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3_TW6_Demultiple.ppt
- 2. DP START Houston 2007 Schlumberger Private At the end of this Session, you will be able to: • Define the terms primary energy and multiple energy as used in data processing. • Define the terms multiple order and period as used in processing. • Define the following water bottom multiple, free surface multiple, peg-leg multiple and internal multiple • Produce a diagram that shows the travel path of each of the following multiple types: water bottom multiple, free surface multiple, peg-leg multiple, internal multiple
- 3. DP START Houston 2007 Schlumberger Private Multiples are events reflected more than once Multiples hinder interpretation False events Incorrect amplitudes Multiples can be very difficult…… to identify to remove Primary and Multiple energy Primary energy is energy which has been reflected only once, and so is a true image of the reflector from which it arose
- 4. DP START Houston 2007 Schlumberger Private General Properties of Multiples • Low velocity (high moveout) – Velocity increases with depth • High amplitude – less geometric spreading • Periodic – Repeated cycles in horizontal layers • Predictable – From primaries
- 5. DP START Houston 2007 Schlumberger Private Multiple Order • This is the number of additional bounces that a seismic wave has undergone in addition to the original primary reflection. • Therefore, a second-order multiple is a signal that has undergone a primary reflection plus two additional bounces off the same layer. P 1st 2nd
- 6. DP START Houston 2007 Schlumberger Private Multiple Period • This is the time between each successive bounce of the multiple series. • Multiple periods are generally classified into three types: 1. Short 2. Intermediate 3. Long
- 7. DP START Houston 2007 Schlumberger Private Multiple Period • Short period multiples generally have a period of 100 or 200 msec. • Long period multiples generally have a period of 1/2 second or longer. • Intermediate period multiples fall directly between these two types.
- 8. DP START Houston 2007 Schlumberger Private Order of Multiples Primary 1st order multiple 2nd order multiple multiple period A B C Event recorded, dependent on receiver position: B - First order multiple C - Second order multiple A - Primary
- 9. DP START Houston 2007 Schlumberger Private Water Bottom Multiple Water-bottom multiple • Extremely common source of multiple reflection. • This is because both the water/air interface and the water bottom are characterized by having a large change in acoustic impedance. • High proportion of the seismic energy is trapped in the water layer, and little signal is transmitted into the earth.
- 10. DP START Houston 2007 Schlumberger Private Water Bottom Multiple The reflection coefficient of the water/air interface approximates to -1, so as can be seen, the polarity of the signal changes after each reflection from this interface (ie. there is a 180° phase change at each reflection).
- 11. DP START Houston 2007 Schlumberger Private Pegleg Multiple Water-bottom Pegleg multiple Pegleg Multiple • The water layer acts as a source of multiple reflections for primary reflections which arise within the earth itself. • The generated multiples are referred to as peg-leg multiples, because of their characteristic ray paths. • Their paths are asymmetric.
- 12. DP START Houston 2007 Schlumberger Private Free Surface Multiple Free-surface multiple • An impedance contrast occurs at the land/air boundary in land acquisition just as at the water/air boundary in marine acquisition. • The reflection coefficient of the air/land surface interface approximates to -1, so the polarity of the signal changes after each reflection from this interface (ie. there is a 180° phase change at each reflection).
- 13. DP START Houston 2007 Schlumberger Private Internal Multiple Inter-bed or internal multiple • The only prerequisite for multiples to occur is that there should be at least two strong reflectors. • Multiples may arise within the earth at any interface, provided that the two reflectors have "large" reflection coefficients
- 14. DP START Houston 2007 Schlumberger Private Removal of multiples The removal of multiples generally relies on either or both of two recognisable characteristics. • Multiples will generally go on and on, repeating with the same time interval and gradually decreasing in amplitude. • Multiples will generally appear on our CDP gathers with a velocity slower than the primary velocity at the same time.
