The Research On Variable Velocity Mapping Method With Imaging Ray Tracing

The underground velocity is a critical media attribute parameter in geophysical exploration,and calculating the velocity accurately is one of the core geophysical problems.As only the accurate velocity has been obtained can we determine the reflected depth,stratigraphic dip angle and stratum location.The most common method to calculate the interval velocity from stacking velocity or migration velocity is using Dix formula.It is based on the assumption that no lateral variation in velocity field.However,with the increased exploration difficulty,this method is hardly to meet the requirement of nowadays oil and gas exploration.Then we in the actual situation how to more accurately get the formation of the following speed data,which is geophysical exploration for the most direct and fundamental one of the core issues.For 3D seismic data,velocity data is often obtained from velocity spectrum or velocity scanning in stacking processing.It's accuracy can be affected by many factors,including various random errors(lateral local noise,shallow conductive inhomogeneity,insufficient statics correction,etc.)and subjective errors.Therefore it is hard to get a good result in depth-time conversion only by simple calculation of stacking velocity data.How to improve the stratum velocity accuracy,and keep the spatial velocity tendency,thereby avoiding wrong structural interpretations caused by velocity traps is the starting point of variable velocity time-depth conversion.Obtaining accurate underground velocity,building high-accuracy velocity field and variable velocity mapping are some of the most important contents in geophysical exploration research.Variable velocity technique is the process of obtaining accurate interval velocity using velocity inversion along horizon,and then doing time-depth conversion of time structural map using migration methods.It's an effective method in the research of lowamplitude structural areas and complex fault blocks.According to different work areas,specific variable velocity mapping methods could be different.Presently,the four widely applied methods are the average velocity method,controlled horizon method,model chromatography method and template method.Moreover,in seismic data processing,prestack depth migration can be undertaken using pre-stack time migration field in order to obtain depth data.And then depth structural map could be created by depth domain interpretation.Variable velocity technique is a widely used and efficient method currently.It could help improve the mapping accuracy of depth structural map.In the process of velocity depth model building,ray migration plays an important role.It can convert time horizons to depth horizons.Before the concept of wave equation migration was proposed,people had used ray migration to deal with horizon homing problems,i.e.homing time horizons to depth horizons.Presently people are using the concept to build depth horizons,utilizing imaging ray to convert horizons from time migration profiles to depth domain horizons,correct the velocity field according to geometric dilution,build ideal depth domain velocity model,and then obtain structural map of target stratum.It mainly used to solve the problem of structural inaccuracy in high-steep areas.Imaging ray is a concept of geometrical optics.The equation that it follows is an approximate solution of the wave equation,which is valid only in the high frequency part of seismic waves.However,ray theory helps us recognize the characteristics and migration of seismic waves.Imaging ray connects scattered points in depth domain to imaging points in time domain.The deviation off the vertical line shows the common time migration error.If the value is great,which means there are deviations in length,dip angle and horizontal locations in reflecting object,relevant conversion is necessary to get the depth domain profile.In the actual work,the author has been engaged in the research and development of the DoubleFox series software.In this article,we designed a procedure of model chromatography method with the combination of DoubleFox structural mapping and depth conversion.Using the Visual C ++ development tool under Windows operating system,combined with visual interface design and object-oriented programming technology to design and implement the software,the software module of imaging ray tracing variable speed mapping is developed.The module uses the program application mode,so that users can quickly get started to speed mapping work in the software testing and practical application process has achieved good results,and according to the user to improve the use of the upgrade to meet the current user's Actual demand.In the end,using the software,we applied the method to a real work area of west China,and made remarkable achievements.As the structural features which we got after the procedure well met the expectation of log data,as well as with high accuracy.It can help in the future works of oil gas exploration.