L21 Course Summary
Transcript of L21 Course Summary
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Seismic Imaging of subsurfacegeology
Course Summary
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Seismic Imaging
Objective
Provide a framework to understand the underlying seismic model -how seismic data are affected by structure and stratigraphy and when
a seismic section is a good representation of geology
Course graduate should be able to
Communicate effectively with specialists in seismic acquisition,
processing, and interpretation
Understand how the earth responds to a seismic source and how
synthetic seismograms are generated in the computer.
Assess effects of earth filtering, data acquisition, and data
processing on seismic sections.
Appreciate Seismic data quality criteria
Resolution, Signal-to-noise ratio, Image integrity
Recognize whether appropriate technology has been applied to
your exploration project
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Overview of Seismic Reflection Prospecting
Display
Seismic
Source
Seismic
signal is
generated
Seismic signal reflects
and transmits through
subsurfaceEarth
Filtering
DataAcquisition
Seismic signal isdetected and recorded
Data
Processing
(filtering)
Seismic signal is
enhanced
Imaging
(multi-dim.
Filtering)
Seismic signal is
positioned properly
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Basic Seismology
Huygens principle is an extremely powerful wavefront tracking tool
By reconstructing wavefronts at closely spaced intervals, we can
propagate waves in uniform media
On prestack (CMP) gathers,
primary and multiple reflections are hyperbolicshape gets flatter (less moveout) with depth and increased
velocity
beyond critical angle, reflections turn into refractions
On poststack (zero-offset) sections,
Unmigrated events are mispositioned and look distorted;
diffractions abound, occurring at reflection discontinuities
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Frequency domain, Linear systems,
& seismograms
Time and space signals can be expressed as sums of sinusoids
Time-space signals are represented by f-k Fourier transformsSignals must be sampled at least twice per period to avoid
aliasing
The output of a Linear Time Invariant System
has no frequency not present in input
is independent of order of operations
is convolution of system impulse response and input
can be obtained by multiplying Fourier transforms Seismic reflections are generated by convolving the earths
reflection coefficient train with a seismic wavelet.
The seismogram is further modified by ghosting (at source andreceiver) and attenuation
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Fourier Transform Terminology
Bandwidth = frequency range where amplitude is
greater than 70% of maximum amplitude
Peak frequency = frequency at which amplitudespectrum has maximum value.
Broad band wavelets are those with large
bandwidths (typically more than 40 Hz for seismicdata).
Narrow band wavelets are those with smallbandwidths (typically less than 15 Hz for seismic
data).
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-1.5
-1
-0.5
0
0.5
1
1.5
0 4 8 12 16 20 24 28
150 Hz Sine wave 100 Hz Sine wave
Aliasing: 4 ms sample rate (125 Hz Symmetry)
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Importance of Fourier Transforms in
Acquisition and Processing
Traces can be filtered to remove low- and high- frequency noises.
Traces can be deconvolved to eliminate multiples or to enhanceresolution.
Resolution and dip control frequency needs, which controls
sample rate Field arrays filter surface waves and other noises in spatial domain
Space-time filters can eliminate surface waves, multiples, etc
Migration is a type of filtering, often performed in frequency
domain
Each frequency component can be treated independently
Signals and noises often have different frequencies
FFT (Fast Fourier Transforms) are computationally very fast
differential-equation solvers
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Linear Systems Summary
The output of a Linear Time Invariant System has no frequency not present in input
is independent of order of operations
is convolution of system impulse response and input
can be obtained by multiplying Fourier transforms
Applications include filtering, synthetic (and real)
seismogram generation, attenuation and ghosting.
== dxxtvxhthtvty )()()(*)()(
)()()( fHfVfY =
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Synthetic Seismogram Summary
Synthetic seismogram for vertically incident plane waveand horizontally layered media
Hang a seismic wavelet on each reflecting interface
Convolve the seismic wavelet w with the reflection coefficienttrain r
Multiply the FT ofw times the FT of r.
Ghosting plays a large role in wavelet shaping
Source and receiver each provides a ghost
Ghost is a 2-point (1,-1) filter separated by 2-way traveltimeto S or R depth. Notch in frequency domain at 0 and 1/ .
Attenuation increases with reflector depth
High frequencies are attenuated more than low frequencies Attenuation is constant for each fT (freq times 2-way travelT)
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Seismic Data Quality Criteria(What does an interpreter want from seismic data?)
