Page 177 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

166 Fundamentals of Ocean Renewable Energy

An ADCP works on the assumption that there is horizontal homogeneity;
otherwise the trigonometric relations that are used to calculate the velocity
components are invalid. Because there are four beams, but only three current
directions, one of the beams could be considered to be redundant. However, this
fourth beam is used to measure the vertical velocity a second time, and the error
between the two measurements gives an indication of the quality of the data (i.e.
a measure of how valid the assumption of horizontal homogeneity has been).
The water column is partitioned into vertical bins. The ADCP listens to
the reflected echos at different time intervals, which correspond to different
distances from the transducer (because v = d/t), that is different bin depths.
This process is known as range gating (Fig. 7.6). For example, if the time
between the sent and return signal is 8 ms, then the distance to the bin is
0.5 × 1500 m/s × 0.008 s = 6 m (where 1500 m/s is the speed of sound in sea
water, but clearly, in practice, an ADCP will use an accurate local value for the
speed of sound, based on a local reading of water temperature and a constant
value for salinity).
The echo from a hard surface such as the sea bed or sea surface is much
stronger than the signal from scatterers in the water column, and so can dominate
the signal. For this reason, data close to the surface (for an upward looking,
moored, ADCP) or close to the bed (for a downward looking, hull-mounted,
ADCP) is rejected. Also, because the ADCP will be at some height or depth
in the water column, there will clearly be a further region of the water column
which cannot physically be profiled. Further, the region close to the instrument
is affected by ringing—an effect where the energy of a transmitted pulse lingers

FIG. 7.6 Range-time plot showing how ADCP transmit pulses and echos travel through space.