Post on 08-Sep-2018
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Land AcquisitionLand Acquisition
Important acquisition components
SeismicSource
Seismicreceiver
surface
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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
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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
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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)
gelatin dynamite -> nitroglycerin + nitrocottonAmmonium nitrate
cheaper, less dangerous
BUT more difficult to detonate
Explosive Sources
Deep shot hole techniques ≈ 20m depth, single shot
Advantages: - reduces source-generated noise groundroll- improved coupling, less explosive used- below weathered layer, less absorption
Disadvantages: - cost of survey dependent in shot hole depth
Shallow shot hole techniques 2 to 3m depth, multi-shot
May not be possible or desirable due to cost, logisticsto drill deep holes. Use a source array pattern to reduceground roll
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Explosive sources
Advantage: Perfect source
1) High cost2) Time and expense of drilling3) Potential damage4) Restrictions on locations of holes
Non Explosive sourcesAdvantage: Flexible location and array design
1) Less powerful2) Need to use stacking techniques3) Propagation through weathered layer twice4) Non-ideal pulse shape
Other Surface Sources
Weight Dropper (Thumper): early source, still used in desertstimes between releases not constant, no more than one source at a time
Gas Guns: explosive gas mixture
Land Air Gun: pan with water and air gun –impulse on groundcoupling poor
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Vibroseis
Vibrator exerts variable pressure on steel plate pressed ground, sweep 7-30 sec, oscillating manner upsweep or downsweep, frequency range 6-50 Hz
Low energy source requires the simultaneous use ofseveral units plus the summation of records fromseveral ‘sweeps’ in the form of an array.
Surface Wave energy 67% Shear wave energy 26%P-wave energy 7%
Vibroseis
Images from SNORCLE experiment (Canada)http://www.litho.ucalgary.ca/transect_info/snorcle/photos/vibtruck.html
Truck during“sweep”
Several unitsworking in phase
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- harder spring ⇒ higher frequency- larger mass ⇒ lower frequency
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1Hz
M
KF
natπ
=Natural Frequency
K = “spring hardness”M = mass of coil
Measures velocity of ground motion
Detectors
Land Geophones
Attenuating horizontally traveling surface wavesand preferentially records vertically traveling reflectionsignals
Output of all 5 detectors = 0 + 1 + 0 + (-1) + 0= 0 for surface waves
= 1 + 1 + 1 + 1 + 1= 5 for reflections
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