World-class training for the modern energy industry

Understanding Seismic Data: Time, Depth and Geology (G082)

  • TypeType: Classroom, Virtual
  • TypeDiscipline: Geophysics
  • TypeDuration: 4 days

Tutor(s)

David Kessler: President of SeismicCity

Ron Kerr: Independent seismic processing QC consultant

John Byrd: President and Principal Geoscientist, ByrdGEO

Overview

The objective of the course is to deliver a broad understanding of seismic data processing and imaging to interpreters and geologists working with seismic data. The course is based on various case studies that are connected to geological fundamentals. Upon completion, course participants will be able to successfully understand depth migrated seismic data, know how to differentiate signal from artifacts, understand industrial methods and workflows, and connect seismic data to geological settings for prospect evaluation and generation.

Duration and Logistics

A 4-day in-person classroom course, consisting of lectures and exercises. A digital manual will be provided for the course.

Level and Audience

Intermediate. The course is intended for seismic interpreters and geologists involved in the use and evaluation of seismic data.

Objectives

You will learn to:

  1. Establish the fundamentals of marine- and land-based seismic from acquisition to pre-processing.
  2. Examine the processing steps leading to post- and pre-stack time migration, and post-stack depth migration.
  3. Evaluate various migration parameters used in the application of pre-stack depth migration and how they affect the PSDM image.
  4. Gauge the accuracy of time to depth conversion by application of pre-stack depth migration, as well as seismic to well tie and residual depth correction.
  5. Demonstrate the fundamental differences between depth and time migration and the improved imaging results when depth migration is utilized to resolve lateral velocity variations.
  6. Evaluate the link between the pre-stack depth image and the underlying geological settings.
  7. Analyze the complex structural geometries associated with salt tectonics and their significant associated imaging challenges.
  8. Differentiate signal from artifacts.
  9. Assess the construction of geological models utilizing our common understanding of velocity estimation, anisotropic parameters and different geologic settings.
  10. Connect seismic data to geological settings for prospect evaluation and generation.

Course Content

Session 1

General Review: Seismic processing, imaging and geological motivation

The course starts with a general description of basic concepts and methods in both time and seismic depth imaging to set the foundation for the course. A brief review of the implications of different geologic settings, subsurface geometries and exploration – exploitation targets will help to establish our motivation. Building on examples from various case studies, we will introduce and review fundamentals of velocity and anisotropy, seismic acquisition and the basic processing steps leading to post- and pre-stack time migration, and then introduce post-stack and pre-stack depth migration (PSDM). The main objectives of depth imaging: correct image, correct depth and correct dip are stated and explained. We will also establish the relation between the PSDM image and well drilling results.

Forward modelling, marine data acquisition and time domain preprocessing

We will demonstrate how waves propagate in elastic media and describe the connection between wave propagation physics and the math used in seismic data processing. Starting with a basic review of wave propagation and ray theory in isotropic and anisotropic media, we will progress to examination of ray diagram curves produced in cases of both simple and complex geology, and the application of pre-stack depth migration. We will progress into the field of numerical approximations to illustrate various wave and ray solutions in geophysical studies, and computer applications for solving the underlying wave equations in seismic processing. Seismic forward modeling describing wave propagation in the sub-surface is inherent in processing. Utilizing examples from the main processing of marine seismic data, we will show how the recorded data are pre-processed to produce more interpretable data. Examples will include OBN dual sensor summation, denoise, deghost and SRME demultiple.

Session 2

Land time domain processing

We will start with the basics and fundamentals of land seismic pre-processing by building on our experience from the marine examples in the previous session. Land-based seismic data presents different challenges, so we follow the processes from acquisition, pre-processing, detailed velocity analysis and pre-stack time and depth migration, i.e. how do we get from raw field shots to data that interpreters will be able to use. This behind-the-scenes look is important; decisions made during pre-processing can affect any prospect. For land pre-processing, this includes refraction and residual statics, deconvolution, denoise and 5-D interpolation. We will use practical examples, demonstrating the basic concepts of how/why these routines are used and give tips as to what to look out for when processing a land seismic dataset.

