Introduction to seismic interpretation
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This document provides an introduction to seismic interpretation. It begins with an overview of seismic acquisition methods both onshore and offshore. It then discusses key concepts in seismic data such as common depth points, floating datum, two-way time, and the relationship between time and depth. The document also covers seismic resolution, reflection coefficients, and examples of calculating tuning thickness. Finally, it discusses important steps for seismic interpretation including checking the line scale and orientation and interpreting major reflectors and geometries.
![Seismic acquisition
• Example 1:
V = 7,000 m/s
F = 50 Hz
l= V/F
= 7,000/50
[(m/s)/(cycles/s)]
= …….m
• Example 2:
V = 3,000 m/s
F = 50 Hz
l= V/F
= 3,000/50 [(m/s)/(cycles/s)]
= ……….m](https://image.slidesharecdn.com/introductiontoseismicinterpretation-171126161353/85/Introduction-to-seismic-interpretation-20-320.jpg)
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Introduction to seismic interpretation
- 1. Introduction to Seismic interpretation By: Amir I. Abdelaziz Assistant Lecturer , Geology Department, Faculty of Science Helwan University, Egypt. 2017
- 7. Seismic method Use acoustic waves (sound) to image the subsurface • Measure • time for sound to get from surface to subsurface reflectors and back - Two-way travel time (twt) • Amplitude of reflection • Wanted: • Depth - Need to know subsurface velocities • Rock properties (porosity, saturation, etc.)
- 10. Seismic acquisition • Some energy will be reflected, some will be transmitted where there is a change in AI • Amount reflected (amplitude of reflection) will depend on the relative difference in physical properties across the interface V11 V11 V22 V - V V + V 2 2 2 2 11 11 – Define reflection coefficient (RC) RC = AI2 – AI1 AI2 + AI1 – If AI2 > AI1 – positive RC – If AI2 < AI1 – negative RC
- 14. Seismic resolution • Convolutional theorem just described has interesting implications for vertical resolution – Each interface produces a distinct reflection. – If reflections are widely enough spaced, each will be recognizable. – Once reflections start to get closer, they start to interfere with each other. – At some point adjacent reflections could be so close that they completely cancel each other out.
- 20. Seismic acquisition • Example 1: V = 7,000 m/s F = 50 Hz l= V/F = 7,000/50 [(m/s)/(cycles/s)] = …….m • Example 2: V = 3,000 m/s F = 50 Hz l= V/F = 3,000/50 [(m/s)/(cycles/s)] = ……….m
- 22. Seismic acquisition From the following Tuning Curve: The following seismic section / Tuning curve showing zone between TWT (0 to 200 ms) with dominant frequency = 45 HZ. If the velocity of this interval is 5000m/sec. -Compute the tuning thickness. - Can you see/pick this interval in the seismic section if its thickness is 15 meter? Why?
- 23. Understanding the Data • Common Depth Points (CDPs) • Floating datum • Two way time (TWT) • Time versus depth
- 24. Common Depth Points (CDPs) CDPs are defined as ‘the common reflecting point at depth on a reflector or the halfway point when a wave travels from a source to a reflector to a receiver’.
- 25. Floating datum The floating datum line represents travel time between the recording surface and the zero line (generally sea level). This travel time depends on rock type, how weathered the rock is, and other factors. The topographic elevation is the height above sea level of the surface along which the seismic data were acquired.
- 26. Two Way Time (TWT)
- 27. Time versus depth • Two way time (TWT) does not equate directly to depth. • Depth of a specific reflector can be determined using boreholes. • For example, 926 m depth = 0.58 sec. TWT
- 31. Seismic Processing Energy within a seismic trace consists of – Signal: that carries desired information Noise: that contains unwanted information
- 33. Random Noise • energy which does not exhibit correlation from trace to trace • not generally source generated (the noise would be present whether we were shooting or not) Examples: Swell Noise Instrument Noise Shark bites Powerline Noise Traffic (vehicles, people, animals) Wind Falling Debris Earthquakes
- 34. Coherent Noise • predictable from trace to trace across a group of traces i.e. have a phase relationship between adjacent traces • often source generated ( if we weren’t shooting the noise would not occur) Examples: Multiples Direct arrivals Ground roll Refractions Airwave Cable Jerk Autofire Ship’s propeller noise
- 35. Noise
- 36. Sources of noise in marine surface seismic
- 38. Basic steps in Seismic Processing
- 39. Basic steps in Seismic Processing
- 40. Basic steps in Seismic Processing
- 41. Basic steps in Seismic Processing
- 42. Basic steps in Seismic Processing
- 43. Basic steps in Seismic Processing
- 44. FK filter • FK filter: Is a dip or velocity filter. Where filtering is performed on data after it has undergone a 2D fourier transform into the FK domain. •Produce a diagram that shows how each of the following events in the XT domain are represented on an FK spectrum: shallow dip, steep dip, flat event. •Define each of the following terms : pass zone, reject zone, taper zone. The Fourier Transform Time domain Frequency Domain
- 46. Know Your Events Frequency ( Hz ) Wavenumber, K = ---------------------------- X Dip ( ms / tr ) Trace Interval ( m ) We can now separate events on the basis of their dips and/or frequencies
- 47. FK-PLOT of a shot record with cable jerk noise
- 48. FK-PLOT of a shot record with no cable jerk noise
- 49. XT-PLOT of a shot record with cable jerk noise
- 50. FK-PLOT of a shot record with cable jerk noise
- 51. Basic steps in Seismic Processing
- 52. Basic steps in Seismic Processing Deconvolutional Model
- 53. Statistical Deconvolution We can now use Statistical deconvolution techniques to remove the system wavelet, by compressing it to a spike, and hence derive the reflectivity.
