Seismic Noises

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.

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.

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.

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.

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

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

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".

Multiple Reflections

Multiple Reflections

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.

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.

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.

an event appears not as a short pulse in time but as a long train of pulses.

When this happens, events become obscured.

Water Bottom Multiples

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

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.

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