Seismic Exploration Methods

298 Pages·1983·14.223 MB·English

by  R. L. Sengbush (auth.)

Seismic Exploration Methods
L
Seismic
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Exploration
Methods
R.L. Sengbush
President
Pexcon International Inc.
IHRDC
BOSTON
Library of Congress Cataloging in Publication Data
Sengbush, R. L. (Ray L.), 1921-
Seismic exploration methods.
Bibliography: p.
Includes index.
1. Seismic prospecting. I. Title.
TN269.s42· 1983 622'.159 82-81559
ISBN-13: 978-94-011-6399-6 e-ISBN-13: 978-94-011-6397-2
DOl: 10.1 007/978-94-011-6397-2
Copyright © 1983 by International Human Resources Development
Corporation. All rights reserved. No part of this book may be used or
reproduced in any manner whatsoever without written permission of
the publisher except in the case of brief quotations embodied in critical
articles and reviews. For information address: IHRDC, Publishers, 137
Newbury Street, Boston, MA 02116.
Softcover reprint ofthe hardcover 15t edition 1983
Interior design by Outside Designs
Cover design by Diane Sawyer
Preface
This book describes the seismic methods used in geophys
ical exploration for oil and gas in a comprehensive, non
rigorous, mathematical manner. I have used it and its
predecessors as a manual for short courses in seismic
methods, and it has been extensively revised time and
again to include the latest advances in our truly remark
able science. I once called it, "Advanced Seismic Inter
pretation," but the geophysicists who attended the
courses always wondered when I was going to start dis
cussing interpretation. They discovered at the end that I
never did discuss interpretation as they knew it. No men
tion was made of reflection picking, posting times, map
ping, contouring, and things they already knew perfectly
well. Instead, I discussed Fourier transforms, sampling
theory, impulse responses, distortion operators, Wiener
filters, noise in f-k space, velocity spectra, wave-equation
migration, and direct detection of hydrocarbons as each
of these topics appeared on the seismic scene. I wanted
the geophysicists to think beyond the routine ofinterpre
tation, to develop a better understanding of why seismic
sections look as they do, to have a better feel for what
digital processing is doing, for good or evil, to the seismic
data. I attempted to stretch their minds. Whitehead said
it best: "A mind once stretched by a new idea can never
shrink to its former dimension." May this book be a suc
cessful mind-stretcher.
R. L. Sengbush
v
Dedication
I dedicate this book to the many geophysicists with
whom I have worked in the 35 years that I have spent
learning and applying the art and science of geophysics
in the search for oil and gas, and to my family, who has
traveled with me along the sometimes tortuous and al
ways rewarding path toward knowledge in my chosen
field.
Perhaps the reader will be interested in the ,story of
how I became a geophysicist. The seed was planted in
my mind by Bill Dabney, a friend and former classmate
at the Naval Radar School at M.I.T., who wrote me a
letter in November 1946 telling about his job with the
Magnolia Petroleum Company as observer on a seismic
crew in Athens, Texas. I had no idea what a seismic crew
did for an oil company, so I went to the public library in
Racine, Wisconsin. (I was in Racine working as a chem
ist, developing auto waxes after having been unsuccessful
in getting ajob in radar.) In the library, I took down the
volume of the Encyclopedia Brittanica containing the S's
and looked up seismic. The first thing I saw was the
picture of the wavefronts being reflected by the rock lay
ers. 'Just like radar," I thought. "I'm going to try to get a
job like Dabney's," I vowed. In his next letter, he said he
liked his job but was going to quit and sell ball bearings.
So I wrote to Magnolia, was interviewed by Dayton
Clewell in Chicago, and was soon on my way to Dallas as
Dabney's replacement on the seismic crew in Athens,
Texas. That was the beginning of 25 years as a research
geophysicist for Magnolia and its parent, Mobil Oil.
vii
viii
During that time, I worked with many persons who
helped shape my career. I wish to acknowledge some of
them now:
My first field crew: Randy Simon, party chief; Stan
Heaps, computer; Carl Richards, observer; Charley Bal-
lard, shooter; and Van Funderburk, surveyor. .
My successors as observer: Warren Hicks,J. W. Miller,
and Joyce Wiler.
My supervisors and leaders in research projects: Ed
White, Milt Dobrin, Dan Feray, Hal Frost, Frank
McDonal, and Manus Foster.
My fellow research workers: Bob Watson, Bill Ruehle,
Phil Lawrence, Frank Angona, C. D. McClure, Norm
Guinzy, Clyde Kerns, and Al Musgrave.
In 1972, I left Mobil and formed the consulting com
pany PEXCON with Norris Harris, a fellow geophysicist
who, first among those at Mobil, recognized and used
bright spots. In my ten years of consulting, the geophysi
cists with whom I was associated who stand out in my
memory include Harry Barbee, George Ball, Jim Ducas,
Bert Pronk, Ray Doan, Jim Lyon, Enders Robinson, Bill
Voskamp, and Charles Saxon.
It has been my pleasure and a distinct honor and
privilege to have worked with this talented cross section
