The effects of refractor topography and overburden anisotropy on time-term solutions of refractor anisotropy

Abstract

Summary It is generally held that a non-iterative time-term solution gives a satisfactory model if refractor dips are everywhere less than 10|Mo. Although this is a reasonable assumption for a refractor with a constant dip, for structures with strong directional characteristics such as anticlines and syn-clines the solution can be seriously degraded and will exhibit a refractor velocity that could be erroneously interpreted as anisotropic. By ray tracing through a continental model of an isotropic crust and upper mantle separated by an anticlinal Moho nowhere dipping more than 8|Mo, it is demonstrated that the Pntravel time can be well fitted by a non-iterative time-term solution with a 4.4 per cent upper mantle anisotropy. The effect is smaller for oceanic cases. These studies illustrate the importance of carrying out an iterative sequence of solutions and indicate that convergence at the correct solution will occur when a good azimuthal distribution of observations is available. It is further demonstrated that an anisotropic crust can perturb the travel times in a similar way to an anisotropic upper mantle. The correct separation of crustal and upper mantle effects requires the independent utilization of Pn and crustal phases.