Three‐dimensional seismic raytracing for the forward modelling and direct inversion of teleseismic delay times

Summary This paper describes a simple, robust technique of 3‐D seismic raytracing through anomalous receiver structure. The technique uses a shooting approach and a vectorial description of both the ray and the anomalous structure through which it passes. By representing the receiver structure as a network of blocks, each with a constant velocity anomaly, the technique may be applied to the newly developed least squares inversion method (Aki, Christoffersson & Husebye) to define the required travel times of each ray through each block. The technique enables subvertical and non‐parallel block boundaries to be handled and positioned to mimic known tectonic contacts, such as the boundaries of descending plates or craton margins. An inversion using such a network will better define the velocity contrasts than one using a regular grid of arbitrarily positioned block boundaries. Alternatively, delay times, slowness and azimuth anomalies may be modelled by raytracing through assumed anomalous receiver structures. As the anomalies will deflect a ray away from its ‘mean‐earth’ path an iterative procedure is necessary to achieve the required end point. A novel, efficient procedure is presented, which involves making a search for the point through which the ray passes at the base of the receiver structure. The slowness/azimuth anomaly of each trial ray is determined as a mislocation vector. Trial rays with significantly different mislocation vectors can initially be regarded as possible different phases. Convergence of more than one phase to the required end point indicates the presence of multipathing. Lack of convergence of all these phases indicates the presence of a shadow zone.