Reflection detection (signal-to-noise ratio,
overburden distortion)
Resolution (vertical: map thin beds, lateral: place faults,detect stratigraphic changes)
Fidelity (similarity of the seismic section to a geologiccross-section: accuracy of amplitudes, structural positions)
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Important Concepts / Terms
Spread = arrangement of geophones relative to sources Off-end all receivers are on one side of the source Split-spread receivers are on both sides of the source
Group interval = receiver spacing= distance between geophone array centers
Channels = number of receiver stations recorded per shot
Shot interval = distance between shots
Bin is the collection of all traces with the same CMP (+/-) Fold is the number of traces in the CMP bin
CMP spacing = stack bin interval= 1/2 the group interval (usually)
Inline = direction of shooting
Crossline = perpendicular to Inline
3-D terms
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Vertical Resolution: Key Points
Resolution is the ability to distinguish distinct events Short-duration seismic wavelets are required
Broad bandwidth, zero-phase wavelets are best
Seismic bandwidth is approximately = center frequency
Potential resolution
Bandwidth can be increased by deconvolution Frequencies to be included must have adequate S/N
Phase coherence or Wiener spectra determine S/N
Deconvolution must
Increase the bandwidth
and / or Align frequency components to be in phase
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Stratigraphic exploration: Amplitudes
Thin Beds
Reflection shape changes little with bed thickness
Reflection amplitude is proportional to bed thickness Amplitudes
Amplitudes convey useful information about rock properties
Statistical AGC makes data useful for structural
interpretation
Long-gate AGC better than short gate for amplitude fidelity
Controlled amplitude - deterministic - most accurate
approach but difficult to remove all amplitude contaminants
S i i I i
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Seismic Imaging
Vertical Resolution
Good vertical resolution is needed to isochron thin beds
Resolution requires broad-band, zero-phase seismic data
Statistical deconvolution broadens the bandwidth & simplifiesphase, subject to acceptable signal-to-noise ratio
Deterministic approach is an excellent alternative (when source
and instrument characteristics are available.)Lateral Resolution
Good lateral resolution is needed to map small structural features
faults, pinnacle reefs, contorted beds
Fresnel zone smearing causes poor lateral resolution at depth
Migration can improve lateral resolution significantly
Prestack migration is especially helpful in improving lateral
resolution
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Seismic Visibility of a bed
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Vertical Resolution / Gain Enhancement Deconvolution / AGC
Applies a different filter / gain to each trace
Assumes random reflection amplitude & depth Statistical correction
Controlled Amplitude & Phase (CAP)
Requires coordinated acquisition & processing Applies same filter / gain to all traces
Assumes all relevant parameters are known
Deterministic corrections
Use CAP whenever possible
Requires special acquisition & processing
Requires fairly high-quality (good S/N) data
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Rock Physics Summary
Seismic response depends upon velocityand density
Velocity and density depend upon rock
properties
lithology, porosity, fluid content, age, depth of
burial, pore pressure, heat flow Seismic amplitude variation with offset also
depends upon shear-wave velocity
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Summary:
Stratigraphic modeling / inversion
Used to extend well-log information away
from the well
Matching synthetic seismic data to real datapermits checking geologic hypotheses
Result is an estimate of short-periodvelocities
used to infer rock properties away from wells
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Velocity Summary
Velocities are the most important factor in seismic explorationLong period estimates
Obtained from traveltime information
Processing uses: NMO Stack, MigrationInterpretation uses: Well-seismic ties, depth estimation,
overpressure prediction
Short period estimatesObtained from reflection waveforms and amplitudes
Interpretation uses: rock property prediction away from
wells
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Velocities and CMP stack
Velocities are at the heart of seismic analysis
NMO velocities can be used to estimate interval
velocities and aid in time-to-depth conversion
NMO velocity errors cause stack to act as a filter -
eliminating high frequencies
Fold is extremely important in enhancing S / N
Offset / azimuth distribution also very important
Shooting geometries to achieve CMP stack often dictated by
equipment availability / field terrain
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Basic Acquisition and Processing
Objectives of acquisition and processing:
reflection identification, resolution, and fidelity
CMP Shooting forms the basis of acquisition and processing 3 - D fold = (Channels per streamer / 2) * (Group Int / eff Shot Int)
Acquisition, the most expensive step in seismic exploration, tries
for the best data quality at a reasonable cost
Illuminate target with sufficient source energy
Minimize recorded noise
Basic Processing comprises
Editing and sorting into CMP order
NMO velocity analysis and NMO application CMP stack
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Basic 2-D Processing Stream
Filtering, corrections
Sorting, Labeling, Editing
CMP Stack
Filtering, migration
Velocity
analysis
Pre-
Stack
Post-
Stack
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Signal-to-Noise Ratio
CMP stack enhances S / (random noise) by n, where n =fold
enhances S / (coherent noise) with stack array
f-k filters suppress dipping noises
Multiples are most significant marine data quality
problem In very shallow water, deconvolution is effective
In very deep water (where sufficient differential
moveout is present), velocity filtering techniques areeffective : f-k filters, weighted stack, radon filters
Surface-related multiple suppression is a recent
development for complex geometries
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Seismic Arrays and CMP Stack
Virtually all exploration seismic data are detected
by receiver arrays The alternative, a Multi-component detector, is in itsinfancy
Seismic Arrays are effective at reducing short-wavelength coherent noises in the inline direction
Long arrays attenuate reflection signals
Field arrays are anti-alias filters
Spatial frequencies that would alias are attenuated
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Signal-to-noise enhancement
Full-fold Stack Array reduces surface-wave noiseswithout significantly attenuating reflections.
