VẬT lý địa CHẤN a2 01 FUNDAMENTALS f SEISMIC EXPLORATION

2D Seismic Line 2D Seismic Survey Reflection Point Subsurface imaging along a seismic line Example of 2D seismic imaging... Borehole Seismic MethodsAdvantage – Detailed subsurface imagi

Fundamentals for Seismic Exploration

Schedule

 Introduction

 Fundamentals of Seismic Wave

 Seismic Wave Propagation

 P wave and Shear wave

 Reflection Coefficient

 Outline of Seismic Data Processing

 Topics of Seismic Data Processing

Schedule

 Introduction

 Fundamentals of Seismic Wave

 Seismic Wave Propagation

 P wave and Shear wave

 Reflection Coefficient

 Outline of Seismic Data Processing

 Topics of Seismic Data Processing

Exploration Seismology Introduction-1

Artificial generator : Seismic energy source

Detector : Receiver (Sensor)

 Goal :

 Imaging of detailed subsurface structure and obtaining information related to rock properties.

 Method :

 Artificially generate seismic waves and observe the

seismic waves (detection).

 Analysis of observed seismic waves

Classification of Exploration Seismology with regard to the geometry of source and receiver

 Surface Seismic Method

for oil and gas exploration)

 Refraction Seismic Method (rare case for oil &

Geometry of Seismic Methods

S

R

R R

S R

Reflection Seismic Method

Acquisition

1 Observe reflected waves

2 Recording (A/D conversion)

Data Processing and Analysis

1 Standard Data Processing

2 Special Data Processing (Pre-Stack Migration etc.)

3 Attribute Analysis (AVO, Inversion)

Seismic Filed Observation

 Digital telemetry sysstem

 24bit A/D convesion

Layout of Land Seismic Survey

図 4 － 1 反射法地震探査の概念図の概念図概念図

JGI

Recording Track

Wir eles

s Sig nal

Reflected wave

Reflector

Direct wave Geophone

Vibrator

Seismic Wave

Remote Station Unit

Source

Receiver

Recorder

Seismic Energy Source

Seismic Energy Source - Vibrator

Generate seismic energy

by continuous vibration ,

starting with a low

frequency and gradually

increasing frequencies.

Concept of cross correlation

for vibroseis system

Reference Sweep Reflector

Reflection (1) Reflection (2) Reflection (3) Reflection (4) Reflection (5) Observed data

After cross correlation with sweep signal

Seismic Energy Source - Impactor

Wight-drop type seismic

energy source

Seismic Energy Source - Airgun

Generate seismic waves

by releasing compressed

air into water.

Shots and Receivers are on the

same line.

2D Seismic Line

2D Seismic Survey

Reflection Point

Subsurface imaging along a

seismic line Example of 2D seismic imaging

１ ２ ３ ４ ５ ６ ７ ８ ９ １０ １１ １２ １３ １４ １５ １６ １７ １８ １９ ２０ ２１ ２２ ２３ ２４ ２５ ２６ ２７

JGI

JGI

Source Geophone

Receiver Location

Relationship between Source & Receiver Position

CMP (Common Mid Point)

Reflection Points

Relationship between Source

& Refection Points

Layout of 2D seismic reflection survey

3D Imaging by reflection points covering on

the whole survey area.

Observation at numerous receivers located on multiple receiver lines for each shot.

t

Distribution of Reflection

points

Geometry of Land 3D Survey

Swath type (Shot lines and Receiver

lines are perpendicular)

Reflection Points

geometry, all reflection points are exactly located at each bin center However, in the case of irregular geometry, reflection points distribute around bin center

Survey area is divided into small rectangle cells,

named “ bin ” and the bin center is defined as CMP

in 3D seismic survey.

point

Field Operation of Marine 3D Survey

Sail Line

Obtain several CMP lines by multiple streamer cables and

alternative shooting of twin airgun strings through a single sail

line

Example of 3D Volume (Marine 3D)

