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Overview ta3520Introduction 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: desiredNoise: 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: noise
Reflection: 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 gather
Mid-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: xs + xr = constant
Common-Offset gathers (COG): xs − xr = constant
Multiplicity = Fold: Nrec
2 Δxs / Δxr
Nrec = Number of receiversΔxs = Spacing between subsequent shotsΔxr = 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
cx = source-receiver distanceR = total distance travelled by rayd = thickness of layerc = 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
Δ 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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Δ TNMO = T − T0 = T0 ( 1 +x2
c2 T02
)1/2 − T0
• Larger ΔTNMO for larger offset
• Smaller ΔTNMO for larger T0(deeper layers have smaller move-out)
• Smaller ΔTNMO for larger wave speed c (deeper layers usually larger velocities so smaller move-out)
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Normal Move-Out (NMO)
Input CMP-gather NMO-corrected CMP gather(with right velocity)
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NMO: effect velocity
NMO with right velocity
NMO with too smallcorrection: too highvelocity
NMO with too largecorrection: too smallvelocity
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NMO: 1-layer approximation
Δ TNMO = T − T0 = T0 ( 1 +x2
c2 T02
)1/2 − T0
Use Taylor expansion of square root:
Δ 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 Snell’s 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
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Velocity Analysis: Original CMP gather
Starting CMP gather
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T2-x2 analysis
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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 gatherwith velocity
1300 m/s
CMP gatherwith velocity
1700 m/s
CMP gatherwith velocity
2100 m/s
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Velocity as function of time
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Velocity panels:real data example
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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 = amplitudeSum Σm over traces m in CMP
S (t , c) =( Σm A (xm , t , c) )2
Σm A2 (xm , t , c)
1M
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Semblance for CMP gather
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Velocity panels:real data example
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Velocity panels:real data example
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Velocity panels:real data example
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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
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Effect spread length on velocity estimation
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Effect spread length on velocity estimation
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Effect spread length on velocity estimation
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Effect spread length on velocity estimation
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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 model: RMS model
Tim
e (m-seconds)
CMP location
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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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Applying NMO
Amount x2/(c2 T02) never exactly on a sample:
INTERPOLATION
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NMO stretch(via picture)
T
T’
Since T0 is larger:shift is smaller
Since T0 is smaller:shift is larger
Hyperbolic shift
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NMO stretch(mathematically)
Due to differential working on T as function of T0:
Δ 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
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Mute: too much NMO-stretch
We do not want too much distortion: setting it zero.
This called muting
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NMO-stretch on field data
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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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Stacking
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