# Migration and Velocity Model Building

 Instructor Mr Piet Gerritsma (Gerritsma Geophysical Training and Consultancy, Netherlands) Duration 2 days Disciplines Geophysics – Surface Imaging Level Advanced Language English EurGeol 10 CPD points Keywords IMAGING   TIME MIGRATION   DEPTH MIGRATION   INVERSION   TOMOGRAPHY   TRAVELTIME   VELOCITIES   WAVE EQUATION

### Course description

The process of migration, whereby a proper image in time or depth of the subsurface is obtained, is directly related with the velocity model that both serves as input for the migration process as well as is the result of such a migration. Therefore migration and velocity model building are intimately related processes. The implementation of migration is characterized by a multitude of methods and algorithms; there is also a great variety of methods to build a velocity model. This course provides an overview of the migration principles, methods and algorithms and an overview of velocity model building principles and methods and algorithms. Examples and case studies will conclude this course.

### Course objectives

At the end of the course the participants will have obtained a complete overview and thorough understanding of the many alternative methods and algorithms that are currently in use in imaging and velocity model building.
The course emphasizes for each method the underlying geophysical model together with its assumptions and strengths and weaknesses; many examples will be shown to illustrate the material; theory with references will be included; a handout that covers all course material will be made available.

### Course outline

The following steps in Migration, DMO and Velocity Model Building will be discussed:

1. Migration or imaging

• Migration, modelling and inversion
• Geometric approach to migration
• Examples
• Resolution before and after migration
• Aliasing
• Ray definitions
• The Dix equations
• Definition of time migration and depth migration
• The acoustic wave equation
• Factorization of the wave equation
• Forward and inverse wavefield extrapolation in depth
• Migration principles; the imaging conditions
• Migration of various data sets:
• Shot profile migration
• Survey sinking or redatuming
• Zero-offset data migration
• Extended imaging conditions (time-shift and/or space-shift)
• Migration algorithms:
• (k,f)-migration (Stolt)
• Phase-shift migration (Gazdag)
• Phase-shift-plus-interpolation (PSPI) migration
• Split-step-Fourier (SSF) migration
• Extended split-step Fourier (ESSF) migration
• The Kirchhoff integral, the Rayleigh integral and Green’s functions
• Kirchhoff (= summation or diffraction stack) migration
• Migration by double focused array synthesis
• Gaussian beam migration
• Reverse time migration — RTM
• Migration and demigration

2. Velocity model building

• Minimal data sets and common image gathers — CIG’s
• Iterative velocity model building with CIG’s
• The migration conditions
• Migration and traveltime inversion
• Migration and demigration
• Normal incidence wavefront curvature and stacking velocity
• Velocity model parameterization
• Velocity model building methods:
• coherency inversion or model based stack
• map migration
• dynamic map migration (DMM) or curvature inversion
• stereotomography
• traveltime inversion (TTI)
• traveltime inversion in the migrated domain (TIMD)
• common focus panel (CFP) analysis
• tomographic velocity model building
• depth focusing analysis (DFA)
• WEMVA: wave-equation migration velocity analysis
• differential semblance optimization (DSO)
• full waveform inversion (FWI)

4. Case studies — Examples

• Tomography
• Full Wave Inversion
• Velocity Model Building
• Parametric Velocity Estimation

### Participants' profile

ocessing and interpretation ` geologists and petrophysicists who wish to understand how the various types of velocity information can be derived from seismic data and who wish to understand how subsurface images are generated.

As the material covers the theory and practical implementations of present day practices, this course is relevant for those who are fresh from university as well as for those who wish to be updated on the newest developments. Participants should have a basic understanding of seismic acquisition and processing practices.

### Prerequisites

Course participants should have a basic understanding of seismic acquisition and processing practices.

Piet Gerritsma (1942) graduated in physics at the University of Groningen. He joined Shell in 1969 as a research geophysicist in Rijswijk (The Netherlands) and Houston (USA). He was actively involved in the development of programs for statics, velocity analysis, synthetic seismograms and raytracing, deconvolution, multi-component seismic, shear waves and anisotropy, AVO and migration. He acquired operational experience as processing and special studies geophysicist in Brunei and in Canada. He was Shell"s representative in international research consortia: SEP (Stanford), DELPHI (Delft University of Technology) and IFP (Institut Francais du Petrole); he also served as associate editor of Geophysical Prospecting on Migration, Modelling and Inversion. During his Shell career he has always lectured at both basic as well as advanced level covering a broad range of topics. He left Shell in 1999 after 30 years of service. Since that time he is a lecturer at CTG (Center for Technical Geoscience) at the Delft University of Technology. He also teaches regularly courses for national and international oil companies and service companies, both as an independent teacher as well as on behalf of geoscience training alliances. He has presented this course several times on behalf of the EAGE and CSEG.

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