A Petroleum Geologist's Guide to Seismic Reflection

This book is written for advanced earth science students, geologists, petroleum engineers and others who want to get quickly ‘up to speed’ on the interpretation of reflection seismic data. It is a development of material given to students on the MSc course in Petroleum Geology at Aberdeen University and takes the form of a course manual rather than a systematic textbook. It can be used as a self-contained course for individual study, or as the basis for a class programme. 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 computer-aided (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 2-D structural interpretation. Part II concentrates on the current seismic reflection contribution to reservoir studies, based on 3-D data.

Table of Contents:
Preface xi 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 space-dependent 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 phase-shift 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 mid-point (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 P-wave 24 4.3 Controls on P-wave velocity 24 4.4 P-wave waveforms 26 4.5 Shear waves and surface waves 26 4.6 P-wave attenuation 27 4.6.1 Spherical spreading 27 4.6.2 Frequency-dependent attenuation 27 4.7 P-wave 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 Ray-tracing in seismic modelling 29 Tutorials: 30 Tutorial 4.1 P- and S-wave 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 Water-layer multiples spoil a deep reservoir interval: program CMPGATHER 39 6 Velocity analysis, CMP stacking and post-stack 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 seismic-derived velocities 44 6.7 Multiple suppression by CMP stacking 45 6.8 Stacking the whole section: a make-or-break 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 Full-waveform 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 ray-tracing 52 7 Interpretation of two-dimensional (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 Time-depth 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 Time-depth conversion strategies 59 7.9 Examples of time-depth 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 well-log 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 Common-offset 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 Velocity-depth model based on layers 87 8.4.2 Velocity-depth model based on tomography 87 8.5 Anisotropy: the ultimate refinement in velocity 89 8.6 Velocity-depth 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 mis-location from time migration 96 Tutorial 8.3 3D data: vertical section and time-slice 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 Cross-correlation and auto-correlation 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 Frequency-domain 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 Multi-attribute 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: ant-tracking 119 10.4.5 Curvature 120 10.4.6 Applications of curvature 121 10.4.7 Structure-oriented filtering 122 10.5 Seismic forward modelling 122 10.5.1 One-dimensional modelling: the synthetic seismogram 123 10.5.2 Mis-match 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 Sparse-spike, model-based 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 Intercept-gradient cross-plots 135 11.6 Fluid factor 136 11.7 AVO inversion to rock properties lr and mr 137 11.8 AVO inversion to P- and S-wave 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

About the Author :
After gaining his geology degree, Dr Ashcroft worked with Seismograph Service Ltd on seismic parties in the Middle East and Nigeria,  then went on to do research at Birmingham University. He  joined the Department of Geology, University of Aberdeen in 1966, teaching Applied Geophysics to BSc students and to the MSc class in Petroleum Geology.  His research interests included large-scale magnetic surveys in the Scottish highlands and studies based on seismic data released from the petroleum industry.  He retired as Senior Lecturer in 1999, but has remained active, teaching geophysics at the University of Glasgow and carrying out magnetic surveys over the Rhynie Basin as well as writing this book and developing its tutorial software.

Review :
"The book also includes extensive references, numerous high-quality illustrations, and a CD-ROM." (Book News, 1 August 2011)