hydrographic survey 11
may be significantly deeper than the depth towed through marine sediments. Importantly,
indicated by the first reflection of the echo the sensing element consists of a single short
trace. This is an important consideration when probe around which fluid can flow easily with
deciding whether or not dredging is required. no risk of material entrapment. Unlike some
Density measurements based on core other types of density meter the MudBug
or grab samples are slow and unreliable, uses no radioactive materials, so there are no
especially in unconsolidated sediments where environmental hazards involved. Using the
the sample is unlikely to be representative MudBug, an ABP survey team in the north-east
of the bulk material by the time it is tested. of England was able quickly and accurately to
Over the years, therefore, various techniques detect the depth at which the mud reached a
have been developed for measuring fluid mud density of 1240-1250kg/m
3
, generally regarded
density in situ. as the maximum density through which a ship
These techniques depend on: the degree can safely navigate. The MudBug was rigged
to which the mud absorbs radioactivity; on a davit and towed through the waters of the
the degree to which the mud attenuates an river Humber at Immingham Outer Harbour.
ultrasonic signal; or the variation in frequency The chart (Fig. 1) shows a typical bathymetric
of vibration of a ‘tuning-fork’ style sensor. survey produced by running the MudBug
Unfortunately, all these approaches have simultaneously with a standard 33 kHz echo
disadvantages. sounder. There was an average 0.5m difference
The first ‘nuclear’ or ‘nucleonic’ density between the results from the MudBug and
gauges were originally developed by the UK those obtained from the echo sounder. “This run
Atomic Energy Authority at Harwell in the early alone shows us that we can save around £35 000
1970s. They contain a radioactive source and a of dredging,” commented hydrographer Mike
detector. The degree to which the radiation is Abbey. “The run we did a few weeks ago
absorbed depends on the density of the material already saved us about £71 000.”
around the probe. Two types of probe are in The MudBug (Fig. 2) also enabled the
use: (1) the ‘transmission’ type, an H-shaped Fig. 2. MudBug is survey to be completed much more quickly
instrument with the radioactive source in one used to determine than was possible with earlier density
leg and the detector in the other; and (2) the the navigable depth measurement methods. Using the MudBug, it
‘backscatter’ type, where both source and in ports, harbours took no more two hours to gather the data for
detector are contained in a single probe. and estuaries. the chart and another two hours to process it.
These nuclear density gauges have serious The MudBug’s multifunctional transducer
drawbacks. Since hazardous radioactive materials incorporates density, temperature and pressure (depth)
are involved they must be handled with care, and sensors plus all interface electronics. The unit is
their use raises significant environmental and marine calibrated at the factory, so on-site calibration is not
safety concerns. They are usually very expensive and necessary. Once at the desired location the transducer is
unwieldy, and require periodic recalibration. What is deployed using a support line, and measurement can start
more, accuracy may be seriously affected by pollutants as soon as the probe is submerged. The weight of the
in the silt, including heavy metals, organic materials or transducer ensures that it will easily sink into silt layers.
dissolved gases. Readings are taken approximately five times a second
The ultrasonic method has been found to be unreliable and, as a large number of measurements can be made
in the presence of air bubbles, and gives varying results very quickly, it is possible to cover a wide area in a short
depending on the composition of the mud. In field tests, time. Density is measured in the range 800 to 1600kg/m
3

for example, it was found that organic materials increased with an accuracy of ±1 per cent at depths up to 100metres.
the attenuation of the signal. Density, depth and temperature measurements are
In the ‘tuning fork’ device, the density of the output by the MudBug as serial data which can be
intervening medium is derived from the vibration imported directly by third-party hydrographic surveying
frequency of the exposed prongs or ‘tines' of the fork. software such as HYPACK. Alternatively, the DataPod
However, granular material can easily become trapped Connection Unit can be connected to a USB port on a
between the tines, producing errors in the readings. PC or laptop, in which case no separate power supply is
The Hydramotion MudBug is used to determine required as the instrument is powered simply through the
the navigable depth in ports, harbours and estuaries USB connection. NaviTrend software supplied with the
by measuring the density of ‘fluid mud’. It is an easy- system shows real-time measurements in both tabular
to-use plug-and-play system consisting of a rugged, and graphic form on the PC/laptop, while logged data can
towable transducer connected by an umbilical cable to be exported to Microsoft Excel for further analysis. l
DataPod Connection Unit on the surface. The heavy-duty
device is made from 316 stainless steel using all-welded Hydramotion Ltd is based in Malton, York, UK.
construction, and is robust enough to withstand being www.hydramotion.com
www.engineerlive.com
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