • Seismic exploration using explosives 23 24 • Geophysics marine acquisition seismic geology
  13. 13. 4/26/2014 13 25 •Airguns and Marine acquisition What Are the Major Benefits of 3D versus 2D? The first major benefit is GEOPHYSICAL. Essentially, the seismic signal is reflected from a large region around the seismic traverse, known as the Fresnel zone, as well as from below the traverse. 26 We can think of the Fresnel zone as a disc with the reflection point at its center. Energy being reflected from inside the disk “adds up” to provide the recorded event on the seismic trace
  14. 14. 4/26/2014 14 Fresnel Zone Cont…. Reflections are radiated from a large disc, known as the Fresnel zone, rather than from a point in the subsurface. RECALL that we hear acoustic echoes from the sides of large buildings, rather than from trees or narrow posts. Migration or imaging collapses the disc to either a dot with 3D data or to an ellipse with 2D data, and re-positions the reflections into their correct position in a 3D (or 2D) sense. Post-Migration Fresnel zone Pre-Migration Fresnel zone 27 Other important benefits are GEOLOGICAL. Geology is 3D! That is, there can be very many significant changes in the geology between the wide line spacing of 2D surveys. Many comparisons demonstrate that 2D results can produce an incorrect, rather than an incomplete picture of the subsurface. Read that sentence once more! 28
  15. 15. 4/26/2014 15  What Is Problem Number Two? The second important problem is that the vertical resolution of seismic reflection images is not as good as we would expect. It is generally accepted that the limit of resolution must be a quarter of a wavelength. 𝑊𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡𝑕 = 𝑆𝑒𝑖𝑠𝑚𝑖𝑐 𝑤𝑎𝑣𝑒 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 29 For seismic velocities 2000 m/s to 4000 m/s and seismic frequencies 10Hz to 80Hz, the wavelengths range from ~20 m to 400 m. These wavelengths are barely able to resolve many reservoirs. Other geophysical methods have either less resolution or less penetration to the depths of most reservoirs. 30
  16. 16. 4/26/2014 16 The Seismic Objectives • Make the image CLEAR, that is, improve signal-to- noise ratios through stacking, filtering, etc. • Improve the VERTICAL resolution through source effort, deconvolution, etc. • Improve the LATERAL resolution through smaller station intervals, migration, etc. • RESOLUTION is the only game in town! 31 Summary • Geophysical methods provide a cost effective method for imaging the subsurface. • Better geophysical images → Better geological models → More successful exploration & production. • 3D and 4D seismic reflection methods, developed in the last few decades, have had a spectacular impact on petroleum E & P. 32
  17. 17. 4/26/2014 17 • Most geophysical methods have problems with resolution and/or penetration i.e., with signal-to-noise ratios. There are ongoing comparisons with boreholes. • With seismic data acquisition, we must increase bandwidth for maximum resolution, and minimize noise. 33 34 LECTURE 2
  18. 18. 4/26/2014 18 Reservoir Management The decision to develop a new field offshore, or a deep play onshore, or a field in a remote location requires accurate appraisals of oil and gas in place, potential production rates, and ultimate recovery. If developed, economic pressures further require that these high-cost fields be brought on stream quicker and that RECOVERY BE INCREASED. 35 Reservoir management is maximizing the economic value of a reservoir by optimizing recovery of hydrocarbons while minimizing capital investments (drilling, seismic surveys, etc) and operating expenses (staff costs, taxes, etc). Reservoir management is an economic process of raising the worth of a oil reservoir to its highest possible value. 36
  19. 19. 4/26/2014 19 Economic value generally increases when more reserves are proved or when the reservoir's producing rate increases. Capital investments and operating expenses must be incurred to find and develop reserves. These expenditures offset value. 37 Development Strategies Development strategies must meet five basic objectives: 1. Reduce the cost of field development, which often translates into minimizing the number of wells. 2. Optimize total reserves. 3. Optimize production recovery. 4. Reduce operating costs of the developed field. 5. Enhance recovery if economically justified. Expenditures which drain present worth of a field must be balanced against the chance of increasing present worth by adding reserves and/or increasing production. 38
  20. 20. 4/26/2014 20 Maximizing the Net Present Value (NPV) In essence, the aim of reservoir engineering is to maximize the NPV. In simple terms, 𝑁𝑃𝑉 = (𝑅𝑒𝑣𝑒𝑛𝑢𝑒−𝐶𝑜𝑠𝑡𝑠) 𝑇𝑖𝑚𝑒 . Geophysics can have an impact on the NPV, by helping define a reservoir so that production can be optimized, costs contained, all within a minimum of time. 39 The Technical Challenges Reservoir management must face the technical challenges of: 1. The early and accurate characterization of the reservoir in terms of volumetrics, fluid properties, lithology, and continuity. 2. Improve reservoir surveillance techniques (to monitor pressure changes and fluid movements) so that fields under production may be accurately monitored and efficiently managed. 40
  21. 21. 4/26/2014 21 Conventional engineering data, such as; core analyses, well logs, and production history for reservoir characterization or surveillance CANNOT provide the complete information required to meet these challenges. The aim of this course for this reason is to illustrate that: “a key to improved characterization and surveillance is the use of high resolution geophysical measurements integrated with conventional data within a geological model of the reservoir”. 41 Geophysical methods can, therefore, provide quantitative information to enhance or constrain reservoir simulation models. 42
  22. 22. 4/26/2014 22 43 LECTURE 3 Propagation of Seismic Energy Seismic energy propagates in wavefronts. You can see wavefronts when you throw a stone into a river. Can you visualize a wavefront in 3D in the ground? For a constant seismic velocity, a hemispherical wavefront will propagate away from the shot. 44
  23. 23. 4/26/2014 23 A wavefront is defined as the surface or the boundary between the region where the seismic energy is or has been, and the region where the seismic energy has yet to reach. 45 46 The wavefront represents a surface of equal travel time from the seismic source. The passage of a wavefront is marked by a rapid increase in amplitude and its velocity is measured in the direction of the normal to the wavefront.
  24. 24. 4/26/2014 24 Raypaths-1 We can define raypaths as the path of a very small interval of the wavefront. Raypaths can be viewed as the seismic equivalent of a laser beam. While seismic raypaths DO NOT really exist, nevertheless, they can be extremely useful for visualizing many seismic phenomena. 47 Raypaths are generally equated with the wavefront normal, but this is only the case in isotropic rocks. Anisotropy, which is the variation in seismic velocities with angle, is common in the earth. 48
  25. 25. 4/26/2014 25 Raypaths-2 The raypath is the trajectory of a particular package of energy. It contains the information on the subsurface structure. 49 Seismic Waves Seismic waves can be categorized into; (i) body waves, which propagate throughout the subsurface, and (ii) surface waves, which usually propagate in the weathered layer. Body waves can be further categorized into P-waves, and S-waves. Currently, the vast majority of seismic surveys use only the P-wave energy. 50
  26. 26. 4/26/2014 26 Relatively easy to generate. The seismic velocity of P-waves is a function of both the rock matrix and the fluids within. VP is the compressional wave velocity, K is the bulk modulus, μ is the shear modulus and ρ is the density. P-Waves 51 P-Wave Particle Motion The particle motion with P-waves is essentially parallel to the direction of propagation. With anisotropy, the principal axes of particle motion do not necessarily coincide with principal axes of propagation. Therefore, the P-wave often has a small amount of S-wave motion.52
  27. 27. 4/26/2014 27 S-Waves Shear waves can be generated with either special S- wave sources, or as a by product of P-wave sources. Frequently, 60+% of the output of P-wave vibrators can produce S-waves. Since fluids cannot support shearing, the S-wave velocity is only affected by the rock matrix and not the presence of any fluids. 𝑉𝑠 = 𝜇 𝑝 53 S-Wave Particle Motion The particle motion with S-waves is essentially orthogonal to the direction of propagation. Can you describe an S-wave-like activity at many sporting events? S-wave particle motion can have both horizontal and vertical components, which are known as SH and SV waves. 54
  28. 28. 4/26/2014 28 Since the axis of symmetry with most earth’s anisotropy is generally vertical, the SH and SV components can have quite different seismic velocities. Traditionally, the SH wave has been preferred, because it is reputedly less prone to mode conversion. 55 P-Waves vs S-Waves The measurement of both P- and S-wave velocities provides a means of separating the effects of the rock matrix (e.g. porosity) from the fluids. P-wave results can however be essentially useless where there are “gas chimneys”, because of the severe attenuation of the P-wave energy. In such cases, S-wave surveys are used, because S-waves are not affected.56
  29. 29. 4/26/2014 29 57 A gas chimney is a subsurface leakage of gas from a poorly sealed hydrocarbon accumulation, clearly visible in the center of the lower seismic section P-P but not as apparent in the upper seismic section P-S. Section P-P displays conventional P-wave data. Section P-S, however, includes S-wave energy, which improves seismic imaging in areas where the acoustic impedance contrast is small, such as in a gas chimney, because the presence of gas has little effect on S-wave propagation. 58
  30. 30. 4/26/2014 30 Applications P-Waves and S-Waves 59  Reducing of strong P-wave multiples.  Fracture density and orientation.  Detection of gas seepages.  Direct hydrocarbon and lithology indication.  Investigations into quantitative saturation and pressure changes.  Identifying drilling hazards.  Improved illumination. 60 Reducing of strong P-wave Multiples The combination of the signals recorded by the hydrophone and the Z-component geophone (4C) can help to reduce water- borne multiple contamination. Multiples are internal reflections in a layer, which occur when exceptionally large reflection coefficients are present. WILL DISCUSS THIS MORE LATER IN THE COURSE
  31. 31. 4/26/2014 31 61 Multiples Multiples are seismic energy that reverberates within a layer. The most important in marine seismic is the water multiple. This multiple is strong since the reflection coefficient on the seafloor is generally large (R = 0.3) and the reflection from the surface is close to total (R = -1) Multiples can occur within layers. The figure above shows an example of a peg-leg multiple 62 Detection of Fracture Density and Orientation As a result of S-wave anisotropy, S-waves usually split into two waves, a fast and a slow mode, these split S- waves are very sensitive to fractures and can provide information about fracture density (fracture porosity) and orientation (directions of preferred permeability).
