module 03 seismic reflection
TRANSCRIPT
-
7/27/2019 Module 03 Seismic Reflection
1/53
Seismic
Reflection
1
Module 3
Seismic Reflection
Learning Objectives
Know ledge Level:
Properties of seismic traces
Vertical and horizontal resolution
Amplitude behavior and recovery
Factors affecting amplitudes, detection, and
resolution
-
7/27/2019 Module 03 Seismic Reflection
2/53
Seismic
Reflection
2
TOPICS
Reflection Coefficients
Convolutional Model
Zero Phase Wavelet
Vertical and Horizontal Resolution
Thin Bed Tuning
Geometric Spreading
Factors that affect Recorded Amplitudes
-
7/27/2019 Module 03 Seismic Reflection
3/53
Seismic
Reflection
3
WHY IMPORTANT
Interaction of propagating source wavelet withinhomogeneous subsurface is fundamental
Understanding resolution is critical to final
interpretation and in design/QC of acquisition andprocessing
Quantitative amplitude analysis is based onunderstanding amplitude changes duringpropagation
-
7/27/2019 Module 03 Seismic Reflection
4/53
Seismic
Reflection
4
Normal Incidence Reflection Coefficient
A
AR12
1V12V2
A(1-R12)
2V2 - 1V12V2 + 1V1
V = Acoustic impedance
Where = density in gm/cc and V= velocity unit length/sec
R12 =
-
7/27/2019 Module 03 Seismic Reflection
5/53
Seismic
Reflection
5
Ideal Seismogram
The seismic data recorded should give us the
Earths reflectivity sequence:
Surface
Depth
Time
Reflection Coefficient
-
7/27/2019 Module 03 Seismic Reflection
6/53
Seismic
Reflection
6
Definitions and Basic Relationships
for Sinusoids
Phase refers to position
of zero crossing.
-
7/27/2019 Module 03 Seismic Reflection
7/53
Seismic
Reflection
7
Exercise
Given the above wavelet,
1. What is the frequency of the wavelet?
2. What is the wavelength in a layer of velocity 3800 m/sec?
-
7/27/2019 Module 03 Seismic Reflection
8/53
Seismic
Reflection
8
Exercise
Most of the energy in a seismic wavelet is contained in a band of
frequencies centered about the dominant frequency. The dominant
period can be defined as the time between two major crests. The
dominant frequency is the reciprocal of the dominant period. The
equation for wavelength, , is:
= velocity/frequency
Calculate wavelengths for the following cases:a. Shallow rocks: V=2000 m/s, f=50 Hz;
b. Deep rocks: V=6000 m/s, f=25 Hz.
-
7/27/2019 Module 03 Seismic Reflection
9/53
Seismic
Reflection
9
Convolutional Model
Source Input Earth Noise Seismic traces
+ =
Time
*
-
7/27/2019 Module 03 Seismic Reflection
10/53
Seismic
Reflection
10
Normal Incidence Synthetic
Wavelet contains
source pulse, instrument
distortion, near surface effects
Rocks V LogRC Log
(depth)
RC Log
(time)
Z
t
Reflectivity R(t)
Plus
The Wavelet
-
7/27/2019 Module 03 Seismic Reflection
11/53
Seismic
Reflection
11
Composing a Wavelet by Superposition
-
7/27/2019 Module 03 Seismic Reflection
12/53
Seismic
Reflection
12
Synthetic Seismogram by Superposition
-
7/27/2019 Module 03 Seismic Reflection
13/53
Seismic
Reflection
13
Constructing the Synthetic Trace
REFLECTIVITY
WAVELET
REFLECTIVITY
WAVELET
SUPERIMPOSED
REPLACEMENTS
EACH REFLECTION STICK
IS REPLACED BY A
PROPORTIONAL VERSIONOF THE WAVELET
THE OVER-LAPPED WAVELETSARE SUMMED TO PRODUCE
THE SEISMIC TRACE
t
t
t
R1 R4 R6
R2R4
R5
-
7/27/2019 Module 03 Seismic Reflection
14/53
Seismic
Reflection
14
Constructing the Synthetic Trace
Zero Phase
-
7/27/2019 Module 03 Seismic Reflection
15/53
Seismic
Reflection
15
TOTAL SEISMIC
RESPONSE
(SYNTHETIC SEISMOGRAM)
INDIVIDUAL
RESPONSE
100 MS
ACOUSTICIMPEDANCE
WATER
GAS
WATER
LITHOLOGY
Mixed Phase
-
7/27/2019 Module 03 Seismic Reflection
16/53
Seismic
Reflection
16
Pinchout?
