MARINE SURVEY EQUIPMENT FOR SALVAGE, ARCHAEOLOGY & WRECK LOCATION

MX500 Digital Magnetometer.

    MX500 Active Tow-Fish

The MX500 Digital Proton Magnetometer is our latest design of  marine proton precession  magnetometer which uses one of the new generation of  ultra low power microprocessors to create an active tow-fish. All the electronics needed to compute the magnetic field are located in the tail of the tow-fish. The proton precession signal generated in our  toroidal  sensor which is located in the head of the tow-fish, is then processed and measured by the microprocessor situated in the tail. Once the magnetic field intensity has been computed, it is combined with the depth of the tow-fish read from the on board pressure sensor, to form a data packet. This packet is then transmitted digitally along the tow cable to the control unit on the boat. The control unit is a small die-cast aluminum box which provides the interface between the PC software and the tow-fish. It processes the data packet received from the tow-fish and forwards it on to the PC. Control commands generated by the PC software are transmitted to the control unit which then relays them to the microprocessor in the tow-fish. The control unit also provides an interface for connecting the data output from a GPS navigator and an depth sounder to the PC.  The MXSurvey II software will then display on the PC screen, the change of magnetic field, the seabed depth, tow-fish depth, boat position and track. 

The MX500 Digital Magnetometer has many advantages over the more common analogue magnetometers. First and most important is the use of digital telemetry along the tow cable, which virtually removes  errors created by radiated interference from other equipment on the boat. Second, a pressure sensor is connected to the low power microprocessor in the tow-fish so that the depth of the tow-fish is computed simultaneously with the magnetic field intensity. The Depth display window on the PC shows the seabed depth directly under the boat along with the depth of the tow-fish. This form of graphical display makes it simple for the person steering the boat to control the height of the tow-fish above the seabed by using the speed of the boat. 

To keep the cost of the system low, the magnetometer is supplied with a tuning range of 6000nT calibrated to the customers geographical locality. However, it can also be supplied with worldwide tuning range of 22000nT to 65000nT if required. The MX500 Digital Magnetometer uses our own omidirectional toroidal sensor to ensure a good signal is obtained in all parts of the world. The tow-fish has also been specially designed to allow the sensor to be rotated in the head to ensure maximum signal level is always generated. For transportation the tow-fish can easily be disassembled so the whole magnetometer can fit in a large suitcase.

The new MX500 Digital magnetometer has been specifically designed for searching for small targets in depths up to 70 meters. When searching for such targets i.e. wooden shipwrecks, cannon, pipelines etc, a major problem is making the tow fish run at the required depth to guarantee the target will be detected when the tow-fish passes nearby. For targets in depths greater than a few meters, it is necessary to force the tow fish deeper by either adding a heavy weight to the cable, or using some form of  depth depressor. In both cases, the pull on the cable is of the order of tens of kilograms.  As most of the off the shelf copper conductor cables cannot take such load without damage, a separate rope is usually taped to the cable to take the strain. This  approach makes deploying the tow-fish very difficult and in addition, the extra rope increases the drag which requires even more weight to be added. When designing the  MX500 Digital Magnetometer this problem was addressed from the start, a new cable was designed which has an integral strain relief Kevlar membrane. To reduce the drag caused by the cable, the diameter  was kept to a minimum,  just 7.75 mm, while the integral strain membrane was designed to withstand the towing force  encountered when using heavy weights  or a depth depressor to force the tow-fish deeper.  A polyurethane outer sheath was chosen to give the tow cable a very high level of  abrasion resistance. 

A cable termination unit is attached to the cable 8 metres ahead of the tow-fish. This stainless steel termination unit is used when attaching heavy weights or a depth depressor to the cable to force the two-fish deeper. It ensures that any force applied to the tow-cable is taken up by the Kevlar braid built into the cable. 

MX500 Dual Tow-fish Magnetometer

This is a development of the single tow-fish magnetometer that allows two tow-fish to be attached to the control unit. Each tow-fish is in effect, a self contained magnetometer with its own integral depth sensor. The MXSurvey II  PC software has been upgraded to display the output of both tow-fish simultaneously. The major advantage of using two tow-fish is that the natural (dirunal) variation in the Earth's magnetic field caused by the influence of the Sun  can be easily seen as it affects both sensors simultaneously. Observing the output from both tow-fish minimizes the possibility of false targets which are sometimes mistakenly identified when only one tow-fish is used. A second advantage is that a wider detection swathe is created if the two fish are towed side by side thus reducing survey time. The two tow-fish can also be run in gradiometer mode were the difference between the magnetic field affecting each tow-fish is displayed. This mode of operation can be very effective in geographical locations  where magnetic rock make the use of a single tow-fish magnetometer very difficult.

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Last modified: December 14, 2008