1 Land Acquisition Land Acquisition Important acquisition components Seismic Source Seismic receiver surface Seismic Source Ideal Characteristics 1) Strength - require enough energy to produce reflection amplitude at target horizon above noise 2) Bandwidth - strength over sufficient bandwidth to meet resolution requirements ie. pulse most like a spike 3) Phase - pulse should be minimum phase for processing 4) Signal-to-Noise - source generated noise should be minimised 5) Consistency - down-going pulse should be consistent from shot to shot 6) Cost 7) Environmental impact
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Land AcquisitionLand Acquisition
Important acquisition components
SeismicSource
Seismicreceiver
surface
�
Seismic Source
Ideal Characteristics
1) Strength - require enough energy to producereflection amplitude at target horizon above noise
2) Bandwidth - strength over sufficient bandwidth tomeet resolution requirements ie. pulse most like a spike
3) Phase - pulse should be minimum phase for processing4) Signal-to-Noise - source generated noise should be
minimised5) Consistency - down-going pulse should be consistent
from shot to shot6) Cost7) Environmental impact
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LAND SOURCES
In 1982:
45% dynamite 42% vibroseis13 % others.
Today:> 70% vibroseis< 30% dynamite
3D surveys are more flexible with vibroseis
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Explosive Sources
Scaling Laws
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AMAM
=
03
1
tMtM
=
Where AM=amplitude of pulse of mass M times larger than that giving an amplitude A0.t is the length of the pulse in time
Explosives
US Bureau of Mines in “ Blasting vibrations and their effect on structures”
velocity of ground motion correlates best with damage:< 5 cm/s = peak velocity: safe criterion< 12 cm/s : minimum damage
⇒
60 m from pipelines12 m from telephone lines30 m from railroad tracks90 m from water wells, cisterns, masonry buildings
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Explosive sources in boreholes:
drill hole, dynamite placed below low velocity layerhole: 8-10 cm diameter, 6-30 m deep (by rotary drill)