Geometrics Inc. G-882 Cesium Marine Magnetometer Page
Retrieval is best performed with the vessel at full stop (propeller not turning) or
maintaining just enough headway to keep the cable from going under the
vessel. The intent is to prevent the tow cable from becoming entangled in the
propellers. Nothing destroys surveys and propeller shafts quicker than
entangling the tow cable in the propellers.
Retrieve the cable as quickly as possible to help prevent the magnetometer
from striking the bottom. Again, neat tow cable management as described
above will greatly facilitate survey execution.
It is recommended that some form of quick communications be provided
between the vessel captain and the magnetometer handlers. This will greatly
ease handling and make the operation safer. Radios work well.
The tow cable length is generally determined by a number of factors. The most
important factor is the desired depth of tow that must be achieved for the
survey. The longer the cable, the greater the depth that can be achieved. The
depth of tow may also be improved by the type of towing method selected for
the magnetometer tow fish.
For shallow surveys on short tow cables (30 to 60 meters), the Nose Tow
method works the best. One man can manage deployment and retrieval with a
second man to manage the tow cable. The depth of tow graph below for a
nose towed G−882 is derived from actual tests. However, it DOES NOT
provide exact numbers. It is just intended to provide an indication of the tow
depths that might be expected. The actual depth of tow can be affected by
water current speed and turbulence. Also, turns to change course greatly slow
the speed of tow increasing the tow depth. If you know you are towing in an
area that may have potential snags upon which the tow fish may hang, retrieval
of tow cable in the turns may be required to manage the tow fish depth
guarding against snagging. This graph is provided as a suggestion of what tow
depths might be expected using a nose towed G−882 magnetometer.
Figure 43. Kellems grip installed on the tow cable.
Use a strong line to attach
this loop to the vessel.
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