MICROCO 2018

Type	Test
Ship	Téthys II
Ship owner	CNRS until 2019 - IFREMER since 2020
Dates	10/04/2018 - 16/09/2018
Chief scientist(s)	LE BOT Philippe
	LABORATOIRE D'OCÉANOGRAPHIE PHYSIQUE ET SPATIALE - UMR 6523 Centre Ifremer Bretagne ZI Pointe du Diable CS 10070 29280 Plouzané +33 (0)2 98 22 42 76 dirumr6523@ifremer.fr http://www.umr-lops.fr/
DOI	10.17600/18000601
Objective	Several technical tests were carried out as part of the development of innovative oceanographic measurement systems. Tests of a microRider (MR, Rockland) working in "Yoyo" mode along a mooring line to assess its noise level. In fine, this MR on a mooring should provide a measure of the time/spatial variability of very small scale turbulence. Testing and qualification of a new RtSys acoustic source, within the framework of the Cognac project. The project aims at measuring the fine-scale oceanic dynamics thanks to the acoustic monitoring of a cloud of low-cost floats. First tests of a low-cost acoustic float prototype. Three-headed sensor intercomparison mounted on a Deep Argo float.

Scientific context

Main results

This campaign of technological tests enabled the acquisition of a large amount of data that helped to advance the concepts of certain instruments (MicroRiyo), leading to the deployment of a MicroRiyo mooring in summer 2025 for a period of one year. It was also an opportunity to validate the proof of concept for the COGNAC float and to test and validate acoustic communication with the RtSys source. The first deployment of the three-headed ARVOR float was an opportunity to compare several sensors and to define the most suitable one.

The main findings and results of the various tests are as follows:

MicroRiYo:

The difference in response between the two channels made it possible to optimise the orientation of the MR-6000 head carrying the velocity shear sensors used to calculate dissipation. In fact, the sensor on channel 1, which is worse than channel 2, could be in the drag of the sensor on channel 2.

On the other hand, it remains difficult to explain why channel #2, although consistent with VMP measurements in the subsurface, suddenly overestimates the signal above a certain depth (which is never the same). This is all the more so as examination of the accelerometer signal for each of the profiles shows no increase in vibration associated with the change in the velocity fluctuation signal. An examination of the compass doesn't show anything remarkable either. It is possible that the sensor itself was sensitive to pressure above a certain threshold. Replacing it on the last two profiles was inconclusive as the sea/wind conditions were much less favourable.

The 8 profiles carried out on leg 2 highlight one of the difficulties of this measurement (measuring speed fluctuations to quantify the dissipation of kinetic energy): since the line on which the vehicle is travelling is ballasted directly above the vessel, as soon as the wind/waves are sufficiently strong, the vessel has to correct its course and trajectory, which affects the line and the speed fluctuation measurements (this is the case for the last two profiles). Nevertheless, the first 6 profiles confirmed the approach by highlighting the consistency of the speed fluctuation signal (and therefore dissipation) between the MR-6000 profiles (channel #2 subsurface) and those of the VMP-6000.

Figure 1- Comparison of the kinetic energy dissipation profiles measured by the VMP-6000 (black) and the MicroRiYo vehicle carrying the microrider (blue: sensor 1, red: sensor 2). For most of the profiles, sensor 2 is in good agreement with the VMP measurements above 200 dbar (here up to 330 dbar). Sensor 1 shows a systematic overestimation of the power loss, probably related to the positioning of the sensor on the head of the microrider.

COGNAC float and acoustic source :

The tests carried out during this campaign have enabled progress to be made in the development of underwater floats by identifying the following areas of work :

Perfecting the ballasting protocol and the control methods.
The acoustic tests carried out enabled us to gain experience in the operation of an acoustic source, which is now used for underwater positioning exercises.

These technical advances were then implemented as part of the CNFC COGNAC2020 campaign.

The experience gained during MICROCO is now being used to continue the COGNAC project, currently supported by the ID-GF project of the Banque Publique d'Investissement (BPI) and the EMOI project of the Office Français de la Biodiversité (OFB), both in collaboration with the robotics unit of ENSTA-Bretagne.

