Update on Ifremer technological developments and operational feedbacks with underwater systems
This paper reports on the status of Ifremer Underwater systems. It will focus on recent innovations, improvements and operational references of the remotely operated vehicle “Victor 6000”, of the autonomous underwater vehicles “AsterX” and “IdefX”, of the new “Sysif” seismic tool and some hints about RanD in the domain of Advanced Intervention AUVs.
Design and Experimental Validation of a USBL Underwater Acoustic Positioning System
This paper presents the steps for developing a low-cost POrtableNavigation Tool for Underwater Scenarios (PONTUS) to be used as a localization device for subsea targets. PONTUS consists of an integrated ultra-short baseline acoustic positioning system aided by an inertial navigation system. Built on a practical design, it can be mounted on an underwater robotic vehicle or be operated by a scuba diver. It also features a graphical user interface that provides information on the tracking of the designated target, in addition to some details on the physical properties inside PONTUS. A full disclosure of the architecture of the tool is first presented, followed by thorough technical descriptions of the hardware components ensemble and the software development process. A series of experiments was carried out to validate the developed prototype, and the results are presented herein, which allow assessing its overall performance.
Compared geochemical signatures and the evolution of Menez Gwen (37°50′N) and Lucky Strike (37°17′N) hydrothermal fluids, south of the Azores Triple Junction on the Mid-Atlantic Ridge
During the DIVA 1 cruise in May 1994, a series of 19 dives was conducted using the French submersible Nautile at the topographic highs of three volcanic segments centered at 37°17′N, 37°50′N and 38°20′N, respectively, south of the Azores Triple Junction (ATJ) on the Mid-Atlantic Ridge (MAR). Hydrothermal fluids were collected at the Lucky Strike (LS) hydrothermal field, discovered at 37°17′N in 1993, and at a new hydrothermal site called Menez Gwen (MGw) discovered at 37°50′N during this cruise. Both systems are relatively shallow compared to other MAR systems with seafloor depths of 1700 and 850 m, respectively, characteristics which make them unique among the already known sites on the MAR. The characteristics of the LS fluids are the same as in 1993 and include temperatures ranging from 170°C to 324°C, variable chlorinities lower than seawater, low hydrogen sulfide (<3.0 mmol/kg), high Ba concentrations (up to 80 μmol/kg), low metal concentrations and high gas contents, and distinct chemical end-members indicative of significant geographic control of the venting system. In contrast, the very clear MGw fluids at 37°50′N show a rather uniform exit temperature (285°C) and chemical compositions with chlorinities (360–380 mmol/kg) lower than at LS and corresponding lower concentrations of cations, H2S (1.8 mmol/kg), metals and silica (8–11.5 mmol/kg), due to the lower temperature (T) and pressure (P) of the system. In the two systems, fluid chemistry is strongly affected by phase separation. At the low-pressure conditions of these sites, phase relations in the NaCl–H2O system dictate production of an extremely low salinity. The higher salinities observed in fluids thus indicate that mixing of extremely low salinity vapor with single-phase hydrothermal seawater is the dominant process controlling Cl concentrations. Depletions and enrichments of elements in solutions are also explained by the involvement of fluid-rock reactions. The enrichment of Ba, K, Cs and Rb in the fluids is linked to the enriched character of these elements in basalts. Relatively high pH and Ca, low Li, Sr and metals are related to the highly altered basaltic substrate in the reaction zone. Low Sr/Ca and high Ca/Na ratios are explained by albitization within the crust. The increase of degassing when approaching the Azores may be related to the carbon-enriched basaltic crust near the Azores hot spot. Isotopic ratios show that CO2, CH4 and helium gases have a magmatic signature. Compared to other deeper sites on the MAR, all fluids collected at LS and MGw are gas-enriched, except for H2S and helium, whose both concentration and isotopic ratio compare well with other hydrothermal fluids. The high CH4 concentration associated with unsaturated hydrocarbons and the high CH4/3He ratios also suggest a contribution of CH4 generated by serpentinization of ultramafic rocks by Fischer–Tropsch catalysis of CO2 reduction.