Practical Migration, deMigration, and Velocity Modeling

by J. Bee Bednar

Duration: Two to three days

Intended Audience: Entry and Intermediate level

Prerequisites (Knowledge/Experience/Education Required): The course is designed to be followed by anyone with a broad geoscience background: no specific detailed foreknowledge is required, although a familiarity with geophysical terminology will be useful. The average student should be at the level of a good interpreter. Someone who regularly interprets seismic data in the effort to find hydrocarbons - this would of course include seismic data processors and first year geophysical graduates.

Summary:
The fundamental objective of this course is to provide a broad, practical and complete understanding of the crucial seismic imaging issues necessary for optimal exploration, development, and production of hydrocarbon bearing reservoirs. A secondary objective is to provide this understanding without the need for complex rigorous mathematical analysis and symbolism. A third objective is to reveal the migration/de-migration toolkit so as to empower the student to rapidly accept and appreciate future algorithmic advancements.

Course Outline:
The course is organized into five interrelated chapters. The first chapter discusses the fundamental issues associated with rocks of different ages. The focus in this chapter is on the expected Earth variation one might expect to observe. The second chapter reviews the fundamental principles of the more popular migration and de-migration algorithms currently available. Chapter three discusses the impact of various acquisition and processing schemes. Chapter four is devoted to Migration Velocity Analysis and its limitations. Chapter five attempts to provide the knowledge necessary for making economically realistic decisions with regard to acquisition and migration technology. The material in this chapter is based on case studies and when time permits limited hands on data analysis. In order the chapters are:

• The Rocks
  • Expected velocity variation
  • Isotropic, elastic, and anisotropic
• Two Migration/deMigration Styles
  • With rays and without rays
  • Migration exercise
  • Amplitude and velocity sensitivity issues
• Acquisition and Processing
  • Data acquisition exercise
  • Pre and post processing
  • Aliasing issues
• Migration Velocity Analysis
  • Elastic vs. anisotropic semblance analysis
  • Tomography
  • Full waveform inversion
  • Land velocity analysis case study
  • Marine velocity analysis case study
• Case Studies
  • An onshore Texas data set
  • A onshore Chinese data set
  • A North Sea case study

Each issue in these chapters is discussed in a practical, data-focused manner. Conclusions are supported through examples using carefully crafted synthetic or real seismic data. Geometrical descriptions and explanations are given preference to precise mathematical analyses. To the extent possible, geometric images and case studies are used in place of complex and cryptic mathematical analysis.

The pros and cons of migration algorithms are evaluated. Deficiencies are illustrated through examples and practical understanding of why limitations are present. The proper use of specific algorithms in both land and marine environments is made plain.

Acquisition geometry is shown to have a profound effect on precisely what can and cannot be achieved. This results in immediate answers to questions concerning optimization of acquisition geometries. While the answers may not be economically feasible they are the only ones that are fully consistent with mathematical assumptions.

Emerging and concurrent velocity analysis techniques are explained, compared, and quantified. The impact of geology and its inclusion in the imaging process is discussed.

The course ends with a detailed looked at the impact of focused business principles on data acquisition, processing, and imaging.

Instructor Biography:
J. Bee Bednar