Underwater Metal Detectors
Most underwater metal detectors are pulse induction detectors, which means the detector operates by delivering short bursts of energy to the antenna. This is followed by a passive period when the antenna can sense the decaying electromagnetic field induced by the target. Pulse induction detectors are able to ignore black magnetic sand as well as salt water, which makes them ideal for underwater use. The main features of an underwater metal detector are:
Electronics and control housing - Many detectors used for beach combing are designed for shallow water (six feet or less) while others can only protect against the occasional splash. What you want is the real thing, which is an underwater detector that can be totally submerged and can withstand pressures at depths up to 200 feet. Also, make sure the control knobs are simple to use and easy to get to.
Batteries - Some underwater detectors use standard batteries while others use rechargeable ones. I prefer to use a standard battery detector because I can grab the detector at a moment's notice, slip in some new batteries and be ready. But there are others who prefer the rechargeable batteries because they are sealed in the housing, which protects them and minimizes the chances of water entering the system.
Searchcoils - Most underwater searchcoils range in size (diameter), from seven to ten inches. The target response of the different sizes is basically the same. However, pinpointing the target is easier with the smaller sizes. A smaller searchcoil is also easier to maneuver underwater but will not penetrate as deep as a larger one.
Underwater headphones - There are two types of underwater audio signaling devices: headphones and bone conductors. I prefer to use headphones because they cover both ears, giving you dual sound and reducing outside noises, which helps you concentrate. You can also use headphones both in and out of the water. Bone conductors are held in place by a mask strap or hood and are less cumbersome than headphones but can only be used effectively underwater.
Short and long stems and handle grip - Make sure your underwater detector comes with a short and a long stem as well as a handle grip. The short stem usually comes with an armrest and gripping handle and is used mainly for underwater searching while the long stem (after being attached to the short stem) allows you to conduct shore or shallow water searches while standing. For underwater searches I prefer using the handle grip with searchcoil attached, allowing you to put the housing on your body. This minimizes strain on your arms, as you have to move only the handle grip and searchcoil back and forth rather than the entire metal detector. Moving the whole thing can be pretty tiresome after a long search.
Underwater metal detectors are configured in two ways:
1. Housing is attached to the stem - This makes the unit compact and easy to handle. The advantage is that the entire unit is together, which makes operating it quick and easy. The disadvantage is that the diver has to carry and move the entire unit when searching.
2. Convertible - It mounts the control housing to the diver's body (usually around the waist). The advantage is that it takes the weight of the housing off the diver's arms and places it on his body. This allows the diver to carry and sweep only the stem and searchcoil. The disadvantage is that the unit's cable that connects the searchcoil to the housing can get entangled. I have found that if you shorten the cable by wrapping it and tying it off, you can minimize the entanglements.
Now that we understand a little more about underwater metal detectors, what can we do with them? Well, it's limited only by our imagination. But to give you an idea, they come in very handy when we are searching shallow, grassy and swampy areas that can be hard to search by hand. They are also needed when we are dealing with current and are searching for items such as guns or knives. The current could have easily moved bottom composition over the item and buried it out of sight.
In normal condition and solid bottoms, I prefer using a metal detector underwater only as a backup to a diver who is searching by hand. This can be done on a jackstay search pattern (more on this pattern coming up). The lead diver can then move and control the search pattern with both hands free. If searching an area on an arc search pattern, the diver can start off searching by hand and then re-search it with a metal detector. Areas with tall grass and other underwater plants sometimes can only be searched thoroughly with a metal detector.
When searching with a metal detector in shallow water, some means of keeping the diver oriented and on line is needed. This can be done by using a grid line (jackstay with no up-and-down lines) to ensure every square inch of the search area has been covered and searched properly. When using a grid, the diver stays to one side of the pattern (usually the side from which the pattern is being moved) and walks the line, searching from side to side in front of him. When the diver gets to the end, he moves the pattern in the direction of the search, then turns around and continues searching, this time in the other direction.
Most underwater metal detectors can be used on land as well. This is important in searching for metal items that might be found around a crime scene such as bullets, keys, jewelry, etc. Many underwater criminal investigators fail to consider the land crime scene and what it might contribute to the investigation. If the water's edge is searched, a grid line search is a great way to keep yourself oriented and ensure that you have properly cleared the area of possible contributing evidence.
When using a metal detector underwater or on land, the diver holds the stem or handle grip and moves the searchcoil slowly from side to side in a straight line, making a grid with his search. The searchcoil should be held off the ground or bottom approximately two inches. This method of scanning covers the area more thoroughly and helps keep the searchcoil at a constant height.
The following companies can help you with your underwater metal detector needs: