Measurements of Spatial Coherence of Bottom-Interacting Sound in the Tagus Abyssal Plain
Coherence of bottom-interacting sound in the Tagus Abyssal Plain was determined as a function of bearing and frequency (20 to 2000 Hz) for grazing angles between 11° and 13°. An acoustic experiment was performed in which SUS charges were dropped in a circular pattern of 28 km radius around a deep hydrophone. The bottom-interacting arrival was isolated and processed to remove the decorrelating effects of varying bubble-pulse periods. A spatial coherence function was calculated between shot pairs corresponding to 5, 9, and 13 km separation. For frequency bands of 20 to 500 Hz and 1200 to 2000 Hz, the spatial coherence of the bottom-interacting arrival is high. For the frequency band of 500 to 1200 Hz, the spatial coherence is lower and more variable. The high coherence values are consistent with the Eckart theory for scattering from an interface having rms roughness less than 0.1 m. The sharply tuned nature of the low coherence values in a discrete frequency region suggests that the effect is due to interference from sediment multi-paths rather than scattering from bottom roughness. Bandpass filtered impulse responses show that the energy of the sediment-refracted arrival predominates at low frequencies and the reflected arrival predominates at high frequencies. The interference effects occur in the middle frequency region where the two types of arrivals have nearly equal amounts of energy, and may be expected to depend on sediment properties, bottom topography, measurement geometry, and oceanographic environment.
KeywordsSpatial Coherence Grazing Angle Ocean Bottom Coherence Spectrum Shot Number
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