Three-dimensional depth-velocity modeling is a very important step in three-dimensional pre-stack depth migration. In the initial depth-velocity model set up process, under constraints such as geology, well logging, seismics, etc., when the velocity-depth pairs are available, what is left is to resolve the tectonically constrained velocity field interpolation issues. In this paper, a depth-velocity field construction method based on a seamless three-dimensional block model is described. The method is completed in two steps: first we set up a three-dimensional geometrically and topologically consistent seamless block model, then we interpolate velocity-depth data for the depth-velocity model with three-dimensional block model constraints. This paper first introduces a seamless block model building method, followed by a description of the velocity field interpolation method with constraints from the seamless block model, and finally gives an example.
Usually the set up of a pre-stack depth migration interval velocity model requires a repeated iterative modification process: first set up a relatively reliable initial depth-velocity model, then disturb the model by residual velocity analysis and a new round of migration until the convergence of the velocity model is achieved (Luo and Li, 1999).
The initial depth-velocity model building process using multiple constraints (Liu, personal communication, 2005) is divided into two steps: (1) The first step is to take full advantage of the results from geological interpretation, logging information, and the RMS velocity field of the prestack time migration of the current exploration area to estimate the depth-velocity data of the critical lines and cross-lines.(2) The second step is the use of the depth-velocity data to set up an initial three-dimensional depth-velocity field under structural constraints. With the depth-velocity data known, structural constraints are used to set up an interval velocity model. We have developed a three-dimensional depth-velocity modeling approach based on the seamless three-dimensional structure block model.
The depth-velocity field building method based on a three-dimensional block model has two steps. First, horizon and fault data are used in the computational geometry algorithm to create a geological interface intersection framework model represented by a triangular mesh. Then a three-dimensional geometrically and topologically consistent seamless block model based on this framework model is set up. The seamless block model is the bridge between the triangle mesh surface model and the entity property model, which is represented in the seismic grids (Jiang et al,2002). This step is the core and key of the entire method. Finally, the depth-velocity model constrained by the block model is interpolated from the velocity-depth data. This process makes full use of the advantages of the seamless feature of the geometrically and topologically consistent block model. The final initial model is a three-dimensional velocity field in the seismic grid representation.
The steps to set up a three-dimensional geometrically and topologically consistent seamless block model from horizon and fault data of the seismic interpretation are as follows:
1. Setting up the intersection framework of fault planes
In order to overcome the multi-valued situation in the z direction, this paper follows the following detailed method:
