Page 201 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 189

main disadvantage of a single beam is that only measurements directly under
the vessel are obtained, and that the resolution of the data depends on the speed
of the vessel. Therefore, it is rather time-consuming to generate a 2D grid of a
region using a single-beam echo sounder, and this instrument tends to be more
suited to obtaining multiple 1D transects. This shortcoming is addressed by the
use of a multibeam echo sounder.
In a multibeam echo sounder, the acoustic signal is generated in the shape of
a fan through a wide angular lateral aperture transducer, and reflections of the
sea bed are received along multiple narrow beams. Water depths are extrapolated
along a wide band called a swath. This swath varies with water depth, but is
typically around two to four times the water depth. For example, in water of
depth 10 m, the swath will be relatively narrow (e.g. 20–40 m), and so sequential
ship tracks will have to be fairly closely spaced to provide contiguous coverage
of the sea bed. However, in deeper water (e.g. 100 m), the swath will be in the
range 200–400 m, and so ship tracks can be spaced further apart. Multibeam
echo sounder surveys therefore represent a considerable improvement of sea-
bed coverage compared with single-beam systems, but at a cost of (a) the need
for expensive specialist calibrated equipment, and (b) considerable expertise
in deployment and postprocessing. In addition, multibeam surveys generate a
considerable amount of data, and allowance must be made for data storage.
Note that, as with the single-beam system, resolution in the direction of the
ship’s transit still depends on the speed of the vessel. Lateral resolution depends
on the water depth. In some circumstances, it may be necessary to disable other
acoustic instruments on the vessel (e.g. ADCPs) to prevent contamination of the
multibeam signal.

7.5 OTHER FORMS OF MEASUREMENT
In addition to the in situ and remote methods of data collection that have been
discussed so far in this chapter, complementary data that could be useful for
resource characterization can be obtained from a variety of sources. Probably
the most important of these is meteorological data, for example, time series
of wind speed and direction, and atmospheric pressure. The best source of
such data are synoptic weather stations, which record such variables to a
consistently high standard. However, often a marine energy site will be far
from a synoptic weather station, or will be influenced by local topographic
effects (e.g. wind that is channelled through a narrow strait); therefore, an
alternative (e.g. local existing personal weather stations that report daily means,
or a project-specific weather station) may be preferable, or could be used to
complement neighbouring synoptic observations.
Other sources of data that are useful for resource characterization include
local authority beach profile surveys [32], useful for quantifying the natural vari-
ability of nearshore systems that may be influenced by marine energy extraction.
In addition to single-beam and multibeam echo sounder surveys (Section 7.4.3),