  32. 32. 4/26/2014 32 Detection of Gas Seepages P-wave reflections may be disturbed by gas trapped in the subsurface. S-waves can be used to help clarify the subsurface image because they are unaffected by pore fluids, an important attribute that can improve seismic imaging and highlight information valuable for reservoir characterization, reservoir monitoring, and well planning. 63 Direct Hydrocarbon and Lithology indication S-waves can provide valuable insights into the nature of subsurface lithologies and pore saturating fluids, highlighting reservoirs not previously visible using only P-waves. 64 WILL DISCUSS THIS MORE LATER IN THE COURSE
  33. 33. 4/26/2014 33 65 Investigations into Saturation and Pressure changes S-waves can help monitor Time-lapse variations. During production or injection, reservoir fluid saturation and pressure can change dramatically. WILL DISCUSS THIS MORE LATER IN THE COURSE 66 Time-lapse or 4D seismic has opened new horizons for monitoring reservoir properties such as fluids, temperature, saturation and pressure changes during the productive life of a field. It is based on the analysis of repeated 3D seismic data.
  34. 34. 4/26/2014 34 67 Detection of areas with significant changes or with virtually unchanged hydrocarbon- indicating attributes helps to determine new drilling sites in an already existing production field. For this method, it is critical that the observed seismic changes can be related to the fluid flow. Identifying Drilling Hazards 4D seismic can also be applied in prediction of pore-pressure which can highlight the presence of shallow gas. 68
  35. 35. 4/26/2014 35 69 Improved illumination Subsurface image is often improved through wide azimuth illumination, multicomponent technology offers a cost effective means of acquiring such data in an offshore environment. Swath designPatch design 70 In swath designs, the source lines are parallel to receiver lines, while in patch designs, source lines are perpendicular to receiver lines. WILL DISCUSS THIS MORE LATER IN THE COURSE
  36. 36. 4/26/2014 36 Surface and Guided Waves As the names imply, these waves are generated either at a free surface, generally within the weathered layer, where they are known as ground roll (Rayleigh waves). These waves can be viewed as being generated by multiple reflections within a layer bounded by other layers with strong contrasts in seismic properties. 71 They are characterized by low frequencies, low velocities, dispersive (the velocity changes with the frequency) and frequently very high amplitudes. 72 Accordingly, these waves are usually treated as noise, and where possible, efforts are made to minimize the recording of these signals in routine data acquisition.
  37. 37. 4/26/2014 37 Surface Waves 73 Rayleigh waves shake the ground both in the direction of propagation and perpendicular (in a vertical plane) so that the motion is generally elliptical – either prograde or retrograde. Love waves shake the ground perpendicular to the direction of propagation and generally parallel to the Earth’s surface. Rayleigh waves Love waves Ways of attenuating Surface Waves 1) The most effective method is to place the source BELOW the base of the weathering. This is not practical with surface seismic sources, such as Vibroseis. 2) Alternatively it may be possible sometimes to limit the amount of signal frequency generated by the vibrator. 74
  38. 38. 4/26/2014 38 3) Extended geophone arrays (Δx > 100m) for each trace. This is the traditional approach, but it usually results in a significant loss in resolution. 4) Processing techniques, such as velocity and frequency filtering. 5) STACKING! High fold stacking essentially generates geophone arrays as long as the spread! 75 First Arrival Refraction Signals The near-surface weathered layers are important because they generally exhibit major changes in seismic or acoustic properties (reductions in seismic velocities and densities). With a reduction in seismic velocities, any variations in the thicknesses of the weathered layer results in significant increases in the travel times through that layer. 76
  39. 39. 4/26/2014 39 As a result, the seismic reflections are de- focused, in much the same way that frosted glass de-focuses the image through a window. The first arrival refraction data provides one source of information for defining and in turn, for correcting for the effects of the weathered layer. These corrections are known as “statics” corrections. 77 78 Static corrections – a bulk time shift applied to a seismic trace, are typically used in seismic processing to compensate for these differences in elevations of sources and receivers and near-surface velocity variations. WILL DISCUSS THIS MORE LATER IN THE COURSE Seismograms showing differences between events on adjacent seismograms due to the different elevations of shots and detectors and the presence of the weathered layer. The same seismograms after the application of elevation and weathering correction showing good alignment of the reflection events.
  40. 40. 4/26/2014 40 79 Statics remove the irregularities in travel-times caused by the variations in topography and weathering, so that standard processing methods, such as NMO, can be applied automatically. Statics represent a major, if not THE major single, limitation on the resolution of land seismic reflection methods. While P-wave statics are often a major challenge, S wave statics are an even greater challenge. 80
  41. 41. 4/26/2014 41 Seismic Waves – Summary • Seismic waves propagate in wavefronts. • Raypaths are an alternative approach for visualizing the propagation of seismic energy. • Useful seismic energy include P and S waves. 81 • S waves can generate additional useful images of the earth. • Surface waves are generally considered to be a source of “noise” and various strategies are employed to attenuate them. • The near-surface weathered layers are a cause of loss of resolution with seismic reflection data. • Statics, the corrections for the near-surface layers, are frequently computed from the first arrival refraction data. 82
  42. 42. 4/26/2014 42 83 LECTURE 4 Effects at Interfaces 1 – Snell’s law 2 – Zoeppritz equations 3 – Mode conversion (P↔S) 84
  43. 43. 4/26/2014 43 Wavefronts at Interfaces When a seismic wavefront encounters an interface between rocks with different seismic properties, three effects can occur. 1 – There can be a change in direction of the wavefront. This effect is described with Snell’s law. 85 2 – Part of the energy is reflected and most of the energy is transmitted, or passes right through. 3 – Mode conversion between P and S also occurs. 86 The relative proportions of the reflected and transmitted components are given by the Zoeppritz equations (also known as the Knott’s equations).
  44. 44. 4/26/2014 44 Snell’s Law – 1 When a seismic wavefront encounters an interface between rocks with different seismic velocities at an angle, there can be a change in direction of the wavefront. Why? Because different parts of the wavefront are traveling at different velocities. In general, Snell’s law only applies where there are plane interfaces. 87 Snell’s Law – 2 88
  45. 45. 4/26/2014 45 Use velocities of 2000 m/s and 5000 m/s. incident angle I, refracted angle r • 0 • 10 • 15 • 20 • 23.578 • 25 • 30 Snell’s Law Calculations 89 Snell’s Law – 3 Snell’s law does not apply to diffractions which occur with irregular interfaces. 90
  46. 46. 4/26/2014 46 The Zoeppritz Equations The Zoeppritz equations are quite complicated, mainly because of the need to accommodate the mode conversion effects. 91 As a result, the most commonly used form is the normal incidence approximation. 92 The normal incidence approximations are quite reasonable up to the critical angle. Beyond the critical angle, mode conversion between P and SV becomes more significant.
  47. 47. 4/26/2014 47 Normal Incidence Zoeppritz Equations V is velocity and ρ is density. Layer 1 is above layer 2. 93 The Zoeppritz Equations cont…. The normal incidence approximation is reasonable, up to the critical angle. Mode conversion from P to S waves becomes more extensive beyond the critical angle. 94
  48. 48. 4/26/2014 48 Exercise – Reflection Coefficients Sketch an anticlinal sand reservoir with a shale seal. Shale: 8500 ft/s 2.5 tonnes/m3 Gas Sand: 6400 ft/s 2.16 tonnes/m3 Oil Sand: 10800 ft/s 2.29 tonnes/m3 Water Sand: 12100 ft/s 2.33 tonnes/m3 95 Compute reflection coefficients for: Shale/gas Gas/oil Oil/water Water/shale Shale/oil Shale/water 96
  49. 49. 4/26/2014 49 Mode Conversion There is more mode conversion beyond the critical angle. Mode conversion does not occur with SH waves in isotropic media. Mode conversion is used to generate S wave data with P wave sources, such as with air-guns in the marine environment. 97 Mode conversion is readily accommodated with Snell’s law, where the appropriate P and S wave velocities are used. Mode conversion is readily accommodated with the Zoeppritz equations. 98
  50. 50. 4/26/2014 50 Mode Conversion – Raypaths Mode conversion occurs at most interfaces. 99 Incident P Reflected S Reflected P Refracted P Refracted S V1 V2 > V1 θ Mode Conversion – Marine Sources Mode conversion is employed in marine operations to generate S waves with P wave sources. 100
  51. 51. 4/26/2014 51 Snell’s Law and Mode Conversion Where the incident signal is a P wave and the reflected signal is an S wave, then the angle of reflection will not be the same as the angle of incidence. 101 𝑆𝑖𝑛(𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑃 𝑤𝑎𝑣𝑒 𝑎𝑛𝑔𝑙𝑒) 𝑉𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑃 𝑤𝑎𝑣𝑒 = 𝑆𝑖𝑛(𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝑜𝑟 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑃 𝑜𝑟 𝑆 𝑤𝑎𝑣𝑒 𝑎𝑛𝑔𝑙𝑒) 𝑉𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝑜𝑟 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑃 𝑜𝑟 𝑆 𝑤𝑎𝑣𝑒 Wavefronts at Interfaces – A Summary When a seismic wavefront encounters an interface between rocks with different seismic properties: 1 – There can be a change in direction of the wavefront. This effect is described with Snell’s law. 2 – Part of the energy is reflected, and most of the energy is transmitted, or passes right through. The relative proportions of the reflected and transmitted components are given by the Zoeppritz equations. 3 – Mode conversion between P and SV also occurs. 102
  52. 52. 4/26/2014 52 103 LECTURE 5 Seismic Sources 1 – Dynamite. 2 – Vibroseis. 3 – Air-guns. 104
  53. 53. 4/26/2014 53 Drill Rigs for Dynamite Sources Dynamite sources are often employed where Vibroseis vehicles cannot obtain access. In such cases, portability of the shot hole drilling rig is an important consideration. 105 Vibroseis Sources Vibroseis sources are low power units which achieve high energy levels by vibrating the ground over several seconds. 106
  54. 54. 4/26/2014 54 107 Vibroseis Sources Vibroseis sources sweep a pad of approximately 1m2 through a range of frequencies, using an hydraulic system. 108
  55. 55. 4/26/2014 55 Air-guns – Introduction These create a seismic signal through the rapid discharge of compressed air at 2000 psi into the water. It is an environmentally friendly alternative to explosives. Air-guns generate an oscillating bubble pulse in addition to the primary pulse. Arrays of many air-guns of various sizes are used to cancel the bubble pulse and to improve signal- to-noise ratios. 109 110 Marine seismic surveys use air-guns to send out the seismic signal. An air-gun works by releasing air under high pressure (140 bar) into the water. Air-guns – Operation The air-gun is towed, usually in an array with other guns, 5-15m depth behind the ship.
