Characteristic Analysis Of P-wave Reflection Time-distance For The Case Of3D Seismic Curved-interface

In recent years, seismic numerical modeling of3D complex structures, especially3D curved-interface (anticline, syncline etc) becomes a research focus in3D seismicreflection. It is very difficult and time-consuming to calculate the precise locations ofthe reflection points, in order to analyze the P-wave reflection time-distancecharacteristics systematically and accurately, the work regards3D curved interface asan envelope of3D dipping interfaces with different tendencies and inclinations, andpresents a new method of calculating P-wave reflection time-distance for3D curvedinterface using3D dipping interface reflection principle. Based on the dip CDP theory,the surface imaging points, that are surface normal direction projection points, can beacquired from the reflection points, the locations of shot points are obtained from thepositions of receivers and surface imaging points, and then the distributions of shotgrid and receiver grid are known. Finally3D P-wave reflection time-distances of shotgather are calculated.The paper discusses the relationship of surface imaging points with differentinterface shapes, changeable inclinations and inclinations. With the curvature of3Dcurved-interface increases, the distribution of surface imaging points becomesirregular. This problem can be solved by extracting a regular imaging grid formintensive projection points, in order to imaging quickly and accurately in subsequentdata processing.In this paper,3D curved-interface is simplified as a rotating paraboloid, whichhas an analytical solution. The overlying medium of the curved-interface is equivalentto a uniform medium. Firstly, the work analyzes the time-distance characteristics ofthree types of interfaces (level interface,3D dipping-interface and curved interface),and then systematically analyzes the changes of reflection time-distance with differentshot positions, orientations of survey lines and curvatures of curved-interface.The reflection time-distance feature comparison of3D curved interface and3Ddipping interface shows that it is difficult to distinguish time-distance charactersbetween two types of interfaces only from the same orientation of survey line, andthis problem can be solved by comparing time-distance differences between differentsurvey orientations. Numerical simulation results of3D convex interface show that reflection pointsare more concentrated, and then the projection points are more scattered, reflectingthe divergent characteristic of the convex interface with the curvature of3Dconvex-interface increases. The Concentrated distribution of reflection points makesincident angle and reflected angle change, and then the slopes of the time-distancecurves become larger compared to level interface and3D dipping-interface.In the numerical simulation of3D concave interface, the reflection points aremore scattered, and then the projection points are more concentrated, reflecting theconvergence characteristic of the concave interface with the interface curvatureincreases. The increase of concave curvature makes time-distance curve become“leveled” gradually, and then change to “upturned” form. When the interface isconvex interface or3D dipping interface, time-distance curve does not appear theseforms. Therefore, we may put the straight or upturned forms of shot gathertime-distance curves as the criterions of subsurface concave interface.The form of time-distance curve changes and vertex of time-distance moves withthe change of shot point location. Characteristics of time-distance curve are affectedby relative position of shot and survey line, interface morphology and survey lineorientation comprehensively. When shot point is far away from survey line, part ofthe receivers cannot get records from the interface reflection, therefore empty tracesappear in the survey lines affected by the critical distance of ray propagation.Numerical simulation results of shot gather show that this method has highcomputing efficiency and can be applied to analysis of the P-wave reflectiontime-distance characteristics with different positions of shots; especially it can revealthe relationship between reflection points and the time-distance curve. Finally, Iconcluded that the reflection time-distance of one shot is only partial response of3Dcurved-interface; numerical simulation starting from the reflection points can controlthe uniform distribution of the reflection points well, and can obtain the reflectioninformation of the imaging key parts from reflection records of observing systems.