Bottom Detection - Kongsberg EM 1002 Operator's Manual

EM 1002 / Base version
17.2

Bottom Detection

344
Conventional techniques
A single beam echo sounder obtains depth by measuring the time
a sound pulse takes to propagate to the bottom and multiplying this
time value by the mean speed of sound in the water column. Good
accuracy depends on:
· small beamwidths in the echo sounder transducer
· a good estimate of sound speed
· a well-designed pulse receiver and detector
A single beam echo sounder normally only measures the depth
vertically. To measure to the side, one could imagine using a
mechanically tiltable transducer or multiple transducers, but
electronic beamforming, i.e. the forming of multiple beams from
a single transducer array, achieves the same results in a more
cost-effective way.
beamforming on the receiver side must be fast enough to allow
simultaneous detections in all beams. The transmit transducer in
a multibeam echo sounder must ensonify the area to be measured
within the coverage of the receive transducer, which in principle
does not require the use of multiple transmit beams.
The accuracy of a multibeam echo sounder depends on the same
factors as those of the ordinary echo sounder. In addition, it is not
enough merely to know the mean sound speed in the water column,
but a detailed knowledge of how the sound speed varies with depth
is also required. This is because the sound path is usually not
straight when it is not vertical, and the resulting so-called "ray
bending" must be calculated by the use of the actual sound speed
profile to find the correct depth and location of soundings taken to
the side.
Both single beam and simpler multibeam echo sounders use only
amplitude detection to determine the travel time of the sound pulse.
When the sound has a zero angle of incidence to the seabed, a good
detection principle is to estimate the time of arrival of the leading
edge of the returned echo. As the angle of incidence increases with
beams measured to the side or when the seabed directly below the
vessel is not horizontal, the returned echo loses its sharp leading
edge. When the angle of incidence becomes sufficiently large, the
echo will be so smeared out in time that an accurate simple
amplitude detection is no longer possible. One is then required to
use more sophisticated methods, for example using the variation
in echo strength over the beam.
Sidescan sonars provide a "photographic" image of the seabed,
and this may give an indication of the bathymetry. In some
For good measurement density, the
850-160977/E