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 Deployment is generally the most problematic issue in static acoustic
studies, as loss of monitors can occur from being caught up by trawlers, from
theft and from severe storms. In addition, there are regulatory conditions
that may represent a significant workload in some areas.
Various approaches have been tried and are described below. However, please
note that no responsibility is accepted by Chelonia Limited for any
consequences of their use. All users must carry out their own risk
assessments.
Calculating the maximum deployment time
Maximum deployment time is dependent upon the following factors:
- The capacity of the D cells (usually specified in milliamp hours,
mAh). For example, using two stacks of four D cells, each with a 15000
mAh capacity, the maximum deployment time is expected to be
approximately 4 months.
- The capacity of the SD card.
Deployment environments are very variable and therefore you should
consider carrying out preliminary tests of battery consumption and memory
use at sea, before full deployment.
Very long deployments are not
recommended, unless maintenance and retrieval are particularly difficult. Due to the challenging environment in which PODs are usually
deployed, the possibility of damage, loss or theft means that it is wise to
check and download data on a more frequent basis.
Depth limits
The depth limit for C-PODs is at least 100 metres.
A deep water version of the C-POD is being designed.
Directionality format
All PODs show lower sensitivity directly along the long axis of the
housing, especially in the battery pack direction.
 Lost and found
Write your contact details on the outside of the POD using indelible ink.
Place a label inside the POD as well.
If you prefer not to write your contact details on the POD, the address of
our contact web site, www.phonehome.org.uk
is embossed in the end cap. This is a
simple web site that will allow someone who finds a POD to contact us so that
we, in turn, can contact you to arrange its return.
Moorings
Heavy moorings
These have been used to deter trawlers in some
locations and to prevent movement of moorings on mobile sand banks exposed to
large waves. As trawler deterrents, they do not seem to have been highly
effective, as trawlers often try to go close to anything that may protect an
area of sea bed in the hope of a larger catch from such areas. The heaviest
moorings have used massive weights rather than conventional anchors and have
weighed over 12 tonnes.
Light moorings
Substantial anchors holding a line to a smaller
anchor of weight with the POD
attachment and a line up to a small surface marker, seem to have achieved
equally good, or better, results. But regular inspection and replacement is
essential.
Concealed surface markers
Light moorings in tidal areas have been
successfully used with a short line to a small buoy that is pulled below the
surface and can only be seen at slack water. The main disadvantage of this
method is that it greatly limits the
number of PODs that can be serviced on a single tide cycle.
Navigational buoys
Where permission can be obtained, PODs may be attached to navigational
buoys. This
is a practical option, but carries some risks of theft, poor quality data due to the
proximity to a noise source and loss of PODs when buoys are lifted for
servicing, etc. An effective rig consists of a chain from the buoy to a rope
with a small weight securely attached to it and of just sufficient weight for
the POD to lift it. The POD is attached a short distance from this. The
effect is that the POD is carried away from the chain by any current and does
not have the chain dropping on to it with every fall of the buoy. When the
buoy falls, the small weight and POD will be pulled down by the weight of the
end of the chain and as the buoy rises the POD and small weight will float
slowly upwards. If their net buoyancy is small, the POD will not have gone
far upwards before the next fall of the buoy, so the movement is greatly
damped.
Despite this, the stress on the system over months can be high and noise
levels in stormy weather are a problem. Also, it may be difficult to obtain
permission.
Buoyancy
A C-POD containing 8 alkaline cells has a positive buoyancy of
approximately 0.5 kg. This means that it can be moored by its own mooring line
and allowed to float with the hydrophone housing upwards.
The C-POD mooring line is attached at the centre so that in tidal streams
the forces on the upper and lower halves of the POD are balanced so the
greater weight at the lower end keeps it vertical.
Location
Moorings on the sea bed are generally a better option as they are further
from the noisy surface. For most deployments the aim is to have the POD
either 3 metres up from the bottom or about 5 metres down from the surface.
The surface is a source of a lot of tonal ultrasound from rain and breaking
waves, so deeper deployments are preferred in most locations.
Security
The main problems with moorings are theft and being moved by trawlers or
storms.
Using anchors
A few concrete blocks are not adequate on a sandy sea bed as these are
dynamic and move with tidal currents. Massive concrete anchors, digging metal
anchors or heavy metal anchors are needed. Manhole covers with a central
attachment are effective but can be hard to lift.
Buoys
Try to use the smallest buoy that you will be able to find on retrieval.
This helps to minimises drag in storms. However, some users have deployed
large surface markers with radar reflectors and 4 tonne anchors.
You might prefer to use a short line that results in the buoy spending most
of the time submerged due to the tide stream or tide height, appearing only
briefly at the surface when the tide is slack or at low water. However, make
sure the float will not be collapsed by the pressure.
Cordage
Design the mooring using buoyant and sinking ropes so that no loops of
cordage float at the surface to entangle propellers or lie on the bottom and
suffer excessive abrasion.
Use knots or fixings that grip the POD attachment rope so that
there is no possibility of continual movement making noise. If you use a
shackle you can tape this with sticky tape. If the POD attachment rope
becomes damaged, replacements are available from us.
Theft
This is a serious risk in many areas. Some possible solutions are suggested
below:
- Get cooperation and advice from local fishermen, who may allow you to use
their gear.
- Use a very small marker, for example, a single fishing float on a thin
line, that is only at the surface at low slack tide. The disadvantage is that
it may wind itself around weed or anything else.
- Do not use buoys. Use a line between two anchors that can be located via
GPS using a grapple.
- Use pop-ups. These are commercially available acoustic releases. However,
currently they are very expensive.
- Use C-links that dissolve after a set time in the water and then release
a recovery buoy from the bottom.
- Use divers to deploy and recover your PODs.
Towing PODs
Towing PODs is not normally recommended, as it is often impossible to
obtain quantitatively useful data for the following reasons:
- Animals are more or less undetectable unless they face the hydrophone.
- Porpoises tend to move away from moving boats and become undetectable
over a width and distance that varies both with boat characteristics and with
porpoise familiarity with boat sounds.
- Detection well beyond the range of responsive movement is unlikely with
the C-POD so no undisturbed data set is available.
- Dolphins find towed PODs very interesting and will follow for long
periods and get detected easily, but again the variables controlling this
interaction are almost impossible to determine. Towing a long way behind a
yacht might give a usable way of filling weather-gaps in survey data for
porpoises, and crude dolphin-encounter data.
Other options
The following designs are free for you to build for your own use.
Vertical grapple
A rig to capture a sub-surface buoy, as an aid to
concealed moorings for use in areas in which theft is a major problem.
Find out more...
POD tow rigs
Chelonia does not support POD towing
but some users have obtained useful results and these designs may help you do
it more successfully. Find out more...
Feedback
What is your experience of deploying PODs? As each deployment situation is
unique, we welcome messages about your own solutions.
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