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Introduction to ocean particle velocity and measurement techniques

S.M. Jesus sjesus(at)ualg.pt and P.J. Santos pjsantos(at)ualg.pt
LarSys, Universidade do Algarve, Campus de Gambelas, PT-8005-139 Faro, Portugal.

Comments: training session zip.
Ref.: EMSO Time Series Conf. Observing Ocean Sound, Gran Canaria (Spain), October 20-22 2021.

At the macroscopic level the relevant parameter for studying ocean sound is acoustic pressure. However, at the sub-wavelength level and close to shores / acoustic surfaces, the particle velocity field shows significant departures from the acoustic pressure field. So, particle velocity and acoustic pressure are two manifestation of the same phenomena at two different spatial scales. There are two important aspects related to particle motion/velocity: one is that many marine animals, including fish and cetaceans, are extremely sensitive to it, and the other is that while acoustic pressure is scalar, particle velocity is vectorial and therefore allows for determining not only its strength but also the wavefield direction, which is of paramount importance in many underwater applications. This training session explores these two aspects with the objective of providing the necessary understanding of the role of particle velocity to ocean observation, including the impact of sound on marine life. For many years particle velocity measurements in the ocean was impossible or extremely difficult due to either too sensitive sensors picking up noise and interference, or due to bulky or delicate sensors difficult to deploy and operate in the field. Nowadays, the situation has significantly improved with vector sensor measurements being widely accessible, and applications starting to appear in many diverse fields. These will be reviewed with practical example highlights, as well as suggestions for deployment strategies and data analysis techniques.