Sea Technology

AUG 2012

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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The EcoMapper has three different operating modes: con- stant depth, constant height or undulation between the sea sur- face and a maximum depth (yo-yo mode). The different operating modes were tested in this campaign, with the yo-yo mode chosen to generate the integrated vertical profiles for the entire perimeter of the farm. This mode is the best way to ac- quire data in an extensive area (horizontal variability), reaching from the surface to the seafloor (vertical variability) in a short period of time and obtaining data sinopticity. The maximum depth of this area is 20 meters, but during the survey, the extent of the vertical profiles was limited from 0.5 meters below the water's surface to 2 meters above the seafloor to ensure the AUV was secure. To complement the data, additional depth- temperature-salinity profiles were conducted with a SonTek (San Diego, California) YSI CastAway CTD, deployed from the safety boat. Fortunately, during fieldwork, the weather conditions were adequate for AUV deployment. The mussel farm was closed temporarily during the survey due to a red tide alert, which eliminated the risk of the AUV colliding with service boats, al- though there were many small fisherman boats around the platforms that presented a risk. Deployment Problems, Precautions Several problems occurred during AUV deployment. The ef- fect of the mussel platforms' iron anchors on the EcoMapper's magnetic compass caused navigation errors. Floating debris was found on the field due to the strong winds that occurred a few days before the survey. For most of the survey, the weather conditions were good. However, on the last day, the weather changed, and the safety boat had to recover the AUV prema- turely to prevent it from crashing against the rocks. As a precaution, the presence of seaweed had to be moni- tored to prevent entanglement with the propeller, which could cause the vehicle to go adrift at the surface, with the additional risk of crashing into the cliffs. For safety, a close supervision of the surface was necessary. As another precaution, it was important to talk in advance with farmers and fishermen to agree on a survey plan. Their input was needed to plan the survey by sectors, with no-enter zones established for the farm's support vessels during short periods of time. An alternative was to create AUV tracks during nonworking hours on afternoons and weekends. Water Quality Monitoring The profiles for the physical-chemical parameters are being analyzed. However, the ADCP data obtained using the yo-yo mode is more complicated to process due to the continuous variation of the AUV pitch and roll. For the next samplings, horizontal surveys at a middle depth will be performed to solve this problem and to measure the current velocity. The farm has several moorings located on two of the rafts situated in the central part of the farm. In addition to meteor- ological data, they acquire fluorescence, turbidity and current velocity at depths of 1 and 6 meters. In contrast to the mooring data, the AUV measurements were dynamic, which will enable water quality monitoring to be extended to a wider area. The expanded data will help to extrapolate the long-term data col- lection of the moorings to the whole farm, and to inform phys- iological and ecological models. www.sea-technology.com AUGUST 2012 / st 25

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