The theory of post stack and pre-stack depth migration

Using both wave and ray equations that were examined in the previous sessions, we move on to discuss how these equations are used for application of depth migration. We start with ray-based depth migration and cover the basic concepts of Kirchhoff summation migration. Next we tackle the building blocks of wave equation depth migrations: Downward Continuation and Imaging Condition. We describe the formulation of one-way wave equation depth migration and two-way wave equation depth migration, as well as depth migration applied for both isotropic and anisotropic media. Step by step, we examine various depth migration impulse responses, migration of single shots and stacking of migrated shots to produce the final stacked image. We also investigate the numerical artifacts that are associated with each type of PSDM used in the industry. The objective is to understand how depth migrations (ray based and wave based) work and to cover the limitation of each depth migration algorithm. Special attention will be given to a demonstration of how Reverse Time Migration (RTM) works. We will conclude demonstrating the differences between RTM based on the acoustic wave equation and the elastic wave equation.

Session 3

Migration parameterization

Here we progress from theory to practice and discuss the implementation of pre-stack depth migration to understand the critical role of various parameters used in the application of PSDM and how they affect the PSDM image. Progressing from a review of the main parameters needed for pre-stack depth migration, to a detailed discussion of the key migration parameters: operator dip, migration aperture and frequency range. We continue with a detailed review of the way image gathers are constructed by various pre-stack depth migration algorithms, their advantages and limitations. This includes variable offset gathers, offset vector tiles, variable offset common azimuth gathers, common offset variable azimuth gathers, common shot gathers, common surface location gathers and reflection angle gathers. We conclude by discussing PSDM stacking procedures, including straight stack, controlled stacking and vector image partitioning.

Seismic velocities and velocity estimation techniques

This session is designed to familiarize participants with industrial use and theoretical aspects of velocity estimation techniques and to analyze the advantages and limitations of each. We start with a review of the various definitions of velocity fields used in seismic processing and depth imaging, including stacking velocities, RMS velocities, NMO and DMO velocities, Dix conversion, interval velocities, vertical velocities and residual velocities. This is followed by review and explanations of various velocity analysis techniques used in the industry, from the simplest to the most advanced, progressing to explaining and demonstrating how ray-based reflection tomography and wave-based Full Waveform Inversion (FWI) work. We conclude with a discussion on the accuracy of time-to-depth conversion done by application of pre-stack depth migration, as well as seismic to well tie and residual depth correction.

Session 4

Anisotropy and time-to-depth conversion

The geological models constructed for the application of pre-stack depth migration incorporate the subsurface geometry, velocity and anisotropic fields. We start by reviewing the various anisotropic models used by the industry and explain the parametrization for complex orthorhombic anisotropic media. This is followed by an introduction to the commonly used Thompsen parameters for describing anisotropy and their use in industrial implementation. Utilizing various geological settings, we carefully examine pre-stack depth migration positioning of key reflectors in various anisotropic cases to determine whether anisotropy was correctly applied. We conclude by discussing the relations between azimuthal anisotropy and the estimation of fracture orientation and density, as well as the objectives of application of PSDM using complex anisotropic models.

Practical aspects including post processing and multiple attenuation

Several examples of synthetic and real data cases of application of PSDM will be used to demonstrate the fundamental differences between depth and time migration and the improved imaging results when depth migration is utilized to resolve lateral velocity variations. This fundamental difference affects key aspects of interpretation: structure, dip, depth to target, as well as location of anticlines and synclines. Linking back to the input velocity model, we will explain why accurate velocities are extremely critical for depth migration and the importance of post-migration post-processing, including multiple attenuation. More importantly, we will demonstrate why inaccurate depth velocities might jeopardize the sub-surface image. The link between the PSDM image and the underlying geological settings will be reviewed and explained.

