Techniques in coastal seismic oceanography: An example in the Adriatic Sea

Abstract

The rapidly developing field of seismic oceanography (SO) uses frequencies far lower (10–200 Hz) than tradition acoustic oceanography, and is not a measure of particulate scattering strength, but rather a direct, quantitative measure of vertical temperature gradient. The temperature gradient is typically a very weak signal (reflection coefficients of 0.001 or less) in the presence of higher amplitude coherent noise, such as the direct wave, and the reverberation of the ship noise (in shallow water). Towing the system faster than 4–5 kts to cover greater distances quickly increases streamer noise. Our objective to develop SO into a useful oceanographic tool is, therefore, a signal-to-noise problem, with mostly coherent noise. Using data acquired in the first coastal application of SO (ADRIASEISMIC), we show the magnitude of the noise sources and how they have been mitigated to result in quantitative (albeit band-limited) measures of temperature gradient from a few tens of meters below the sea surface to just meters above the seafloor. The profiles allow the detailed tracking of very small ocean features, among them warm thermohaline intrusions and dense, cold, bottom water masses, both in places only 10 m thick.