4D Seismic Survey

Definition : 4D = 3D + time (Time-lapse 3D)

– Repeat 3D seismic survey several times

– Compare the monitor 3D survey with the baseline 3D 　 survey

– Same specification as the baseline survey

– Same weather condition as the baseline survey

– Same noise level as the baseline survey

Baseline 3D survey : the first survey

Monitor 3D survey : on and after the second survey

Example of 4D Seismic Survey

(After David E Lumley, 2004)

Seismic Survey in Transition Zone

Geological boundary (reflector)

Land : Geophone

Seismic Survey in Transition Zone

Seismic survey Layout in Transition zone with Digital OBC System

Example of original shot record acquired in a transition zone

Example of Migrated Time Section in Transition Zone

(From Ikawa, et al.,(1996), Yokokura, et.al.(1996)

Granit

Sediment

Osaka Bay Rokko Mountain City Area

Example of Record Section in Transition Zone

Energy Source Marine : Airgun Land : Vibroseis Receiver

Marine : Hydrophone Land : Geophone

Vertical Horizontal (H1)

H1

3-Component Geophone

Vertical Horizontal (H1)

S-Wave (Refraction) 34 0m

/s ec

PS Converted Wave (Reflection) ?

S-Wave ( Reflection )

Borehole Seismic Methods

Advantage

– Detailed subsurface imaging

High resolution Avoid weathering layer

– Accurate velocity information

Travel time of Direct wave

Disadvantage

– Restricted survey area

Around borehole (VSP) Interwell zone (Crosswell method)

– Well distance < about 1000m for Crosswell

Limitation of energy source

(After Harris, 1995)

(After Harris et al., 1995)

Schedule

 Introduction

 Seismic Wave Propagation

 P wave and Shear wave

 Reflection Coefficient

 Outline of Seismic Data Processing

 Topics of Seismic Data Processing

 Static Correction

 Polarity

 DMO (Dip Moveout)

Basic Concept of Wave Propagation

A progress disturbance propagates from point to point in a medium The disturbance is generated

by a pushing and pulling of material particles around the baseline

Note: Particles do not flow along the wave.

Displacement of particles is limited around the baseline.

t=t 1

t=t 2

t=t 3

Direction of wave propagation

U U

K t

) (

) 3

By solving the above equation, the following

two types of seismic wave are derived.

S

V p

V 

: density : Bulk modulus : shear modulus

P-Wave and S-Wave

SV

• Definition

– The particle motion is parallel to the direction of propagation.

Acoustic waves, Primary waves, Longitudinal

waves, Compressional waves

Propagation of Volume change

V V-V1 V+V2 V-V3

Direction of Wave

Propagation

Note : Shear wave is usually polarized (SH or SV type) In homogenous media, velocities of both types are equal.



Density vs Water Saturation

Density (g/cc) matrix: 2.5 gas: 0.001 oil: 0.8 water:1.0

Porosity : 30%

Property of P-Wave

f d

d d

K K

K K K

K K

2

0

1 1

Bulk modulus K and

density are functions of porosity and pore fluid.

K 0 : Bulk modulus of mineral

K d : Bulk modulus of dry rock

K f : Bulk modulus of pore fluid

P-wave velocity is sensitive

to the property of pore fluid.

Bulk Modulus

f d

d

d

K K

K K K

K K

w

S K

S K

S-wave velocity is not sensitive

to the property of pore fluid.

