This document discusses different types of noise and signals that can appear in seismic data. It defines noise as unwanted information that is not useful for purposes like petroleum exploration. Common types of noise include ground roll, multiples, reverberations, and random noise caused by things in the environment. Signal is the desirable seismic energy, such as reflections from geological formations. The ratio of signal to noise is used to describe data quality. Noise can be divided into random and coherent categories. Random noise is made up of sporadic pulses and can be reduced by summing multiple traces, while coherent noise aligns across traces and is more difficult to overcome.

2.  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.

3.  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.

4.  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.

5.  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.

6.  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

7.  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

8.  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".

10. Multiple Reflections

12.  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.

13. • 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.

14.  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.

15. • an event appears not as a short pulse in time but as a long train of pulses.
• When this happens, events become obscured.

16. In marine shooting
• the arrival of one particular primary
• and its accompanying water-bottom multiple reflections.

17.  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.

18. 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