introduction to seismic inversion methods || 1. part 1 - introduction

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Page 1: Introduction to Seismic Inversion Methods || 1. Part 1 - Introduction

Introduction to Seismic •nversion Methods Brian Russell

PART I - INTRODUCTION

Part 1 - Introduction Page 1 - 1

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Page 2: Introduction to Seismic Inversion Methods || 1. Part 1 - Introduction

Introduction to Seismic Inversion Methods Brian Russell

I NTRODUCT ION TO SE I SMI C INVERSION METHODS , __ _• i i _ , . , , ! • _, l_ , , i.,. _

Part i - Introduction _ . .

This course is intended as an overview of the current techniques used in

the inversion of seismic data. It would therefore seem appropriate to begin by defining what is meant by seismic inversion. The most general definition is as fol 1 ows'

Geophysical inversion involves mapping the physical structure and

properties of the subsurface of the earth using measurements made on the surface of the earth.

The above definition is so broad that it encompasses virtually all the

work that is done in seismic analysis and interpretation. Thus, in this

course we shall primarily 'restrict our discussion to those inversion methods

which attempt to recover a broadband pseudo-acoustic impedance log from a band-1 imi ted sei smic trace.

Another way to look at inversion is to consider it as the technique for

creating a model of the earth using the seismic data as input. As such, it

can be considered as the opposite of the forwar• modelling technique, which involves creating a synthetic seismic section based on a model of the earth

(or, in the simplest case, using a sonic log as a one-dimensional model). The

relationship between forward and inverse modelling is shown in Figure 1.1.

To understand seismic inversion, we must first understand the physical processes involved in the creation of seismic data. Initially, we will

therefore look at the basic convolutional model of the seismic trace in the

time and frequency domains, considering the thre e components of this model: reflectivity, seismic wavelet, and noise.

Part I - Introduction

_ m i --.

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Page 3: Introduction to Seismic Inversion Methods || 1. Part 1 - Introduction

Introduction to Seismic InverSion Methods Brian Russell

FORWARD MODELL I NG i m ß

INVERSE MODELLING (INVERSION) _

, ß ß _

Input'

Process:

Output'

EARTH MODEL

,

MODELLING

ALGORITHM

SEISMIC RESPONSE i m mlm ii

INVERSION

ALGORITHM

EARTH MODEL i ii

Figure 1.1 Fo.•ard ' andsInverse Model,ling

Part I - Introduction Page I - 3

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Page 4: Introduction to Seismic Inversion Methods || 1. Part 1 - Introduction

Introduction. to Seismic Inversion Methods Brian l•ussel 1

Once we have an understanding of these concepts and the problems which

can occur, we are in a position to look at the methods which are currently ß

used to invert seismic data. These methods are summarized in Figure 1.2. The

primary emphasis of the course will be

the ultimate resul.t, as was previously

on poststack seismic inversion where o

Oiscussed, is a pseudo-impeaance section.

We will start by looking at the most contanon methods of poststack

inversion, which are based on single trace recursion. To better unUerstand

these recurslye inversion procedures, it is important to look at the

relationship between aleconvolution anU inversion, and how Uependent each method is on the deconvolution scheme Chosen. Specifically, we will consider

classical "whitening" aleconvolution methods, wavelet extraction methods, and

the newer sparse-spike deconvolution methods such as Maximum-likelihood deconvolution and the L-1 norm metboa.

Another important type of inversion method which will be aiscussed is model-based inversion, where a geological moael is iteratively upUated to finU

the best fit with the seismic data. After this, traveltime inversion, or

tomography, will be discussed along with several illustrative examples.

After the discussion on poststack inversion, we shall move into the realm

of pretstack. These methoUs, still fairly new, allow us to extract parameters

other than impedance, such as density and shear-wave velocity.

Finally, we will aiscuss the geological aUvantages anU limitations of

each seismic inversion roethoU, looking at examples of each.

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Page 5: Introduction to Seismic Inversion Methods || 1. Part 1 - Introduction

Introduction to Selsmic Inversion Methods Brian Russell

SE I SMI C I NV ERSI ON

.MET•OS ,,,

POSTSTACK

INVERSION

PRESTACK

INVERSION

MODEL-BASED I RECURSIVE INVERSION • ,INVE SION

- "NARROW BAND

TRAVELTIME

INVERSION

!TOMOGRAPHY)

SPARSE- SPIKE

WAV EF I EL D NVERSIOU i

LINEAR

METHODS ,,

i i --

I METHODS ]

Figure 1.2 A summary of current inversion techniques.

Part 1 - Introuuction Page 1 -

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