Page 200 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

188 Fundamentals of Ocean Renewable Energy

Static Properties
To characterize sea-bed sediment, grab samplers are often deployed from
vessels. Grab samplers are quick and relatively easy to deploy (provided the
operating team is skilled) and provide a sample of the sea bed that is suitable
for subsequent grain size analysis in the laboratory. Typical grab samplers are
the Shipek, Van Veen, and the day grab (Fig. 7.19). However, the fine-grained
sediments are washed out in grab samples, and there is a risk of blockage of the
jaws from coarser particles. Further, a grab sampler produces a mixed sediment
sample, and so no information about the structure of the sea bed is preserved,
and the grab only provides information about the sea-bed surface. Under many
of these circumstances, more specialist equipment is required, such as corers
(e.g. gravity corer, piston corer, box corer, and vibro corer), or drilling. Drilling,
in particular, requires a specialist vessel with dynamic positioning.

7.4.3 Multibeam
All vessels are fitted with a single-beam echo sounder, which measures the
double way transit time of an acoustic beam that has been reflected from the sea
bed. Because we know the speed of sound in water (and can correct using local
measurements of temperature and salinity, etc.), this transit time can be used
to calculate the water depth under the vessel. In real-time, this is essential for
navigation, for example, to monitor the depth under the vessel in shallow waters,
but the data can also be logged to create an xyz (scatter) dataset of water depths,
which can later be corrected for temporal changes in tidal elevation, for example,
by obtaining measurements of a local tide gauge to reduce the data to mean sea
level (MSL) during the postprocessing stage. The resulting corrected xyz dataset
can then be gridded and used to generate model bathymetry, etc. However, the

FIG. 7.19 Day grab being deployed from the RV Prince Madog.