Static Corrections for Seismic Reflection Surveys

Transcription

1 Static Corrections for Seismic Reflection Surveys MIKE COX Volume Editors: Series Editor: Eugene F. Scherrer Roland Chen Eugene F. Scherrer Society of Exploration Geophysicists Tulsa, Oklahoma

2 Contents CHAPTER 1 INTRODUCTION 1.1 Definitions Historical Background Near-Surface and Static Corrections Chapter Synopses Future Trends Summary 7 References 8 CHAPTER 2 NEAR-SURFACE TOPOGRAPHY AND GEOLOGY 2.1 Introduction Topographic Variations Weathered Layer Near-Surface Irregularities Sand Dune Topography Highly Irregular Weathered Layer Youthful Topography Mature Topography Permafrost Topography Mountain Front Topography Time-Variant Changes in Near-Surface Layers Temperature Precipitation Tides Ice Movement Wind Recent Erosion and Deposition Volcanic Activity and Earthquakes Human Activity Near-Surface Velocities Velocities in Consolidated Rocks and Unconsolidated Sediments Velocities in Muds Velocities in Water, Snow, and Ice Velocities in Permafrost Anisotropy 31 References 32 ill

3 iv Contents CHAPTER 3 COMPUTATION OF DATUM STATIC CORRECTIONS 3.1 Introduction Reference Datum Datum Choice Computation of Intermediate or Floating Datum Replacement Velocity Spatial Control and Interpolation Numerical Computation Using Formulas Source or Receiver at Surface Deep-Hole Data with Source or Receiver Below Weathered Layer Deep-Hole Data with Source or Receiver Within Weathered Layer Marine Datum Static Corrections Shear-Wave Datum Static Corrections Methods Available Uphole Surveys Refraction Surveys Simple Elevation Correction Method Sand Thickness Curve (Compaction) Method Shallow Reflection Surveys Recording Parameters Sources Depth Conversion and Datum Static Computation Other Geophysical Techniques Ground-Penetrating Radar (GPR) Surface Wave Dispersion Gravity and Magnetic Surveys Electrical Methods Data Integration and Near-Surface Model Generation Miscellaneous Time-Invariant Time Shifts Display of Near-Surface Information Near-Surface Data Storage 93 References 94 CHAPTER 4 UPHOLE SURVEYS 4.1 Introduction Data Acquisition Methods Source in Borehole and Receivers at Surface Source Receivers Source at Surface and Receivers in Borehole (Downhole Survey) Source Ill Receivers Ill 4.3 Sample Interval Ill 4.4 Interpretation Picking and Timing Data Arrival-Time Corrections Time-Depth Display Spatial Consistency 122

4 Contents v 4.5 Lateral Changes Predicted from Nonzero-Offset Data Wavefront Diagrams Tomography Uphole Times from Deep Dynamite Shots Shear-Wave Uphole Surveys 135 References 137 CHAPTER 5 REFRACTION SURVEYS 5.1 Introduction Refraction Theory Huygens' Principle and Refraction Across an Interface Transmission of Refracted Waves Time-Distance Curves for Layered Media Two Layers Separated by a Horizontal Interface Three Layers Separated by Horizontal Interfaces Two Layers Separated by a Dipping Interface Multilayer Case Velocity Increase with Depth Hidden Layers, Velocity Inversions, and Blind Zones Two Layers Separated by a Faulted Interface Data Acquisition of Refraction Data Low-Velocity Layer (LVL) or Weathering Surveys Refraction Data Recorded on Seismic Reflection Records Factors Affecting Shape and Amplitude of Refraction Arrivals Refraction Arrival Picking and Arrival-Time Adjustments Manual Picking of Refraction Arrivals Automated Methods Second Arrivals Additional Requirements for Vibroseis Data Crosscorrelation Techniques Display of Refraction Picks Refraction Arrival-Time Adjustments Surface-to-Surface Times Array-Length Correction Source-Offset Correction Crooked-Line and 3-D Recording Static Corrections for Refraction Data Refraction Interpretation Techniques Intercept-Time Method Delay Time Concept ABC Method (Method of Differences) Hagedoorn (Plus-Minus) Method Generalized Reciprocal Method (GRM) Gardner Method and Other Delay Time Techniques Blondeau Method Single-Layer Method Two-Layer Method Wavefront Methods Single-Layer Method Multilayer Approach Raypath Techniques 216

