Page 197 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 185

and so can remain on station for extended periods of time, sampling around the
clock.
Various forms of vessel sampling are introduced in the following sections.
However, one form of vessel measurement that has already been covered is a
hull-mounted ADCP (Section 7.1.4). Therefore, this is not considered further in
this section.

7.4.1 Vertical Profiling

In many circumstances, it is useful to make measurements that resolve the
vertical structure of the water column. The most widely used instrument for
vertical profiling is the conductivity, temperature, and depth (CTD) (Fig. 7.18).
A CTD consists of a cluster of sensors that measure properties of the water
column and either log internally (for subsequent download) or, more frequently,
transmit the data directly to the ship through a data cable attached to the
instrument package. The CTD is lowered on a steel cable to the sea bed,
and variables continuously sampled throughout the water column. A CTD
measures the electrical conductivity and temperature of the water, and from
these two properties salinity can be calculated. The depth of each measurement
is determined by a pressure sensor. Although regions of strong wave and tidal
activity that could be suitable for generating electricity are generally well-
mixed, there will be some amount of periodic (e.g. due to regions of freshwater
influence) or seasonal stratification. Further, the density of the water is required
to make accurate calculations of the marine energy resource and to provide
a more thorough characterization of the resource than velocity measurements
alone will provide.
It is common for other instruments to be integrated into a CTD cluster, such
as a transmissometer or optical backscatter (OBS) (both measure suspended
sediment concentrations), or instruments that measure biological properties
such as dissolved oxygen or chlorophyll fluorescence (an indication of the
concentration of phytoplankton in the water column). In addition, an instrument
of the design shown in Fig. 7.18 allows water sampling bottles to be added to the
CTD. The operator can fire these bottles at predetermined water depths to collect
water samples for subsequent processing; for example, the water samples can be
filtered for either SPM or chlorophyll concentrations to calibrate the electronic
SPM and chlorophyll sensors, respectively.

7.4.2 Point Sampling
Ships are useful platforms for making point measurements of dynamical water
properties that vary both temporally and spatially (e.g. plankton), and properties
of the sea bed that could be considered relatively static over longer timescales
(e.g. sea-bed sediments).