N172 Depth Conversion Methods and Pitfalls

Depth conversion is approached as part of the geological-geophysical interpretation process, starting during seismic time interpretation and ending with maps for STOIIP/GIIP calculations and well planning. Tips and techniques in data QC, practical velocity modelling methods, and recognition of geological and geophysical pitfalls are emphasised. The course is equally appropriate for those with sophisticated depth conversion tools who must understand the ‘black box’, and for those who have to depth convert using only mapping software and a spreadsheet. Attendees will learn how to maximise use of their velocity data, see how greater control on the velocity model can be achieved by incorporating geological understanding, and recognise the importance of appropriate well tying. A wide range of experience will be catered for in the course which is biased towards worked examples.

Introduction

• Understand the relevance of depth conversion in the era of Pre Stack Depth Migration
• Know the defining characteristic of a good depth conversion

Geological factors influencing velocity

• Understand the intrinsic properties of rocks that influence velocity
• Understand how geological processes control macro velocity variations in the subsurface

Sources of velocity data and their problems

• Be aware of the different sources of data providing a measure of velocity
• Understand the limitations of the data and know checks to help ensure data integrity
• Be familiar with velocity terminology

Synthetic seismograms and time-depth relationships

• Understand how interpretation systems use time-depth data and how this influences calculated velocities
• Learn how to optimally calibrate synthetic seismograms for use in depth conversion

Analysing velocity data

• Learn the importance of looking at all of the data, including logs and seismic velocities
• Learn data analysis techniques which help determine the best approach to velocity modelling and depth conversion
• Understand the importance of scale in displaying quantitative data

Velocity model building methods

• Understand when simple functions (e.g. t-d polynomials) and average velocity are poor velocity models
• Appreciate benefits of layering even with simple velocity models
• Be comfortable implementing the common velocity modelling methods, including linear velocity functions (‘Vok’), and understand their limitations
• Know techniques to condition seismic velocity products

The process of depth conversion

• Be aware of generic workflows for depth conversion
• Understand the influence of data availability and geological complexity on choice of workflow
• Be aware of potential pitfalls in implementing the processes

Tying to wells

• Know mapping techniques that make depth conversion easier by minimising misties
• Be aware of pitfalls in the use of check-shots and well picks
• Learn the importance of QC for assessing the effects of tying
• Understand when it is best to tie in depth, velocity or time domains
• Appreciate the implications and value of mapping Vo, and learn how to do it
• Know when it is best not to tie

Depth domain seismic techniques

• Understand the benefits and limitations of tomographic velocity models
• Understand how anisotropic PSDM results are matched to well depths
• Recognise the importance of being aware of/involved in seismic processing of your dataset
• Learn how to adapt good depth conversion practice to depth domain seismic horizon corrections

Quality control: tips and techniques

• Know techniques to QC the time model – time grids and correction surfaces
• Learn techniques to QC the velocity model through ‘time conversion’
• Learn workflows to assess the impact of adding sophistication to velocity modelling
• Be aware of quick and effective technique for QC’ing others’ work

Dealing with geophysical pitfalls

• Be aware of the reasons why the seismic image is not a vertical scaled version of the real earth
• Understand the impact of channels and other shallow anomalies on seismic reflection times and how they limit the accuracy of seismic velocities
• Learn what solutions can be implemented in vertical depth conversion, and which require another approach

Dealing with geological pitfalls

• Understand that, even when properly implemented, there are times when some velocity modelling methods will not work (presence of hydrocarbons, deep sea floor channels etc)
• Learn solutions or work-arounds for use when the recommended techniques need modification (variable water depth, discontinuities, overpressure etc)

Quantifying uncertainty

• Learn the fundamentals of combining uncertainty measurements
• Be aware of the techniques available to quantify mapping uncertainty