GEOSPHERE INC -- Magnetometry and Metal Detection: Metal Detection

an active technique, unlike magnetometers which are passive. (Some pipe/cable locators are a combination of a small fluxgate magnetometer and signal detector).

Geosphere uses the single coil and transient type instruments and the newer pipe/cable locators, selecting the appropriate instrument for the specific task. Our transient system is waterproof to a depth of 20 feet and can use several different size coils from two inches to six feet in diameter. Typical metal detectors have a relatively shallow range of operation; their response to a given target decreases at the rate of the target's depth to the sixth power (1/D^6). Small objects like quart containers may be found to a depth of 3 feet while larger objects with greater surface area, like 55 gallon drums, may be detected to depths of 4-9 feet, dependent on the size of the primary coil. The performance of many instruments will be limited by high concentrations of iron-rich minerals in the soil, conductive fluids (salt water), or small metallic debris. Again, such limitations may be minimized or completely avoided by employing the proper equipment and field procedures.

Metal detectors respond to the high electrical conductivity of metal targets contained within the normally low conductivity of soil. Targets with greater surface areas will cause greater responses. Targets may consist of any metallic materials which cause a local conductivity anomaly, such as iron, steel, copper, brass, bronze, tin, aluminum, silver, gold, and lead.

Various types of instruments measure this response in different ways. With the two box (coil) system, a transmitter coil generates an alternating magnetic field around itself; this field is "balanced" with the receiver coil, oriented perpendicularly to the transmitter. Any metal target encountered during the survey causes eddy currents which interact with the transmitter field to upset the original balance condition. At this time, the meter or beeper signals the event. The treasure hunter-type systems combine the transmitter and receiver functions in one coil which responds to the field emanating from the target in different ways. Thus, metal detectors are classified as