  56. 56. 4/26/2014 56 Each air-gun is armed with high pressure > 2000 psi, compressed air. Each air-gun is discharged by bleeding air under the flange of the shuttle in the upper chamber. 111 112
  57. 57. 4/26/2014 57 113 114 The high pressured air is generated by a compressor on the ship, and the timing of the shot comes from the navigation system via a gun controller. The high-pressured air is stored in two chambers inside the air-gun (see figure above).
  58. 58. 4/26/2014 58 115 The firing signal is sent as an electric signal to the magnetic sensor on the air-gun. Air is released under the upper piston causing the air in the lowermost chamber to be released instantaneously as an explosion. When the shot has been fired, a signal is sent from the magnetic sensor to the gun controller. If the shot was not fired at exactly zero time, the gun controller will adjust the shot-time for the next shot. Air-Gun Signatures Air-guns are typically 10 to 20 cm in diameter and from 10 in3 to 500 in3 in size. Usually, operating air pressure is 2000 psi and guns are deployed at depth of 5-15 m. Signature consists of (1) direct arrival from air-gun ports, (2) ghost or reflection from surface of the water, and (3) the bubble pulses produced by the expansion-collapse of the air bubble. Signature is given by strength and bubble period.116
  59. 59. 4/26/2014 59 117 Although the initial energy burst is reasonable, a complex pressure interaction between the air bubble and the water causes the bubble to oscillate as it floats towards the surface. Output of air-guns 118 This effect produces the extraneous bursts of energy following the initial burst. The period of the bubble oscillations is given approximately by the modified Rayleigh-Willis formula 𝑇 = 𝑘 𝑃 1 3 𝑉 1 3 𝑃𝑎𝑡𝑚 + 𝜌𝑔𝐷 5 6 Where P is the gun pressure, V is the gun volume, Patm is atmospheric pressure, ρ is the density of water g is gravitational acceleration and D is the depth of the gun, and k is a constant whose value depends on the units.
  60. 60. 4/26/2014 60 119 From the bubble period of one gun of known volume, pressure, depth and bubble period, it is possible to determine the constant k. It follows directly from this formula that the bigger the capacity of the gun fired, the longer the period of oscillation. Air-guns – Bubble Pulse Reduction Each air-gun produces a bubble, upon firing of the gun. The bubble period is proportional to the cube root of pressure and the cube root of gun volume. 120 The figures show a comparison of the source signatures: (a) a single air-gun (peak pressure 4.6 bar metres) and (b) a seven gun array (peak pressure 39.9 bar metres) note the effective suppression of the bubble pulse in the latter case.
  61. 61. 4/26/2014 61 Air-Gun Arrays The signature of a single air-gun is unsatisfactory, because it is too weak to produce good signal-to-noise ratios at large target depths, and because the bubble pulses are difficult to remove with deconvolution. Both problems can be overcome with tuned air-gun arrays in which many guns of different carefully selected volumes are fired simultaneously. Arrays improve the primary-to-bubble ratio (PBR). Arrays can have up to 100 air-guns, but 25-50 is more typical. 121 Signature Measurement Sound pressure created by a single air- gun is inversely proportional to the distance. If the source signature is measured close to the array, the signal is found to be very distorted. This is because the influence of the individual airgun is too big. 122
  62. 62. 4/26/2014 62 123 This is why the source signature is measured in the far-field, which is the region where the shape of the pulse does not change with distance. The far-field signature represents the output of the total array. 124 It is obtained by towing a hydrophone at a depth of >300m below the centre of the array.
  63. 63. 4/26/2014 63 Strength of an Air-Gun Array The SEG-approved unit for far-field strength is the bar-m. A bar is a unit of pressure equal to 14.5 psi or 1 atmosphere or 1011 μPa (micro-Pascal). 125 The bar-m is obtained by multiplying the measured pressure expressed in bars by the distance between the source and the sensor. The advantage of the bar-m is that source strength is characterized by a single number. Average air-gun array signal is about 10-20 bar-m. 126
  64. 64. 4/26/2014 64 Measuring Signal Levels Sound levels are usually measured with the decibel scale: Air Water Comments re 20 μPa re 1 μPa 0 62 Hearing threshold 60 122 Office environment 120 182 Feeling threshold 140 202 Pain threshold 160 222 Damage threshold127 Marine Sound Sources Large tanker 170 db re 1 μPa @ 1m Fishing trawler 150 – 160 db Air-gun arrays 210 – 250 db 1 kg explosives 270 db Sperm whales 200 - 225 db 128
  65. 65. 4/26/2014 65 CHABA specification for impulse noise versus continuous noise on humans state “no protection required” below 208 db in water environment. CHABA specifications indicate that if there are fewer than 1000 impulses per day then the sound level can be increased by another 20 db. 129 Sound and Marine Life 50 fish families have sound-producing species, while all mammals are vocal underwater. Signal levels exceeding 230 – 240 db are necessary to cause damage to fish eggs and lavae. Sound levels of 220 db caused fish to side skip. Damage to marine life is considered low. Major impact is considered to be on communication, avoiding predators, catching prey, migration paths, resting areas, etc., i.e. the ability to survive issues.130
  66. 66. 4/26/2014 66 Air-guns on Mammals Air-gun design, underwater acoustics, animal behavior, and marine mammal physiology are complex subjects and interactions between them are even more complex. Can interpret the same data in quite different ways, eg, whales breeching. Escaping or enjoying? 131 Anecdotes of whales being attracted by air- guns. Mating whales have ignored seismic vessels under survey. With no clear consensus, many organizations recommend mitigation practices. 132
  67. 67. 4/26/2014 67 Marine Acquisition Sources are air-gun arrays. Receivers are usually multiple streamers. Hydrophones, which are pressure sensitive, are the receiving elements. 133 Arrays 1 – Receiver arrays 2 – Source arrays 134
  68. 68. 4/26/2014 68 Receiver Arrays In the days of analogue recording, ground roll could often over-power reflections. Therefore, receiver arrays were employed mainly to attenuate ground roll. 135 Summing up a number of receivers in an array can increase the strength of the reflected signal. However, arrays which effectively attenuate ground roll, must be long, usually >100m. This however reduces resolution! 136
  69. 69. 4/26/2014 69 Response of Receiver Arrays The response of receiver arrays is a function of the number of elements in the array. The greater separation between the elements, the greater the improvement of the attenuation of longer wavelengths is. 137 Receiver Arrays with Data The receiver array, which is 140m long, has greatly attenuated the ground roll. 138 Noise test to determine the appropriate detector array for a seismic reflector survey. (a) Seismic record obtained with a noise spread composed of clustered geophones. (b) Seismic record obtained over the same ground with a spread composed of 140m long geophone arrays.
