By measuring the differences in phase angle of Fourier spectral densities across a fixed array, apparent velocities may be determined at a number of frequencies. Because the analysis is carried out in the frequency domain, determination may be objectively restricted to regions of the spectrum giving consistent phase relations between array elements.

Using an array of approximately 1.25 km length, the apparent velocities of the first arrivals from the 1962 North Carolina experiment were determined for shots from 45 to 155 km. No indication of an increase in velocity with range between 55 and 140 km was found.

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