Current velocity in deep-sea areas must be measured during physical environmental assessments of exploration areas for cobalt-rich ferromanganese crusts. However, the use of an acoustic Doppler current profiler (ADCP) for velocity measurement in the abyssal zone has technical disadvantages:
Insufficient acoustic backscatter due to the low density of suspended matter; and
difficulty in detecting low abyssal velocity.
Therefore, an appropriate method for confirming the validity of velocity measurement results is needed. We examined physical aspects of the reliability of ADCP velocity measurements made during 1 year, by verifying the Coriolis effect, validating the velocity power spectrum, and comparing the ADCP results with second version of Japan Coastal Ocean Predictability Experiment (JCOPE2) reanalysis velocity data.
Cobalt-rich ferromanganese crusts, composed of manganese, cobalt, nickel, platinum, and rare earth elements (REEs) occur on the tops and slopes of seamounts. Their thickness varies from a few centimeters to ten centimeters in the depth range from 1000 to 5000 m. In the 21st century, cobalt-rich ferromanganese crusts have attracted attention as an important resource for metals presently in short supply or whose terrestrial sources have become depleted (Narita et al., 2015).
The International Seabed Authority (ISA) regulates mining areas in the high seas in accordance with the "Mining Code" under the 1982 United Nations Convention on the Law of the Sea. To avoid or reduce as much as possible the environmental impacts from the point of view of biodiversity conservation during prospecting and exploration for marine minerals, ISA recommends that contractors conduct environmental impact assessments and environmental monitoring surveys (ISA, 2013a). Additionally, contracts between ISA and contractors for mineral exploration in the license area require the contractor to collect oceanographic and environmental baseline data and to establish baselines against which to assess the likely effects of its programme of activities under the plan of work for exploration on the marine environment and a programme to monitor and report on such effects. The ISA Legal and Technical Commission issued these recommendations as document ISBA/19/LTC/A (ISA, 2013a). In this document ISA emphasizes the importance of environmental baseline studies as follows: "It is important to obtain sufficient information from the exploration area to document the natural conditions that exist prior to test mining, to gain insight into natural processes such as dispersion and settling of particles and benthic faunal succession, and to collect other data that may make it possible to acquire the capability necessary to make accurate environmental impact predictions. The impact of naturally occurring periodic processes on the marine environment may be significant but is not well quantified. It is therefore important to acquire as long a history as possible of the natural responses of seasurface, mid-water and seabed communities to natural environmental variability." Additionally, ISA (2013a) lists seven classes of baseline data requirements:
physical oceanography,
geology,
chemical oceanography,
sediment properties,
biological communities,
bioturbation, and
sedimentation.
Baseline physical oceanography data collection should include the following activities (ISA, 2013a):
"Collect information on the oceanographic conditions, including the current, temperature and turbidity regimes, along the entire water column and, in particular, near the sea floor;
Adapt the measurement programme to the geomorphology of the seabed;
Adapt the measurement programme to the regional hydrodynamic activity at the sea surface, in the upper water column and at the seabed;
Measure the physical parameters at the depths likely to be impacted by the discharge plumes during the testing of collecting systems and equipment;
Measure particle concentrations and composition to record distribution along the water column".
