Learning to properly sweep a metal detector coil is important. Poor coil techniques lead to missed objects and inefficient use of the metal detector. Four of the more significant aspects of good coil control are proper side to side swing, distance from the coil to the ground, overlapping and mineral chasing.
Metal detectors are swept in a side to side motion. Too often, a new detectorist will swing the metal detector in a pendulum pattern. If you were to view this motion from a distance, you would see that the sweep would start high on one side, closer to the ground in the middle of the sweep and then return to a high position on the other side. At the middle point of the sweep, the metal detector will achieve maximum metal detection depth. The larger gaps on the sides of the sweep will result in a loss of depth for most of the sweep. The correct way to swing the coil is to keep the coil the same distance from the ground during the entire sweep. This may take some getting used to, but it is important.
The distance between the coil and the ground also affects metal detection depth. New detectorists often keep the coil too far from the ground. If you are worried scratching or soiling your coil by scraping the ground, then you can purchase a protective coil cover or accept the fact that the coil will get scratched and dirty. It is a part of the hobby. Keep in mind that if your coil is six inches from the ground, you will have lost up to six inches of depth. Some metal detectors such as the Minelab Explorer, tend to work best if the coil is as close to the ground as possible.
To efficiently cover a given area of detectable ground, it is important to overlap your coil sweeps. Your coil needs to pass over or be in close proximity to an object in order to find it. If your coil sweeps do not overlap, then you are likely missing objects in the gaps. However, there may be times where overlapping is not as important. When detecting a new location, for example, it may be advantageous to search a larger area faster by not overlapping in order to gain a better understanding of how the objects are concentrated. Once you have a broad understanding of what areas are producing finds and what areas have had little use, then you can focus your efforts on searching the more promising areas with a more time consuming, but proper coil sweep technique.
In many instances, mineral chasing will not matter, especially in areas with little mineralization. In areas with higher concentrations of ground minerals, mineral chasing is a way to improve metal detector stability. If you have been to a beach, you may have noticed that black iron sands and other minerals are concentrated into streaks often creating a pattern similar to the grain in a piece of wood. In gullies and stream beds, heavy minerals will form depositional layers that vary in mineral composition. Using the piece of wood as an analogy, it is much easier to cut wood with the grain than across the grain. Similarly, it is more effective to metal detect with the mineral streak than across it. When cutting across the mineral deposits, the metal detector must repeatedly adjust and compensate for the minerals which can cause feedback noise and instability. By detecting with the flow of the minerals, the metal detector will not have to work as hard to maintain stability. As a result, it can be helpful to walk toward and away from the water at the beach and to search back and forth across a stream bed instead of up and down the stream.
By developing proper metal detector coil sweep techniques, you will increase your effectiveness as a detectorist and you will be less likely to accidently pass up objects worth digging. Do not be afraid to scrub the ground with your coil as they are typically quite durable and as mentioned before, coil covers can be used to protect the coil from scratches and wear.