1887

Principles of Seismic Velocities and Time-to-Depth Conversion

image of Principles of Seismic Velocities and Time-to-Depth Conversion
  • By M. Al-Chalabi
  • Format: EPUB
  • Publication Year: 2014
  • Number of Pages: 491
  • Language: English
  • Ebook ISBN: 9789462820098

Despite its fundamental importance for acquiring an accurate picture of the subsurface, the topic of time-to-depth conversion on the basis of true propagation velocities has never been addressed in the form of a comprehensive, dedicated book. The present book has long been overdue for bridging this obvious gap in geoscience. Geophysicists proficient in data processing fully appreciate that depth imaging, despite the excellence achieved in data quality and lateral positioning, does not amount to true or accurate depth conversion. This is because modelling “velocities” in processing (described in the book as “pro-velocities”) can often be quite different from the actual propagation velocities.
The many forms of velocities, their interrelationships, their derivation, their assessment and the various factors that could affect their stability and accuracy, are all thoroughly covered. Common misconceptions are brought to notice. The main thrust is for the use of these velocities for time-todepth conversion. Extensive details of depth conversion procedures are established, together with velocity models, each to suit the various depth conversion situations arising in practice. Factors affecting depth conversion results and estimation of the associated uncertainty are extensively covered. Clarity and simplicity of presentation and practical applicability are a prime consideration. Demystifying some topics that are vague to a majority of geoscientists (e.g. anisotropy) also receives ample attention.
The book is written for the interpretation geophysicist working in exploration and development, for the seismic processor seeking a wider perspective on the quality of output and on the provision of the data to help the “frontline” geoscientist, for the geologist using geophysical methods as a tool at various levels of detail in the evaluation of the subsurface, and for the geoscientist at large.

Table of Contents

Cover
Title Page
Copyright Page
TABLE OF CONTENTS
DEDICATION
THIS BOOK
ACKNOWLEDGEMENTS
1 VELOCITY IN ROCKS – BASIC ISSUES

1.1 Velocity
1.2 Main Factors Affecting Rock Velocity
1.2.1 Rock constituents
1.2.2 Porosity
1.2.3 Secondary porosity
1.2.4 Overburden pressure
1.2.5 Fluid content
1.2.6 Tectonic history
1.2.7 Age
1.2.8 General remarks: Velocity versus rock type

2 METHODS OF DIRECT MEASUREMENT OF VELOCITY IN ROCKS
2.1 Overview
2.2 Acoustic Measurements on Cores and Rock Samples
2.3 Sonic Measurements
2.3.1 The sonic device: Principles and brief historical note
2.3.2 Later developments in the sonic device
2.3.3 The sonic log (also called acoustic log)
2.4 Check-shot
2.4.1 General comment
2.4.2 Equipment
2.4.3 Adjustment of check-shot data
2.4.4 Calibration procedure
2.5 Downhole Acoustic Imagers

3 OVERVIEW OF VELOCITIES IN SEISMIC WORK
3.1 Basic Description of Instantaneous, Average and Interval Velocities
3.1.1 Preamble
3.1.2 Simple illustrative example of true velocities
3.1.3 Representations of the velocity of propagation in the ground
3.2 Velocities in Seismic Processing
3.2.1 Processing “velocities” and true velocities
3.2.2 Traditional and modern methodologies
3.3 The Family of Physically Representative Velocities and Related Quantities 3.3.1 Instantaneous velocity
3.3.1 Instantaneous velocity
3.3.2 Average velocity
3.3.3 Root mean square (RMS) velocity
3.3.4 The heterogeneity factor
3.3.5 Interval velocity
3.3.6 The “genetic” inter-relationships between the physically meaningful velocities
3.4 Time Versus Offset Relationships
3.4.1 The time-offset equations
3.4.2 The Taner-Koehler equation
3.4.3 Normal moveout
3.5 Dipping Layers
3.5.1 The case of a single dipping layer
3.5.2 Layers with arbitrary dips

