The UlyXDemo-24 technical cruise was conducted in the Thyrrenian sea to a) transfer the AUV UlyX to the GENAVIR for routine operations by the French Oceanographic Fleet, to b) test and evaluate the conditions, performances, and limits of operation to achieve scientific goals defined by a science Party while c) scientifically validate its sensors and acquired data. The mission modes tested during the cruise may differ from those finally validated by GENAVIR after the AUV UlyX transfer, based on the analysis of cruise results, vehicle security, and other constraints, resulting in different limitations. The interactions and team work between the GENAVIR, IFREMER and Science Teams was key cruise success, advancing on UlyX improvements while gathering scientifically valuable data. A hybrid mode of scientific/engineering cruise is viewed as optimal to accelerate and facilitate the evolution of UlyX to achieve its full capacity (instrument integration, and full-depth operations to 6000m).
UlyX is equipped with instrumentation for acoustic surveying (multibeam, reflectivity, water column), optical inspection (APN, i.e., still camera with flashes; Figures 2,5), and other physicochemical sensors (oxygen, pH, redox, magnetometers (3), fluorimeter, nephelometer). At the time of its transfer, UlyX has not integrated yet two core instruments, the interferometric sonar system (SAMS) and the sediment profiler (ECHOES), and thus a flight mode of 20-30 m for the interferometric sonar (on flat seafloor). The integration of these remains a priority for the scientific community to have an AUV that is both operational and flexible for a wide range of scientific missions (including P1 and other cruises).
The UlyX Demo-24 cruise targeted areas in three types of terrain (Figure 1), with depth of test sites ranging between 500 and ~3700 m, for UlyX dives including both inspection and survey modes:
a) Sedimented areas with gentle relief (e.g., abyssal plain, Italian margin), <10°,
b) Areas with relief typically <20°,
c) Rugged terrain with scarps, slopes (>30° locally) and complex morphology.
Some of these areas also host hydrothermal vents and gas plumes, that allowed the development and evaluation of different surveying and inspection strategies, as well as the data acquired during the dives.
AUV UlyX conducted 17 dives (Table 1). Dives during Leg 1 included several technical and engineering operations to correct issues with the AUV and improve performance and reliability. From dive #93-07, the AUV operated in what can be considered a nominally routine mode, with dives typically lasting about 14h (deck to deck), conducting surveying and seafloor inspection in proportions depending on scientific objectives, and with no dive interruption for technical issues.
Regarding the navigation over different kinds of terrain the UlyXDemo-24 cruise confirms the capacity of the AUV UlyX to conduct multibeam acoustic surveying under all terrain conditions (including the most extreme) at a nominal altitude of ~70 m above seafloor at 3 knots (Figure 3, 4).
Analyses onboard show that the bathymetry is of high quality, owing to vehicle dynamics, highquality navigation, and improvements implemented to solve prior problems (e.g., pitch/heave noise, DVL problems, etc). In this configuration, bathymetric grids have a resolution of ~1 m. Bathymetry from optical inspections (9-6 m of altitude) also yield excellent bathymetry (10 cm resolution).
Optical seafloor inspection with parallel tracks can be done in flat zones (slopes <10° at the scale of the inspection). Optical inspection in rugged areas was tested successfully with a hover mode that provides ground-truthing over areas of ~100-200 m2 or more; this mode of inspection (Figure 7) is extremely promising and of interest for ground-truthing in all types of terrain. Systematic inspections (mosaicking) in extremely complex terrains (e.g., hydrothermal field with chimneys >5 m-high, large boulder files) are more suited for ROV, complementing the hover mode of AUV successfully conducted during UlyXDemo.
Optical inspection at 6 m altitude provides high-quality data. Dives at 9m altitude provided underexposed images that are exploitable scientifically (Figures 5,8). The APN camera shows several issues (Figure 9): it does not trigger consistently; interrupts acquisition; shows uneven shooting intervals (PL#16). AUV immersion at 6m was not maintained in some inspections, flying instead at 9m. Unmounting of the APN in PL#16 revealed scaling of the dome. The APN was removed for security on PL#17. Reliability of the APN and solving the dome fitting are highest priorities for UlyX in the immediate time.
The multibeam system records the water column return for plume detection at all altitudes (including inspections 6-9m above seafloor, Figures 5,6). This allows a multi-scale, nested survey strategy combining large area acoustic surveys (~70 m altitude) and optical inspection (6-9 m altitude), correlating imagery and water column data to pinpoint plume sources in optical mosaics and 3D reconstructions (Figures 5,6). This survey strategy was particularly successful for sites in flat-lying areas (e.g., gas plumes, pock mark fields, etc.).
All sensors recorded data that were validated onboard, within the limitations of the Science Party expertise. The Science Party also evaluated and improved the protocols for some of the instruments (e.g., pH, Eh), to establish the mode of operation during cruises, with calibrations secured by the science team (for all UlyX science cruises as an obligation of the PI). A communalized instrumentation and associated protocols at the level of the FOF and the deep submergence vehicles would greatly facilitate interoperability, management, and data consistency among platforms. Ascent and descent may show an offset for given sensors; similar offset is observed in the CTD acquisitions from the board.
Deployment and recovery operations were efficient and well-run. AUV operations are intended in tandem with ROV or HOV, one of the main assumptions and requirements of the CORAL/ UlyX Working Group. These operations cannot be conducted simultaneously onboard L’Atalante. This inability to operate two deep-sea submersibles will impact the flexibility of future operations for the French National Fleet, as only Pourquoi Pas? can conduct these operations. Cruises on L’Atalante would require instead two legs in the case of a two-vehicle strategy, which is not sustainable nor optimal.
During UlyXDemo-24, L’Atalante escorted the AUV through almost all the dives. The scientific community requests operations conducted during AUV dives, as conducted in the past with an external AUV and ROV Victor diving simultaneously. As a preliminary test of this type of strategy, we conducted two CTD casts during AUV dives. L’Atalante was also positioned away from the AUV during a survey to test the range limit to position the AUV (~3 km at 700 m immersion, >4 km at 3000m immersion) and to get telemetry (~1.5 km at 700m immersion). Operations during AUV dives remains a priority of the science teams.
At the end of the UlyXDemo Cruise, the preliminary analysis of dives and operations shows that AUV UlyX can fly multibeam survey missions in all kinds of terrain. Optical inspection can be conducted with strategies tailored for the terrain conditions so as to secure the AUV, but require further reliability of the camera system as a priority. Future operations and field experience, together with feedback from operators and scientists, will surely result in improving its capacities and reliability in coming years, and hopefully developing the possibility to carry out operations during AUV dives (e.g., CTD, coring, dredges, ROV deployment, etc).
AUV UlyX requires in the short-term a) securing the reliability of the optical payload, b) the integration of the sonar and sediment profiler instruments, identified by the CORAL/UlyX Working Group as key for polyvalent scientific operations, and c) tests at >5000 m in ‘routine mode’ to confirm its full depth operability: King’s Trough dives (ESSULYX23B) did not operate UlyX in fully scientific mode, and maximum depths in the Thyrrenian (UlyXDemo) are limited to <3800 m).