  70. 70. 4/26/2014 70 Receiver Arrays – The Realities Receiver arrays were developed to attenuate ground roll when analogue recording systems had limited dynamic range. 139 Ground roll is currently more effectively attenuated in data processing through high fold stacking. The “stack array”, formed with the CMP gather, is an array as long as the receiver spread. 140 Below is an example of a CMP-gather. The figure shows that increasing the shot-receiver distance, increases the travel-time. Processing – NMO
  71. 71. 4/26/2014 71 141 The difference between (assumed) vertical two-way travel-time and observed travel-time is called normal- move-out (NMO). 142 Processing stages of seismic traces
  72. 72. 4/26/2014 72 Receiver arrays can minimize spatial aliasing, which was a major concern with large trace spacings. 143 However, arrays reduce resolution, because of differential moveout. The current trend is towards reduced trace spacings and ultimately, towards point receivers. Aliasing Source Arrays Source arrays achieve much the same as receiver arrays. Source arrays are common with Vibroseis sources, in order to: (i) increase signal into the ground and in turn, signal-to-noise ratios and (ii) attenuate ground roll. 144
  73. 73. 4/26/2014 73 145 The new generation 90,000lbs vibrators are reputedly as effective as three 60,000lb units. „It is extremely likely that source arrays will become less common in the future. Arrays – A Summary Arrays for sources and more commonly receivers have seen extensive use as a means of reducing ground roll, and spatial aliasing. Receiver arrays are most effective when they are of a comparable length to the wavelength of the ground roll.146
  74. 74. 4/26/2014 74 Long receiver arrays reduce resolution. Ground roll is currently most effectively attenuated in the processing stages with the “stack array.” Point receivers and point sources are seeing greater use, and permit even better attenuation of ground roll through digital group forming. 147 148 LECTURE 6
  75. 75. 4/26/2014 75 Common Midpoint Methods Common Midpoint (CMP) methods can be viewed as the acoustic equivalent of the lens in optics. By recording sufficient redundant data and then processing it so that it focuses on the target, other “extraneous” signals, such as multiples and ground roll, are attenuated because they are out of focus. An essential factor for the success of CMP methods is sufficiently high fold, which has been facilitated with sources such as air-guns in the marine environment and Vibroseis on land. 149 CMP Data Acquisition The essential feature of CMP data acquisition is to obtain a multiplicity of reflections from the same point(s) in the subsurface, with a multiplicity of source points. 150
  76. 76. 4/26/2014 76 151 Therefore, source points can be as regular as every receiver interval. The shot Gather Operation The data are acquired as “shot gathers”, i.e., each trace has the same shot point. 152
  77. 77. 4/26/2014 77 153 The data are then reordered or sorted into “CMP gathers”, that is, each trace has the same midpoint. Incidentally, the shot and CMP gathers appear to be very similar. Shot and CMP Gathers Shot and CMP gathers appear very similar! 154
  78. 78. 4/26/2014 78 A CMP Gather The raypaths for a single CMP gather cover a range of source-to- receiver offsets. Can you visualize a lens equivalent? 155 Shot & CMP Gathers – Differences There are TWO major differences between shot and CMP gathers. The first is that the interval of each interface sampled is reduced from half the spread length, to essentially a point. 156
  79. 79. 4/26/2014 79 The second difference is that the reflection hyperbolae are always symmetrical about the midpoint, EVEN WITH DIPPING INTERFACES. Symmetrical hyperbolae facilitate automatic processing, in particular, velocity analyses of the data. 157 158
  80. 80. 4/26/2014 80 Raypaths for Shot and CMP Gathers 159 CMP Methods – A Summary CMP methods are the standard method for acquiring seismic reflection data. CMP methods acquire HIGHLY REDUNDANT data. Redundancy is used to reinforce primary reflections and to attenuate noise with stacking. CMP gathers generate symmetric hyperbolae, which have major conveniences in the processing of the data. 160
  81. 81. 4/26/2014 81 161 LECTURE 7 Noise 1 – Coherent noise 2 – Random noise 162
  82. 82. 4/26/2014 82 What Is Noise? Noise is everything other than primary reflections, also known as single bounce reflections. Noise can be coherent, such as multiple reflections. Often multiples can be strong in marine surveys with reverberations in the water column. 163 164 Coherent noise is unwanted seismic energy that shows consistent phase from one seismic trace to another. With land operations, ground roll or surface waves, especially with surface energy sources are another major source of coherent noise. Here the waves travels through the top of the surface layer, also known as the weathering layer.
  83. 83. 4/26/2014 83 Multiple Reflections Multiples can be generated in many ways. Multiples constitute one of the principal sources of “noise” with many seismic operations. 165 166 This is energy trapped within a layer which is another form of coherent energy. Multiples are internal reflections in a layer, which occur when exceptionally large reflection coefficients are present.
  84. 84. 4/26/2014 84 Random Noise Noise can be random, such as wind noise, cultural noise from infrastructure, vehicles, boats, etc. Random noise such as wind noise, streamer noise or sea noise is usually monitored during acquisition. When the noise levels rise above the contractually agreed levels, acquisition is usually stopped. In many cases, slashing or rolling the vegetation can reduced the effects of wind.167 Random noise is usually reduced in processing by stacking. 168 Essentially, random noise is reduced by the square root of the number of traces in the stack.
  85. 85. 4/26/2014 85 Improving S/N Ratios with CMP Stacking Stacking improves signal-to-noise ratios as the square root of the number of traces in the CMP stack. Below are seismic sections showing how stacking of seismic traces can improve the signal-to-noise ratio. The horizontal scales are different. 169 170 A single-fold section obtained in 1965.
  86. 86. 4/26/2014 86 171 A 4-fold stacked section obtained in 1967 172 A 12-fold stacked section obtained in 1981 along the same traverse.
  87. 87. 4/26/2014 87 Scattered Noise Scattered noise or diffractions are common in rocks like carbonates. Often, scattered noise is signal which belongs somewhere else, rather than in the plane of the seismic section. When in doubt, filtering it out can be a common approach. 173 174 Effect of f-k filtering of a seismic section. Left is a stacked section showing steeply dipping coherent noise events, especially below 4.5s two-way reflection time. Right showing same section after rejection of noise by f-k filtering
  88. 88. 4/26/2014 88 Noise – A Summary Noise is everything other than primary reflections, also known as single bounce reflections. Noise can be coherent, such as multiple reflections. Often multiples can be as strong as primaries, as in marine surveys with reverberations in the water column. With land operations, ground roll or surface waves, especially with surface energy sources are another major source of coherent noise. 175 Noise can be random, such as wind noise, cultural noise from infrastructure, vehicles, boats, etc. Acquisition system noise is rarely an issue. Noise is usually addressed in data processing. 176
  89. 89. 4/26/2014 89 177 LECTURE 8 Amplitudes All seismic systems currently in use have 24- bit recording and therefore, they have sufficient dynamic range to record every seismic signal. The limiting factor in data acquisition is usually the dynamic range of the receivers. 178
  90. 90. 4/26/2014 90 The processing of the seismic data utilizes the full dynamic range of the recorded data. The real limiting factor is the dynamic range of the human eye, that is, the display of the data is the issue. 179 MEMS (micro-electro-mechanical-system) receivers have greater dynamic range than the standard geophone. Spherical Divergence The major cause of the dramatic variations in seismic amplitudes down the seismic record is spherical divergence. This results from the apparent loss of energy from a wave as it spreads during travel. Spherical divergence decreases energy with the square of the distance 180
  91. 91. 4/26/2014 91 181 Spherical divergence and attenuation of seismic waves causes a Fresnel zone, shown in this 2D sketch as length A-A'. In 3D seismic, the Fresnel zone is circular and has diameter A-A'. The Fresnel zone is the area in the subsurface which contributes to each reflection. The diameter of this zone, which can be quite large, can be reduced through seismic migration. 182 The area of a hemisphere is proportional to the square of the radius, that is, double the radius, quadruple the surface area. Therefore, the seismic amplitudes systematically decrease with recording time, simply because the energy is spread over a larger area.
  92. 92. 4/26/2014 92 As a result, the dynamic range of the seismic data is greater than the human eye can accommodate. The dynamic range of the human eye is about 42db to 48db. Therefore, seismic data must be gained to facilitate convenient examination by observers during acquisition and geophysicists. 183 During processing, corrections for spherical divergence is usually made. These corrections are based more on appearance of being true or real, than on science. 184
  93. 93. 4/26/2014 93 185 Before After Amplitudes – A Summary Seismic amplitudes can exhibit very large dynamic range, often > 96 db, largely because of geometric spreading. Current 24 bit acquisition systems with 144 db of dynamic range are adequate to record most seismic signals. The human eye has a limited dynamic range of 42 db – 48 db. 186
  94. 94. 4/26/2014 94 187 Quiz 8 188 LECTURE 9
  95. 95. 4/26/2014 95 189 Datum Statics In marine seismic, the sea surface defines a datum for further processing. Hence, only minor static corrections are introduced to compensate for the source- and streamer-depths. 190
  96. 96. 4/26/2014 96 However, in land seismic static corrections play a much more important role, since variations in topography may cause severe distortions if not corrected for. 191 192 Statics aim to replace the irregular topography and weathered layer with a flat surface at the datum.
  97. 97. 4/26/2014 97 Statics – The Results Statics remove the irregularities in travel-times caused by the variations in topography and weathering, so that standard processing methods, such as NMO, can be applied automatically. 193 Seismic Data Processing 1 – Velocity analysis 2 – Stacking 3 – Deconvolution 4 – Migration 194
  98. 98. 4/26/2014 98 Data Processing – Objectives The aims of data processing are to: (i) Improve signal-to-noise ratios, mainly through CMP stacking. (ii) To improve vertical resolution, mainly through deconvolution. (iii) To improve lateral resolution, mainly through migration a.k.a imaging. 195 Seismic data processing CMP Gathers: The data are recorded in the field as files for each shot. These are known as common shot gathers. 196 The data are then re-arranged within the processing computer into common midpoint or CMP gathers, i.e. all of the traces from various shots with the same mid point (i.e., with the same station number on the ground mid way between the shot and the geophone) are gathered together.