4 PRO-VELOCITIES AS A SOURCE OF TRUE VELOCITY ESTIMATES
4.1 Preliminary Remarks
4.2 Velocity Analysis – The Hyperbolic Time-Offset Case
4.2.1 Review of principles and process details
4.2.2 Measures of coherence
4.2.3 The relation between MC stacking pro-velocity and true propagation velocity
4.2.4 MC stacking pro-velocity as a biased estimator of RMS velocity
4.2.5 Methods of correcting for the bias
4.3 Forms of Velocity Analysis Displays
4.4 Picking of Velocity Analysis Coherency Build-ups
4.4.1 Manual picking
4.4.2 Automatic picking – HDHR velocity (moveout) analysis
4.5 Tomography
4.5.1 Preliminary notes
4.5.2 Types of inverted data
4.5.3 Other aspects of tomography
4.5.4 Outline of modelling and procedure
4.6 Velocity Analysis – Higher-Order Time-Offset and Pre-Stack Migration
4.6.1 General comment
4.6.2 Pro-velocities in pre-stack time migration (PreSTM)
4.6.3 Pro-velocities in pre-stack depth migration (PreSDM)
4.7 Concluding Remarks

5 INSTABILITY OF THE PRO-VELOCITY FIELD
5.1 Introduction
5.2 The Case of Hyperbolic Normal Moveout
5.2.1 Introductory remarks
5.2.2 The step model
5.2.3 The trough model
5.2.4 Anomalous feature at depth
5.2.5 Practical implications of the depth of the anomaly level
5.3 Nature of the Pro-velocity Response
5.4 Instability of the MC Pro-velocity Field in Modern Processing Methods
5.4.1 General remarks
5.4.2 Tests in Area No.1
5.4.3 Tests in Area No.2
5.5 Methods of Dealing with Instability in the MC Pro-velocity Field
5.5.1 Applying a running average filter
5.5.2 Modelling techniques
5.5.3 Tomography and full waveform inversion
5.5.4 Dynamic and static corrections in an iterative loop
5.5.5 Deconvolving the pro-velocity field
5.6 Summary, Conclusions and Supplementary Comments

6 FACTORS AFFECTING THE ESTIMATE OF VELOCITIES DERIVED FROM PRO-VELOCITIES
6.1 Overview
6.2 Acquisition Factors/Errors
6.2.1 Offset errors
6.2.2 Ship motion
6.2.3 Streamer feathering
6.2.4 Recording and onboard processing delays
6.3 Processing Factors
6.3.1 Factors arising before velocity (moveout) analysis
6.3.2 Factors arising during velocity (moveout) analysis
6.3.3 Datum Correction
6.4 Noise
6.4.1 Coherent Noise
6.4.2 Random Noise
6.5 Factors/Errors Relating to Wavelet Form
6.5.1 Offset-related changes
6.5.2 Onset time of the wavelet
 
6.6 Factors Relating to Wave Propagation
6.6.1 Multiples
6.6.2 Diffractions
6.6.3 Mode conversion
6.6.4 Anisotropy
6.7 Geological Factors
6.7.1 Preamble
6.7.2 Lithology
6.7.3 Structure
6.8 Subjective Errors
6.9 Summary and Conclusions

7 INTRODUCTION TO TIME-TO-DEPTH CONVERSION
7.1 Preliminary Remarks
7.2 Velocities Derived From Direct Measurements
7.3 Derivation of Velocity Estimates From Pro-velocity Data
7.3.1 RMS velocity as the link between velocities and pro-velocities
7.3.2 Aspects relating to factors/errors affecting pro-velocities
7.4 Pseudowells
7.4.1 Introductory remarks
7.4.2 Guidelines for the construction of pseudowells
7.4.3 Additional and concluding notes on pseudowells
7.5 Depth Imaging and Depth Conversion
7.5.1 General comment
7.5.2 Three fundamental issues in geoscience
7.5.3 Critical review of PreSDM in relation to true depth
7.6 Basic Velocity Modelling Options
7.6.1 General remarks
7.6.2 Average velocity (Chapter 8)
7.6.3 Interval velocity (Chapter 8)
7.6.4 Instantaneous velocity (Chapter 9)
7.7 Forms of Available Data
7.8 Vertical and Non-vertical Depth Conversion
7.9 Accuracy of Interval Velocity Estimate Derived from Pro-velocities
7.9.1 Quantifiable errors
7.9.2 General interval velocity accuracy

