A comparative study of 2D and 3D crosswell tomography

Abstract

Crosswell tomography can generate high-resolution velocity images that can be used directly in interpretation or as inputs to migration processing. Two-dimensional tomography is computationally efficient and has been widely used to invert crosswell data. Real geological structures, however, are usually 3D, and it is important to assess 3D effects on first arrival traveltimes and account for these effects in traveltime tomography. In this paper, we investigate how 3D structures affect first arrival traveltime data in crosswell surveys, and present a 3D tomography algorithm that minimizes a model objective function consisting of the second order directional derivatives of the model parameters. Our 3D forward modelling results show that 3D effects can become significant in a 2D crosswell survey when the 3D structure is close to the 2D survey plane. Furthermore, the 3D effects in the data can be propagated to the final inversion results. Three dimensional tomography counts for off-plane ray-paths and hence can produce better velocity images over 2D inversion in 3D environments. Because of the restriction from the geometry, 3D tomography of crosswell data from 2D surveys cannot provide any lateral resolution.