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F.A.Q.
Is dual frequency required?
Dual frequency echosounders were originally designed for use by sea going vessels to report reliable, low
frequency depths in deep water situations, and more accurate, high frequency navigation within shallow areas.
Low frequency is of limited use in shallow hydrographic surveys Reasons are as follows:
• physical accuracy is outside IHO specifications
• power consumption prohibits true portable use
• minimum depth possible outside survey requirements
Can mud thickness be measured with dual frequency?
The residual difference between low and high frequency shown on an echogram gives the impression that mud
thickness can be measured. In fact, the trace gives an impression of soft sediment. However, in most sounders,
this is the difference in reflected energy as a result of simple penetration of higher power low frequency signals
plotted against the low power high frequency 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.
Is a bar checking required?
Many older technology echosounders 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 echosounders gave the user facilities to ‘’fiddle’’ with many parameters during the survey,
modern digital sounders do not expose settings such as gain and threshold.
• 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 with several locations within the survey area which
have a known elevation, normally derived from GPS and a weighted tape. These points should be used as a
reference throughout the duration of the survey.
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SonarMite MILSpec/M8 User Manual | Rev 07/22/2016