- 54. Basic steps in Seismic Processing
- 56. Basic steps in Seismic Processing
- 57. Basic steps in Seismic Processing
- 58. Basic steps in Seismic Processing
- 59. Sea Bottom Multiple Basic steps in Seismic Processing
- 60. Basic steps in Seismic Processing
- 61. Basic steps in Seismic Processing
- 62. Basic steps in Seismic Processing
- 63. Basic steps in Seismic Processing
- 64. Basic steps in Seismic Processing
- 65. Basic steps in Seismic Processing
- 66. Basic steps in Seismic Processing
- 67. Basic steps in Seismic Processing
- 68. Basic steps in Seismic Processing
- 69. Basic steps in Seismic Processing
- 70. Positioning Problems Energy Source The seismic ray hits an inclined surface at 90º and reflects back 0.4 s - The reflection is displayed beneath the source-receiver midpoint Bounce Point Migration
- 71. Time for an Exercise 1 2 3 4 65 Where would the reflection lie? 90º Migration
- 72. Time for an Exercise 1 2 3 4 65 Where would the reflection lie? Compass Migration
- 73. Time for an Exercise 1 2 3 4 65 Where would the reflection lie? Migration
- 74. Exercise Answer 1 2 3 4 65 The reflection is downdip and its dip is less than the interface Migration
- 75. Migration
- 76. Migration
- 77. Migration
- 78. Migration
- 79. Migration
- 80. Migration
- 81. Migration
- 82. Diffraction
- 83. Diffraction
- 84. Diffraction
- 85. Pre Stack migration (2D marine) Field data in Mute NMO correction Migration Stack Amplitude scaling Filtering Final products Energy spreading corrections Amplitude recovery Low-cut / velocity filtering Noise rejection Wavelet compression / demultiple Deconvolution Velocity Analysis Near/far offset noise removal Summing traces within a cmp. Lowcut / highcut / signal enhance Time variant amplitude balancing Cosmetic plotting test:bias/gain Final velocity field Initial velocity field Migration Data re-ordering Possible additional velocity Analysis CMP gather Remove source wavelet from data Signature decon. Define positions and relationships Geometry DemultipleRemoving multiples Final velocity field SHOT SORTED CMP SORTED OFFSET SORTED STACK SORTED
- 86. Processing Sequence (3D marine) Field data QC & edit Signature Decon. Amplitude recovery Noise rejection Deconvolution NMO correction Apply automatic field edits QC navigation data and select binning strategy 2:1 data reduction Temporal resample Initial velocity field DMO on velocity lines Final velocity field Nav./seismic merge Mute 3D DMO & Stack Migration Migration vel. field Demultiple Time variant filtering Time variant scaling final products field data QC shot data As early as possible! QC stack displaysPre Stack Migration
- 87. Processing Sequence (2D land) Basic Processes Field data in Geometry Amplitude recovery Noise rejection Deconvolution CMP gather Mute NMO correction 2D DMO & Stack Migration Amplitude scaling Filtering Final products Residual statics Field statics loop Initial velocity field 2D DMO Final velocity field
- 88. Virtually all processing projects will require the following: • Accurate locating of shots and receivers and their relationship • Time-correction to a known datum • Amplitude recovery • Noise attenuation • Signal enhancement • Cmp gathering • Demultiple • Normal move-out correction (including muting any noise introduced) • Residual time-corrections (land data) • Correction for smearing within a cmp - dip-moveout correction • Data reduction - stacking • Repositioning / imaging - Migration
- 89. Seismic interpretation • Check line scale and orientation. • Work from the top of the section, where clarity is usually best, towards the bottom. • Distinguish the major reflectors and geometries of seismic sequences.
- 91. Understanding the data (1) CDPs are typically marked at intervals along the top of seismic lines and they are regularly spaced to form a horizontal scale. Here, 80 CDPs represent about 1 kilometre (km).
- 92. Understanding the data (2) Signals from farther away will provide information for deeper horizons Gaps in land seismic data are due to omissions where data could not be acquired For example, it is not always possible to transmit the signal above pipes, in sensitive areas and above buildings
- 93. Understanding the data (3) • Two way time (TWT) is recorded on the vertical axis of the seismic line in fractions of a second. Sometimes it is more convenient to express time as milliseconds. • TWT is the time required for the seismic wave to travel from the source to some point below the surface and back up to the receiver.
