returns.
To measure mud thickness and avoid litigation on wrong results the surveyor
should use equipment specifically designed for geophysical measurements such
as a sub-bottom profiler, penetrometer, seismograph or a simple bottom sample
grab
Is a Barcheck required ?
Many older technology echo sounders needed to "Warm up" before they
became stable, in addition to their internal frequency/timing circuits varying
with ambient conditions the physical parameters that effect speed of sound in
water also varied with location. The accepted solution was a ''Barcheck'' where
a large plate was lowered in a specified sequence where the depth measured
by the echo sounder to the plate was compared with an absolute stave or tape
measurement. This method ensured that all variable parameters were included
in the calibration, as a gross check the Barcheck was normally also performed
at the start and end of a survey.
There are several problems with this method ....
• The location of the barcheck is only relevant to the water column at that
particular barcheck location and time.
• Older analogue echo sounders gave the user facilities to ''fiddle'' with many
parameters during the survey, modern digital sounders do not expose settings
such as gain and threshold.
• The older instruments do not record changes to instrument settings during the
survey.
• Narrow beam sounders with bottom detection algorithms can misdetect the
moving plate.
• Modern digital electronic timing components are very accurate and stable.
There is still a requirement for calibration of sound velocity but if required this
should be measured using a calibrated Sound Velocity Probe lowered through
the water column to build a Velocity Profile.
A simple equivalent of the Barcheck is to accurately visit several locations within
the survey area which have a known elevation (normally derived from GPS and