- 15. DP START Houston 2007 Schlumberger Private Primary and Multiple Velocity • As the primary and multiple energy has both travelled through the same layer the multiple just spent longer in the layer, then what’s their velocity relationship? P 1st 2nd They have the same velocity
- 16. DP START Houston 2007 Schlumberger Private Velocity 1st order multiple multiple period We can recognise a multiple as having the same velocity as the primary. On semblance displays multiples will appear directly below primaries. Velocity should increase the further into the earth you go so multiples will have a slower velocity than the events surrounding them. They will appear to be under corrected on your cmp gathers. Constant Velocity Stack (CVS) 2nd order multiple primary
- 17. DP START Houston 2007 Schlumberger Private CMP gather and stack after NMO correction using primary velocity function CMP stack of nmo corrected gathers will result in enhancement of the primary energy and degradation of the multiple energy. Stack
- 18. DP START Houston 2007 Schlumberger Private Stack Easy Cheap Improves S/N Incomplete multiple suppression Output is poststack - limited AVO available
- 19. DP START Houston 2007 Schlumberger Private CMP gather and stack after NMO correction using primary velocity function Removing the near traces from our cmp gather results in further degradation of the multiples. As the multiple event appears flattest on the near traces and so stacks up best for the near traces. Near Trace Mute
- 20. DP START Houston 2007 Schlumberger Private CMP gathers after NMO correction using multiple velocity function primary multiple 1. The normal procedure is to overcorrect the primaries while leaving the multiples undercorrected. 2. Transform to FK domain. 3. Remove all events at K > 0 i.e. all multiple energy muted. primary multiple FK demultiple
- 21. DP START Houston 2007 Schlumberger Private FK demultiple x t CMP gather f k FT Mute K + - x t NMO NMO correction Multiple Primary Using velocity between primary and multiple
- 22. DP START Houston 2007 Schlumberger Private FK demultiple Easy Cheap Requires regular offsets Poor on near offsets - amplitude effects Suffers from aliasing
- 23. DP START Houston 2007 Schlumberger Private Radon Transform • A way of transforming data from the x-t domain into the Tau-p domain. • A multi-channel process which involves summing amplitudes along events in the x-t domain to transform them into the Tau-p domain • The trajectories along which the amplitudes are summed can be Linear, Parabolic or Hyperbolic. Depending on the type of transformation, we give the names: • Linear Radon Transform • Parabolic Radon Transform (PRT) • Hyperbolic Radon Transform
- 24. DP START Houston 2007 Schlumberger Private Tau-p Domain = t - px + p P = -200 -100 0 100 200 300 T X -200 -100 0 +100 +200 +300
- 25. DP START Houston 2007 Schlumberger Private Tau-p Domain A B C A’ B’ C’ = t - px + p Hyperbolae in T-X map to ellipses in - p Linear events in T-X map to single points in - p Plane wave P=0
- 26. DP START Houston 2007 Schlumberger Private • So what is Tau and what is P • Tau represents intercept time at zero offset • P represents dip. The larger the dip of an event in the x-t domain the higher its P value in the Tau-P domain. Flat events (our primaries) appear around P=0 in the Tau-P domain. P = t / x. Tau-p Domain
- 27. DP START Houston 2007 Schlumberger Private Conversion examples - Diagram (tau1, p2) P (tau2, p1) (tau3, p3) Traces (Offset) tau2 tau3 Slant = p2 Slant = p3 Slant = p1 • The zero offset time of the slant path is Tau • The slant (moveout, 1/velocity, slowness) is P • A slant whose time increases with offset is +P • A slant whose time decreases with offset is -P +p 0 -p Slant = p2 (tau3, p2) tau2 tau1 tau1 tau3 p1 p2 p3
- 28. DP START Houston 2007 Schlumberger Private Tau-p Domain Velocity filtering - events with different dips in T-X will have different ‘p’ values which can be muted/filtered. Demultiple –Parabolic Tau-p transform is more commonly used for multiple attenuations. Deconvolution – more effective due to short-period reverberations being more periodic (for any one p value) Several processes are achievable in the tau-p domain, the most common being:
- 29. DP START Houston 2007 Schlumberger Private PRT Demultiple Multiple attenuations via PRT is generally accomplished in the following way: • Input CMP gathered data with NMO applied (primary events corrected with multiples under-corrected) • Transform the data into Tau-p domain • Sum along parabolas as opposed to straight lines. • Primary and multiple energy more focused than in F-K domain. • Multiples muted prior to transform back to t-x domain or, primaries are muted and the multiples transformed back to time domain for subtraction from original data.
- 30. DP START Houston 2007 Schlumberger Private x t PRT Demultiple x t NMO PRT p Mute 0
- 31. DP START Houston 2007 Schlumberger Private PRT Parameters Let’s look a bit more closely at the parameters for prt: • MOVEOUT RANGE: Minimum and Maximum times define a range for conversion to tau-p domain specified at the reference (maximum) offset. • P TRACES: This range is split into evenly spaced parabolas which is defined by the number of P-traces specified in the setup. • MUTE: Area in the tau-p domain that is going to be muted ie area with either primaries or multiples.