8 AVERAGE AND INTERVAL VELOCITY MODELLING
8.1 Introduction
8.2 General Guidelines for Generating Average Velocity Maps
8.3 Average Velocity Modelling on the Basis of Available Data
8.3.1 Direct measurements, [Direct]
8.3.2 Well depths and seismic section times, [Vhyb]
8.3.3 Velocities estimated from pro-velocity data [Provel]
8.3.4 [Direct] + [Vhyb]
8.3.5 [Provel] + [Direct] and/or [Vhyb]
8.4 Introductory Comments on Interval Velocity Modelling
8.5 Interval Velocity Modelling on the Basis of Available Data

9 INSTANTANEOUS VELOCITY MODELLING
9.1 Introduction
9.2 The Linear Function (used as an illustrative example)
9.2.1 Aspects of the linear function
9.2.2 The time-depth relationship
9.3 Faust and Hypertan Functions
9.3.1 The Faust relationships
9.3.2 The Hypertan function
9.4 Generation of Function Parameters
9.4.1 General remarks
9.4.2 Basic function generation procedure in the velocity-depth domain
9.4.3 Basic function generation in the time-depth domain
9.5 Parameter Nonuniqueness
9.5.1 The basis of nonuniqueness
9.5.2 Exact data
9.5.3 Real data
9.6 Instantaneous Velocity Modelling With Multi-Solution Trough Displays
9.7 The Solution Trough as a Valuable Geological and Geophysical Tool
9.7.1 Slope and intercept
9.7.2 Instantaneous velocity modelling as a versatile geological and geophysical tool
9.8 When Does Geological Change Amount to a Change?
9.9 Supplementary Modelling Procedures
9.9.1 Composite V-Z or T-Z datasets
9.9.2 Superimposed parameter space
9.10 Summary and Concluding Remarks

10 TIME-TO-DEPTH CONVERSION PROCEDURES
10.1 Introduction
10.2 Depth Conversion Formuls and Procedures
10.2.1 General comment
10.2.2 Depth conversion with average velocity
10.2.3 Depth conversion with interval velocity
10.2.4 Depth conversion with instantaneous velocity
10.3 Miscellaneous Special Methods for Velocity Modelling and Depth Conversion
10.4 Selection of an Appropriate Velocity Model – The Management of a Depth Conversion Project
10.4.1 General Remarks
10.4.2 Factors in the Choice of an Appropriate Velocity
10.4.3 Overall comment
10.5 Direct Time-to-Depth Conversion With T-Z Numerical Functions
10.5.1 Preliminary remarks
10.5.2 Numerical function generation in the time-depth (T-Z) domain
10.5.3 Numerical function generation in the velocity-depth (V-Z) domain
10.5.4 Time-to-depth conversion procedure with numerical functions
10.6 General Notes on Depth Conversion in Specific Situations

11 EVALUATION AND ADJUSTMENT OF DEPTH CONVERSION RESULTS
11.1 Introductory Remarks
11.2 Major Factors Affecting Depth Conversion Results
11.2.1 Velocity modelling efficiency
11.2.2 Well-to-seismic data correlation
11.2.3 Detail and adequacy of imaging processes
11.2.4 Overburden effects
11.2.5 Data quality
11.3 Application of Depth Residual Corrections
11.3.1 Residual corrections as a vital step for accurate depth conversion
11.3.2 Guidelines for minimising depth residuals
11.3.3 Complementary Notes
11.4 Aspects of Accuracy and Uncertainty in Depth Conversion Results
11.4.1 General remarks
11.4.2 Deterministic approach to depth uncertainty
11.4.3 Stochastic approach to depth uncertainty
11.4.4 Concluding remarks

APPENDIX A: TIME-AVERAGE AND BACKUS AVERAGE (AND INTERVAL) VELOCITIES

APPENDIX B: AVERAGE, RMS AND INTERVAL VELOCITIES AND THE HETEROGENEITY FACTOR FOR THE GENERAL LINEAR VELOCITY FUNCTION

APPENDIX C: DERIVATION OF EQUATIONS [4.13] AND [4.14]

APPENDIX D: DETAILS OF CORRECTION METHODS FOR THE RMS VELOCITY BIAS

GLOSSARY

REFERENCES

INDEX

Back Cover

References

http://instance.metastore.ingenta.com/content/books/9789462820098
Loading
/content/books/9789462820098
dcterms_title,dcterms_subject,pub_author,pub_keyword
-contentType:Journal
10
5
This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error