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)