In the summer of 1754, the VOC ship Geldermalsen, on her way from Nanking to Batavia, sailed into a coral reef in the South China Sea and sank immediately. The ship was loaded with 343 tonnes of tea, 239,000 pieces of precious Chinese earthware and more than 45 kilograms of gold. The china replaced the normal ballast of bricks and stones.

In 1985, treasure hunter Michael Hatcher, assisted by the Swiss geophysicist Max de Rham started searching for the wreck in an area of some 260 km2, littered with hundreds of coral reefs. Following a dense search pattern with side scan sonar and magnetometer, they finally located the wreck at a depth of 40 meter. Thanks to the fact that both the on-board gold and two large iron cannons gave a clear indication on the magnetometer that something ‘was there’, the treasure hunters were able to recover, gold, artefacts and all the undamaged blue and white china, handdecorated with Chinese bridges, river landscapes, fisherman, farmers and ornate fences. Half the treasure was sold at Christies in Amsterdam in 1986 for some 15 million Dollars.

Hundreds of ship wrecks from bygone era’s are still buried or locked in coral reefs, completely overgrown and hidden from multi-beam, side scan sonar or sub bottom profiler systems. Only magnetometer may be able to locate these wrecks due to on-board metallic material….

When was the marine magnetometer system developed?
We could locate it’s ‘birth date’ to 1955, when the Survey Ship Pioneer surveyed the U.S. West Coast from San Diego to Cape Flattery.. Shortly into the project, the Scripps Institution of Oceanography requested that it be allowed to tow a newly
developed marine magnetometer from the Pioneer. Permission was granted, leading to what the great marine geologist H.W. Menard called “one of the most significant geophysical surveys ever made”. The Pioneer Survey, as it came to be called, discovered long, linear magnetic stripes on the sea floor. Ultimately, this striping led to the ability to date the age of the sea floor, as well as to compare magnetic patterns across fracture zones and from one side of an oceanic ridge to another. Because of these factors, the recognizable magnetic patterns associated with the sea floor were a major element in formulating
the Theory of Plate Tectonics.

For surveyors, the magnetometer has maybe been an instrument of doubtful reputation. It does not give us a clear picture, like an echo sounder or side scan sonar. Only some readings and funny spikes on the recorder are the only visible output of the device. But properly used, specially in combination with sidescan sonar and Subbottom profiler systems the magnetometer
can provide a useful tool for all kinds of detection surveys and developments of the various types of magnetometer systems have given it a well earned place in the range of offshore hydrographic survey tools.
We would like to thank the manufacturers who contributed to this Product Survey and responded to the Questions about their products.

Participating suppliers:

• Geometrics: G-882
• IXSEA: Magis
• JW Fishers: Diver Mag I