Three-head float :

These initial tests of the three-head float highlighted the following main points:

The SBE61 and RBR pressure sensors were largely in agreement, with the exception of the surface zones, which tends to show that they are accurate.
The SBE41 pressure measurement deviated from the other two during the cycle, reaching a bias of -5 dbars at the surface. This bias may seem high, but it was not out of specification (7000 dbars sensor, 0.1%xFS=7 dbars). This surface bias was not stable; it gradually decreased towards the pressure offset measured at the start of the next cycle.
There were differences between the 3 sensors in the areas close to the surface.

Data acquired and analyses carried out at sea and on shore

During this mission, a series of technological tests were carried out to qualify concepts or systems under development, which will eventually be used under operational conditions in scientific projects and campaigns.

MicroRiyo tests: The term MicroRiyo used in this document refers to the use of a microstructure measurement instrument (microrider) that moves in yo-yo mode (up/down) along a wetting line. The microrider is an instrument designed to measure velocity shear and temperature variability on vertical scales of less than one millimetre.

Tests on the COGNAC float prototype: The Cognac float is a low-cost acoustic float developed jointly by teams from LOPS, ENSTA Bretagne, the RDT (Technological Research and Development) department and Ifremer's NSE/ASTI (Underwater Acoustics and Information Processing Department).

Qualification of the Deep Argo 'Tri-head' float: The qualification of this ARVOR float, equipped with several CTD sensors, will make it possible to compare the different CTDs and ultimately define the sensor that could be installed on future ARGO floats.

MicroRiyo tests: the ultimate goal is to have a system for observing the oceanic microstructure on a mooring, capable of studying the dissipation of kinetic energy and the intensity of mixing by carrying out repeated vertical profiles autonomously.

The aim of these tests was to test version 2 of the vehicle and to identify a number of critical points with a view to moving the project forward. These tests follow on from the ESSLOPS campaign (2017), which used version 1 of the vehicle and whose preliminary results were encouraging. The vehicle has been modified into a version 2, the main differences of which are

- the use of a MicroRider (MR)-6000, which replaced the MR-1000 borrowed in 2017 from our Norwegian colleagues at UIB (Bergen). The MR-6000 weighs almost 6 kg in the water, compared to 0 kg for the MR-1000. The choice of an MR-6000 (purchased by LOPS at the end of 2017) was necessary to deploy the mooring on deep bathymetric structures to study current-topography interactions.

- Removing the external buoyancy that surrounded the top of the vehicle on version 1 to reduce the drag of the vehicle as it moved along the line. Reducing drag is a never-ending quest, as it affects vibrations, the speed at which the instrument falls and rises, the ballast required to achieve a given fall speed, etc.

- Replacement of the rings through which the mooring line slides: use of Teflon rings instead of stainless steel rings.

Profiling with a VMP6000 was carried out in parallel with the MicroRiyo measurements. Several tests were performed:

1/ To compare the intrinsic noise level of the vector with the VMP6000 when the MicroRiYo vehicle is in near free fall without the constraint of the mooring line.

2/ To validate the ability to cycle (ascend and descend) along a Dyneema cable of a MicroRider installed in a buoyant HDPE tube.

3/ To validate the ballast recovery system.

The sea trials revealed that

demonstrate an intermittent fault in the MR-6000 inclinometers, which subsequently necessitated the replacement of one of the electronic cards under warranty.
o show that one of the two channels (#1) measuring velocity shear almost systematically gives a stronger signal than the other channel (#2) (as well as in comparison with the VMP measurements) when the vehicle is forced to move along the line (a problem that does not exist when it is in quasi-free fall). This overestimation of the signal is independent of the number of sensors used.
To show that the trace (#2), which compares very favourably with the VMP measurements in the underground, suddenly becomes bad at a given moment below 200 m.
Adjust the ballast recovery system located in the lower part of the mooring line. Initial tests showed that if the distance between the ballast release point and the recovery net was too great, the ballast could not be recovered effectively.