- 94. Well to Seismic tie
- 95. Objectives of Well-Seismic Ties • Well-seismic ties allow well data, measured in units of depth, to be compared to seismic data, measured in units of time. • This allows us to relate horizon tops identified in a well with specific reflections on the seismic section. • We use sonic and density well logs to generate a synthetic seismic trace. • The synthetic trace is compared to the real seismic data collected near the well location.
- 96. Measurements In Time and In Depth
- 97. Check Shot Data • Check shots: measure the vertical one-way time from surface to various depths (geophone positions) within the well. – Used to determine start time of top of well- log curves – Used to calibrate the relationship between well depths and times calculated from a sonic log
- 98. Pulses Types • Two options for defining the pulse: A. Use software that estimates the pulse based on a ‘window’ of the real seismic data at the well (recommended) B. Use a standard pulse shape specifying polarity, peak frequency and phase: • Minimum phase • Zero phase • Quadrature
- 99. We ‘block’ the velocity (sonic) and density logs and compute an impedance ‘log’ Velocity Density Impedance =x Shale Sand Shale Sand Shale Lithology Reflection Coefficients • We calculate the reflection coefficients at the step-changes in impedance * Wavelet • We convolve our pulse with the RC series to get individual wavelets • Each RC generates a wavelet whose amplitude is proportional to the RC Synthetic • We sum the individual wavelets to get the synthetic seismic trace The Modeling Process
- 104. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Impedance
- 105. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Impedance
- 106. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Impedance
- 107. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Impedance
- 108. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Impedance
- 109. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Impedance
- 110. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Recorded Trace Impedance
- 111. Forward Process Earth Reflection Coefficients Wavelet Wavelet Superposition Recorded Trace Seismic Section Impedance
- 112. Our Example
- 113. Tying Synthetic to Seismic Data Tying Synthetic to Seismic Data
- 114. Tying Synthetic to Seismic Data
- 115. 2D Seismic data
- 116. 2D Seismic data X1 X2 Y1 Y2 2 D Seismic acquisition
- 117. 3 D Seismic acquisition
- 118. Bad data Good data Very Good data 3 D Seismic acquisition
- 122. Top down approach - Picking intersted Horizon • Start at the top of the section, where definition is usually best • Work down the section toward the zone where the signal to noise ratio is reduced and the reflector definition is less clear
- 124. Interpret Faults Inline 500 Inline 400 Inline 300 Inline 200 Inline 100
- 125. Interpret Horizon Inline 500 Inline 400 Inline 300 Inline 200 Inline 100
- 126. Create and Associate Fault Polygon Set
- 127. Interpret Horizon
- 128. Two Way Time Map: (TWT)
- 129. velocity from check-shot survey • The velocity required for the map is Average velocity • In case of determining velocity from check-shot survey, the result velocity will multiplying by 2 (to convert it to one way time).
- 130. Depth contour map • We extract the depth map values from the velocity & one way time map.
- 131. Depth Conversion-Simple Average Velocity • Convert depth to depth below seismic reference datum, and convert TWT to a one-way time.
- 132. Depth Conversion-Simple Average Velocity
- 133. Types of Petroleum Traps ❖ Several geologic structures may act as petroleum traps, but all have two basic conditions in common: 1) Porous, permeable reservoir rock that will contain quantities of oil and gas that make it worth drilling. 2) Impermeable cap rock that traps oil and gas preventing it from escaping to the surface. ❖ Types of Petroleum traps include: 1) Anticline Trap 3) Fault Trap 2) Salt Dome Trap 4) Stratigraphic Trap
- 134. Types of Petroleum Traps 1) Anticline Trap: • If a permeable rock like sandstone or limestone is located between impermeable rock layers like shale and the rocks are folded into an anticline, oil and gas can move upward in the permeable reservoir rocks, and accumulate in the upper region of the anticline.
- 135. How to find oil: Source rock, reservoir rock, traps
- 136. Types of Petroleum Traps
- 137. Types of Petroleum Traps 2) Fault Trap : ➢ If faulting can shift permeable and impermeable rocks so that the permeable rocks always have impermeable rocks above them, then an oil trap can form. ➢ Note that both normal faults and reverse faults can form this type of oil trap.
- 138. Types of Petroleum Traps
- 139. Types of Petroleum Traps 3) Salt Dome Trap : ➢ Here we see salt that has moved up through the Earth, punching through and bending rock along the way. ➢ Oil can come to rest right up against the impermeable salt, which makes salt an effective trap rock.
- 140. Types of Petroleum Traps
- 141. Types of Petroleum Traps
- 142. Types of Petroleum Traps 4) Limestone Reef Trap : ➢ Limestone reef trap is a type of stratigraphic trap. ➢ When coral reefs become buried by other impermeable sediments they can form excellent oil sources and reservoirs.
- 143. Example 1 Which petroleum trap would be formed by a simple fold? (A) anticline (B) fault (C) salt dome (D) stratigraphic The limestone reef trap belongs to which type of petroleum trap? (A) anticline (B) fault (C) salt dome (D) stratigraphic
- 144. Example 2 Based on the diagram below, which is the correct match between the force and petroleum trap produced?
- 145. Gas Chimney
- 146. Gas Chimney
- 147. Seismic acquisition