Research On Time-depth Conversion Method Of Seismic Data In Complex Surface Area

The intelligent mining of coal resources and the CBM development mode dominated by horizontal wells put forward higher requirements for the control of micro morphological coal seam.Three dimensional seismic technology has been widely used in the control of coal seam occurrence form.Conventional seismic data are mostly in time domain.During seismic data interpretation,the target layer should be transformed from time domain to depth domain,that is time-depth conversion,so as to obtain the occurrence form of the target layer in depth domain.Traditional time-depth conversion methods are usually based on common datum.When the surface and geological conditions are complex,the accuracy of the traditional time-depth conversion method is low.Pre-stack depth migration processing technology can directly provide interpretation results in depth domain.However,due to the fact that the technology under undulating surface heavily depends on the rationality of formation velocity structure model,the technology is not perfect yet.Therefore,the research on high-precision time-depth conversion still has strong theoretical and practical significance.In seismic data processing under complex surface conditions,in order to improve the accuracy of time-depth conversion,researchers have carried out the research on time depth conversion method based on floating datum.Among them,how to obtain the floating datum in depth domain corresponding to time domain and how to obtain the high-precision average velocity information of target layer from pre-stack migration velocity field are the key to restrict the accuracy of time-depth conversion.Xinjing mining area is a complex mountainous landform,and the micro amplitude structure in the main coal seam is relatively developed.Taking the No.15 coal seam in this area as an example,the accuracy of time depth conversion method based on fixed datum and surface smooth floating datum under different borehole density constraints is compared.It is found that when the number of available boreholes in the study area is 2 boreholes per square meter,1.5boreholes per square meter and 1 borehole per square meter respectively,after the test of reserved boreholes,the error distribution of floor depth of coal seam 15 obtained based on time depth conversion of fixed datum and surface smooth floating datum is 1.34%,1.73%,2.03%and 1.30%,1.69% and 1.87%.The time-depth conversion accuracy based on the surface smooth floating datum has been improved,but both methods are difficult to meet the error accuracy requirement of about 1% required by the trajectory design of CBM horizontal wells in this area.Firstly,a method to obtain the floating datum in depth domain by using the shallow surface velocity structure is proposed.When calculating the floating datum in depth domain,this method uses the shallow surface velocity structure model obtained in seismic data processing.Because this method has homology with the floating datum in time domain,it has better matching with the floating datum in time domain.In addition,a spatial correction method of average velocity field considering the influence of formation shape is proposed.Through the analysis of the average velocity error of the target layer obtained from the pre-stack migration velocity at the well point and its influencing factors,it is found that the error can be approximately fitted as a binary function of the buried depth and dip angle of the target layer.Therefore,a spatial error correction method for the average velocity field obtained from the pre-stack migration velocity field is established.The application in the study area shows that,when the number of available boreholes in the study area is 2 boreholes per square meter,1.5boreholes per square meter and 1 borehole per square meter respectively,through the test of reserved boreholes,the average depth errors of coal seam 15 obtained by time depth conversion using this method are 0.75%,1.05% and 1.08% respectively.It basically meets the error accuracy requirement of about 1% required by the trajectory design of CBM horizontal wells in this area.The method has certain practical value in improving the control accuracy of micro morphological target layer in 3D seismic data interpretation under complex surface conditions.