Seismic velocity vs Water saturation

Gas Reservoir

Drastic change of P-wave velocity

Seismic velocity vs Water saturation

Oil Reservoir

Poisson’s Ratio and Seismic Velocity

0

1

2 2

S

P

V V

Definition

Relationship

Stress

Lateral strain Vertical strain

Poisson’s ratio vs Water saturation

Vp/Vs ratio vs Water saturation

A I : Amplitude of Incident Wave

A R : Amplitude of Reflected Wave

Definition of Reflection Coefficient

1 1 2

2

1 1 2

2

V V

V

V R

1 1 2

2

V V

V

V R

Relationship between  and V,

known as ¼-power law

4 1

23

A I : Amplitude of Incident Wave

A T : Amplitude of Transmitted Wave

Definition of Transmision Coefficient

1 1 2

2

1 1

2

V V

V T

2111

Conservation of Energy

Incident Wave

Reflected Wave

Transmitted Wave

Transmission Coefficient

Reflection Coefficient

Reflected Incident

(1) Calculate acoustic impedance of each layer

(2) Calculate reflection and transmission coefficient at the boundaries

(boundary-1 and 2)

(3) Calculate travel time of transmitted wave at each layer.

(4) Calculate travel time of reflected waves observed at the upper boundary.

(5) Calculate amplitudes of reflected waves, assuming that the amplitude of

incident wave=1.0

(H i : Thickness of layer)

Seismic velocity derived from

velocity analysis

velocity analysis

P- w

av e

P-W ave

p s

V

trn s p

V

trn p s

V

ref s p

V

ref p p

, sin 1

, sin 1

, sin

1

,

P : Ray Parameter (constant)

Seismic wave refracts at the boundary having a velocity contrast (acoustic impedance contrast).

Boundary Condition

Continuity of displacement Continuity of stress

(Function of bulk modulus

Amplitude of reflected and transmitted waves

Amplitude of P-wave is affected by

velocities of both P and S waves at

Critical Angle

Refracted P-Waves (Head wave)

1 , V 1

2 , V 2

Reflected P-Waves (Wide Angle Reflection)

If incident angle is larger than the critical angle, almost all energy of the wave, excluding the transmitted S-wave

reflects at the boundary.

S: Source R: Receiver

(After Hilterman, 1995 )

(After Hilterman, 1995 )

Schedule

 Introduction

 Fundamentals of Seismic Wave

 Seismic Wave Propagation

 P wave and Shear wave

 Reflection Coefficient

 Topics of Seismic Data Processing

 Static Correction

 Polarity

 DMO (Dip Moveout)

Basic Flow of Seismic Data Processing

Off-Shore

On-Shore Transition Zone

Ｘ Ｘ

Ｘ

Ｔ０

ＴＴ

Ｖ

Original data (CMP Gather)

NMO Stacking

Concept of CMP Stacking

１ ２ ３ ４ ５ ６ ７ ８ ９ １０ １１ １２ １３ １４ １５ １６ １７ １８ １９ ２０ ２１ ２２ ２３ ２４ ２５ ２６ ２７

JGI

JGI

Source Geophone

Receiver Location

Relationship between Source & Receiver Position

CMP (Common Mid Point)

Reflection Points

Relationship between Source

& Refection Points

Layout of 2D seismic reflection survey

Basic Flow of Seismic Data Processing

Off-Shore

On-Shore Transition Zone

Amplitude Recovery

Example of Original Seismic Data

Refracted Wave (First Break)

Reflected Wave

(Reflection)

Attenuation of Seismic Wave

Spherical Divergence Anelastic Absorption Scattering Loss

Signal Loss at Reflectors

Spherical Divergence

Anelastic Absorption

Scattering Loss Reflection

Elements of Signal Decay

Comparison of Amplitude Recovery Method

Principle of

Automatic Gain Control (AGC)

Frequency Components of Observed Seismic Data

Effective Frequency Component

Basic Flow of Seismic Data Processing

Off-Shore

On-Shore Transition Zone

Deconvolution

Convolution Model of Seismic Trace

Wavelet

Shot Receiver

Layer boundary

Shot Receiver

Layer boundary

Primary Reflections Multiple Reflections

Seismic signal contains primary and multiple

reflections.

