Modeling of frequency-division velocity and its application in igneous rock area in G structure, Xihu Sag

The G structure in the Xihu Sag of the East China Sea Shelf Basin is influenced by tectonic inversion and volcanic activities, leading to significant lateral velocity variations. The frequency information of the seismic velocity field obtained by traditional processing method is limited, which is a challenge to acquire specific information of igneous rocks of different scales and to predict accurate structural feature. A new method for frequency-division reconstruction of the seismic velocity field and a hybrid interval velocity model for time-depth conversion was proposed, which is applicable to the areas with structural inversion and the development of igneous rocks. First, igneous rocks were identified and classified by scale through multi-attribute analysis. For large-scale igneous rocks and lateral velocity trends, low-frequency velocity components were derived via high-resolution nonlinear grid tomography combined with pre-stack depth migration, which was further enhanced by anisotropic velocity modeling to improve complex structural imaging. For small-scale igneous rocks, high-frequency velocity components were obtained through pre-stack simultaneous inversion. Secondly, the high- and low-frequency velocities were then integrated to reconstruct a seismic interval velocity field, which is not only consistent with the structural trend, but also can reflect the local details. Finally, under structural model constraints, well-derived velocities were used to calibrate the frequency-division velocity model to build a hybrid interval velocity model for time-depth conversion. Based on this method, we found the migration of structural high in the deep and shallow layers of the G anticline. The finding was supported by the drilling of an exploration well: the pre- and post-drill structural interpretations was highly consistent and the depth prediction errors were within 10 m, which validated the reliability of the method. Inspired by the new understanding of structure G, we found that the structural-high migration in other anticlinal structures in Xihu Sag was common through variable velocity mapping. The early high points in the deep layer are more conducive to the early convergence and accumulation of oil and gas, which is favorable exploration direction for the next-stage working.