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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

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)

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

][2

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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