of geophysicists. Each has contributed to my knowledge,
and each, directly or indirectly, has made a contribution
to this book that cites me as the author.
I also dedicate this book to my family: to my wife,
Earlene, who packed up and left pastoral Wisconsin with
me and our eight-month-old daughter, Lynn, for the
wilds of Texas and the adventurous life of a doodle
bugger; and to our children, Lynn, Bill, and Larry, who
never stopped believing in me. Finally, I wish to remem
ber my father, now gone, and mother, and brothers Glen
and Bob, with whom I shared the good life growing up
on a small Wisconsin farm. To all of them, I say thank
you from the bottom of my heart.
I wish to thank Bonnie McFerren, who typed and re
typed the final stages of the book, and Debbie Dureell,
who drafted and redrafted many of the final figures.
R. L. Sengbush
Houston, Texas
September 18, 1982
Contents
Preface v
Dedication vii
CHAPTERS
1. Introduction to Seismic Methods 1
Introduction
RIiflection Method
Rtfraction Method
Summary
2. Seismic Wave Propagation 13
Introduction
Elasticit}'
Elastic Waves
Raypath Theory
Diffractions
Summary
3. Seismic Signatures 31
Introduction
Land Sources
Marine Sources
Shear Sources
Spherical Spreading
Attenuation
Summary
4. Analysis and Suppression of Seismic Noise 43
Introduction
Suppression of Random Noise
Horizontally Traveling Noise
Coherent Noise Analysis
Patterns: Space-Domain Filters
Pattern Design
Velocit}' Filters
Summary
ix
x
5. The Seismic Signal Process 89 APPENDIXES
Introduction
A. Transform Theory 231
Tile Linear Filter Model of the Seismic Rtiflection Process
Simple Acoustic Impedance Functions Introduction
Correspondence Between· Depth and Time Fourier Transforms
Comparing Synthetics with FielLi Records Laplace Transjorms
Summary z-Transforms
Hilbert Transforms
6. Multiple Reflections 103
B. Linear System Theory 241
Introduction
Ghosts and Reverberations Introduction
Multiple Generation Within the Seismic Section Linear Systems
Multiple Suppression Impulse Response
Summary Filtering in the Time Domain
Types of Linear Systems
7. Distortion Operators and Deconvolution 125 AIU1log and Digital Filters
Introduction
C. Sampling Theory 257
Distortion Operators and Their Deterministic Inverse
Operators Introduction
Wiener Inverse Filter Time-Domain Sampling
Wiener Processing of Play Data Aliasing: Sampling Is Not Filtering
Wiener Processing of FielLi Data Frequency-Domain Sampling
Wiener Deconvolution and Nonminimum-Phase Data Time-Limited and Band-Limited Functions
Other Types of Deconvolution Mathematical Operations with Time Series
Summary
D. Random Processes 267
8. Velocity Estimation 151
Introduction
Introduction Theory of Probability
Shooting a Well Random Variables
Continuous Velocity Logs Ensemble Averages
Dix's Expanding Spread Technique Time Averages
Velocity Spectra Obtained in Conjunction with Horizontal Spectral Considerations
Stacking Estimation of Power Spectrum
Applications of Velocity Spectra in Predicting Lithologr Random Inputs to Linear, Time-Invariant Systems
Summary
E. Mathematics of Wave Propagation 279
9. Migration Techniques 171
Stress and Strain
Introduction Hooke's Law
Migration by the Hagedoom Method Wave Equation
Migration by Summation Methods Boundary Conditions: Plane Waves with Normal Incidence
Migration by Finite Differences Energr Relations
Migration Using Fourier Methods
Three-Dimensional Migration References 287
Summary
Index 293
10. Direct Detection of
Hydrocarbons on Seismic Data 187
Introduction
Direct Detection Techniques
Theoretical Velocity and Density Calculations
Model Studies
Case Histories
I'?formation-Preserving Processing
Summary
Seismic Exploration Methods
Chapter 1 Introduction to
Seismic Methods
INTRODUCTION
The seismic method is based on the propagation of elas
tic waves in the earth and their reflection and refraction
due to changes in the earth's velocity-density distribu
tion. Active sources of energy are required; dynamite, air
guns, and chirp signal generators are the most widely
used sources. Detection of faint pressure or particle mo
tion at or near the surface is achieved by use of sensitive
pressure gauges or geophones. The received signal is
amplified, recorded (usually by digital methods), pro
cessed (usually in a digital computer), and displayed in a
form that is interpretable in terms of geologic structure,
stratigraphy, and hydrocarbon content.
Two seismic methods coexist -one based on reflec
tions and the other based on refractions. In the reflection
method, which is the most explicit, displays of reflection
data look like slices through the earth. It is important,
however, to understand why such a simplistic viewpoint
often fails. The refraction method peels off the layers,
divulging the gross features of the velocity distribution
with depth.
REFLECTION METHOD
A pictorial view of the reflection method shows wave
fronts of seismic energy progressing outward from the
source, with reflections at discontinuities in the geologic
section. The received energy is recorded at or near the
surface on a spread of detectors located at various dis
tances from the source. The arrival times of reflections
are observable, and the distances traveled can be cal-
1