A 3D Shallow Velocity Model Building Method Using the Joint Inversion of First-Arrival and Reflection Traveltimes Constrained by Well Data

Prestack depth migration requires a velocity model that extends from the surface through shallow to deep layers. Generally, the shallow velocity model is built using first-arrival wave, under which the velocity model is built from reflected wave. These two models are integrated to form a continuous velocity model spanning from the surface to deep layers. Conventional shallow velocity model building methods are restricted by non-uniqueness of inversion, limited inversion depth and poorly defined reflection interface, which hinders their integration with deeper reflection-derived velocity models. Errors in the shallow velocity model can compromise the imaging quality of mid-deep. In this article, we propose a 3D shallow velocity model building method using the joint inversion of first-arrival and reflection traveltimes constrained by well data. This approach enables simultaneous inversion of velocity and reflection interfaces, thereby facilitating the integration of shallow and mid-deep velocity models. Combining the first-arrival and reflected waves enhances ray coverage angles and folds. By constraining the inversion for velocity and reflection interface with logging velocity and drilling depth, respectively, the method effectively mitigates the non-uniqueness of inversion, thereby improving the accuracy of shallow velocity model. The inverted reflection interface can also be used for integration of shallow velocity model and its underlying velocity model. The theoretical model test proves the feasibility of the method, and the field data application verifies the effectiveness of the method.