Measurements of coherence of sound reflected from ocean sediments

Spatial coherence of sound reflected from the ocean floor at grazing angles 50 °–70 ° was determined from sound received by a hydrophone array suspended at midocean depths. An explosive source and the sensor geometry allowed isolation of the bottom reflections from other paths. A coherence function was calculated as a function of frequency 20–250 Hz and vertical hydrophone separation up to 280 m. Generally coherence decreases with increasing frequency and increasing sensor separation. The rms roughness of the sea floor was estimated from the coherence data to be about 0.5 m for the Tufts Abyssal Plain and 0.5 m for the Cascadian Plain. Differences between measured coherence spectra and theoretical coherence spectra for rough surface scattering might be due to effects of subbottom arrivals and/or changes in effective scattering area with frequency.