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
Contents CHAPTER 1 INTRODUCTION 1.1 Definitions 1 1.2 Historical Background 2 1.3 Near-Surface and Static Corrections 4 1.4 Chapter Synopses 5 1.5 Future Trends 6 1.6 Summary 7 References 8 CHAPTER 2 NEAR-SURFACE TOPOGRAPHY AND GEOLOGY 2.1 Introduction 9 2.2 Topographic Variations 10 2.3 Weathered Layer 11 2.4 Near-Surface Irregularities 14 2.4.1 Sand Dune Topography 16 2.4.2 Highly Irregular Weathered Layer 17 2.4.3 Youthful Topography 19 2.4.4 Mature Topography 19 2.4.5 Permafrost Topography 20 2.4.6 Mountain Front Topography 21 2.5 Time-Variant Changes in Near-Surface Layers 22 2.5.1 Temperature 22 2.5.2 Precipitation 22 2.5.3 Tides 23 2.5.4 Ice Movement...23 2.5.5 Wind 23 2.5.6 Recent Erosion and Deposition 23 2.5.7 Volcanic Activity and Earthquakes 24 2.5.8 Human Activity 24 2.6 Near-Surface Velocities 24 2.6.1 Velocities in Consolidated Rocks and Unconsolidated Sediments 25 2.6.2 Velocities in Muds 26 2.6.3 Velocities in Water, Snow, and Ice 29 2.6.4 Velocities in Permafrost 31 2.6.5 Anisotropy 31 References 32 ill
iv Contents CHAPTER 3 COMPUTATION OF DATUM STATIC CORRECTIONS 3.1 Introduction 39 3.2 Reference Datum 42 3.2.1 Datum Choice 43 3.2.2 Computation of Intermediate or Floating Datum 45 3.3 Replacement Velocity 49 3.4 Spatial Control and Interpolation 49 3.5 Numerical Computation Using Formulas 53 3.5.1 Source or Receiver at Surface 53 3.5.2 Deep-Hole Data with Source or Receiver Below Weathered Layer 55 3.5.3 Deep-Hole Data with Source or Receiver Within Weathered Layer 56 3.6 Marine Datum Static Corrections 59 3.7 Shear-Wave Datum Static Corrections 64 3.8 Methods Available 67 3.8.1 Uphole Surveys 69 3.8.2 Refraction Surveys 69 3.8.3 Simple Elevation Correction Method 70 3.8.4 Sand Thickness Curve (Compaction) Method 70 3.8.5 Shallow Reflection Surveys 70 3.8.5.1 Recording Parameters 72 3.8.5.2 Sources 72 3.8.5.3 Depth Conversion and Datum Static Computation 74 3.8.6 Other Geophysical Techniques 75 3.8.6.1 Ground-Penetrating Radar (GPR) 76 3.8.6.2 Surface Wave Dispersion 78 3.8.6.3 Gravity and Magnetic Surveys 80 3.8.6.4 Electrical Methods 82 3.8.7 Data Integration and Near-Surface Model Generation 86 3.9 Miscellaneous Time-Invariant Time Shifts 88 3.10 Display of Near-Surface Information 90 3.11 Near-Surface Data Storage 93 References 94 CHAPTER 4 UPHOLE SURVEYS 4.1 Introduction 103 4.2 Data Acquisition Methods 106 4.2.1 Source in Borehole and Receivers at Surface 108 4.2.1.1 Source 108 4.2.1.2 Receivers 110 4.2.2 Source at Surface and Receivers in Borehole (Downhole Survey) 110 4.2.2.1 Source Ill 4.2.2.2 Receivers Ill 4.3 Sample Interval Ill 4.4 Interpretation 112 4.4.1 Picking and Timing Data 112 4.4.2 Arrival-Time Corrections 116 4.4.3 Time-Depth Display 117 4.4.4 Spatial Consistency 122
Contents v 4.5 Lateral Changes Predicted from Nonzero-Offset Data 125 4.5.1 Wavefront Diagrams 127 4.5.2 Tomography 129 4.6 Uphole Times from Deep Dynamite Shots 131 4.7 Shear-Wave Uphole Surveys 135 References 137 CHAPTER 5 REFRACTION SURVEYS 5.1 Introduction 141 5.2 Refraction Theory 142 5.2.1 Huygens' Principle and Refraction Across an Interface 142 5.2.2 Transmission of Refracted Waves 143 5.3 Time-Distance Curves for Layered Media 145 5.3.1 Two Layers Separated by a Horizontal Interface 145 5.3.2 Three Layers Separated by Horizontal Interfaces 146 5.3.3 Two Layers Separated by a Dipping Interface 147 5.3.4 Multilayer Case 148 5.3.5 Velocity Increase with Depth 149 5.3.6 Hidden Layers, Velocity Inversions, and Blind Zones 150 5.3.7 Two Layers Separated by a Faulted Interface 152 5.4 Data Acquisition