Acoustic Wave Propagation in Stochastic Marine Environments

113 σ.Παράγονται οι βασικές στοχαστικές ακουστικές εξισώσεις και γίνεται μια επισκόπηση στις μεθόδους μοντελοποίησης της διάδοσης σε στοχαστικά ακουστικά μέσα. Η μέθοδος που είναι γνωστή ως smoothing perturbation και η ανάλυση πολλαπλών κλιμάκων, εφαρμόζονται σε ένα μονοδιάστατο πρόβλημα ακουστικής διάδοσης, όπου το μέσο έχει μία τοπική, ασθενώς στοχαστική, ανομοιογένεια. Δείχνουμε ότι οι δύο μέθοδοι είναι ισοδύναμες στο συγκεκριμένο πρόβλημα, καθώς, για ειδικές μορφές της συνάρτησης αυτοσυσχέτισης των στοχαστικών ανομοιογενειών του μέσου, καταλήγουν σε κοινές αναλυτικές λύσεις. Μελετάται η επίδραση της ανομοιογένειας στα ακουστικά κύματα και σχολιάζονται οι δύο μέθοδοι.The main acoustic equations are derived while introducing the appropriate stochastic dependencies. The area of modeling propagation of scalar waves in stochastic media is surveyed. Approximations of the smoothing and multiple-scale methods are introduced and then applied on an acoustic field propagating through a 1-D weakly stochastic slab. Analytic solutions, using a specific form of the acoustic medium correlation function, point out that both methods are equivalent for this specific problem. The physical effects of the stochastic inhomogeneity on the propagating waves are analyzed and finally the two methods are compared.Ζαχαρίας Γ. Καπελώνη

An integrated wave modelling framework for extreme and rare events for climate change in coastal areas – the case of Rethymno, Crete

Coastal floods are regarded as among the most dangerous and harmful of all natural disasters affecting urban areas adjacent to the shorelines. Rapid urbanization combined with climate change and poor governance often results in significant increase of flood risk, especially for coastal communities. Wave overtopping and wave run-up are the key mechanisms for monitoring the results of coastal flooding and as such, significant efforts are currently focusing on their predicting. In this paper, an integrated methodology is proposed, accounting for wave overtopping and wave run-up under extreme wave scenarios caused by storm surges. By taking advantage of past and future climatic projections of wind data, a downscaling approach is proposed, utilizing a number of appropriate numerical models than can simulate the wave propagation from offshore up to the swash zone. The coastal zone of Rethymno in Greece is selected as a case study area and simulations of wave characteristics with the model SWAN for the period 1960–2100 in the offshore region are presented. These data are given as boundary conditions to further numerical models (MIKE21 PMS and HD) in order to investigate the spatial evolution of the wave and the hydrodynamic field in intermediate and shallow waters. Finally, the calculated wave height serves as input to empirical formulas and time dependent wave propagation models (MIKE21 BW) to estimate the wave run-up and wave overtopping (EurOtop). It is suggested that the proposed procedure is generic enough to be applicable to any similar region

Effects of sampling intensity and biomass levels on the precision of acoustic surveys in the Mediterranean Sea

WOS:000744169400002Acoustic surveys represent the standard fishery-independent method worldwide for evaluating the biomass and spatial distribution of small pelagic fish populations. Considering the peculiarities of the spatial behaviour of pelagic fishes, the efficiency of the survey design in determining their biomass and spatial distribution is related to the ability to capture the portion of the patches accounting for a larger part of the total biomass. However, the spatial structure of the patches could be strongly influenced by ecosystem characteristics as well as by changes in total biomass related to a density-dependent mechanism. This is of particular interest for anchovies and sardines, which are known for their wide fluctuations and high sensitivity to the environment. In this study, we analysed the efficiency of acoustic surveys targeting European anchovies (Engraulis encrasicolus) and European sardines (Sardina pilchardus) in 10 different areas of the Mediterranean Sea spanning three years of different biomass levels. Using the geostatistical coefficient of variation (CVgeo) of the average occurrence probability of high/medium density values, we showed different patterns in terms of survey design efficiency among areas and species. Anchovies usually showed a lower CVgeo than sardines in the Alboran Sea. In 4 out of 20 cases, CVgeo values showed a consistent decrease with increasing biomass, while in the remaining cases, the CVgeo did not follow any clear pattern, suggesting the presence of important environmental effects. Higher survey design efficiency was found in highly productive sectors influenced by river run-off, allowing us to hypothesize that higher productivity along with the presence of well-localized enrichment mechanisms could favour a spatially consistent distribution and coherent organization of fish populations, leading to higher precision estimates with a given transect design. While most surveys displayed CVgeo close to 10% or less even at low biomass, indicating generally good performances of the survey design, a few areas exhibited higher CVgeo, yielding a potential need to decrease the intertransect distance, always keeping in mind that any survey should be as synoptic as possible

First description of the shelf epipelagic plankton layers at a Mediterranean basin-scale

This research represents the first approach to the study of the shelf zooplankton community at a Mediterranean scale, using acoustic standardized data collected in June-July in 10 different geographical sub-areas (GSAs) established by the General Fisheries Commission for the Mediterranean (GFCM) within the framework of the MEDiterranean International Acoustic Survey (MEDIAS). The analysis of the zooplankton layers based on their acoustic characteristics has revealed the potential of these surveys for the study of zooplankton at a Mediterranean basin scale and, also, the need to collect biological samples to interpret the acoustic records in terms of species. The fish population’s direct assessment is established in the MEDIAS framework, but the integration of zooplankton community data would constitute a qualitative step for the understanding of the fluctuations of fish populations and therefore to achieve the objective of an ecosystem-based management. Results have revealed the ubiquity of the zooplankton layers and its ability to form layers detectable by scientific echosounders throughout the Mediterranean at the common fish assessment frequency (38 kHz). In addition, the use of two frequencies (38 and 120 kHz) has allowed to apply the dB difference method, observing changes in the difference of the Mean Volume Backscattering Strength (∆MVBS) at 38 and 120 kHz frequencies, which would be related to changes in the composition of the zooplankton community