HaoRaoZhao Aear Low Amplitude Structure Seismic Velocity Analysis

The number of low amplitude hydrocarbon reservoirs dis- covered is climb- ing as exploration goes into depth.The key to detect this kind of reservoirs is to draw up a high precision strcture map. Time-depth con- version is critical to the strcture map . Based on a theoretical analysis of the relation between time-depth conversion and precision of drawing,we study the feasibility and methods of the low amplitude structures during data interpretation.Seismic exploration is based on finding the rule of seismic wave in the layers.Velocity is most important parameter .From the velocity analysis ,we can know the rule of velocity distribution in the target area.And it is useful to avoid the interpretation mistake and improve the degree of reliability and precision in the mapping.Usually ,we obtain the velocity field from the seismic velocity spectrum ,VSP data and sonic logging data.The high precision of mean velocity can be obtained from the VSP and sonic logging data.However the cost of VSP is too expensive,especially the limited VSP data.And the sonic logging only get the high precise critical velocity .Fold velocity spetrume method only obtain the high precise lateral velocity.So we should flexibly use these datas to get the high precise velocity field.Seismic velocity is the important for time-depth conversion in the seismic layers interpretation.the precision of velocity is the key factor to mapping of depth resolution .So how to obtain the seismic velocity field is very important .At the present time ,there are lots of method to gain the velocity field.This paper presents three methods : 3-D ray tracing method , mean velocity method, Layer-by-layer stack method3-D ray tracing method is very useful for locating the layer,chromatography and offset .the precision of trace and velocity are impact on the resolution of interpretation.Thus ,the fast and precise 3-D ray tracing method is very important in geophysical field.In the light of 3-D model made on the complex geological structures in HQ area,an observation and analysis of the wavefield using the ultrasonic analog experiment are made .As a result ,the ultrasonic model records of the complex wavefield in multi-layer media are obtained.It can be seen from the ultrasonic analog time sections obtained under the complex geological structures and the actual geological sections.A single reflection horizon may reveal a time section of"multi-layer like"lineups,so it follows that the migration technique and 3-D seimci survey should be conducted in the complex structural area.In addition ,efforts should be made to the study of"unusual waves"during interpretation.In the seismic exploration ,velocity is the key to the interpretation,especially in complex area.low signal-to-noise ratio and shortage velocity spectrum ,it is difficult for geophysicists and gelogists to find the interval velocity of geological target.For the gentleness fold,mean velocity method provide a easy way to resolve this problem.In the 3-D space-time domain Layer-by-layer stack method calculate and add each layer velocity and depth.As a result ,we realize the time-depth conversion .there is difference between geological layers and seismic layers .At this time ,we can reduce the influence of reflected interface by using this method .While the high velocity layer and low velocity layer can be easy to find .For example,in the uniform reflected interface ,we can find the reliable low amplitude structures. This paper discussed methods how to get the time-depth conversion to study the low amplitude structures during data interpertation in the HaoRaozhao area .And we obtain the good result.