Ebook
A Petroleum Geologist's Guide to Seismic ReflectionISBN: 9781444397864
176 pages
March 2011, WileyBlackwell

The book clarifies those aspects of the subject that students tend to find difficult, and provides insights through practical tutorials which aim to reinforce and deepen understanding of key topics and provide the reader with a measure of feedback on progress. Some tutorials may only involve drawing simple diagrams, but many are computeraided (PC based) with graphics output to give insight into key steps in seismic data processing or into the seismic response of some common geological scenarios. Part I of the book covers basic ideas and it ends with two tutorials in 2D structural interpretation. Part II concentrates on the current seismic reflection contribution to reservoir studies, based on 3D data.
Acknowledgements xiii
Part I Basic topics and 2D interpretation.
1 Introduction and overview 3
1.1 Exploration geophysics in petroleum exploration 3
1.2 The principle of seismic reflection surveying 3
1.3 Overview of the seismic reflection industry 4
1.4 A brief history of seismic surveying 5
1.5 Societies, books and journals 5
2 Geophysical signal description 6
2.1 Overview 6
2.2 Cosine waves 6
2.3 Signals and spectra 8
2.4 Periodic waveforms: Fourier series 8
2.5 Seismic wavelets 9
2.6 Wavelet characteristics: time and frequency domains 10
2.7 Digitization of signals 12
2.8 Fourier description of spacedependent quantities 13
Tutorials: 13
Tutorial 2.1 Consolidating ideas of frequency and phase shift 13
Tutorial 2.2 Fourier summation of a periodic waveform 14
Tutorial 2.3 The transition from periodic waveform to wavelet: program FOURSYN 15
Tutorial 2.4 Amplitude and phaseshift changes in the wavelet: program FOURSYN 15
3 Data acquisition 17
3.1 General points 17
3.2 Seismic sources and receivers 17
3.3 Static corrections 18
3.4 Recording and presentation of data 18
3.5 Common midpoint (CMP) shooting 19
3.6 The attack on noise 20
3.7 3D surveys 22
4 Seismic wave propagation 24
4.1 Introduction 24
4.2 Pwave 24
4.3 Controls on Pwave velocity 24
4.4 Pwave waveforms 26
4.5 Shear waves and surface waves 26
4.6 Pwave attenuation 27
4.6.1 Spherical spreading 27
4.6.2 Frequencydependent attenuation 27
4.7 Pwave transmission paths 27
4.7.1 Wavefronts and raypaths for a single interface 28
4.7.2 Reflection coefficient and wavelet polarity 29
4.7.3 Raytracing in seismic modelling 29
Tutorials: 30
Tutorial 4.1 P and Swave particle motion on screen: program PSWAVE 30
Tutorial 4.2 Basic measurements of time, velocity and depth 30
Tutorial 4.3 Drawing the reflection wavefront from Huygens's Principle 30
Tutorial 4.4 Calculating typical reflection coefficients from well data 31
5 The process of reflection 33
5.1 Introduction 33
5.2 Fresnel zones 33
5.3 Fresnel zones and the seismic reflection 33
5.4 Faults and diffractions 34
5.5 Hyperbolae on stacked time sections 35
5.6 The reflection as a summation of hyperbolae 36
5.7 Resolution of the seismic reflection method 36
5.8 Multiple reflections: common modes 37
5.9 Multiples: the scale of the problem 38
Tutorials: 39
Tutorial 5.1 Seismic expression of a ‘point’ reflector 39
Tutorial 5.2 Waterlayer multiples spoil a deep reservoir interval: program CMPGATHER 39
6 Velocity analysis, CMP stacking and poststack migration 41
6.1 General points 41
6.2 Definitions of seismic velocity: well data 41
6.3 Velocities from seismic data: Vrms 42
6.4 Velocities from seismic data: Vstack 42
6.5 Velocity analysis 44
6.6 Errors in seismicderived velocities 44
6.7 Multiple suppression by CMP stacking 45
6.8 Stacking the whole section: a makeorbreak process 45
6.9 Some stacking refinements 45
6.9.1 NMO stretch 45
6.9.2 Weighted stacking 46
6.10 Migration: the fundamental idea 46
6.10.1 Map migration 46
6.11 Fullwaveform migration 47
6.11.1 Migration by Kirchhoff diffraction stack 47
6.11.2 Migration by wave equation 47
6.12 Migration example: 2D section 48
Tutorials: 49
Tutorial 6.1 Velocities from well data 49
Tutorial 6.2 NMO correction, CMP stacking and velocity analysis: program NMOSTAK 50
Tutorial 6.3 Picking stacking velocities from a velocity spectrum: programs VELSPEC and SEGY2D 51
Tutorial 6.4 Suppression of multiples by CMP stacking: program NMOSTAK 51
Tutorial 6.5 How multiples appear on a velocity spectrum: program VELSPEC 51
Tutorial 6.6 Migration by raytracing 52
7 Interpretation of twodimensional (2D) surveys for structure 53
7.1 Introduction 53
7.2 Linking well geology to the seismic section 53
7.2.1 Sonic log or continuous velocity log (CVL) 53
7.2.2 Timedepth plot 53
7.2.3 Making the link: synthetic seismogram 54
7.3 Choosing reflections to pick 54
7.4 Picking reflections 55
7.5 Sideswipe 55
7.6 A sideswipe example: fault diffractions 56
7.7 Preparing structure maps in TWT 57
7.8 Time to depth conversion 58
7.8.1 Velocity as an analytical function of depth 59
7.8.2 Timedepth conversion strategies 59
7.9 Examples of timedepth conversion 59
7.9.1 Southern North Sea: Rotliegend sandstone target 59
7.9.2 Central North Sea: Paleocene sands target 60
7.9.3 West Sole field, southern North Sea 60
Tutorials: 60
Tutorial 7.1 Constructing a synthetic seismogram from welllog data: program SYNTH 60
Tutorial 7.2 Matching a synthetic seismogram to seismic data: program IMAGES 62
Tutorial 7.3 Picking reflections along a 2D section from the Moray Firth, northern North Sea basin 62
Tutorial 7.4 Time to depth conversion, West Sole Field, southern North Sea Basin 70
Part II Seismic input to reservoir characterization.
