بسم الله الرحمن الرحيم
1st topic
noises
not assosiated with reflection ,must be suppressed, by geophone arrays and processing technologyincrease signal to noise ratio
types of noises
_surface noise
_near surface noise
_scattered or incoherent noise
_multiple noise
surface and near surface noise
ground roll
horizontal....low velocity of noise.....low frequency....amplitude more than reflections]
solution to it:use [color=red]low cut filters in amplifier circuit ,or use geophones groups to supress horizontal noises and increase (enhance )vertical reflection ]
mechanism of interference
noises have sense of interference ....more complex physical mechanisms for interference said that shear reflected guided wavetrapped in surface and near surface layer with lower speed coherent noiseالنوع ده من التشويش سهل لانه متوقعpredictive noise لانه يظهر في المضاهاه بين الامواجcorrelation سهل التعامل معاi
اما بالفلاتر او بتجمعات للجيوفونات
scattered incoherent noise
near surface irrigularities scattering....when the shot lie over gravel or boulders , vuggy limestone,these cause scattering of waves ,make the seismic noise be obsecured
solution to this:addition of signals contain incohent noises....more signal added more cancel noise (something like geophones arrays)
n=s.r
s number of sources
r number of reciever
n^1/2 (root of n) cancel of noise propotional to it
we can use many sources as many reciever
geophone per trace
example :100 geophone per trace:10 geophone per trace =3 cancel noise :1 cancel noise in practical view
multiple reflection
complex... troublesome
surface multiple most common....combination most complex
there is change in material properties in multiple and brimary reflections , so frequency change and differ the frequency filterand differ in wave pathlength......so,change in frequency filter
ده معناه ان النوع ده من التشويش من الصعب شويه التعامل معاه لانه غير معلوم المصدر
geophones arrays used to cancel noise where the noise wavelength is smaller that lengths of the arraysbut for this, noise length is large than geophones arrays and regardless from its origen
but it appear in the nmo correction we u correct to remove the normal moveout of the primary reflaction , multiple appear
solutions
CDP......common depth point stacking correction....talk about it later
in practical view
these random noise is present while u shoot or not , not assaosiated with source
like well noise, instuments,traffic(vehicles,people,animals ), earthquakes....but coherent noise is peresnt with shoot ,if there isnot shoot,there isnot noise...like.ground roll, air wave...direct arrivals
to remove noises, this is the main task of seismic processing...using frequency filers,amplifier,velocity analysis,two ravel time.
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thanks
عدل سابقا من قبل في 2007-10-08, 11:07 pm عدل 2 مرات
