Application Of Vector Wavenumber Transformation Method In Shallow Layer Exploration

Rayleigh wave exploration is an important branch of the field of geophysical exploration compared with the traditional drilling method,which has the characteristics of non intrusive,non destructive,low cost and high efficiency.It is mainly divided into active source and passive source surface wave exploration,in which passive source surface wave exploration is more convenient and less destructive to site because it does not need artificial source to excite.The so-called passive sources mainly refer to the ambient noise,and the main methods of surface wave exploration based on ambient noise are spatial autocorrelation(SPAC),frequency wavenumber(F-K)method,seismic interference method,passive source surface wave multichannel analysis(MASW).At present,the surface wave frequency obtained by ambient noise method is generally low,and the deep underground(>100m)velocity structure is obtained.In actual engineering exploration,the detection range is generally shallow,so if the method of ambient noise can be used for shallow surface exploration,it will undoubtedly provide a new and efficient and convenient shallow wave exploration method.In this paper,vector wavenumber transform(VWTM)is used to do shallow surface wave exploration.VWTM is a new method for extracting surface wave dispersion curves from ambient noise records.Compared with the traditional passive source surface wave method,its station layout is more flexible and can be applied to a variety of arrangement.In shallow surface applications,VWTM can get the more high frequency part of the surface wave information;VWTM can extract high mode surface wave dispersion curve.The paper uses VWTM to deal with the ambient noise recorded in the field,and successfully extracts the clear higher modes frequency dispersion curve,and its corresponding frequency range is up to tens of Hz.From the theoretical calculation of Rayleigh wave dispersion,we know that Rayleigh wave is a multi-order model.The energy of the surface wave is distributed in all order modes.In different cases,the energy ratio of each order pattern may be different.The higher order model Rayleigh wave is larger than the base order mode penetration depth.When the formation structure is different,the corresponding base order dispersion curves may be similar,but the higher mode dispersion curves may have great differences,that is,the higher mode dispersion curve is more sensitive to the medium parameters.Therefore,it is necessary to consider the constraint effect of the high order dispersion curve in the inversion of dispersion curves.The paper uses the borehole data near the measuring point as the initial model,and uses the quasi Newton method to invert the multi mode dispersion curve extracted from the ambient noise record,and the inversion results show that there are many high pattern dispersion curves in the extracted dispersion curve.Three high frequency dispersion curves are retrieved.The inversion of the three order high frequency dispersion curve is carried out here,and the shallow velocity structure below the measuring point is obtained.