seismic noise ppt
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Seismic Noises
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Noise
Noise is the unwanted information
contained on a record
which one does not wish to use.
For example,
ground-roll
gives information about near surface waves.
Since this information is not useful to us
in petroleum exploration
ground roll is considered as noise.
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Signal
Signal is that seismic energy
which contains desirable information.
For example,
reflections from geological formations.
It should be noted that
certain types of energy,
such as diffractions,
might be considered
noise for one purpose
and signal for another.
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Signal to Noise Ratio (S/N)
The term signal to noise ratio (S/N) is frequently used to describe
the quality of data.
The term is sometimes used to mean
the ratio of signal energy to noise energy.
It is also used to mean the ratio of signal amplitude to noise amplitude,
which is the square root of the first definition.
Using the amplitude ratio definition statistical analysis shows that
S/N increases by square root of n
when 'n' traces containing signal and random noise
are summed.
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Type of Noises Noise is often divided into two categories:
Random It includes energy
which does not align
from trace to trace
record to record.
Coherent
It is seismic energy which aligns from trace to trace
record to record.
This type of noise is often very similar in appearance to signal
usually is more difficult to overcome
than is random noise.
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Random Noises
Random noise is made up of sporadic uncorrelatable pulses which can be caused by
any of the following
Wind Noise
Water Flow Noise
Small movements within the earth
Local Noise (People, traffic etc.)
Bad geophone Noise
Short wave length propagating Noise
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System Noises (Uncorrelateable)
Thermal Noise (Johnson Noise)
random thermal energy
Shot Noise
Semiconductor has discrete current carries
I/F Noise
Imperfect semiconductor surfaces and leakage effects.
(More dominant than shot noise at seismic frequencies)
Magnetic fluctuations (Barkhanson)
discreteness of magnetization
Modulation Noise
imperfect tape coating with signal imposed
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Source of Coherent Noise
Multiple reflections
Refracted events
Diffraction events
Ground roll
Direct arrivals
Reflections from locations outside the vertical plane through thesource and receiver and
from other sources of energy not associated
with the seismic project. Sometimes called
"side-swipe".
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Multiple Reflections
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Multiple Reflections
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Multiple Reflections
Simple Multiple
are those which have paths back and forth between the surface and one interface
Interbed or Peg-Leg Multiple
if they have paths back and forth between the surfaces and more than one interface.
Ghost
This is another kind of coherent noise.
It is common for both marine and land recording
source is placed beneath the water or earth surface
in order to create good coupling of the energy sources
To the surrounding medium.
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Ghost
part of the energy travels upward to the surface and then being reflected downward.
The recorded trace then gives the appearance of two sources slightly separated in time, or it might be misinterpreted as two closely-spaced
reflections.
If the shot is not buried very deep, the ghost pulse merges with the end of the primary, thus, changing its wave shape and adding a tail.
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Reverberation This kind of noise can be caused
by an energy source which emits a long train of pulses.
It also is frequently produced in the earth
by reflection of energy back and forth between beds.
This is especially a problem when recording data In shallow water,
because the air-water and water-rock contracts
are usually very good reflectors
and tend to establish a sort of wave trap.
Reverberation noise is also named
"Ringing" or "Signing".
an event appears not as a short pulse in time
but as a long train of pulses. When this happens, events
become obscured.
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an event appears not as a short pulse in time but as a long train of pulses.
When this happens, events become obscured.
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Water Bottom Multiples
In marine shooting the arrival of one particular primary and its accompanying water-bottom multiple reflections.
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Identification & Suppression of Random Noises
Statistical methods are often used
to overcome random noise problems.
By recording more than one trace
from the same location,
Theoretically the same signal is obtained each time,
with different random noise.
If these data are then summed,
the signal is in phase
enhanced,
while the random noise is
out of phase and is diminished.
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Ambient Noises
Type Description Apperance on Seismogram
Traffic Cars, Animal, People Often Seen as spikes
Wind Air Currents High Frequency
Earthquakes Any Motion produced from within the
earth by other than the seismic source Usually very low frequency
Highline Noise Induce noise from nearby power line 50 or 60 HZ
Shot Hole Debris falling from the blown shot hole Both high & low frequency, fanning from the
source point
Instrument From Recording Instruments Usually high frequency, noise should be very
low level
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