  99. 99. 4/26/2014 99 Statics: Statics are the corrections for the variable near surface weathered layer. They are usually computed with the first arrival refraction data, and they are one of the important factors limiting the resolution of seismic surveys. Like NMO corrections, they are time shifts. Statics and NMO are different sides of the same coin, i.e. they are inter-related. 197 NMO Corrections: Each trace within a gather is corrected in order to remove the NMO, which is the difference between the travel- time for an inclined raypath, over the travel- time for a vertical raypath. The amount of correction is a measure of the average horizontal seismic velocity to that reflector. 198
  100. 100. 4/26/2014 100 Stacking: The NMO corrected traces are added together to form a stacked trace. Not only does this process improve signal-to- noise ratios, but it also reduces the amount of data. Deconvolution: Deconvolution aims at compressing the seismic wavelet, close to an approximation of a spike as possible. 199 Deconvolution cont… It also removes reverberations, Improves bandwidth, sharpens wavelets and removes multiples. 200 Left show figure without deconvolution and right shows figure when deconvolved
  101. 101. 4/26/2014 101 201 Removal of reverberations by predictive deconvolution. The seismic record on the left above is dominated by strong reverberations. Below, same seismic record after spiking deconvolution Spiking deconvolution seeks to whiten the signal, while gapped deconvolution seeks to reduce the number of cycles in a reflection wavelet. The many cycles can be caused by reverberations within the shot point, by the ghost or reflection from the earth's surface, or by reverberations within reflectors. Filtering: The spectrum of the traces is filtered to reduce those frequencies where noise predominates. This process is usually so effective that small faults are often removed! 202
  102. 102. 4/26/2014 102 Migration: Signals from each point in the subsurface are recorded over a large area on the surface (fresnel zone). Seismic migration is the process which collapses the reflection energy back to the source. It sharpens all structures, including faults. 203 Migration cont… Migration is more correctly known as imaging. We plot the reflectors below the CMP. Migration moves the reflections up- dip to their correct position. 204
  103. 103. 4/26/2014 103 Migration - Application Migration: (i) repositions reflections to their correct place in the subsurface. (ii) unscrambles complex reflections. Migration effectively collapses the Fresnel zone. 205 206 Migrated CMP gather after muting Filtered
  104. 104. 4/26/2014 104 Depth Migration Migration in the time domain can be ineffective, where there are large velocity changes, such as where salt pillows occur. In such circumstances, migration in the depth domain is required. 207 208 (a) CMP stack and (b) its migration. Time migration treats the top of the salt “T” properly, while it fails to image the salt base “B” accurately. Depth migration must be done to handle this properly.
  105. 105. 4/26/2014 105 Velocity Spectra – Theory The normal moveout (NMO), is a function of offset x, t0, and velocity. A range of velocities is used and that which produces the best stack is taken as the NMO velocity. 209 210 A set of reflection events in a CDP gather using a range of velocity values. The stacking velocity is that which produces peak cross power from stacked events. i.e., the velocity that most successfully removes the NMO. In this case, V2 represents the stacking velocity.
  106. 106. 4/26/2014 106 211 The NMO velocities are determined from velocity spectra computed at regular closely spaced intervals down the CMP gather. Velocity Spectra – Application Muting for NMO Stretch NMO corrections stretch the seismic trace. Shallow reflections are corrected more than deep reflections. The stretched signals can degrade the stack, but are surgically removed with muting, prior to stacking. 212 NMO correction and muting of a stretched zone on field data. (a) CMP gather, (b) NMO correction and (c) mute
  107. 107. 4/26/2014 107 The purpose of muting is to remove: • Direct waves • Refracted waves (i.e. mainly associated with the waterbottom in marine seismics). 213 Too mild mute function applied Examples of muting Proper choice of mute function Too strong mute function applied 214
  108. 108. 4/26/2014 108 Summary Seismic data processing aims to convert to field data into an image of the subsurface. The volumes of data can be staggering, and one aim of the processing stage is to reduce the amount of data to manageable and practical sizes. 215 The first step in the processing stream is to remove the effects of spherical divergence. The seismic traces are amplified using a gain function which accommodates the loss of signal strength with time and distance. The next step is to re-arrange the data from the common shot files or gathers, into common midpoint gathers. This is very computer intensive. 216
  109. 109. 4/26/2014 109 For land seismic surveys, the corrections for variations in surface topography and thickness of the weathered layer, known as static corrections, are required to re-align the reflections. The reflection time is a hyperbolic function of the source to receiver separation, the reflector depth, and the average seismic velocity to the reflector. 217 This curvature is known as Normal Move Out, and its removal to obtain aligned reflections also provides a measure of the average seismic velocity. The NMO corrected gathers are then added or stacked 218
  110. 110. 4/26/2014 110 Convolution and deconvolution filtering are processes which effect the temporal spectrum of each seismic trace. They are used to improve resolution by sharpening the seismic pulse and by removing reverberations. Velocity filtering operates on sets of seismic traces, in order to remove or enhance data with particular apparent seismic velocities, such as ground roll. 219 Migration is the process of collapsing scattered seismic signals data back to their source in the subsurface. The Fresnel zone is the area in the subsurface which contributes to each reflection. The diameter of this zone, which can be quite large, can be reduced through seismic migration. 220
  111. 111. 4/26/2014 111 221 Quiz 9 222 LECTURE 10
  112. 112. 4/26/2014 112 Marine Systems Known as ocean-bottom cables (OBC), uses four component (4C) receivers – three component velocity geophones and one hydrophone. Often buried in the sea floor. 223 Use pop-up buoys at the end of each line to interface with recording equipment in shallow waters (<1000 ft). Permanent Seismic Monitoring Permanent seismic monitoring is becoming an important tool in the reservoir management toolkit. 224
  113. 113. 4/26/2014 113 It is a 4C fiber-optic advanced seismic acquisition technology, that is installed permanently on the seabed over a producing field. Permanent installation of 4C cables at the sea bottom over a producing field. It improves data quality by ensuring more accurate receiver locations within the repeated 3D surveys over a period of time. 225 It reduces acquisition time and cost. Permanent seismic monitoring helps to improve data quality by employing more accurate survey orientation and acquisition geometry (receiver locations) within the repeated 3D seismic surveys compared to conventional OBS 4D survey. Such a method is important in monitoring a reservoir injection process employed to enhance recovery from a producing reservoir. 226
  114. 114. 4/26/2014 114 Use fibre optic connection to platforms for deep deployments. Need to consider retrieval and repair because of strong sea floor currents, etc. Need to know precise location of sensors. 227 OBC System 228
  115. 115. 4/26/2014 115 The use of 4C OBS recording has several advantages over conventional towed streamer technology, which includes: • Dual-sensor summation (3C geophone + hydrophone) for the suppression of receiver-side multiples. 229 • Utilizing P–S wave conversions for enhanced imaging. A comparison of seismic data acquired by the towed streamer (top) and OBS (bottom) techniques. The OBS survey significantly improves the subsurface image. 230
  116. 116. 4/26/2014 116 • Attenuation of free surface multiples when combined with towed streamer recording. A comparison of the migrated P–S stack versus the P–P stack is shown below. Comparison of P–P stack of conventional 3D streamer data (top) and P–S stack of OBS data (bottom). Note how the OBS data produces a much better deeper image in the presence of gas versus 3D streamer data. 231 The P–S stack is produced from OBS converted wave data whereas the P–P stack is produced from 3D towed streamer P-wave data. From this comparison it is clear that OBS data can be used to successfully image through a gas chimney. 232
  117. 117. 4/26/2014 117 Passive Seismic Monitoring This technique is quite common in mining operations. Here, detectors are cemented into the borehole, between the casing and rock. These measure the microseismic activities associated with production and development. • These can locate fractures using triangulation. • Can often detect if fractures are opening or closing. • Useful in monitoring hydraulic fracturing. 233 Direct Detection of Hydrocarbons DHI Direct Hydrocarbon Indicators 234
  118. 118. 4/26/2014 118 Flat Spots The standard exploration approach seeks to find structural or stratigraphic targets which are favorable for hydrocarbon accumulations. However, it is possible to directly detect hydrocarbons under certain conditions, especially in younger sediments.235 A key diagnostic for the presence of hydrocarbons is a flat spot. In this situation, the hydrocarbon-brine contact produces a flat/horizontal reflection, inconsistent with the lithological reflections from the trap boundaries, and over a limited area bounded by structural contours. 236
  119. 119. 4/26/2014 119 Where it can be reliably detected and mapped, the flat spot can provide a reasonably unambiguous indication and areal extent of a reservoir and an estimate of reservoir thickness. 237 A flat spot can indicate a gas-oil, gas- water, or oil-water interface, with the reflection coefficient for the last interface being substantially lower than that of each of the others. 238
  120. 120. 4/26/2014 120 Flat Spot Examples 239 Bright Spots The amplitudes of the reflected and transmitted signals are described by the Zoeppritz equations. These equations are quite complex, but simplify considerably at normal incidence. 240
  121. 121. 4/26/2014 121 Under most circumstances, these approximations are sufficiently valid up to the critical angle, where phase shifts can occur. In young sediments, the presence of gas in a reservoir usually further reduces the specific acoustic impendence, and therefore, increases the magnitude of the reflection amplitude. These are known as bright spots. However, there can be other effects, such as dim spots and phase changes which depend upon the petrophysical contrasts of the reservoir with the surrounding layers. 241 Effect of Gas on the Poisson’s Ratio Poisson’s ratio, σ, is the ratio of the fractional transverse contraction (transverse strain) to the fractional longitudinal extension (longitudinal strain) when a rod is stretched. 242 It varies between 0 and 0.5. It has a value of 0.5 for fluids and 0.25 for a Poisson solid.