- 32. DP START Houston 2007 Schlumberger Private Moveout Range “write-up”
- 33. DP START Houston 2007 Schlumberger Private Number of P traces The default number of p-traces is calculated as follows: Np = 2 (Dtmax - Dt min) f max where – Dtmax = Maximum moveout (seconds) – Dtmin = Minimum moveout (seconds) – fmax = Maximum frequency (Hz) – With reference offset set to max offset for gather
- 34. DP START Houston 2007 Schlumberger Private Moveout Range and P Traces Zero offset Reference offset • This example shows 7 p- traces • The 7 parabolas all start at the same time for zero offset • They finish at equi-distant times which span the Moveout Range
- 35. DP START Houston 2007 Schlumberger Private Mute Range Zero offset Reference offset • In this case we are muting the primaries (red area) and leaving the multiples to be subtracted from the original data
- 36. DP START Houston 2007 Schlumberger Private Data Example Multiples model is subtracted from the original gather NMO corrected data Multiple Model Primaries
- 37. DP START Houston 2007 Schlumberger Private Stacked Data Example Before demultiple After demultiple
- 38. DP START Houston 2007 Schlumberger Private PRT Demultiple Easy Reasonably AVO friendly Expensive Can be difficult to parameterise Suffers from aliasing and inversion artefacts
- 39. DP START Houston 2007 Schlumberger Private x-t Predictive Deconvolution • Apply predictive deconvolution in time to remove periodic energy Cheap Poor on far offsets where period is not constant Statistical
- 40. DP START Houston 2007 Schlumberger Private -p Predictive Deconvolution • Apply predictive deconvolution in on each p trace to remove periodic energy Period is constant for horizontal layering Cost Period changes with p Statistical
- 41. DP START Houston 2007 Schlumberger Private • Involves the prediction of water-bottom multiples by wave- field extrapolation - implemented in the FK domain • Multiple model subtracted from the input data • Does not predict other types of multiple e.g. inter-bed • Requires specification of water depths and water velocity Wave Equation Multiple Attenuation (WEMA)
- 42. DP START Houston 2007 Schlumberger Private Propagate & subtract Wave Equation Multiple Attenuation (WEMA)
- 43. DP START Houston 2007 Schlumberger Private Wave Equation Multiple Attenuation (WEMA) Only requires knowledge of water layer AVO preserving Requires simple water bottom Only removes water column multiples
- 44. DP START Houston 2007 Schlumberger Private Multiple Attenuation • Three basic classifications of attenuations methods – Advantages / disadvantages – Examples • No single method provides “a silver bullet” – We often apply several – We must avoid attenuating signal – We often have a problem with residual multiple. • Development – a better future?
- 45. DP START Houston 2007 Schlumberger Private General Classification of Multiple Suppression Methods (1) • Separability – Velocity / moveout discrimination Cheap & easy Requires interpretation Incomplete separation
- 46. DP START Houston 2007 Schlumberger Private Method 1 - Separability • Stack – Limited application • FK-demultiple – Fallen out of favor – too many artifacts • Radon-demultiple – Current “workhorse” – Dependent on spatial sampling, geometry, and parameterization – Higher resolution versions implemented – Performs better on far offsets
- 47. DP START Houston 2007 Schlumberger Private Method 1 – Separability – Example
- 48. DP START Houston 2007 Schlumberger Private Method 1 – Separability – Example
- 49. DP START Houston 2007 Schlumberger Private Method 1 – Separability – Example
- 50. DP START Houston 2007 Schlumberger Private Method 1 – Separability – Example
- 51. DP START Houston 2007 Schlumberger Private General Classification of Multiple Suppression Methods (2) • Periodicity – Predictive deconvolution Minimal interpretation required Difficult to control May assume flat geology
- 52. DP START Houston 2007 Schlumberger Private Method 2 - Periodicity • Post-stack – time domain – Limited application • Pre-stack time domain – Only for very short period multiples – Multiples are not periodic in x-t space • Tau-p deconvolution – Current “workhorse” (in shallow water) – Dependent on spatial sampling, geometry, and parameterization
- 53. DP START Houston 2007 Schlumberger Private Method 2 – Periodicity – example Stack with time domain deconvolution
- 54. DP START Houston 2007 Schlumberger Private Method 2 – Periodicity – example Stack with 1-pass tau-p deconvolution
- 55. DP START Houston 2007 Schlumberger Private Method 2 – Periodicity – example Stack with 2-pass tau-p deconvolution
- 56. DP START Houston 2007 Schlumberger Private General Classification of Multiple Suppression Methods (3) • Modelling – Prediction and subtraction of multiples based on primary information Minimal interpretation required Usually 2D formulation Expensive = +
- 57. DP START Houston 2007 Schlumberger Private Method 2 - Modeling • “2D SMP” (Surface Multiple Prediction) is WesternGeco’s implementation of a “SRME” (Surface Related Multiple Elimination) approach. – Dependent upon geometry – Dependent upon cross-line dip (water bottom) – Benefits from reduced feathering (presented at ASEG and EAGE workshops) – Usually performs better on near offsets – 2D assumptions
- 58. DP START Houston 2007 Schlumberger Private Method 3 – Modeling - STACK before 2D SMP
- 59. DP START Houston 2007 Schlumberger Private Method 3 – Modeling - STACK after 2D SMP
- 60. DP START Houston 2007 Schlumberger Private Method 3 – Modeling - CMPs before 2D SMP
- 61. DP START Houston 2007 Schlumberger Private Method 3 – Modeling - CMPs after 2D SMP
- 62. DP START Houston 2007 Schlumberger Private Method 3 – Modeling - Common Offset 2D SMP Model
- 63. DP START Houston 2007 Schlumberger Private Method 3 – Modeling - Common Offset before 2D SMP
- 64. DP START Houston 2007 Schlumberger Private Method 3 – Modeling - Common Offset after 2D SMP