5 vi Contents Hagedoorn Method Downward-Continuation Procedure Inversion, Time-Term, and Tomographic Methods Inversion Time-Term or Decomposition Method Tomography Crooked-Line Surveys D Surveys Marine Surveys Applicability and Choice of Refraction Interpretation Methods Velocity Control and Depth Conversion Near-Surface Velocities Uphole Surveys Direct Arrivals Shallower Refractor Information Delay Time Analysis Reflection Surveys Refractor and Subweathered Layer Velocities Estimates from the Refraction Interpretation Method Conversion to Vertical Velocity Uphole Surveys Reflection Surveys Depth Conversion and Impact of Errors Refractor Depth and Near-Surface Velocity Ambiguity Observational Errors and Uncertainties in Refraction Interpretation Errors in Datum Static Corrections Derived from Refraction Data Shear-Wave Surveys 244 References 244 Chapter 5 Appendix: Glossary of Refraction Interpretation Methods 252 CHAPTER 6 STATIC CORRECTIONS LIMITATIONS AND EFFECT ON SEISMIC DATA PROCESSES 6.1 Introduction Raypaths Through Near-Surface Layers Limitations of Static Corrections Offset and Depth (Time) Dependence Offset Source Locations and Crooked-Line Recording Downward Continuation and Wave-Equation Datuming Dynamic or Ray-Traced Static Corrections Short- and Long-Wavelength Variations Identification of Surface and Subsurface Features Surface or Subsurface Anomaly Combined Surface and Subsurface Anomalies Time-Variant Effect Stacking Charts Separate-Surface or Partial-Offset Stacks 292

6 Contents vii 6.5 Interaction with Stacking Velocity Reference Datum Impact of Static Anomalies Sensitivity of Seismic Data Processes to Errors in Static Corrections Multichannel Coherent Noise and Multiple Attenuation Dip Moveout (DMO) Stack Response Migration Static Corrections and Multiples Deep Anomalies 317 References 319 CHAPTER 7 RESIDUAL STATIC CORRECTIONS 7.1 Introduction Data Processing Flow Chapter Outline Basic Observations and Equations Basic Equations Crosscorrelations Common-Surface-Location Methods Common-Surface-Location (Prestack) Method Time Shift Estimation Formation Cumulative (Integrated) Profile Estimation Residual Static Corrections Common-Surface-Stack Method Processing Flow Estimation of Residual Static Corrections Common-Midpoint-Based Methods Traveltime Equations Decomposition of Traveltime Equations Weighting Schemes General Linear Inverse Methodology Independent Solutions and Decoupling Gauss-Seidel Method Zero Mean Approach Estimation of Observed Reflection Times or Time Shifts Data Conditioning Selection of Data for Analysis Crosscorrelation Procedure Model, Pilot, or Reference Traces Summation of Crosscorrelations Stack Power Methods Common Offset-Based Methods Monte-Carlo and Global-Optimization Techniques Simulated Annealing Two-Step Method One-Step or Heat-Bath Method Data Examples Genetic Algorithms 405

7 viii Contents D Surveys Seam Statics or Decoupled Solution Marine Surveys Poststack Approach Long-Wavelength Residual Static Corrections Extension of Conventional Techniques Comparison of Partial-Offset or Separate-Surface Stacks Analysis or Inversion of Stacking Velocities Velocity-Depth Ambiguity Computation of Stacking Velocity Profile Manual or Interactive Inversion Automated Inversion Inversion of Prestack Reflection Times Nonsurface-Consistent Residual Static Corrections Partitioning by Time, Offset, and Source-to-Receiver Azimuth Trim Static Corrections Shear-Wave Surveys Surface-Consistent Phase Corrections Residual Refraction Static Corrections Quality Control of Residual Static Corrections Residual Static Correction Profile Analysis Source and Receiver Similarities and Differences Medium- to Long-Wavelength Corrections Cycle Skips Seismic Data Analysis Stack Response and Event Continuity Cycle Skips Analysis to Determine Processing Sequence Changes Impact of Long-Wavelength Residual Static Corrections Section Ties Reconciliation with Near-Surface Model 473 References 477 CHAPTER 8 INTERPRETATION ASPECTS 8.1 Introduction Short- and Long-Wavelength Static Corrections Identification of Remaining Surface Anomalies Correlation of Horizon Times with Static Profiles Analysis of Residual Surface Anomalies and Updating the Near-Surface Model Data Examples Sensitivity Analysis Mis-Ties Data Acquisition Factors Instrumentation Source Receivers Positioning Static Correction Factors Datum Static Corrections 503

8 Contents ix Residual Static Corrections Data Processing Factors Deconvolution and Phase Crooked-Line and 3-D Processing Stacking Velocity Normal Moveout Stretch Migration Filters Display Use of a Marker or Reference Horizon 505 References 507 INDEX 509