  122. 122. 4/26/2014 122 Classical elasticity determines compressional and shear wave velocities with the equations: These equations can be combined to obtain the ratio of the compressional and shear wave velocities in terms of Poisson’s ratio: 243 𝑉𝑝 = 𝜆 + 2𝜇 𝜌 = 𝐾 + 4 3 𝜇 𝜌 𝑎𝑛𝑑 𝑉𝑠 = 𝜇 𝜌 𝑉𝑠 𝑉𝑝 = 0.5 − 𝜎 1 − 𝜎 The s-wave velocities largely depend on the fluid content of rocks, whereas the p-wave velocities are significantly affected. However, there are significant inconveniences with shear wave acquisition and processing. Therefore, the measurement of the p-wave velocity and Poisson’s ratio, provides an alternative means of determining fluid saturates in a reservoir. 244
  123. 123. 4/26/2014 123 Amplitude Variation with Offset (AVO) The Zoeppritz's equations are usually adequate for large angles of incidence. Where the angle of incidence is other than normal, both P and S-waves are generated. The reflection coefficient depends upon the ratio of the P and S-wave velocities, or what is equivalent to the Poisson's ratio. 245 The Shuey approximation of the reflection coefficient for non-normal incidence is given by: If there is no contrast in Poisson's ratio across an interface, the second term is zero and the variation with angle is simply the cosine factor, which causes a decrease of amplitude with increasing angle.246 𝐴 𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝐴𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 = 𝜌2 𝑉2 − 𝜌1 𝑉1 𝜌2 𝑉2 + 𝜌1 𝑉1 𝑐𝑜𝑠2 𝜃 + 2.25 𝜎2 − 𝜎1 𝑠𝑖𝑛2 𝜃
  124. 124. 4/26/2014 124 If there is a significant contrast in the Poisson's ratio, as normally occurs at the boundary of gas sands, then the second term becomes important and the amplitude generally increases with increasing angle. The increase of amplitude with increasing angle of incidence, or recording offset, can be used as a diagnostic in the identification of gas reservoirs. 247 AVO – Case Study Coal and gas sands both have low seismic velocities and low densities, and therefore, they generate strong reflection amplitudes. 248 How can they be differentiated?
  125. 125. 4/26/2014 125 Chronostratigraphy and Lithostratigraphy The Shuey approximation of the reflection coefficient for non-normal incidence is given by: 249 𝐴 𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝐴𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 = 𝜌2 𝑉2 − 𝜌1 𝑉1 𝜌2 𝑉2 + 𝜌1 𝑉1 𝑐𝑜𝑠2 𝜃 + 𝜎2 − 𝜎1 1 − 0.5 𝜎2 + 𝜎1 2 𝑠𝑖𝑛2 𝜃 The normal incidence reflection coefficient is the chronostratigraphic reflection coefficient. The Poisson’s ratio reflection coefficient is the lithostratigraphic reflection coefficient. 250
  126. 126. 4/26/2014 126 AVO Cross- Plot Poisson’s ratio provides a means of differentiating gas sands from wet sands and shales. 251 DHI – A Summary • Gas-liquid contacts can be recognized as flat spots. • The large reductions in seismic velocities and densities with gas sands can produce high amplitudes or bright sands with young sediments. • With older sediments, the occurrence of gas is detected with AVO which is a measure of the change in Poisson’s ratio. • Most current methods of seismic inversion include the AVO response and invert for P and S wave velocities and density. 252
  127. 127. 4/26/2014 127 253 LECTURE 11 254 Land Acquisition
  128. 128. 4/26/2014 128 A complication in land acquisition is that, unlike marine data, a seismic line is rarely shot in a straight line because of the presence of natural and man-made obstructions such as lakes, buildings and roads. 255 The shot points and the receivers may be arranged in many ways. Many groups of geophones are commonly used on a line with shot points at the end or in the middle of the receiver array. The shot points are gradually moved along a line of geophones. 256
  129. 129. 4/26/2014 129 The variations in ground elevation in land acquisition causes sound waves to reach the recording geophones with different travel-time. The Earth’s near-surface layer may also vary greatly in composition, from soft alluvial sediments to hard rocks. 257 This means that the velocity of sound waves transmitted through this surface layer may be highly variable. 258 Static corrections, just like in marine seismic, involves applying a bulk time shift to a seismic trace during seismic processing to compensate for these differences in elevations of sources and receivers and near-surface velocity variations.
  130. 130. 4/26/2014 130 Vertical Seismic Profile (VSP) VSP is a technique of seismic data acquisition, whose data is used for correlation with conventional seismic data (land or marine seismic). 259 The defining characteristic of a VSP is that either the energy source, or the receivers (or sometimes both) are in a borehole. VSPs include the zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, salt- proximity VSP, shear-wave VSP, and drill- noise or seismic-while drilling VSP. Read more about each VSP 260
  131. 131. 4/26/2014 131 VSP involves a series of measurements in which a seismic signal generated at the surface is recorded by geophones secured to the side of a borehole, at various depths. The receiver interval is commonly 15m, although a 7.5m interval has been employed for greater resolution. VSP is a modernization of the earlier “check shot” survey. 261 262 A check shot survey differs from a VSP in the number and density of receiver depths recorded. Geophone positions may be widely and irregularly located in the wellbore, whereas a VSP usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore.
  132. 132. 4/26/2014 132 Initially, only a single channel sonde was used. However, Reservoir Seismic 2020 are deploying up to 1200 channels within the borehole. The arrays can be deployed in horizontal as well as vertical boreholes 263 264 Initially, VSPs were used to obtain an accurate time-depth correlation, and to separate upward travelling signal from downward propagating signal, in order to optimize deconvolution, recognize multiples, etc.
  133. 133. 4/26/2014 133 In the most common type of VSP, hydrophones, or more often geophones in the borehole record reflected seismic energy originating from a seismic source at the surface. Acquisition of VSP. The downhole geophones record important structural and stratigraphic data generated by a surface energy source. The VSPs vary in the well configuration, the number and location of sources and geophones, and how they are deployed. 265 VSP – Land Source 1 The most common land source is Vibroseis. However, air-guns, (both truck mounted and mud pit located) have been employed. Dynamite provides good energy levels. 266
  134. 134. 4/26/2014 134 VSP – Land Source 2 267 Cross-well seismic Detailed understanding of reservoir flow and barrier architecture is crucial to optimizing hydrocarbon recovery. Cross-well seismic, that is using seismic sources in a wellbore and recording the wave propagation in another wellbore has the potential of giving high- resolution images of features like faults, unconformities, sequence porosity and fracturing. 268
  135. 135. 4/26/2014 135 269 Cross-well data currently are expensive to acquire and the technique is almost solely employed onshore. Use of cross-well seismic in marine environments is difficult because of the large distances between the boreholes and the complicated geometrical shape of the (deviated) wells. 4D Seismic The acquisition of 4D or time-lapse seismic has opened new horizons for monitoring reservoir properties such as fluids, temperature, saturation and pressure changes during the productive life of a field. 270
  136. 136. 4/26/2014 136 4D seismic is based on the analysis of repeated 3D seismic data. The differences in seismic attributes over time are caused by changes in pore fluid and pore pressure associated with the drainage of a reservoir under production. 271 Detection of areas with significant changes or with virtually unchanged hydrocarbon indicating attributes helps to determine new drilling sites in an already existing production field. For this method, it is critical that the observed seismic changes can be related to the fluid flow. 272
  137. 137. 4/26/2014 137 Differences in data acquisition, survey orientation, processing, and data quality can introduce significant noise in a 4D analysis. Hence, such differences must be corrected for as best as possible. 273 The known applications of 4D seismic can be summarized as: • Monitoring the spatial extent of steam injection used for thermal recovery. • Monitoring the spatial extent of the injected water front used for secondary recovery. • Imaging bypassed oil or gas. • Determining the flow properties of sealing or leaking faults. • Detecting changes in oil-water contact. 274 Read more about 4D seismic
  138. 138. 4/26/2014 138 275 Enhanced Oil Recovery (EOR) Seismic Processing Seismic technology has achieved amazing achievements in exploration and production activities in the past few decades. What we record in the acquisition stage is called raw seismic data, which contains real signals, together with noise and multiples. 276
  139. 139. 4/26/2014 139 This raw data must then be processed by employing advanced methods within signal processing and wave-theory to get better images of the subsurface. 277 The prime objective in the processing stage is to enhance the signal and suppress the coherent and non-coherent noises and multiples. Raw seismic data with coherent and non-coherent noise. 278 Noise attenuation image after autocorrelation, deconvolution and trace muting.
  140. 140. 4/26/2014 140 Coherent noise is unwanted seismic energy that shows consistent phase from one seismic trace to another. 279 This may consist of waves that travel through the air at very low velocities such as airwaves or air blast, and ground roll that travels through the top of the surface layer, also known as the weathering layer. The energy trapped within a layer known as multiples is another form of coherent energy. Multiples are internal reflections in a layer, which occur when exceptionally large reflection coefficients are present. In marine seismic, the water-bottom multiples normally dominate. 280
  141. 141. 4/26/2014 141 Non-coherent energy is typically non-seismic- generated noise, such as noise from wind, moving vehicles, overhead power line or high- voltage pickup, gas flares and water injection plants. 281 It has been stated earlier that seismic processing is the alteration of seismic data to suppress noise, enhance signal and migrate seismic events to the appropriate location in space. Seismic processing facilitates better interpretation, because subsurface structures and reflection geometries become more apparent. 282
  142. 142. 4/26/2014 142 The actual sequence of the seismic processing will be determined by (a) the purpose of the investigation, (b) extensive testing on selected parts of the dataset and (c) a trade-off between quality and cost. The 2D seismic processing steps typically include static corrections, deconvolution, velocity analysis, normal and dip moveout, stacking and migration.283 Amplitude losses Seismic amplitude losses are caused by three major factors: 1. Geometrical spreading. 2. Intrinsic attenuation. 3. Transmission losses. 284
  143. 143. 4/26/2014 143 Geometric spreading: Progressive diminution of amplitude (proportional to the inverse of propagation distance) caused by increase in wavefront area. 285 Intrinsic attenuation: energy losses due to internal friction. 286
  144. 144. 4/26/2014 144 Transmission losses: reduction in wave amplitude due to reflection at interfaces. 287 Amplitude recovery This stage attempts to correct for amplitude losses that are unrelated to the reflection coefficient, such as; wave attenuation and source variations. Both: Deterministic and Statistical approaches are used. 288
  145. 145. 4/26/2014 145 Deterministic approach: A popular deterministic model is the t-square model, where the data is multiplied by 𝑡2 (t being the two-way travel-time). It is based on the following assumptions: • Multiplication with t to compensate for geometrical spreading. • An attenuation model of the type where the total losses are given as an integration over all frequencies. 289 Statistical approach: Automatic gain control (AGC) is the most common class of routines. They are based on these principles: Let 𝑋𝑖 denote the amplitude at time-sample number i (i.e. corresponding to time 𝑡𝑖 = 𝑖∆𝑡) of a seismic trace. Introduce a tie-window of length 2𝐿 + 1 and compute the weighted amplitude value around this sample point. 𝑥𝑖 = 1 2𝐿 + 1 𝑤𝑖 𝑥𝑖+𝑙 , 𝑤𝑖 𝑤𝑒𝑖𝑔𝑕𝑡 − 𝑓𝑎𝑐𝑡𝑜𝑟𝑠 𝐿 𝑙=−𝐿 290
  146. 146. 4/26/2014 146 Examples of amplitude recovery Raw data to the left and amplitude recovered data to the right employing AGC. 291 Raw data to the left and amplitude recovered data to the right employing 𝑡2 Examples of amplitude recovery cont.. 292
  147. 147. 4/26/2014 147 Raw data to the left and amplitude recovered data to the left employing both AGC and 𝑡2 Examples of amplitude recovery cont.. 293 Still during the processing stage, bad measurements are edited, datuming applied and corrections of wave-energy decay introduced. 294 The true amplitude recovery is applied to increase the amplitude at large travel times.