of Refraction Data 152 5.4.1 Low-Velocity Layer (LVL) or Weathering Surveys 153 5.4.2 Refraction Data Recorded on Seismic Reflection Records 156 5.4.3 Factors Affecting Shape and Amplitude of Refraction Arrivals 157 5.5 Refraction Arrival Picking and Arrival-Time Adjustments 159 5.5.1 Manual Picking of Refraction Arrivals 160 5.5.2 Automated Methods 161 5.5.3 Second Arrivals 164 5.5.4 Additional Requirements for Vibroseis Data 166 5.5.5 Crosscorrelation Techniques 167 5.5.6 Display of Refraction Picks 170 5.5.7 Refraction Arrival-Time Adjustments 177 5.5.7.1 Surface-to-Surface Times 177 5.5.7.2 Array-Length Correction 179 5.5.7.3 Source-Offset Correction 179 5.5.7.4 Crooked-Line and 3-D Recording 180 5.5.7.5 Static Corrections for Refraction Data 180 5.6 Refraction Interpretation Techniques 182 5.6.1 Intercept-Time Method 185 5.6.2 Delay Time Concept 187 5.6.3 ABC Method (Method of Differences) 189 5.6.4 Hagedoorn (Plus-Minus) Method 191 5.6.5 Generalized Reciprocal Method (GRM) 195 5.6.6 Gardner Method and Other Delay Time Techniques 200 5.6.7 Blondeau Method 206 5.6.7.1 Single-Layer Method 208 5.6.7.2 Two-Layer Method 210 5.6.8 Wavefront Methods 211 5.6.8.1 Single-Layer Method 211 5.6.8.2 Multilayer Approach 214 5.6.8.3 Raypath Techniques 216
vi Contents 5.6.8.4 Hagedoorn Method 217 5.6.8.5 Downward-Continuation Procedure 218 5.6.9 Inversion, Time-Term, and Tomographic Methods 219 5.6.9.1 Inversion 219 5.6.9.2 Time-Term or Decomposition Method 221 5.6.9.3 Tomography 221 5.6.10 Crooked-Line Surveys 223 5.6.11 3-D Surveys 225 5.6.12 Marine Surveys 227 5.6.13 Applicability and Choice of Refraction Interpretation Methods 228 5.7 Velocity Control and Depth Conversion 231 5.7.1 Near-Surface Velocities 231 5.7.1.1 Uphole Surveys 232 5.7.1.2 Direct Arrivals 232 5.7.1.3 Shallower Refractor Information 233 5.7.1.4 Delay Time Analysis 233 5.7.1.5 Reflection Surveys 234 5.7.2 Refractor and Subweathered Layer Velocities 236 5.7.2.1 Estimates from the Refraction Interpretation Method...236 5.7.2.2 Conversion to Vertical Velocity 237 5.7.2.3 Uphole Surveys 237 5.7.2.4 Reflection Surveys 237 5.7.3 Depth Conversion and Impact of Errors 237 5.7.3.1 Refractor Depth and Near-Surface Velocity Ambiguity 238 5.7.3.2 Observational Errors and Uncertainties in Refraction Interpretation 239 5.7.3.3 Errors in Datum Static Corrections Derived from Refraction Data 241 5.8 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 255 6.2 Raypaths Through Near-Surface Layers 256 6.2.1 Limitations of Static Corrections Offset and Depth (Time) Dependence 257 6.2.2 Offset Source Locations and Crooked-Line Recording 262 6.2.3 Downward Continuation and Wave-Equation Datuming 263 6.2.4 Dynamic or Ray-Traced Static Corrections 268 6.3 Short- and Long-Wavelength Variations 275 6.4 Identification of Surface and Subsurface Features 277 6.4.1 Surface or Subsurface Anomaly 279 6.4.2 Combined Surface and Subsurface Anomalies 281 6.4.3 Time-Variant Effect 286 6.4.4 Stacking Charts 290 6.4.5 Separate-Surface or Partial-Offset Stacks 292
Contents vii 6.5 Interaction with Stacking Velocity 299 6.5.1 Reference Datum 301 6.5.2 Impact of Static Anomalies 303 6.6 Sensitivity of Seismic Data Processes to Errors in Static Corrections 310 6.6.1 Multichannel Coherent Noise and Multiple Attenuation 311 6.6.2 Dip Moveout (DMO) 312 6.6.3 Stack Response 314 6.6.4 Migration 315 6.7 Static Corrections and Multiples 317 6.8 Deep Anomalies 317 References 319 CHAPTER 7 RESIDUAL STATIC CORRECTIONS 7.1 Introduction 323 7.1.1 Data Processing Flow 326 7.1.2 Chapter Outline 327 7.2 Basic Observations and Equations 329 7.2.1 