Session 5

Salt tectonics

The complex structural geometries associated with salt tectonics pose significant imaging challenges. Here we will briefly review the genesis of salt structures and their varying structural domains to provide a geological foundation for different seismic acquisition and processing strategies. Our discussion will include a review of salt basin geometries and deposition, and the fundamental mechanisms driving salt deformation. Outcrop and sub-surface examples will be discussed to help us decipher the progression from autochthonous to allochthonous salt bodies, and then the evacuation of salt and the associated structural geometries. Particular attention will be given to identifying key stratal and structural geometries associated with salt deformation and kinematics.

Integration of interpretation and model building

Here we discuss the construction of geological models utilizing our common understanding of velocity estimation, anisotropic parameters and different geologic settings. Model building invariably introduces interpretation into the processing workflow and an integrated effort between the seismic interpreter and seismic processor is key to successful model building. We start with the construction of salt bodies by reviewing the link between velocity models and the pre-stack depth-migration images and examining various workflows using several case studies. We explain exploration targets related to overthrust geology and demonstrate how to build a velocity model to correctly image these target layers. We then cover model building of faulted geology. We explain ‘fault shadows’ and what the optimal way to construct a velocity model is to resolve them. The last geological setting to be covered is related to unconventional plays. The geology in this case is ‘simple’, however the imaging objectives are different and require accuracy and resolution. PSDM has an important role in resolving seismic challenges to assist unconventional drilling programs. This will be discussed and demonstrated.

Session 6

The model and image quality

We will examine the relationship between the anisotropic models used as input to pre-stack depth migration and the corresponding image they produce. Various algorithms may perform better if the anisotropic models used as input are slightly modified; this includes operations such as smoothing of the velocity models, optimization of salt bodies, as well as applying limitations on the anisotropic fields. We will review the sensitivity of each depth migration algorithm to the input velocity / anisotropic models, including ray-based PSDM, downward propagation PSDM and two-way wave equation (RTM) PSDM. We will also examine how accurate the anisotropic model should be to achieve reliable imaging for each type of migration algorithm, and the implications for selecting a drilling location.

Imaging and interpretation of sub-salt sediments

Less sub-salt illumination and the resulting image artifacts create imaging – and therefore interpretation – challenges. For example, subsalt artifacts may come from multiples, converted waves or algorithm noise. To help differentiate coherent signal from coherent noise, we review the imaging of converted waves below salt bodies, prism waves in close proximity to salt bodies, and inner bed multiples, to decipher the sub-salt coherent noise that is part of the depth-migrated data. This enables us to differentiate noise from signal when interpreting the subsalt section. In an effort to improve sub-salt imaging, new seismic acquisition and depth-imaging methods are continuously being introduced to the marketplace. Currently an increasing number of Ocean Bottom Node (OBN) surveys are acquired. The correct processing of data from these surveys requires implementation of Elastic RTM. The different aspects and capabilities of elastic RTM PSDM will be reviewed, including sub-salt imaging by use of converted waves and their impact on the selection of sub-salt drilling locations.

Session 7

PSDM amplitudes

We move on from a focus on correctly imaging the structural geometries, to concentrate on preserving seismic amplitude. This is directly linked to evaluation of amplitude-related prospects, as well as selection of drilling locations. A review of illumination analysis techniques, including both ray-based and full wave equation-based methods will provide insight on amplitudes extracted from depth-migrated data and lead us to provide a workflow for analyzing amplitude maps as part of the prospect generation process. This requires examining the theoretical basis of least-squares RTM (LSRTM) by interrogating data examples where it has been used. Another PSDM objective is to produce PSDM gathers that can be used as input to impedance inversion. We conclude with a case study of a stratigraphic-driven exploration to examine the advantages of using PSDM gathers over time-domain gathers as input for impedance inversion.

Prospect generation

Clear definition of a prospect or play elements can significantly reduce the geologic risk associated with exploration and exploitation. In this final session we will review the primary play and prospect elements in general, and more specifically with relation to illuminating their subsurface geometries. Discussions will include consideration of the geological evolution and some basic structural techniques to assess the viability of the interpreted geometries. Participants will interpret several seismic lines to gain familiarity with identifying these features, the uncertainties associated with their interpretations, and the potential for reducing those uncertainties using the seismic imaging and processing techniques discussed in the previous sessions.

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