  148. 148. 4/26/2014 148 Correlation Cross-correlation is a measure of the similarity or linearity between two waveforms. Cross-correlation involves: (1) cross-multiplication of the individual waveforms, and summation of the cross-multiplication products over the common time interval. (2) progressively sliding one waveform past the other and, for each time shift of lag, summing the cross- multiplication products. 295 296 (a) North-South, East-West particle oscillation components and (b) their particle motion (c) North-South, East-West particle oscillation components and (d) the fastest wave polarization direction. (e) North-South, East-West particle oscillation components rotated into Fast and slow waves (f) Cross correlation between the fast and slow wave.
  149. 149. 4/26/2014 149 The cross-correlation function is the value of the sum, the cross-multiplication products as a function of the lag time. The cross-correlation operation is similar to convolution, but it does not involve folding or reversing one of the waveforms. 297 298 It can be shown that the cross-correlation of two functions in the time domain is mathematically equivalent to the multiplication of their amplitude spectra and the subtraction of their phase spectra in the frequency domain.
  150. 150. 4/26/2014 150 For two similar waveforms, the correlation function will peak at zero lag. For two functions containing only random noise, the cross-correlation function is zero for all lag values. Cross-correlation is used to detect weak signals embedded in noise. 299 300 The width ∆𝜔, is a measure of resolution and the ratio between the side lobes and the main lobe is a measure of the S/N-ratio. An ideal time-window should have: • Narrow and strong main lobe (delta). • As small as possible side lobes
  151. 151. 4/26/2014 151 The width ∆𝜔, is a measure of resolution and the ratio between the side lobes and the main lobe is a measure of the S/N-ratio. An ideal time-window should have: • Narrow and strong main lobe (delta). • As small as possible side lobes 301 It is used to convert Vibroseis field records into correlated shot records. A special case is autocorrelation, which is symmetrical about the zero lag position. It is used to detect hidden periodicities (multiples) in any given waveform such as ghosts and other reverberations in seismic reflection methods. 302
  152. 152. 4/26/2014 152 Vibroseis Correlation 303 Cross correlation of the sweep signal with the field recording generates an output similar to an impulsive source, such as dynamite. The correlated pulse is a symmetrical zero phase Klauder wavelet. 304
  153. 153. 4/26/2014 153 Correlation 305 Autocorrelation is applied to compress the wavelet and to attenuate multiples. Autocorrelation is the cross-correlation of a signal with itself. Informally, it is the similarity between observations as a function of the time lag between them. It is a mathematical tool for finding repeating patterns, such as the presence of a periodic signal obscured by noise. It is often used in signal processing for analyzing functions or series of values, such as time domain signals. 306 Autocorrelation
  154. 154. 4/26/2014 154 Autocorrelation Cont… Autocorrelation is widely used to determine periodicity (multiples) in seismic signals. 307 Convolution Suppose we need to determine the response of a system, such as a stereo system, to an input, such as a track from an audio CD, the input can be viewed as a series of impulses which; (i) are separated by the digitizing interval and (ii) are scaled by the amplitude of the signal. 308
  155. 155. 4/26/2014 155 The output is the sum of the multiplicity of impulse responses which: (i) are time shifted to correspond with the time of the input impulse. (ii) are scaled or multiplied by the amplitude of the input value. Convolution is the mathematical process used to derive the output y(t) from the input g(t) and the impulse response f(t). 309 The symbolic notation for convolution is *, ie., y(t) = g(t) * f(t). Convolution, which is correctly known as an integral transform, is simply a series of multiply and add operations. There are two major applications of convolution in seismic exploration. 310
  156. 156. 4/26/2014 156 The first is its use for filtering and inverse filtering (deconvolution) of seismic data. It is also applied to spatial data, e.g., image processing. The second is the description of the seismic reflection process with “The Convolutional Model.” 311 312 Deconvolution This is a technique that can compress the source signature and eliminate multiples is applied after sorting the data into CMP gathers. Deconvolution and Convolution are different sides of the same coin.
  157. 157. 4/26/2014 157 Earth’s reflectivity series and the convolutional trace model We assume a stratigraphic (e.g. horizontally layered) earth model. The earth’s reflectivity series is then a time series of spikes, where each spike represents the plane-wave reflection coefficient for a given layer positioned at the zero-offset (e.g. coincident source and receiver) two- way travel-time (TWT) (neglecting transmission losses across each interface). 313 The seismic trace x(t) can then be described as a linear convolution between the source pulse s(t) and the Earth’s reflectivity series r(t): 𝑥 𝑡 = 𝑠 𝑡 ∗ 𝑟 𝑡 314
  158. 158. 4/26/2014 158 Pulse shaping and inverse filtering Pulse shaping (or signature processing) transforms the seismic pulse to a more compressed signal that is more optimal for further processing and interpretation. If the source pulse is given by s(t), we want to design a filter with impulse response f(t) that transforms the original pulse into another known pulse b(t). 315 We can describe the problem in terms of a linear convolutional model: 𝑓 𝑡 ∗ 𝑥 𝑡 = 𝑓 𝑡 ∗ 𝑠 𝑡 ∗ 𝑟 𝑡 = 𝑏 𝑡 ∗ 𝑟 𝑡 Assume for a moment that the filter f(t) is known. In the time-domain, the pulse shaping is carried out according to the equation Where, 𝑠 𝑡 ∗ 𝑓 𝑡 = 𝑏 𝑡 Since we assume sampled signals (and filters), we must in practice employ discrete linear convolution. 316
  159. 159. 4/26/2014 159 317 In processing, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing. In general, the object of deconvolution is to find the solution of a convolution equation of the form: 𝑠 𝑡 ∗ 𝑓 𝑡 = 𝑏 𝑡 318 Usually, b(t) is some recorded signal, and s(t) is some signal that we wish to recover, but has been convolved with some other signal f(t) before we recorded it. The function f(t) might represent the transfer function of an instrument. If we know f(t), then we can perform deterministic deconvolution. However, if we do not know f(t) in advance, then we need to estimate it. This is most often done using methods of statistical estimation
  160. 160. 4/26/2014 160 Trace muting is also applied to get rid of unwanted energy. Contributions from the direct waves and possible head waves are removed by trace muting. 319 Too mild mute function applied Examples of muting Proper choice of mute function Too strong mute function applied Due to muting, only a few traces are left at shallow travel-times in the CMP-gather 320
  161. 161. 4/26/2014 161 321 NMO correction and F-K filtering are usually applied to attenuate multiples. Linear coherent noises are also removed by employing F-K filtering. 322 A CMP-gather before the F-K filtering: the primaries dipping up and the multiples dipping down in a time- distance display. The same CMP gather after F-K filtering. The F-K filtering accepted only primary energy (within polygon) and filtered out multiples energy. The F-K domain (top, right) shows energy distributions of both primary and multiples energy, respectively.
  162. 162. 4/26/2014 162 The NMO is the difference between the travel-time for a certain offset (X) and the vertical (zero-offset) travel-time T(0). Normal move out is applied according to the following formula: 323 𝑇 𝑋 = 𝑇2 0 + 𝑋 𝑉 2 where T(X) is the two-way travel time for a seismic event, X is the actual source-receiver offset distance, V is the NMO or stacking velocity for this reflection event and T(0) is the two-way travel time for zero offset. Once the correct velocity function has been interpolated, the exact moveout at each sample is computed based on the actual source-to-receiver offset and velocity at that time sample. NMO stretch is a fundamental and long- standing problem in seismic processing. 324
  163. 163. 4/26/2014 163 After normal moveout correction, the early events are stretched at the far offset. If we stack this unmuted gather, the early events suffer a severe loss of high-frequency energy, and thus resolution. NMO corrected CDP gathers show NMO stretch. 325 This can appreciably reduce the interpretability of the seismic section. There have been many attempts to solve the NMO stretch problem. The most universal is stretch muting, where samples at the beginning of a trace that have suffered severe NMO stretch are zeroed out. 326 Stretch muting at the far offsets. Muting to remove NMO stretch may destroy far offsets information
  164. 164. 4/26/2014 164 In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip-move-out (DMO) processing becomes necessary to reduce smearing or inappropriate mixing of data. 327 328 Effect of reflector dip on the reflection point. When the reflector is flat (top) the CMP is a common reflection point. When the reflector dips (bottom) there is no CMP. A dipping reflector may require changes in survey parameters, because reflections may involve more distant sources and receivers than reflection from a flat layer
  165. 165. 4/26/2014 165 Stacking is an important step in seismic processing. Stacking represents summation of NMO-corrected traces in a CMP family. The collection of stacked traces forms a seismic section which gives an image (slice) of the subsurface. 329 Stacked seismic section. The stacking process has two major advantages: (a) it increases the signal-to-noise (S/N) ratio and (b) it amplifies primary energy relative to multiple energy. This second point depends on a good velocity analysis. 330
  166. 166. 4/26/2014 166 In the case of an accurate velocity model, stacking is the most efficient multiple removal method. A velocity model. 331 332 Seismic migration is the process by which seismic events are geometrically re-located in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface. This process is necessary to overcome the limitations of geophysical methods imposed by areas of complex geology, such as: faults, salt bodies, folding, etc.
