DIFFRACTION EFFECTS UPON FINITE-FREQUENCY TRAVEL-TIMES - A SIMPLE 2-DEXAMPLE

The widespread availability of broad-band digital seismic data makes i t possible to measure traveltime anomalies by cross-correlation with s pherical-earth synthetic seismograms. Finite-frequency diffraction eff ects render such measurements sensitive to wave-speed perturbations of f of the infinite-frequency geometrical ray path. We show, by consider ation of a simple 2-D example, that the Born approximation provides an excellent description of these off-path sensitivity effects, in the a bsence of caustics and for travel-time shifts that are small compared to the wave period. Remarkably, an isolated low-velocity anomaly may p roduce fringing fast travel-time anomalies, as measured by cross-corre lation.