-
7/27/2019 Module 03 Seismic Reflection
17/53
Seismic
Reflection
17
Wavelets & Spectra
Mixed PhaseZero Phase
Wavelets can be decomposed into amplitude
and phase spectra using the Fourier Transform
-
7/27/2019 Module 03 Seismic Reflection
18/53
Seismic
Reflection
18
The Zero Phase Wavelet
Zero phase is optimum
Strongest peak
Symmetry optimizes resolution
Peak at zero reference time
Closest to reflection coefficient spike
-
7/27/2019 Module 03 Seismic Reflection
19/53
Seismic
Reflection
19
Common Types of Seismic Wavelets
Name Shape Spectrum Features
RickerNo side-lobe
overshoot
KlauderWhite from
f1 to f2
Texas
Doublef3 = 4 f2
Ormsby
Controllable
side-loberipple
f1
f1 f2
f1 f2 f3
f1 f2 f3 f4
S
-
7/27/2019 Module 03 Seismic Reflection
20/53
Seismic
Reflection
20
Klauder Wavelet
S i i
-
7/27/2019 Module 03 Seismic Reflection
21/53
Seismic
Reflection
21
Vertical or Temporal Resolution
Seismic ability to define top and
bottom of a rock layer
In general, reflections are composites of thin layer effects.
S i i R l ti Li it
-
7/27/2019 Module 03 Seismic Reflection
22/53
Seismic
Reflection
22
Resolution Limits
Full Wavelength
Resolved Layer
Half Wavelength
Resolved Layer
Quarter Wavelength
Unresolved Layer
(Detected)
Single Reflection
No Layer
Seismic
-
7/27/2019 Module 03 Seismic Reflection
23/53
Seismic
Reflection
23
Ideal Threshold For Vertical Resolution
Dominant Wavelength of Seismic Wave =
Where: V is the velocity in unit distance per second and
f is the dominant frequency in Hz
f
V
V(m/sec) F(Hz) / 4 (m)
2000 50 10
3000 40 19
4000 30 33
5000 20 62.5
Seismic P ti l V ti l R l ti
-
7/27/2019 Module 03 Seismic Reflection
24/53
Seismic
Reflection
24
Practical Vertical Resolution
estimateveconservat idatanoisy2
12=N
estimatenominaldataaverage313=N
est imateopt imist icdatamodel4
14=N
qualitydataondepending4and2betweenisNWhere
fthanveconservat imoreffBANDWIDTHmaxm inmax
BandwidthN
VR intv
Seismic
-
7/27/2019 Module 03 Seismic Reflection
25/53
Seismic
Reflection
25
Vertical Resolution Exercise (N=3)
A shallow feature with velocity of 2000 m/s and
bandwidth of 50 Hz can be resolved if it is as thin
as_______meters.
A deep feature with 5000 m/s velocity and
bandwidth of 20 Hz can be resolved if it is as thin as_________meters.
The thickness of a deep feature must
be__________(greater, less) than that of a shallowfeature in order to be resolvable.
Seismic Lateral Effects
-
7/27/2019 Module 03 Seismic Reflection
26/53
Seismic
Reflection
26
Lateral Effects
Fresnel Zone
Lateral Resolution Before Migration
Lateral Resolution After Migration
Seismic
-
7/27/2019 Module 03 Seismic Reflection
27/53
Seismic
Reflection
27
Fresnel Zone
frequencydominantf
f
t
2
V
R
V
Z2t
wave.aofphasecoherent
common,areflecting
areacirculartheof
radiustherepresentsR
o
oo
Velocity = V
Reflections come from areas (not points) on interfaces. Seismic
wavelength and wavefront travel time result in an area of
constructive interference on the reflector.
