Headwaves in ocean acoustic interferometry

Abstract

In ocean acoustic interferometry, signals measured on two or more receivers are cross-correlated to produces an estimate of the Green's function between these receivers. A “virtual refracted” wave is an early arrival in the time-domain Green's function estimate. This virtual refracted wave is a phenomenon that has been widely reported on in the seismic interferometry literature. This early arrival of energy is also referred to as spurious energy or non-physical arrivals. Although this can be interpreted as a virtual wave it can be a result of the physically propagating headwave. The headwave travels in the seabed and re-radiates into the water column and therefore has important information content such as the seabed sound speed. In seismic interferometry, active sources and horizontal arrays are used but the virtual refracted headwave phenomenon is also observable on vertical arrays and with passive measurements of ocean noise. The signal processing used is a generalization of passive fathometer processing which applies beamforming (including adaptive methods) to the array data. Modeling and experimental data will be presented to show the headwave can be observed and used to estimate the seabed sound speed. The generalized passive fathometer signal processing is compared to the seismic interferometry processing.