Measurements of multi-megameter travel-time bias and coherence

Abstract

The quality of ocean acoustic travel-time measurements depends on the coherence as well as the bandwidth of the signal. Ocean internal-wave fields are thought to be responsible for the loss of coherence in low-frequency acoustic signals. If the coherence bandwidth is less than the signal bandwidth, it is possible to consider sub-bands of the signal as separate measurements. The separate measurements can then be combined incoherently to improve the quality of the travel-time measurement. Theoretical work and computer simulations also predict that the travel-time of acoustic signals is biased by the ocean internal wave field. The path-integral theory for scattering predicts that the size of the travel-time bias depends on the logarithm of the center frequency of the acoustic signal, thus separate sub-bands would have different travel times and incoherent recombination would not be an optimal procedure. A recent experiment, in which two acoustic signals with different center frequencies were transmitted simultaneously, makes it possible to measure the bias experimentally and to determine the efficiency of sub-band averaging. Preliminary results suggest that the bias is about 50 ms at 3500-km range, which is roughly as large as expected.