Position Fixing

When locating a shipwreck, one of the first things you need to know at sea is where you are. If you find a shipwreck, you want to be able to go back to the same spot again.

Near land, you can find your position from landmarks. By measuring the angle to three landmarks and plotting the angles on the chart, you get three lines that cross. That marks your position. This is called "triangulation", and is also used underwater by archaeologists. Away from land, sailors at one time used the sun or the stars to work out their position. Nowadays, sailors use electronics. Older systems like Decca or Loran rely on having a series of transmitters at fixed points. These send out a signal, which can be picked up by the a reciever on the boat. By working out the strength of the signal, the receiver works out your position, which can be plotted on the chart.

A more up to date system uses satellites, and a computer on the boat receives signals from the satellite, like the older systems did from transmitters. This Global Positioning System (GPS) can work out your position to within a few feet anywhere on the earth.

Magnetometers

Magnetometers are very large underwater metal detectors. They work in the same way, by reading differences in the magnetic readings of the earth. This means that if they pass over an area where metal is buried, they let you know, either by sound or by a needle on a dial. It also tells you how strong the reading is.

The same is true for magnetometers, except instead of being carried by hand, they are towed from a boat. They look like long thin metal fish, with a large magnet in their nose. The way they work for wreck searching is simple. The boat sails up and down in a series of lines, a bit like a farmer plowing a field. This way the searchers know that they are being systematic, and covering the search area thoroughly.

Anything found is called a "hit", and a marker buoy is dropped on the spot. The bigger the hit, the more likely that something big lies down there on or just under the ocean floor. Each "hit" is then checked out by divers. Sometimes it is an unexploded bomb, sometimes a lost anchor or just sometimes part of the missing treasure wreck.

Sonar

Sonar works by sending out a pulse of sound that then gets reflected back from either the seabed or anything else down there. It was first developed to help warships look for submarines, but nowadays it can also help us look for shipwrecks.

The basic kind is an echo sounder, and a paper trace records the seabed. If the bottom is flat, a shipwreck might show up as a bump on the flat seabed. Sidescan sonars do the same job, but cover a much wider area, sweeping a large lane over the seabed. Searchers still have to sail their boats up and down, like mowing a lawn, making sure all of their search area is covered.

A development of the echo sounder is the sub-bottom profiler. This is an echo sounder powerful enough to look through the ocean floor and give some idea if anything is buried beneath the sand or mud.

Other tools have been used, from aircraft, towed sledges, unmanned mini-submarines or just simply using scuba divers. The best tool is knowledge. If the searchers know where the ship sank from old documents, then they have a good chance of finding what they are looking for.