Magnetometers, which detect the presence of ferrous metal, have been the main search tool used for locating old shipwrecks, especially those that are buried under sediment or coral, and for shipwrecks where visual search is restricted for whatever reason. Magnetometers are also used for confirming targets, which have been located with sidescan sonar, when the operator is not certain if the target is a shipwreck or a natural anomaly. Any magnetometer, however, is only as good as the skill of the operator running it. Many shipwrecks have been missed by incompetent magnetometer users.
Basically, a magnetometer detects gradients or localized distortions in the earth’s magnetic field produced by local concentrations of ferro-magnetic materials such as cannon, anchors or any other ferrous metal objects on a shipwreck. The larger the object is and the longer it has remained in the same position, the better the chances are of locating it with a magnetometer.
There are numerous types of magnetometers available today, each working on a different principle: Proton Precession, Rubidium, Overhauser, Alkali Vapor, Rotating Coil, Hall Effect, Fluxgate, Caesium Vapor, to name a few. The most common types used for locating shipwrecks are Proton Precession, Overhauser, Caesium, and Fluxgate. I have personally used both the Proton Precession and the Overhauser magnetometers to search for shipwrecks. Magnetometer measurements are made in nanoteslas (nt) which are often called “gamma”. The standard Proton magnetometer achieves a 0.1nt resolution, whereas the Overhauser magnetometer achieves a 0.01nt resolution. The Overhauser magnetometer also has the added benefits of using much less power and it is not affected by location or direction, nor is it as susceptible to noise as the Proton magnetometer.
Marine magnetometer surveys are generally performed by towing the magnetometer sensor (towfish) behind a boat at a speed between 3 and 10 knots. Before the advent of the GPS, survey areas were marked off with buoys and the survey boat attempted to run parallel grid lines based on the buoys and compass readings. Older magnetometers either had no recording capability, or they printed readings on a paper roll. Today, sophisticated computer software is available which interfaces with the magnetometer and a GPS, giving the boat pilot a visual grid to follow, with the boat’s position shown at all times. The magnetometer data is constantly recorded on a laptop or desktop computer along with the GPS data, and may be played back for review at a later time.
The configurations for using magnetometers in shallow water (less than 100 feet deep) and deep water vary considerably. In shallow water, the length of cable required is usually 100 meters or less. This can be readily handled manually and the magnetometer towfish can be deployed and retrieved manually. At greater depths, the cable must be much longer and thus much heavier. The increased weight requires mechanical equipment to pay out and retrieve the cable and towfish. The towfish must also be able to withstand the pressure of the greater depths. Shallow water surveys can generally be accomplished from small skiffs, whereas deep water surveys are generally performed by larger boats with electric winches to pay out and retrieve the towfish.
The grids or lanes used to perform a magnetometer survey are spaced, depending on the target. A 300 ton steel ship can be detected from over 1,000 feet away. So, if you are searching for a target of that size, your lanes could be every 500 feet or even more. An older wooden shipwreck with a large number of iron cannon and anchors may be detected from 200 to 300 feet away, if the iron objects are confined to one small area. When the wreck has been scattered and large ferrous articles broadcast over a large area, each piece will probably be detected at a maximum range of 100 feet. In cases where a shipwreck has only small ferrous items such as cannonballs, tools, weapons, and bits of rigging, the location will be detected only if the sensing probe passes within 30 or 40 feet of the site. If very small items are widely scattered, the probe must come within 10 to 15 feet to detect them.
As you can see, knowledge of your target is very important (Did it have bronze or iron cannon? Did it sink in a storm, and thus would more likely be scattered? Is the area where it sank subject to frequent storms or hurricanes, and if so, it would more likely be scattered?). The time it takes to do a magnetometer survey depends on the speed and the width of the lanes. If you want to make certain you don’t miss targets that can only be detected if you are within 10 or 15 feet, then your lanes must be no more than about 15 feet and your sensor has to be towed within about 10 feet of the bottom. This can be very challenging in an area that is rocky, has large coral growths, or has variable depths. In a fixed depth, flat, sandy bottom environment, a survey is very simple. Otherwise it takes an experienced operator to perform a survey to gather meaningful data while, avoiding damaging the towfish or cable on rocks or reefs.
Several manufacturers produce a small hand held magnetometer that can be utilized by a diver to search for ferrous objects, or to pinpoint an object detected with the towed magnetometer. These have proven very useful when the shipwreck is totally covered in sand since the diver can pinpoint exactly the spot with the handheld device and then bring in the appropriate equipment to excavate.