PETR8510
RECAP LAST WEEK• Seismic reflection method
– Acoustic based method which provides high resolution images of the sub-surface
– Data recorded as shot gathers – many receivers for each source
– Arrivals in data are identified from their character and moveout
– Once identified their travel times can be used to determine depth to features in the sub-surface (within the errors of the input parameters)
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ZERO OFFSET DATA• Shot gathers
– Numerous receivers, one source– Data are recorded at ‘finite offset’
• Zero offset– Equivalent of one source,
one receiver at eachlocation, but in lots of places
– Much easier to interpret
TWT
TWT
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STACKING• Acquisition involves creating a series of shot gathers
– Move-up rate – source vs receiver spacing
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STACKING
• Acquisition involves an inherent data redundancy– Data recorded as shot
gathers are sorted in to common mid point (CMP) gathers
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STACKING• CMP gathers are
corrected for normal moveout (NMO)– This creates equivalent
zero offset traces
• Zero offset traces are plotted so centred on the equivalent source-receiver location
TWT
Location
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STACKING• NMO corrections in
practice– Function of reflector
depth, average velocity and source-receiver offset
– Only offset is known
• Resort to trial and errors and statistics
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STACKING• The zero offset section is
not a depth section and its representation of sub-surface geometry is flawed– It needs to be ‘migrated’
• Post-stack migration only here
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MIGRATION• A reflector is the equivalent of a
series of closely spaced diffractors– Migration = series of diffractor
response corrections
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MIGRATION• Diffraction migration
– Sum along diffraction travel time curve
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MIGRATION• Time migration
– Based on a series of 1D velocity variations
• Depth migration– Based on 2D velocity function
Time Depth
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MIGRATION• Post-stack migration works
provided dips are shallow• If not require pre-stack
migration– More expensive– Based on ellipse rather than
circle (in 2D)
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MIGRATION• Places features in their correct
relative positions• Requires perfect knowledge of the
sub-surface velocity field• Many methods• Pre- or post-stack
– Depends on geological complexity
– dips, lateral velocity variation• Computational intensive
(expensive)
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SUMMARY• Stacking and migration
– Improve SNR– Place features in ‘correct’ relative positions – Depth conversion difficult
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NEXT TIME• Well logging – measurements in a
drill hole– Acoustic and density logging and
synthetic seismograms• Other kinds of logging
– Radioactivity (gamma), electrical (SP, resistivity)