Full-fold data acquisition used only in areas with severe
noise problems, due to cost.
Less than full-fold data acquisition produces stack array
response that attenuates surface-wave noise, but to a
lesser extent.
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Data Processing Terminology
Basic data processing consists of
enhancing signal-to- noise ratio
accounting for traveltime variations (in overburden and
with offset)
compensating for geologic structure
High-end processing is needed
when basic data processing does not meet business needs to compensate for complex structure / overburden
to attenuate complex noise fields
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Advanced Data Processing Summary
Data processing streams must be tailored to source of
data-quality degradation Filtering is often useful in eliminating additive
noises
Efficacy of multiple suppression technique depends
on water depth
Long-period static correction reduces structuraldistortion
Short-period static correction enhances signal-to-
noise ratio
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Addressing Seismic Data Quality
Reflection detection (signal-to-noise ratio, overburdendistortion)
1-D and 2-D filters aimed at attenuating specific noises CMP Stack
Short-period static and dynamic corrections
Resolution
Vertical: Deconvolution / controlled phase processing
Lateral: Migration Fidelity
Amplitudes: Controlled amplitude processing
Structure: Long-period statics, Velocity estimation, Timemigration, Time-depth conversion, Depth migration
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Structural Modeling summary
Structural modeling provides insight into the
relationship between a complex geologic structure
and its seismic response and is often useful in
interpreting complex seismic data.
Raytrace modeling is inexpensive and providesinsight into the seismic-geology relationship.
In recent years, wave equation modeling has
become much more affordable.
Migration is the inverse to structural seismic
modeling.
Marmousi Model
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Marmousi Model
Velocity
Density
Versteeg
TLE, 2004
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Migration Summary - I
Migration makes a seismic section look more like a geologic cross-
section
Flat horizon remains unchanged, if no velocity anomaly above it
Dipping horizon becomes steeper, shallower, and moves laterally
up dip
Synclines become broader and bow ties are eliminated
Anticlines become narrower
Diffractions collapse to points
Migration operationCorrect velocities are key to successful migration
Noise spikes and edges cause migration smiles
Adequate aperture needed to capture dipping eventsA properly migrated section has neither diffractions nor smiles
Migration Summary II
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Migration Summary - II
Depth migration is necessary when velocity varies rapidly(laterally) but is more expensive than time migration
Accurate velocity-depth model is essential
Kirchhoff migration used almost exclusively (adequateaperture is important)
Prestack migration overcomes limitations of stack but is more
expensive than poststack migration
Depth point smear,, conflicting dips
NMO combined with DMO, stack and poststack migration iseffective and less expensive than full prestack time migration
3-D migration does not require all energy to come from directly
below the seismic line
eliminates sideswipe
moves energy in both inline and crossline directionsPrestack 3-D Depth Migration combines all of the above (costly)
Survey Surface Coverage
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Survey Surface Coverage
Shots occupy a distance = prospect length + ApertureLeft + ApertureRight + 1/2 spread
Line length = positions occupied by either sources of receivers = prospect length +ApertureLeft + ApertureRight + 1 1/2 spread
ProspectSeismic line
Aperture Aperture
Full-fold
Shot point 1
Shooting direction
(off-end shooting) Full-fold buildup(1/2 spread)
Full-fold buildup(1/2 spread)
Last shot
point
streamer
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Summary of Migration algorithms
Correct velocities are key to successful migration
Noise spikes and edges cause migration smiles
Adequate aperture needed to capture dipping events
A plethora of migration algorithms are available
Historically, Kirchhoff migration has been most
widely used
Wave-equation methods have now become
affordable
C i h h h i (i li )
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Cross-sections through the reservoir (inlines)
Inline
From well data, AVO analysis, & seismic inversion, obtainpetrophysical information to populate sublayers
Pl i 3 D S
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Planning 3-D Surveys
Pre-design stage
Determine objective
Assemble required informationDesign Stage
Determine resolution
Determine aperture
Determine sampling
Estimate Effort level (fold)
Post-design stage
Feedback results of survey
Iterate design process
Interpreter
Designer
Interpreter
Designer
Processor
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Survey Design: Summary
Important objectives: small vertical and lateral resolution high signal-to-noise ratio
small acquisition footprint
low cost and fast acquisition time