On crustal structure deduced from seismic time-distance curves

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Abstract
The numerical computation of the time-distance curves for several quite different velocity-depth functions shows that first arrivals are not sufficient to determine the velocity-depth function. Minor second-order fluctuations in velocity superimposed on a general velocity gradient in the crust give a travel-time curve with multiple arrivals separated by fractions of a second. These initial bursts of multiple arrivals are observed in explosion seismology for shot-point ranges out to 450 km. With observed seismic data the position and amplitudes of the late arrivals and especially cusp points determine the general details of the crustal structure.