Multiple Reflections

Multiple reflections are noise signals in reflection seismology and should be eliminated

How to eliminate multiples

1 NMO + Stacking : Enhance primary reflections using velocity difference

2 Deconvolution : Remove short-period multiples

3 Special Processing : Multiple Attenuation

Before Deconvolution After Deconvolution

Basic Flow of Seismic Data Processing

Off-Shore

On-Shore Transition Zone

Velocity Analysis

Principle of NMO Correction

Common Mid Point (CMP)

0

rms

V

x t

t(0)

Hyperbolic Curve

Why is the CMP stacking method robust ?

Traveltimes of reflection events are approximately hyperbolic with the offset

Horizontal　two　layered　model

X

V

H T

V

x T

2 2



(exact)

Dipping　two　layered　Model

X

V

H T

V

X T

cos

0 2

2 2

Apparent velocity derived from

conventional velocity analysis is affected by reflector.

X

Horizontal　 Multi-layered　 Model

V

H T

rms V

X T

2 2



(approximation)

Dix’s equation

Principle of NMO Correction

Common Mid Point (CMP)

  0 2   0

2 2

t V

x t

If we know t(0) and Vrms, we can

calculate the value of tnmo

Velocity Analysis

Example of Velocity Analysis

Constant Velocity Scan

Example of Velocity Analysis

Constant Velocity Scan

Example of Velocity Analysis

Constant Velocity Stack

Example of Velocity Analysis

Constant Velocity Stack

Example of Velocity Analysis

Interactive Method on Workstation

-Basic Flow of Seismic Data Processing

Off-Shore

On-Shore Transition Zone

NMO Mute Stack

Principle of NMO Correction

Common Mid Point (CMP)

  0 2   0

2 2

t V

x t

t V

x t

rms





NMO Correction,

Mute & Stack

CMP Gather NMO Mute Stack

Frequency Component after Stack

Filtered Stack Section

Principle of

Migration

Normal Time and Imaging Point for Dipping Reflector

Filtered Stack Section (after Migration)

Schedule

Introduction Fundamentals of Seismic Wave

– Seismic Wave Propagation – P wave and Shear wave

Principle of Static Correction

Remove weathering layer

and shift to the datum plane

Example of Static Correction

Vsw

A

C B

D

ｉｉｃ ｒ

First Break

Vsw

Refraction Analysis for Static

Correction

Time-Term

Sub-weathering Velocity

Weathering Structure

Example of Static Correction

Without static

correction

With static

correction

Schedule

Introduction Fundamentals of Seismic Wave

– Seismic Wave Propagation – P wave and Shear wave

Polarity of Seismic Waves

* )

Observed Data

)) 2 ( cos(

* )

2

Amplitude of reflected waves on free surface

Geophone : Twice of incident wave Hydrophone : Zero (Cancelled)

Schedule

Introduction

Fundamentals of Seismic Wave

– Seismic Wave Propagation

– P wave and Shear wave

S M G

R P

DMO operation creates CRP gather from post-NMO CMP gather

Principle of DMO (Dip Moveout)

2 2 2 2 2

H

L

H V

DMO in Common Offset Domain

sin cos

H

L H

DMO Operator

Reflection point exists somewhere

on the ellipse of DMO operator.

DMO in Common Offset Domain (L= constant)

NMO

 cos

Comparison of Velocity Analysis

　Stacking　Velocity　 　D　M　O　　 Velocity　Analysis　by 　Analysis Velocity　Analysis Pre- Stack　Time　Migration

Influence of Dipping Reflector No Inflence of Dipping Reflector No Influence of Dipping Reflector

( z x V

Stacking　

Velocity

DMO　 Velocit y

Soni c

P-wave Reflection Coefficient vs Incident Angle

Attenuation of Seismic Wave

• Spherical Divergence

• Anelastic Absorption

• Scattering Loss

• Signal Loss at Reflectors

Spherical Divergence

Anelastic Absorption

Scattering Loss Reflection

Elements of Signal Decay

Amplitude Decay Factor

(  Ri