Seismic Fresnel Zone
-
7/27/2019 Module 03 Seismic Reflection
28/53
Se s c
Reflection
28
Fresnel Zone
Where:
V : is the velocity in unit distance per secondT : is time in second
F : is the frequency in HZ
T(sec) V(m/sec) F(hz) R(m)
1 2000 50 141
2 3000 40 355
3 4000 30 632
4 5000 20 1118
f
t
2
V
RRadiusZoneFresnel
Seismic
-
7/27/2019 Module 03 Seismic Reflection
29/53
Reflection
29
Fresnel Zone in 3-D
Map View
3-D Migration reduces (but does not eliminate) fresnel zone in
both x and y dimensions
Seismic
-
7/27/2019 Module 03 Seismic Reflection
30/53
Reflection
30
Implications of Fresnel Zones
Seismic Lateral Resolution
-
7/27/2019 Module 03 Seismic Reflection
31/53
Reflection
31
Lateral Resolution
After migration, lateral resolution (RL) depends upon
the maximum frequency contributing to the migrated
image. This can be computed from the verticalresolution (RV) and maximum ray angle.
angle)raym ax
angleraym ax
sin(BandwidthN
VR
sinRR
intL
VL
Seismic Lateral Resolution
-
7/27/2019 Module 03 Seismic Reflection
32/53
Reflection
32
Lateral Resolution
Lateral resolution after migration
Point diffractors, 30m separation, 10m traceinterval, 3000m/s velocity
Hz100maxf Hz50maxf
After Kirchhoff Migration After Kirchhoff Migration
After Cordsen, Galbraith and Peirce
Planning Land 3-D Seismic Surveys, 2000
Geophysical Development Series, V. 9
RLRL
Seismic Lateral Resolution Concept Based
-
7/27/2019 Module 03 Seismic Reflection
33/53
Reflection
33
p
on Diffraction Properties
Seismic
R fl ti Practical Lateral Resolution
-
7/27/2019 Module 03 Seismic Reflection
34/53
Reflection
34
Practical Lateral Resolution
A pragmatic definition depends upon the interpreters ability to visually
detect the seismic response of the target. An empirical threshold of 3
consecutive traces is often used as the minimum detectable response.
Minimum Detectable Size
Rv = Vertical ResolutionDiffractions Prior to Migration
angle)raymax
angleraymax
sin(BandwidthN
V3R
sin
R3R
intL
VL
3 traces * Rv / sin(max ray angle)
Seismic
R fl ti Exercise
-
7/27/2019 Module 03 Seismic Reflection
35/53
Reflection
35
Exercise
Calculate the size of a feature that is
detectable under the following circumstances: Trace spacing = 25m
Interval velocity Vint = 3000 m/s
Dominant frequency = 40 hz
Maximum ray angle = 30 degrees
angle)raymaxsin(
R3R
BandwidthN
VR
VL
intV
Seismic
Reflection Wedge Model and Tuning
-
7/27/2019 Module 03 Seismic Reflection
36/53
Reflection
36
Wedge Model and Tuning
Horizontal Distance (x)
Depth(Z)
Seismic
Reflection Wedge Model and Tuning
-
7/27/2019 Module 03 Seismic Reflection
37/53
Reflection
37
Tuning alignment at:
bed thickness = /4
wavelet time delay = 1/(2*fdom)
Bed thickness as a % of dominant wavelength
Wedge Model and Tuning
Seismic
Reflection Tuning Amplitude Behavior
-
7/27/2019 Module 03 Seismic Reflection
38/53
Reflection
38
Tuning Amplitude Behavior
AMP
Bed Thickness
1.0
Seismic
Reflection Spherical Divergence and Geometric
-
7/27/2019 Module 03 Seismic Reflection
39/53
Reflection
39
p g
Spreading
Spherical Geometric
Divergence Spreading
CONSTANT
Seismic
ReflectionDecrease in Amplitude and
-
7/27/2019 Module 03 Seismic Reflection
40/53
Reflection
40
Frequency with Time
Amplitude is equal to 100
Amplitude is equal to 10-6
Time
High frequency signal 0.5 sec
Low frequency signal 6.0 sec
Raw observed
seismic data
Seismic
Reflection Changes During Propagation
