| Ultra shallow reflection seismic method has a high resolution,the existing ultra shallow reflection seismic data processing mostly uses mature data processing methods in oil and gas exploration.However,there are differences in the characteristics of deep and shallow data,and many conventional processing methods can not be directly transplanted to the ultra shallow data processing.In this paper,the processing of ultra shallow reflection seismic data focuses on the shallow seismic data of 200 ms,which is of great significance to the development of urban shallow underground space and other fields.The main problems of the ultra shallow seismic data processing are as follows: 1.The ultra shallow seismic wave field is very complex,and the reflection wave is seriously interfered by various kinds of complexity,especially the near offset surface wave and source interference.In order to obtain the near offset reflection signal,these strong noises must be suppressed;2.The near surface strata have large lateral changes,strong anisotropy,and even in some flat terrain There are also static correction problems in this area.At the same time,because the shallow seismic wave velocity is low and the velocity gradient is large,even in the low-speed layer with small thickness change,there will be a large amount of correction.3.The number of effective waves contained in the pre stack trace concentration in the ultra shallow layer is small and the statistical effect is poor,so the resolution of the velocity spectrum is low,resulting in a large error in velocity analysis.In addition,the lower real coverage makes the stack The anti-interference ability of the adding process is poor,the reflection wave in-phase axis is not fully aligned,coherent noise and other phenomena make the signal-to-noise ratio of the superimposed section low,and the authenticity is poor;4.The traditional NMO correction method is based on the arrival time and the selected NMO speed to extend the time interval between single sampling points.This stretching process will distort the reflection wave and lose20-30% of the high-frequency information due to stretching In order to avoid the change of reflected wave characteristics and wrong interpretation,the influence of over stretching on reflected wave must be removed before the stacking process,but a large number of effective signals will be lost if the simple removal method is used.Aiming at the complex interference wave,this paper analyzes the difference of the characteristics of the data in each frequency band through frequency division,designs a frequency division series F-K filter to suppress the interference wave,realizes the high fidelity noise suppression,and effectively recovers the reflection wave in the interference wave area such as near offset surface wave and acoustic wave.In this paper,refraction static correction and tomographic static correction are used to solve the static correction problem caused by the complex medium near the surface of the earth.The results show that tomographic static correction is better in the ultra shallow complex surface medium.After the application of residual static correction,the signal-to-noise ratio of the profile is higher,but it will also cause some minor anomalies.To solve the problem of low resolution of velocity spectrum,this paper uses tomographic velocity model to constrain ultra shallow velocity analysis,which effectively reduces the human error in velocity analysis.In this paper,anisotropic NMO is used to solve the problem that hyperbolic NMO is easy to produce the pull-up effect,reduce the amount of removal,and retain more ultra shallow reflections.This paper studies the evaluation method and weighted stack of prestack gathers based on the third generation coherence algorithm.Using the local coherence value as the evaluation parameter of gathers and then weighted stack according to the coherence value can effectively avoid the influence of interference wave aliasing and obtain more accurate stack results. |
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