The most common form of seismic exploration remains a nearly linear survey with data acquisition lines including the source and receivers. The interpretation of amplitude and waveform information for such linear acquisition requires consideration of 3D seismic wavefields. In many scenarios the structure is approximately 2D, but still modeling is needed for point sources. In recent years 2.5D modeling methods have been developed for the simulation of 3D seismic wavefields in media varying in two dimensions. Although the computer memory requirements are only slightly larger than those for 2D computations, the computation times are too long for ready application to actual surveys. To overcome this problem, we propose a new approach for modeling 2.5D seismic wavefields using a quasi-cylindrical representation. We show an implementation of this approach using the finite-difference method and demonstrate the validity and efficiency of the technique with numerical examples, including a profile across a realistic model of subduction zone structure.