Tests on the COGNAC float prototype: The aim was to generate data to test the acoustic system (RTsys SDA14 source and electronics and loggerhead hydrophone) of a low-cost float and to test the sensitivity of the geolocation quality in different configurations: depth of the sources, distance between source and float. These initial tests were also an opportunity to test various aspects of the float (dynamic behaviour, buoyancy, energy autonomy, transmission capacity, etc.).

The COGNAC tests allowed :

To highlight the lack of autonomy, especially during the servo phases of the float control system, compared to the specifications.
Define the specifications of the new acquisition and regulation system based on a Teensy card (without on-board OS), in particular to significantly increase the autonomy of the float.
To validate the acoustic acquisition system that currently equips the floats and to get to grips with the RtSys acoustic source.
To obtain a first set of acoustic data to validate the RtSys source system, the electronics of the acoustic receiver system and the hydrophone currently fitted to the floats.

In general, these tests allowed the current version of the float to be fine-tuned in terms of mechanics, electronics and software.

Figure 2: Cognac float (2022 version).

Qualification of the Deep Argo 'Tri-head' float:

These tests, carried out with our colleagues from Ifremer's RDT (Research and Technological Development) department, enabled us to carry out the first stage of qualification (end of qualification tests successfully completed in May 2019) of a platform that allows us to make very good in situ comparisons of different CTD sensors, to check their dependence on pressure, their accuracy and their long-term stability.

These deployments lasted only a few days and were limited to a depth of 2000 m. They confirmed all the functional aspects and the acquisition performance of the instrument, but did not provide enough in situ data to draw any conclusions about the performance of the sensors.

Since then, a long-term deployment of these floats has been carried out during the RAPROCAN2020 campaign.

Figure 3: 'Tri-head' float

Figure 4: Location of the different types of sensors

Missions

Bibliography

Publications

Ferron Bruno, Leizour Stephane, Hamon Michel, Peden Olivier (2025). MicroRiYo : An observing system for deep repeated profiles of kinetic energy dissipation rates from shear-microstructure turbulence along a mooring line. Journal of Atmospheric and Oceanic Technology, 42(4), 387-399. Publisher's official version : https://doi.org/10.1175/JTECH-D-24-0009.1 , Open Access version : https://archimer.ifremer.fr/doc/00928/104005/

References of International Seminar Communications

Grangeon, M., Socheleau, F.-X., Ponte, A., Le Courtois, F., Kinda, B., 2023. An Oceano-Acoustic Simulation Framework for the Design of Lagrangian Systems Drifting in Mesoscale and Sub-Mesoscale Currents, in: OCEANS 2023 - MTS/IEEE U.S. Gulf Coast. Presented at the OCEANS 2023 - MTS/IEEE U.S. Gulf Coast, pp. 1-8. doi: OCEANS52994.2023.10337251

References of National Seminar Communications

Mathis Grangeon, François-Xavier Socheleau, Aurelien L. Ponte, Florent Le Courtois, G Bazile Kinda. Estimation du canal acoustique sous-marin pour le positionnement et suivi de plateformes dérivantes sous-marines. XXVIII colloque GRETSI: Groupe de recherche et d'études du traitement du signal, Sep 2022, Nancy, France. hal-03769754

Le Bot P. et al. (2019) - Développement d'un flotteur acoustique à bas coût dans le cadre du projet COGNAC.

Mission	Name	Begin date	End date	Ship	Port of departure	Port of return	Chief scientist(s)
1	MICROCO 2018 LEG1	20180410 00:00:0010/04/2018	20180414 00:00:0014/04/2018	Téthys II	La Seyne-sur-Mer	La Seyne-sur-Mer	LE BOT Philippe
2	MICROCO 2018 LEG2	20180909 00:00:0009/09/2018	20180916 00:00:0016/09/2018	Téthys II	La Seyne-sur-Mer	La Seyne-sur-Mer	LE BOT Philippe