The evaluation of accuracy and precision of the employed forward solution m
ethod (ray tracer) and of its effects on the inverse solution is one of the
key issues in quality and reliability assessment of tomographic images. In
general, an analytical solution to the forward problem does not exist for
arbitrary three-dimensional (3-D) velocity models. To analyze ray tracing p
erformance and to evaluate the effects of numerical inaccuracies and approx
imations inherent to any such forward calculation method, we implement a 3-
D ray shooting algorithm in the widely used SIMULPS software and compare it
to its' standard approximative pseudo-bending method (ART_PB). In a first
step the effect of the different parametrizations of the velocity model req
uired by the two ray tracers is assessed. This leads to the definition of s
pecific 'forward grids' derived from and numerically representing the same
seismic velocity model, thus ensuring that only insignificant differences i
n the ray tracing results originate from the different forward model repres
entations. Hence, the observed differences in travel times and ray paths ar
e then due to inaccuracies and lack of precision of the ray tracers. Precis
ion of each ray tracer is assessed by exchanging source and receiver coordi
nate pairs and comparing the results from ray tracing in both directions, w
hereas accuracy is analyzed by comparing the results from the two ray trace
rs for the same source-receiver pair. Using realistic heterogeneous synthet
ic velocity models, our results indicate that both ray tracers are precise
within +/- 10ms in travel time for ray lengths less than about 60 km, and r
ay paths are identical with respect to appropriate Fresnel volumes. For lon
ger rays, however, the ray shooting method yields significantly smaller err
ors than the ART_PB ray bending. Even with regard to optimal resolution in
real tomographic images, differences between the two ray tracers are insign
ificant up to about 80 km raylength. Noteworthy differences, however, appea
r in the resolution matrix calculated for the same inverse problem with the
two different ray tracers. In addition, differences in the take-off angle
of the ray from the source are significant with respect to calculations of
focal mechanisms. (C) 2001 Elsevier Science B.V. All rights reserved.