8 Better images of the subsurface 81
8.1 Introduction 81
8.2 Reflection point dispersal, conflicting dips and DMO 81
8.3 Prestack time migration (PSTM) 82
8.3.1 Commonoffset sections and the Cheops pyramid 82
8.3.2 PSTM and image gathers 83
8.3.3 The limitations of PSTM: lateral variations in velocity 85
8.4 Prestack depth migration (PSDM) 85
8.4.1 Velocitydepth model based on layers 87
8.4.2 Velocitydepth model based on tomography 87
8.5 Anisotropy: the ultimate refinement in velocity 89
8.6 Velocitydepth ambiguity 90
8.7 Future migration technique: Kirchhoff or wave extrapolation? 91
8.8 3D migration 91
8.9 3D seismic interpretation 93
8.10 Growth and impact of 3D seismic surveys 94
Tutorials: 95
Tutorial 8.1 Reflection point dispersal 95
Tutorial 8.2 Lateral mislocation from time migration 96
Tutorial 8.3 3D data: vertical section and timeslice 97
9 Modifying the seismic waveform 98
9.1 Introduction 98
9.2 Testing an electronic filter: the impulse response 98
9.3 Digital filters: convolution 98
9.4 Crosscorrelation and autocorrelation 101
9.5 Frequency filtering by convolution 102
9.6 The seismogram as a convolution 103
9.7 Deconvolution 103
9.8 Designing deconvolution operators 104
9.9 Predictive deconvolution 105
9.10 Wavelet processing 105
9.11 Frequencydomain processing 105
9.12 Data processing and the fragility of bandwidth 106
Tutorials: 107
Tutorial 9.1 Digital filtering by hand 107
Tutorial 9.2 The power of the Vibroseis technique: program SIGPROC 107
Tutorial 9.3 Testing the seismic response of a geological model: program SYNTH 108
10 Refining reservoir architecture from seismic data 109
10.1 Introduction: the reservoir model 109
10.2 Refining reservoir environment: seismic stratigraphy and facies analysis 109
10.2.1 Sequences and system tracts 109
10.2.2 Picking seismic sequence boundaries 109
10.2.3 Seismic facies analysis 110
10.2.4 Sedimentary units as 3D volumes 110
10.2.5 Multiattribute facies analysis 111
10.2.6 Analysis of seismic facies by trace shape 112
10.2.7 Seismic facies in carbonates 113
10.3 Refining reservoir structure: vertical seismic profiling (VSP) 113
10.3.1 VSP processing and applications 114
10.3.2 Walkaway VSP 115
10.4 Refining reservoir structure: seismic attributes 116
10.4.1 Horizon displays of dip magnitude and azimuth 116
10.4.2 Volumetric dip magnitude and azimuth 118
10.4.3 Coherence 118
10.4.4 Automatic fault extraction: anttracking 119
10.4.5 Curvature 120
10.4.6 Applications of curvature 121
10.4.7 Structureoriented filtering 122
10.5 Seismic forward modelling 122
10.5.1 Onedimensional modelling: the synthetic seismogram 123
10.5.2 Mismatch between synthetic seismogram and section 124
10.5.3 Forward modelling in two and three dimensions 124
Tutorials: 126
Tutorial 10.1 Section limits in walkaway VSP 126
Tutorial 10.2 Forward modelling of fault shadow: program SYNTHSEC 126
11 Seismic input to mapping reservoir properties 127
11.1 Introduction 127
11.2 Reflection amplitude 127
11.3 Acoustic impedance (AI) inversion 128
11.3.1 AI inversion by recursion and trace integration 128
11.3.2 The good and the bad of AI inversion 129
11.3.3 Sparsespike, modelbased and coloured inversion 130
11.4 Amplitude variation with offset (AVO) 130
11.4.1 AVO and poisson’s ratio 132
11.4.2 AVO methodology 132
11.4.3 Angle stacks 133
11.5 AVO intercept and gradient 134
11.5.1 Interceptgradient crossplots 135
11.6 Fluid factor 136
11.7 AVO inversion to rock properties lr and mr 137
11.8 AVO inversion to P and Swave impedance 138
11.9 Elastic impedance: AVO made easy? 139
11.10 Best fluid indicator? 140
11.11 Instantaneous seismic attributes 140
11.12 Usage of seismic attributes 141
11.13 Predicting log properties from seismic attributes 142
11.14 4C and 4D surveys 143
Tutorials: 144
Tutorial 11.1 AVO for typical lithological interfaces 144
Tutorial answers 146
References 147
Index 153