利用联合反演技术进行反射地震的波速成象

本文介绍了根据反射地震数据进行波速成象的一种方法,其基础为多种反演技术的综合。由于要求的波速图象C（x,z）具有间断性,除利用走时数据T（x,t）外,在地层比较水平的情况下,还利用了均方根速度V（x,t）和统计子波W（t）的数据来成象。计算机层析成象过程分为三步:首先重做速度分析,取得与初次反射走时一致的均方根速度数据;然后用反射走时与均方根速度联合反演对应分析道的层速度和界面深度;最后由联合反演结果和反射面走时求波速图象函数的数字化版。文中还给出了波速成象方法在我国西北某沉积盆地上的应用及验证结果。

VELOCITY IMAGING FROM REFLECTION SEISMIC DATA BY JOINT INVERSION TECHNIQUES

This paper introduces a new procedure for P-wave velocity imaging from reflection seismic data based on integrated inversion techniques. After deconvolution, stacking and migration, interpretors may distinguish the events between primary reflections and other events on a seismic section. Therefore, it is possible to obtain traveltime data and a statistical estimate of the wavelet W（t） from both CSP gathers and zero-offset sections by using an interactive workstation. If the dips of formations in the studied area are gentle, then the traveltime data T（x, t） can be picked up from common source-receiver seismic traces on a stacked seismic section.The traveltime data are insufficient for velocity imaging because there exist infinite solutions which can fit the data. We use both the traveltime and RMS velocity data to construct a joint inversion procedure. The first step in this procedure is to redo velocity analysis which keeps consistence between picked primary reflections and RMS velocity data V（x', t）, where x' represents the position of the velocity analysis traces. In order to guarantee the. vertical resolution of velocity imaging, the spacing of the velocity analysis should be less than 21 CDP traces, and depending on the thickness of target layers; the time intervel in the analysis should be less than 21 ms.The second step is joint inversion of traveltime data and RMS velocities for velocity analysis traces, producing intervel velocity and layer thickness via generalized inversion techniques. The equations in the joint inversion procedure can be represented as follows:*=1 wherec1 = interval velocity of the i-th layer,Vj = RMS velocity data at time tj,△T1 = T1 - T1-1, the diffirence of traveltimes between adjacent reflectors,δVi = errors in the RMS velocity data.After this equation is solved by employing a generlized inversion technique, the thickness of each layer can be calculated easily. This joint inversion method has been tested by synthetic models, having vertical resolution of 60 m and interval velocity variance about ± 100 m/s. In the areas lacking wells, the inversion results, acting like acoustic logging data, provide satisfactory constraints for quality control of velocity imaging.The third step in the imaging procedure estimates the velocity variations in each section-block between adjacent velocity-analysis traces via inversion of traveltime T（x, t） and the resulted interval velocity data c（x', z） on the block boundaries. Various techniques in computed tomography and wave equation inversion may be used for velocity reconstruction in a sectio-block. For instance, an 2-D scalar wave equation may be reduced to the eikonal equation via. WKJB approximation, then discrete reconstruction algorithms, e. g. ART or SIRT in computed tomography, may be applied. By employing the "Exploding reflector" model, the 2-D scalar wave equation can be written as:wherevi（x） = continuous effective velocity for the i-th interface, can be calculated via c（x' , z）, P（x, z） = pseudo-source term representing velocity discontinuities on reflectors, W（t） = source signiture obtained from wavelet processing.In order to determine the velocity discontinuity P（x, z） which is actually a singular function of the velocity function c（x, z）, we need singularity inversion techniques to solve the source-term inverse problems of partial differential equations. Finally, a discretized image of c（x, z） can be obtained from P（x, z） and vi（x）.The above mentioned procedure of velocity imaging has been tested by a few drilling wells in a sedimentary basin in western China. The results （see enclosed color photos） shows a positive relation between velocity images and variation of lithology, giving distinguished indications of obscure traps for locating oil and gas deposits.