Metal Detectors at the Crime Scene
Nielsen, Eugene, Law & Order
Metal detectors play a variety of vital roles in law enforcement today. Most of the attention has been directed toward their role in weapons detection. However, metal detectors play an invaluable role as a crime scene management tool. In the hands of a skilled operator, metal detectors can locate metallic evidence at crime scenes better than other methods.
Metal detectors generate an electromagnetic field that penetrates any nonmetallic material. When metal and, in some cases, certain minerals, come into contact with the field, eddy currents and electromagnetic distortion occur. The generation of the eddy currents on the metal object and the electromagnetic distortion cause a loss of power in the electromagnetic field generated by the metal detector. The circuitry in the metal detector senses the power loss and alerts the operator to the presence of metal. Metal detectors can detect both ferrous and non-ferrous metal.
There are five different types of circuitry that have been employed in metal detectors. They are the beat frequency oscillator (BFO), transmitter receiver (TR), very low frequency (VLF), automatic VLF and pulse induction.
BFO metal detectors are no longer manufactured, although they are still in use. The performance of BFO detectors is adversely affected by wetted salt and iron mineralization. They are best suited for use in areas where ground mineralization is at a minimum.
Transmitter-receiver metal detectors are also best suited for areas where ground mineralization is at a minimum. TR detectors cost less than other currently manufactured types of metal detectors. In the proper environment, they have a very good detection depth.
Very low frequency metal detectors can be manually tuned or ground-balanced to eliminate the effects of iron earth minerals. Ground elimination shouldn't be confused with the ability of a detector to ignore salt water. In VLF detectors the effects of wetted salt water can only be eliminated from detection by using the discrimination mode.
Automatic VLF Ground Canceling Metal Detectors
Automatic Very Low Frequency (VLF) ground canceling metal detectors have proven to be the best all-around choice for ground (land) searching in law enforcement applications. This is due to their ease of use and sensitivity.
Automatic VLF detectors don't need to be manually tuned to cancel out interference from ground mineralization, such as iron earth mineralization or wetted salt, although under some circumstances the effects of salt can be noticeable. They automatically tune themselves to the ground that is being searched. This feature is known as automatic ground canceling.
Automatic VLF detectors are "motion" detectors. This means the searchcoil of an automatic VLF detector must be kept in motion to detect a metal object. If the searchcoil is stopped over a signal-producing object, the signal will gradually die out.
Very little motion is required for most of today's automatic VLF ground canceling metal detectors. The operator can almost stop the motion of searchcoil for accurate pinpointing. Many of the better automatic VLF detectors have an electronic pinpointing feature. This feature allows the operator to switch from motion to a non-motion mode.
Two-box Metal Detectors
For extremely deep searching for large objects on land, specialized metal detectors known as "two-box" metal detectors may be employed. These detectors are designed to locate large, deep objects. While they won't detect coin-sized or smaller objects, larger objects may be detected at comparatively great depths. Very large objects like an automobile may be detected as deep as 20 feet.
Underwater Metal Detectors
Metal detectors are absolutely essential when searching underwater due to the extreme environmental conditions. Detectors manufactured for underwater use are specially designed to allow the entire metal detector, including the control housing, to be completely submersible. …