Unlocking the Secrets of Ocean Dynamics: Insights from ALMA

Florent Le Courtois – florent.lecourtois@gmail.com

DGA Tn, Toulon, Var, 83000, France

Samuel Pinson, École Navale, Rue du Poulmic, 29160 Lanvéoc, France
Victor Quilfen, Shom, 13 Rue de Châtellier, 29200 Brest, France
Gaultier Real, CMRE, Viale S. Bartolomeo, 400, 19126 La Spezia, Italy
Dominique Fattaccioli, DGA Tn, Avenue de la Tour Royale, 83000 Toulon, France

Popular version of 4aUW7 – The Acoustic Laboratory for Marine Applications (ALMA) applied to fluctuating environment analysis
Presented at the 186th ASA Meeting
Read the abstract at https://eppro02.ativ.me/web/index.php?page=IntHtml&project=ASASPRING24&id=3673397

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Ocean dynamics happen at various spatial and temporal scales. They cause the displacement and the mixing of water bodies of different temperatures. Acoustic propagation is strongly impacted by these fluctuations as sound speed depends mainly on the underwater temperature. Monitoring underwater acoustic propagation and its fluctuations remains a scientific challenge, especially at mid-frequency (typically the order of 1 to 10 kHz). Dedicated measurement campaigns have to be conducted to increase the understanding of the fluctuations, their impacts on the acoustic propagation and thus to develop appropriate localization processing.

The Acoustic Laboratory for Marine Application (ALMA) has been proposed by the French MOD Procurement Agency (DGA) to conduct research for passive and active sonar since 2014, in support of future sonar array design and processing. Since its inception in 2014, ALMA has undergone remarkable transformations, evolving from a modest array of hydrophones to a sophisticated system equipped with 192 hydrophones and advanced technology. With each upgrade, ALMA’s capabilities have expanded, allowing us to delve deeper into the secrets of the sea.


Figure 1. Evolution of the ALMA array configuration, from 2014 to 2020. Real and Fattacioli, 2018

Bulletin of sea temperature to understand the acoustic propagation
The campaign of 2016 took place Nov 7 – 17, 2016, off the Western Coast of Corsica in the Mediterranean Sea, located by the blue dot in Fig.2 (around 42.4 °N and 9.5 °E). We analyzed signals from a controlled acoustic source and temperature recording, corresponding approximately to 14 hours of data.

Figure 2. Map of surface temperature during the campaign. Heavy rains of previous days caused a vortex in the north of Corsica. Pinson et. al, 2022

The map of sea temperature during the campaign was computed. It is similar to a weather bulletin for the sea. From previous days, heavy rains caused a global cooling over the areas. A vortex appeared in the Ligurian Sea between Italy and the North of Corsica. Then the cold waters traveled Southward along Corsica Western coast to reach the measurement area. The water cooling was measured as well on the thermometers. The main objective was to understand the changes in the echo pattern in relation to the temperature change. Echos can characterize the acoustic paths. We are mainly interested in the amplitude, the time of travel and the angle of arrival of echoes to describe the acoustic path between the source and ALMA array.

All echoes extracted by processing ALMA data are plotted as dots in 3D. They depend on the time of the campaign, the angle of arrival and the time of flight. The loudness of the echo is indicated by the colorscale. The 3D image is sliced in Fig. 3 a), b) and c) for better readability. The directions of the last reflection are estimated in Fig. 3 a): positive angles come from the surface reflection while negative angles come from seabed reflection. The global cooling of the waters caused a slowly increasing fluctuation of the time of flight between the source and the array in Fig. 3 b). A surprising result was a group of spooky arrivals, who appeared briefly during the campaign at an angle close to 0 ° during 3 and 12 AM in Fig. 3 b) and c).

All the echoes detected by processing the acoustic data. Pinson et. al, 2022

Figure 3. Evolution of the acoustic paths during the campaign. Each path is a dot defined by the time of flight and the angle of arrival during the period of the campaign. Pinson et. al, 2022

The acoustic paths were computed using the bulletin of sea temperature. A more focused map of the depth of separation between cold and warm waters, also called mixing layer depths (MLD), is plotted in Fig 4. We noticed that, when the mixing layer depth is below the depth of the source, the cooling causes acoustic paths to be trapped by bathymetry in the lower part of the water column. It explains the apparition of the spooky echoes. Trapped paths are plotted in the blue line while regular paths are plotted in black in Fig. 5.

Figure 4. Evolution of the depth of separation between cold and warm water during the campaign. Pinson et. al, 2022

Figure 5. Example of acoustic paths in the area: black lines indicate regular propagation of the sound; blue lines indicate the trapped paths of the spooky echoes. Pinson et. al, 2022

The ALMA system and the associated tools allowed illustrating practical ocean acoustics phenomena. ALMA has been deployed during 5 campaigns, representing 50 days at sea, mostly in the Western Mediterranean Sea, but also in the Atlantic to tackle other complex physical problems.