Classification and attenuation of Noise in Acquisition Stage
Wei Ji-dong · Progress in geophysics · 2011
According to the typical classification,seismic noises are divided into two types:coherent noise and random noise in the acquisition stage.Coherent noise includes ground roll,refraction wave,acoustic wave etc.Random noises include natural noise and what brought by human being activity.In my opinion,coherent noises not only include the noises mentioned above which come from seismic source directly,but also the scattering noise created by the responding of the inhomogeneous of subsurface to the seismic source's shock,what could travel in any directions.At the same time,the environmental noises what have coherent characteristics in shot gather,such as noise caused by machine s shaking,should be placed in 'coherent noise'.These two kinds of noises could have similar features with random noise when they are very strong and interfere each other,what could result in a misunderstanding that they are random noises.In practices many attempts to attenuate the above noises based on the statistics characteristics failed,on the contrary the longer-length geophone array in cross-line showed much improvement.Most of ambient noises which have random features come from the shaking of wind,grass,trees etc.On the same time,the 'coupling noise' caused by the bad condition is another significant source of random noises.The experiment in this paper show the evidence of existence of 'coupling noise'.which equals to about 5 to 20 percents of ambient noise and affects the weak signal coming from the deep layer seriously.The strength of 'coupling noise' is different even when inserting several geophones of the same type into th same medium,and the subtle difference among various geophones is hard to detected except by using micro-spread.However,to a specific buried geophone.the Iiroportion of 'coupling noise' remains roughly stable.Because of the above,the 'coupling noise' is a secondary noise attached to outside noise and can't be existing independently.For both coherent and random noises,the main measures to suppress are array receiving and multicoverage technique.The longer length geophone array in cross-line can overcome the scattering noise and environmental noise which has coherent characteristics successfully,and the coupling noise' should be paid more attention to than before.Meanwhile the classification of coherent and random noise is not absolute,which largely depends on the size of geological objective what we are pursuing.