-
7/27/2019 Module 03 Seismic Reflection
41/53
Reflection
41
Changes During Propagation
Decay in amplitude:
divergence due to geometric spreading - v2t
Loss of high frequencies due to earth's
attenuation - Q effect
Seismic
ReflectionGeometric Spreading Correction
-
7/27/2019 Module 03 Seismic Reflection
42/53
42
Spherical Divergence:
Constant Velocity = Vc
Ar= A0(r0/r) = A0(VcT0/VcTr)= A0(T0/Tr)
A0
r2 Ar2
r1 Ar1
Source Source
0
2
0
2
000
t
t
V
)t(VS
SSpreadingGeometric
TV
TVAA
V(t)timeand
depthwithincreasesVelocity
:SpreadingGeometric
gs
gs
rrr
Seismic
Reflection Amplitude Recovery
-
7/27/2019 Module 03 Seismic Reflection
43/53
43
p y
Seismic
Reflection Absorption and Scattering
-
7/27/2019 Module 03 Seismic Reflection
44/53
44
p g
Rock Fragments
Seismic
Reflection High Frequency Attenuation
-
7/27/2019 Module 03 Seismic Reflection
45/53
45
A shot record after
correction for geometric
spreading loss is shown
at left
The far left record is not
filtered but narrow passband filters were applied
to the next four
The amplitude is seen to
decay with both time and
frequency
Seismic
Reflection Inelastic Attenuation
-
7/27/2019 Module 03 Seismic Reflection
46/53
46
(Absorption and Scattering)
A(x) = A0e-ax
where A(x) = amplitude as a function of distance
A0 = reference amplitude
x = seismic path length
e = base of natural logarithms = 2.718...a = attenuation constant
= p/Q pf/QV
where Q = Quality Factor, = wavelength,
f = frequency, and V = velocity
-
7/27/2019 Module 03 Seismic Reflection
47/53
Seismic
Reflection Factors That Affect Seismic Amplitudes
-
7/27/2019 Module 03 Seismic Reflection
48/53
48
Superimposed
Noise
Geophone Sensitivity
and Coupling
Instrument
Balance
SourceStrength
and
coupling
Geometrical
Spreading
Reflection
CoefficientVariation of Reflection
Coefficient with angle
of Incidence
Interference of
Different Events
Interbed Multiples
in Thin Beds
Absorption
ArrayDirectivity
Scattering
Reflector Curvatureand Rugosity
Seismic
Reflection Summary
-
7/27/2019 Module 03 Seismic Reflection
49/53
49
Properties of seismic waveforms and traces , f, velocity
Vertical resolution Zero phase wavelet
Resolution limits
Tuning Lateral resolution
Fresnel zones
Detectable object size
Amplitude effects Geometric spreading
Absorption
Seismic
Reflection
-
7/27/2019 Module 03 Seismic Reflection
50/53
50
Answer
40 ms
m
Hz
15225
3800
Frequency
VelocityWavelength
25040.0
1
Period
1Frequency
Seismic
Reflection Answer
-
7/27/2019 Module 03 Seismic Reflection
51/53
51
Shallow rocks: V=2000 m/s, f=50 Hz;
Deep rocks: V=6000 m/s, f=25 Hz.
m4050
2000
m24025
6000
Seismic
ReflectionVertical Resolution Exercise
Answers
-
7/27/2019 Module 03 Seismic Reflection
52/53
52
Answers
A shallow feature with velocity of 2000 m/s and bandwidth of 50 Hz can
be resolved if it is as thin as_______meters.
A deep feature with 5000 m/s velocity and bandwidth of 20 Hz can be
resolved if it is as thin as _________meters.
The thickness of a deep feature must be__________(greater, less) than
that of a shallow feature in order to be resolvable.
m3.13503
2000
BandwidthN
VR intv
m3.83203
5000
BandwidthN
VR intv
Greater
Seismic
Reflection Answer
-
7/27/2019 Module 03 Seismic Reflection
53/53
53
m150)30sin(
253
sin(
R3
m25403
3000
v
angle)raymaxRresolutionlateral
Rresolutionvertical
L
v
Assume N = 3 for normal data
Minimum
Number
of Traces
(empirical constant)
Vertical
Resolution