Using nonseismic data to aid velocity model building and complex imaging: A case study from western China

A workflow for inverting high-velocity conglomerates by simultaneous joint inversion (SJI) of seismic and nonseismic data was developed during a depth-migration project using land 3D data sets from western China. Large localized velocity variations caused by conglomerates severely limited the effectiveness of ray-based seismic tomography in generating an accurate velocity field. Single-domain inversion indicated different responses of particular lithologies for various nonseismic methods, and well-log crossplots enabled estimation of the relationships between models generated from different geophysical measurements. Using the high-resistivity property of the conglomerates, SJI of seismic first-break and magnetotelluric (MT) data was conducted to estimate spatial thickness and velocity. Combining the relatively low-resolution information derived from the MT data with the higher-resolution seismic image, the distribution of conglomerates could be interpreted with the help of well markers and well-logging horizons. The velocity model then was adjusted iteratively. The model that was updated by using the SJI workflow showed better consistency with borehole data compared with the initial tomographic model. The depth-migrated image created by using the updated model showed more geologically plausible structures both at target level and in the overburden.