chapter 5 - processing of seismic reflection data 1 01
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Overview ta3520
Introduction to seismics
Fourier Analysis Basic principles of the Seismic Method
Interpretation of Raw Seismic Records
Seismic Instrumentation
Processing of Seismic Reflection Data Vertical Seismic Profiles
Practical:
Processing practical (with MATLAB)
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Signal and Noise
Signal: desired
Noise: not desired
So for reflection seismology:
- Primary reflections are signal
- Everything else is noise!
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Signal and Noise (2)
Direct wave: noise
Refraction: noise
Reflection: (desired) signal
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Signal and Noise (3)
Direct wave: noise
Refraction: noiseReflection: signal
Multiply reflected : noise
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Signal and Noise for P-wave survey
Noise: direct wave through first layer
direct air wave
direct surface wave
S-wave Multiply reflected wave
Refraction / Head wave
Desired signal:
primary reflected P-waves
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Signal and Noise for P-wave survey
Signal
Noise=
Primary P-wave Reflected Energy
All but Primary Reflection Energy
Goal of Processing:
Remove effects of All-but-Primary-Reflection Energy
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Processing of Signal
(Primary-reflected energy)
Goal of processing:
Focus energy to where it comes from
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Understanding signal and noise:
wave theory
Basic physics underlying signal is captured by wave equation
Ray theory: approximation of wave equation (high-frequency)
Resonances: modes expansion of wave equation
S-waves, P-waves: elastic form of wave equation
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Seismic Processing Basic Reflection and Transmission
Sorting of seismic data
Normal Move-Out and Velocity Analysis
Stacking
(Zero-offset) migration
Time-depth conversion
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Basic Reflection and Transmission
(pdf-file with eqs)
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CMP sorting
shot data
image
NMO correction
(zero-offset) migration
stack
velocity model
velocity model
Processing flow
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Processing
Input: Multi-offset shot records
Results of processing:
1. Structural map of impedance contrasts
2. Velocity model
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Sorting:
Common Shot gather
Seismic recording in the field:
Common Shot data
(Each shot is recorded sequentially)
Nomenclature:
- common-shot gather- common-shot panel
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Common Shot gather
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Sorting:
Common Receiver gather
Gather all shots belonging to one receiver position in the field
Analysis/Processing: shot variations
(e.g., different charge depths)
(Also in common-shot gathers: receiver variations, e.g.,
geophones placed at different heights)
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Common Receiver gather
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Sorting
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Sorting:
Common Mid-Point gather
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Sorting:
Common Mid-Point gatherMid-points defined as mid-points between source and receiver
in horizontal plane
Since reflections are quasi-hyperbolic:
Seismograms not so sensitive to laterally varying structures
Good for velocity analysis in depth
Stacking successful (noise suppression)
In practice, not really a point but an interval: BIN
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CMP gather over structure
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Sorting:
Common Offset gather
Purpose:
Very irregular structures (in which stacking does not work)
Application of Dip Move-Out (correction for dip of reflector)
Checking on migration: small and large offsets should give the
same picture : otherwise velocities are wrong
In practice, not really a point but an interval: BIN
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Zero-offset gather over structure
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SortingCommon-Mid-Point (CMP) gathers:x
s+ x
r= constant
Common-Offset gathers (COG):xs xr= constant
Multiplicity = Fold:N
rec
2 xs/ x
r
Nrec
= Number of receivers
xs = Spacing between subsequent shotsx
r= Spacing between subsequent receivers
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Sorting
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CMP sorting
shot data
image
NMO correction
(zero-offset) migration
stack
velocity model
velocity model
Processing flow
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Reflection 1 boundary
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Normal Move-Out: 1 reflectorT =
R
c=
(4d2 + x2)1/2
c
x = source-receiver distance
R = total distance travelled by ray
d = thickness of layer
c = wave speed
We do not know distance, but we know time:
T = T0 ( 1 +x2
c2 T02
)1/2
where T0 is zero-offset (x=0) traveltime: T0 = 2d/c
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T = T0 ( 1 +
x2
c2 T02 )
1/2
Extra time shift compared to T0 called:
NMO- Normal Move-Out
TNMO = T T0 = T0 ( 1 +x2
c2 T02 )1/2 T0
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Normal Move-Out (NMO)
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Normal Move-Out (NMO)
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TNMO = T
T0 = T0 ( 1 +
x2
c2 T02 )
1/2
T0
Larger TNMO for larger offset
Smaller TNMO for larger T0(deeper layers have smaller move-out)