  167. 167. 4/26/2014 167 333 In general, migration is the process that reverses wave propagation effects to get clear images of the subsurface. The term migration came about because, compared to stack sections, the echoes “migrate” to their true subsurface position. 334 Seismic waves are elastic waves that propagate through the Earth with a finite velocity, governed by the acoustic properties of the rock in which they are travelling. At an interface between two rock types, with different acoustic impedances, the seismic energy is either refracted, reflected back towards the surface or attenuated by the medium.
  168. 168. 4/26/2014 168 335 The reflected energy arrives at the surface and is recorded by geophones that are placed at a known distance away from the source of the waves. When a geophysicist views the recorded energy from the geophone, they know both the travel time and the distance between the source and the receiver, but NOT the distance down to the reflector. 336 In the simplest geological setting, with a single horizontal reflector, a constant velocity and a source and receiver at the same location, the geophysicist can determine the location of the reflection event by using the relationship: 𝑑 = 𝑣𝑡 2
  169. 169. 4/26/2014 169 337 In this case, the distance is halved because it can be assumed that it only took one-half of the total travel time to reach the reflector from the source, then the other half to return to the receiver. 338 The situation is more complex in the case of a dipping reflector, as the first reflection originates from further up the direction of dip and therefore, the travel-time plot will show a reduced dip that is defined the “migrator’s equation” : tan 𝜉 𝑎 = 𝑠𝑖𝑛𝜉 where ξa is the apparent dip and ξ is the true dip.
  170. 170. 4/26/2014 170 339 Zero-offset data is important to a geophysicist because the migration operation is much simpler, and can be represented by spherical surfaces. When data is acquired at non-zero offsets, the sphere becomes an ellipsoid and is much more complex to represent (both geometrically, as well as computationally). 340 Three vertical sections through or adjacent to a salt dome before migration (top) and after migration (bottom), showing the repositioning of several reflections near the salt face. Migration Puts Reflections in their Place!
  171. 171. 4/26/2014 171 Migration is used for several reasons; the most important one is to move reflectors from seismic “apparent” position to their geological “true” position. Another reason for doing migration is to collapse and focus diffractions. 341 Seismic migrations are of four types: Pre-stack time and Pre-stack depth migration, Post-stack time and Post-stack depth migration. Comparison of time domain images from (a) Pre-stack time migration and (b) Post-stack time migration.342
  172. 172. 4/26/2014 172 In time migration, the images are displayed in two- way travel times, and wave-field extrapolation is done in a time stepping way. Pre-stack time migrated. 343 Post-stack depth migrated. In depth migration, the wave-stepping is done with respect to depth, and the images can be represented in a true vertical depth 344
  173. 173. 4/26/2014 173 345 LECTURE 12 Seismic Resolution Seismic resolution is the ability to distinguish separate features, the minimum distance between two features, so that the two can be defined separately rather than as one. The limit of seismic resolution usually makes us wonder, how thin a bed can we see? 346
  174. 174. 4/26/2014 174 Normally, we think of resolution in the vertical sense, but there is also a limit to the horizontal width of an object that we can interpret from seismic data. 347 Horizontal Resolution The horizontal dimension of seismic resolution is described by the Fresnel zone. 348 The Fresnel zone is a frequency and range dependent area of a reflector from which, most of the energy of a reflection is returned and arrival times differ by less than half a period from the first break.
  175. 175. 4/26/2014 175 The size of the Fresnel zone helps to determine the minimum size of the feature that can be seen in a seismic section. 349 A Fresnel zone in 3D seismic is circular and has diameter A–A´ where S is the source position, Z is the depth down to the target and λ is the wavelength. Waves with such arrival times will interfere constructively and so be detected as a single arrival. Subsurface features smaller than the Fresnel zone usually cannot be detected using seismic waves. At spacing greater than one-quarter of the wavelength, the event begins to be resolvable as two separate events. 350
  176. 176. 4/26/2014 176 351 Migration can improve lateral resolution by reducing the size of the Fresnel zone. For a plane reflecting interface and coincident source and receiver, the Fresnel zone will be circular with its radius Rf expressed as: 𝑅𝑓 = 𝜆𝑍 2 where λ is the dominant wavelength and Z is the depth down to the target surface. Horizontal resolution depends on the frequency and velocity of seismic waves. If we introduce the centre frequency fc of the pulse (i.e. representing the most energetic part), we have λ ≈ V/fc, with V being the wave velocity. 352
  177. 177. 4/26/2014 177 353 Hence, we can rewrite the formula for the Fresnel zone as: 𝑅𝑓 = 𝑉𝑍 2𝑓𝑐 Remember, λ ≈ V/fc, Vertical Resolution Vertical resolution is the ability to separate two features that are close together. A seismic wave can be considered as a propagating energy pulse. If such a wave is being reflected from the top and the bottom of a bed, the result will depend on the interaction of closely spaced pulses. 354
  178. 178. 4/26/2014 178 In order for two nearby reflective interfaces to be distinguished well, they have to be about λ/4 in thickness which is called the tuning thickness. 355 This is also the thickness where interpretation criteria change. For smaller thickness, the limit of visibility is reached and positional uncertainties are introduced. The typical recorded seismic frequencies are in the range of 5–100 Hz. High frequency and short wavelengths provide better vertical and lateral resolution. One could argue that we could simply increase the power of our source so that high frequencies could travel further without being attenuated. 356
  179. 179. 4/26/2014 179 However, there is a practical limitation in generating high frequencies that can penetrate large depths. The Earth acts as a natural filter removing the higher frequencies more readily than the lower frequencies (absorption effect). 357 This means the deeper the source of reflections, the lower the frequencies we can receive from those depths and therefore the lower resolution we appear to have from great depths. 358 Filtered seismic data showing frequency content variation with depth.
  180. 180. 4/26/2014 180 Each panel has been filtered to allow a different band of frequencies. As the band-pass rises, the maximum depth of penetration of seismic energy decreases. Lower frequencies penetrate deeper. Higher frequencies do not penetrate to deeper levels. 359 The vertical resolution decreases with the distance travelled (hence depth) by the ray because attenuation preferentially robs the signal of the higher frequency components. Deconvolution can improve vertical resolution by producing a broad bandwidth with high frequencies and a relatively compressed wavelet. 360
  181. 181. 4/26/2014 181 361 As an example, if we introduce the centre frequency fc of the energy pulse (disturbance), we obtain the following simple relationship between the dominant wavelength (λ), the wave velocity (V) and the centre frequency (fc): 𝜆 ≅ 𝑉 𝑓𝑐 The typical values for the dominant wavelength are then (a) λ = 40 m at shallow depth (upper 300–500 m depth), where V = 2,000 m/s and f = 50 Hz, (b) λ = 100 m at intermediate depths (about 3,500 m), where V = 3,500 m/s and f = 35 Hz (c) and (c) λ = 250 m at depths (about 5,000 m), where V = 5,000 m/s and f = 20 Hz. 362
  182. 182. 4/26/2014 182 For thicknesses smaller than λ/4 we rely on the amplitude to judge the bed thickness. For thicknesses larger than λ/4 we can use the waveform. 363 Seismic Interpretation Seismic data are studied by geoscientists to interpret the composition, fluid content, extent and geometry of rocks in the subsurface. Interpretation of seismic data will be based on an integrated use of seismic inlines, crosslines, time slices and horizon attributes. 364
  183. 183. 4/26/2014 183 The seismic sections or images represent slices through the geological model, which can be input to advanced workstations where the actual interpretation can take place. 365 Seismic data can be used in many ways such as regional mapping, prospect mapping, reservoir delineation, seismic modelling, direct hydrocarbon detection and the monitoring of producing reservoirs. Based on the seismic interpretation, one will decide if an area is a possible prospect for hydrocarbon (oil or gas). 366
  184. 184. 4/26/2014 184 If the answer is positive, an exploration well will be drilled. The ultimate goal will be the drilling of production wells if the target area proves to be a commercial reservoir. Seismic data contain a mixture of signal and noise. 367 It is therefore crucial to understand the signature of the noise, whether it is systematic or random, dipping or flat-lying, planar or non-planar. It is also necessary to investigate the origin of the noise. The challenge of seismic interpretation is then to fully utilize all the information contained in the seismic data. 368
  185. 185. 4/26/2014 185 Systematic noise can be related to acquisition procedures, processing artefacts, water-layer multiples, faults, complex stratigraphy and shallow gas. Random noise includes natural noise (e.g. wind and wave motion), incoherent seismic interface and imperfect static corrections. 369 Without a sound understanding of these factors as well as knowledge of the limitation of seismic resolution, there is a danger of misinterpreting noise as real features. 370
  186. 186. 4/26/2014 186 371 END