Basic Equations 329 7.2.2 Crosscorrelations 331 7.3 Common-Surface-Location Methods 334 7.3.1 Common-Surface-Location (Prestack) Method 334 7.3.1.1 Time Shift Estimation 335 7.3.1.2 Formation Cumulative (Integrated) Profile 337 7.3.1.3 Estimation Residual Static Corrections 341 7.3.2 Common-Surface-Stack Method 343 7.3.2.1 Processing Flow 345 7.3.2.2 Estimation of Residual Static Corrections 347 7.4 Common-Midpoint-Based Methods 352 7.4.1 Traveltime Equations 358 7.4.2 Decomposition of Traveltime Equations 361 7.4.2.1 Weighting Schemes 364 7.4.2.2 General Linear Inverse Methodology 365 7.4.2.3 Independent Solutions and Decoupling 366 7.4.2.4 Gauss-Seidel Method 370 7.4.2.5 Zero Mean Approach 377 7.4.3 Estimation of Observed Reflection Times or Time Shifts 379 7.4.3.1 Data Conditioning 379 7.4.3.2 Selection of Data for Analysis 384 7.4.3.3 Crosscorrelation Procedure 387 7.4.3.4 Model, Pilot, or Reference Traces 388 7.4.3.5 Summation of Crosscorrelations 392 7.4.4 Stack Power Methods 393 7.5 Common Offset-Based Methods 394 7.6 Monte-Carlo and Global-Optimization Techniques 395 7.6.1 Simulated Annealing 396 7.6.1.1 Two-Step Method 398 7.6.1.2 One-Step or Heat-Bath Method 399 7.6.1.3 Data Examples 401 7.6.2 Genetic Algorithms 405
viii Contents 7.7 3-D Surveys 407 7.7.1 Seam Statics or Decoupled Solution 408 7.7.2 Marine Surveys 413 7.7.3 Poststack Approach 414 7.8 Long-Wavelength Residual Static Corrections -.417 7.8.1 Extension of Conventional Techniques 419 7.8.2 Comparison of Partial-Offset or Separate-Surface Stacks 422 7.8.3 Analysis or Inversion of Stacking Velocities 427 7.8.3.1 Velocity-Depth Ambiguity 428 7.8.3.2 Computation of Stacking Velocity Profile 428 7.8.3.3 Manual or Interactive Inversion 429 7.8.3.4 Automated Inversion 431 7.8.4 Inversion of Prestack Reflection Times 433 7.9 Nonsurface-Consistent Residual Static Corrections 438 7.9.1 Partitioning by Time, Offset, and Source-to-Receiver Azimuth..439 7.9.2 Trim Static Corrections 441 7.10 Shear-Wave Surveys 441 7.11 Surface-Consistent Phase Corrections 445 7.12 Residual Refraction Static Corrections 449 7.13 Quality Control of Residual Static Corrections 451 7.13.1 Residual Static Correction Profile Analysis 452 7.13.1.1 Source and Receiver Similarities and Differences 454 7.13.1.2 Medium- to Long-Wavelength Corrections 455 7.13.1.3 Cycle Skips 456 7.13.2 Seismic Data Analysis 457 7.13.2.1 Stack Response and Event Continuity 459 7.13.2.2 Cycle Skips 461 7.13.2.3 Analysis to Determine Processing Sequence Changes 470 7.13.2.4 Impact of Long-Wavelength Residual Static Corrections 471 7.13.3 Section Ties 472 7.13.4 Reconciliation with Near-Surface Model 473 References 477 CHAPTER 8 INTERPRETATION ASPECTS 8.1 Introduction 483 8.1.1 Short- and Long-Wavelength Static Corrections 485 8.1.2 Identification of Remaining Surface Anomalies 486 8.2 Correlation of Horizon Times with Static Profiles 486 8.2.1 Analysis of Residual Surface Anomalies and Updating the Near-Surface Model 490 8.2.2 Data Examples 491 8.3 Sensitivity Analysis 497 8.4 Mis-Ties 500 8.4.1 Data Acquisition Factors 502 8.4.1.1 Instrumentation 502 8.4.1.2 Source 502 8.4.1.3 Receivers 502 8.4.1.4 Positioning 503 8.4.2 Static Correction Factors 503 8.4.2.1 Datum Static Corrections 503
Contents ix 8.4.2.2 Residual Static Corrections 503 8.4.3 Data Processing Factors 504 8.4.3.1 Deconvolution and Phase 504 8.4.3.2 Crooked-Line and 3-D Processing 504 8.4.3.3 Stacking Velocity 504 8.4.3.4 Normal Moveout Stretch 504 8.4.3.5 Migration 504 8.4.3.6 Filters 505 8.4.3.7 Display 505 8.5 Use of a Marker or Reference Horizon 505 References 507 INDEX 509