VẬT lý địa CHẤN a2 01 FUNDAMENTALS f SEISMIC EXPLORATION

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
 Static Correction
 Ｐｏｌａｒｉｔｙ　ｏｆ　Ｓｅｉｓｍｉｃ　 wave
 DMO (Dip Moveout)



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
 Static Correction
 Ｐｏｌａｒｉｔｙ　ｏｆ　Ｓｅｉｓｍｉｃ　 wave
 DMO (Dip Moveout)




Exploration Seismology

Introduction-1

 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 .


Artificial generator : Seismic energy source
Detector : Receiver (Sensor)


Introduction-2

Classification of Exploration Seismology with
regard to the geometry of source and receiver
 Surface Seismic Method


Reflection Seismic Method (usual
for oil and gas exploration)



Refraction Seismic Method (rare case for oil &
gas Exploration)

 Borehole Seismic Method



VSP (surface to borehole)
Cross-well (borehole to borehole)


Introduction-3

Geometry of Seismic Methods
Surface

R

S

R

R

R

S

R

R

R

R

Reflector
Reflection Seismic Method
Surface

Refraction Seismic Method

S
R

R

R

R

R

R

S

S

Reflector
VSP (Vertical Seismic
Profiling)

Crosswell Seismic method


Reflection Seismic Method

Introduction-4

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 )
Basement

Results
1. Subsurface imaging
2. Velocity information
3. Information for reservoir characterization

Available to Exploration and
development stages

Introduction-3

Reflection Seismology

 Acquisition




Survey Area
 On-Shore (Land)
 Off-Shore (Marine)
 Transition Zone
Method
 2D method
 3D method
 4D method
 3 Component Observation
 4C Observation
 S-Wave Survey

 Data Processing
 Conventional Processing
 3D Processing
 Special Processing
 Pre-Stack Migration
 Depth Migration
 Multiple Attenuation

 Attribute Analysis
 AVO
 Inversion


Seismic Filed Observation


Energy Source





Receiver






Explosive source
Non-explosive source
Geophone
Hydrophone
Accelemeter

Recording



Digital telemetry sysstem
24bit A/D convesion


Layout of Land Seismic Survey
ss
e
l
e
Wir al
n
Sig

Recording
Track

Remote Station
Unit

Source

JGI

Recorder

Vibrator

Receiver
Geophone

Seismic Wave

Reflected wave

図 4 － 1 反射法地震探査の概念図

Direct wave

Reflector


Seismic Energy Source


Explosive Source




Dynamite

Non-Explosive Source




Airgun
Vibroseis (Vibrator)
Weight drop type

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.


2D Seismic Survey
2D Seismic Line

Shots and Receivers are on
the same line.

Reflection Point

Subsurface imaging along a
seismic line

Example of 2D seismic imaging


Layout of 2D seismic reflection survey
Geophone

JGI
JGI

Source
Reflection Points
１

２

３

４

５

６

７ ８

Receiver Location

９ １０ １１ １２ １３ １４ １５ １６ １７ １８ １９ ２０ ２１ ２２ ２３ ２４ ２５ ２６ ２７

Relationship between
Source & Receiver Position

Relationship between Source
& Refection Points

CMP (Common Mid Point)


3D Seismic Survey
3D Imaging by reflection points covering on
the whole survey area.
Receiver Line

Shot Line

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

X

t

Distribution of Reflection
points


Geometry of Land 3D Survey
Swath type (Shot lines and Receiver
lines are perpendicular)

JGI

Recording
Track

JG

I

Receiver Line

Reflection Points
n
Li
t
o
Sh

Bin

e

Survey area is divided into
small rectangle cells,
named “bin” and the bin
center is defined as CMP
in 3D seismic survey.
CMP (Bin Center)

Note : In the case of regular
geometry, all reflection points
are exactly located at each bin
center. However, in the case of
irregular geometry, reflection
points distribute around bin
center.
Reflection
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)
in
L
n
I

Cr

e

ce
i
l
eS
m
Ti

os
s

- Li
ne

Definitions
In Line : Pararell to sail lines
Cross-line : Perpendicular to
sail lines


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

Purpose
– Reservoir Monitoring
Obtain time –lapse images associated with fluid and pressure
front of reservoir during hydrocarbon production

Ideal condition of 4D seismic acquisition (practically
difficult)
–
–
–

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
Baseline seismic
cross-section

4D Seismic Difference Section
(Monitor – Baseline)
after co2 injection

Horizontal section

(After David E. Lumley, 2004)


Seismic Survey in Transition Zone
Recording
track

Shooting boat
Guard boat

Airgun
Ocea

n

mc
botto

Vibrator

a bl e

Geological boundary (reflector)

Energy Source
Marine : Airgun
Land : Vibroseis and/or Dynamite

Receiver
Marine : Hydrophone in Ocean Bottom
Cable
Land : Geophone


Seismic Survey in Transition Zone