Smaller TNMO for larger wave speed c(deeper layers usually larger velocities so smaller move-out)
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NMO: effect velocity
NMO with right
velocity
NMO with too small
correction: too high
velocity
NMO with too large
correction: too small
velocity
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NMO: 1-layer approximation
TNMO = T T0 = T0 ( 1 +x2
c2 T02
)1/2 T0
Use Taylor expansion of square root:
TNMO = T T0 T0 ( 1 +x2
2 c2 T02 ) T0 =
x2
2 c2 T0
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NMO: 1-layer approximation
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NMO: 2-layer
Position depends on
velocities of layer 1 and 2
(via Snells Law)
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NMO: 2-layer
(pdf-eqs)
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NMO: multi-layer approximation
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Velocity model: RMS model
time
velocity
Interval velocity(velocity of layer)
Root-mean-square velocity
(weighted-average velocity of layers above)
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Velocity Analysis
Approaches:
T2 x2 analysis
Alignment of reflectors: visually or mathematicalexpression of coherence
With T2 X2 analysis we depend on picking travel-times,
and thus signal-to-noise ratio
V l i A l i
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Velocity Analysis:
Original CMP gather
Starting CMP gather
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T2
-x2
analysis
V l it A l i li i
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Velocity Analysis: aligning
reflectors
Starting CMP gather
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Constant-velocity NMO
Velocity too high:
correction too little
Velocity too low:
correction too much
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Velocity panels
CMP gather
with velocity
1300 m/s
CMP gather
with velocity
1700 m/s
CMP gather
with velocity
2100 m/s
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Velocity as function of time
V l i l l d l
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Velocity panels:real data example
h f l i l i
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Coherence measures for velocity analysis
Stacked amplitude (normalized or not)
Cross-correlation (normalized or not)
Semblance:
related to cross-correlation
A = amplitude
Sum m over traces m in CMP
S (t , c) = m A (xm , t , c) )
2
m A2 (xm , t , c)
M
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Semblance for CMP gather
V l it l l d t l
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Velocity panels:real data example
V l it l l d t l
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Velocity panels:real data example
V l it l l d t l
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Velocity panels:real data example
Factors affecting velocity estimation
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Factors affecting velocity estimation
(Yilmaz, 1988)
Spread length
Stacking fold / Signal-to-Noise ratio(fold = multiplicity in CMP)
Choice of coherence measure
Departures from hyperbolic move-out
NMO stretch
Effect spread length on velocity estimation
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Effect spread length on velocity estimation
Effect spread length on velocity estimation
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Effect spread length on velocity estimation
Effect spread length on velocity estimation
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Effect spread length on velocity estimation
Effect spread length on velocity estimation
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Effect spread length on velocity estimation
V l it d l RMS d l
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Velocity model: RMS model
time
velocity
Interval velocity
(velocity of layer)
Root-mean-square velocity
(weighted-average velocity of layers above)
V l it d l RMS d l
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Velocity model: RMS model
Time(m-seconds)
CMP location
h t d tProcessing flow
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CMP sorting
shot data
image
NMO correction
(zero-offset) migration
stack
velocity model
velocity model
g
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Applying NMO
Amount x
2
/(c
2
T02
) never exactly on a sample:
INTERPOLATION
NMO stretch
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(via picture)
T
T
Since T0 is larger:
shift is smaller
Since T0 is smaller:
shift is larger
Hyperbolic shift
NMO stretch
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(mathematically)
Due to differential working on T as function of T0:
TNMO =x2
2 c2 T0
T0
T0
x2
2 c2 T02
=
This is called NMO-stretch
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NMO stretch
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NMO stretch
Amplitude spectra
Mute: too much NMO-stretch
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We do not want too much distortion: setting it zero.
This called muting
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NMO-
stretch on
field data
shot dataProcessing flow
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CMP sorting
shot data
image
NMO correction
(zero-offset) migration
stack
velocity model
velocity model
Stacking
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g
Add traces from NMO-corrected, CMP gather into ONE trace
Number of traces = stack fold
Events that are not hyperbolic, do not add up nicely and
destructively interfere
Goal of stacking : to increase signal-to-noise ratio
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Primaries and multiple
primary
multiple
primary
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