Forward Modeling And Inversion Of Time-distance Of P-wave Reflection For Shot Gathers With Complex Interfaces

In recent years, the seismic exploration under complex geological conditions is a focused and difficult issue. It is necessary to make detailed research on the reflection characteristics of complicated interfaces, which are derived from shotgather records. The advantage of shotgather lies that it contains a series of full information of reflection points. Based on the numerical simulation of P-wave reflection of a 3D complex interface, this thesis presents a more realistic description about the complex models. Firstly, the range of reflection interface is limited, different from the previous research which focused on the infinite extension interface. Under such a circumstance, it is unclear what are characteristics of reflection traveltime, and the relationship between the limited interface and the extent of the traveltime curve. Next, the complex structure is described by a number of interfaces with different properties, such as horizontal, tilted, and curved interfaces. The features of reflection traveltime and the traveltime relationship among these interfaces caused by this kind of complex model is largely not concerned in previous work.With the two limited conditions above, the complex model would be more grounded in reality. Furthermore, as 3D seismic exploration has greater advantages and potential than 2D seismic prospecting, which can provide a major constraint on the complex subsurface structure and reveal the urgent problem about the inclined path of the propagation of seismic waves, even in isotropic media with special complexity. Therefore, this work attempts to simulate the complex structure based on a 3D multi interface embedded model. First, it performs forward numerical simulation of P-wave reflection traveltime characteristics starting from reflection points under the 3D complex structure condition. Then, on the basis of the numerical forward modeling, with the example of 2D seismic data, this work inverts P-wave reflection traveltime using fitting curves, including real data processing and tests of synthetic seismic shotgather records.The results of numerical forward simulation show that there are several curves in shotgather records, in which the features change with the azimuths of survey lines. It means that different curves correspond to varied 3D ray paths, ranges and positions of reflection points and interface features. The next conclusion is that there is a complex position relationship among curves, including cross, intermittent and no-signal traces, all of which are closely linked to the range and position of the interface. At last but not least, this study acquired the correspondence relationship between the range of the reflection area and the receiver by theoretical numerical analysis of a complex interface model. The partial reflection features and the reflection blind zones are proved to be presented in theory, which is in agreement with real seismic data processing.Taking the 2D actual seismic data for example, this work obtained the information of velocity of the medium, interface depth, apparent dip and the extent of the interface by P-wave reflection traveltime inversion under the condition of small seismic arrangement. The results from theoretical data and actual data inversion demonstrate that the inversion of shotgather can be regarded as the initial model of seismic data stack and migration processing. At the same time, it can be used as a constraint on imaging, and can improve the self-inspection ability of geophysics.This research would be helpful to actual survey design, model prediction and data processing and interpretation, and to improve the understanding of 3D complex interfaces in seismic exploration.