Electromagnetic induction has long been used to detect hidden or buried metal objects. Typically a magnetic field generator propagates a primary electromagnetic field that in turn induces a secondary electromagnetic field in a nearby hidden object. A magnetic field sensor spaced from the hidden object senses the induced secondary electromagnetic field and consequently in the process the hidden object is detected.
Detection of presence alone reveals little about the hidden object. If the detection is accurate, that is the target detected is indeed a target and not a false target, about the only conclusion that can be reached is that the detected object is at least partially metal. No conclusions, or even postulations, can be made about the material composition of the hidden object or the object's size and shape. Further, detection alone is incapable of shedding light on other important object characteristics such as conductivity and magnetic permeability.
In the past, simple detection has been used, for example, by the military and others to detect buried land mines and unexploded ordinances. Once detected, it is common practice in certain cases to recover the land mines and buried ordinances through excavation. Because of the nature of conventional electromagnetic induction detection, one can never be sure that the detected object is in fact a targeted land mine or an unexploded ordinance. Thus, to some degree at least, excavation for such objects is not always productive because of detected false targets and because the detected object may turn out to be something other than a land mine or buried ordinance.
In the end, conventional metal detectors are simply not designed to do any more than simply detect the presence of a metal object. This is because most conventional metal detectors operate at a single frequency while a few may have the capability of operating at a small number of discrete frequencies. Because of this frequency limitation, conventional metal detectors are unable to induce a continuous varying frequency response in an object. As such, various metal detector devices have tried other approaches to identify a hidden object. For instance, U.S. Pat. No. 5,506,506 to Candy discloses a metal detector that utilizes a magnetic pulse approach. The Candy device sends a magnetic pulse towards the hidden object and then measures the time decay response of the secondary magnetic field in an attempt to identify whether the hidden object is ferrous or non-ferrous. Such a method is incapable of identifying the size or shape of the hidden object or of determining other material properties of the hidden object.
U.S. Pat. No. 5,642,050 to Shoemaker discloses a metal detector that uses a small number of pre-determined discrete frequencies in an attempt to identify a hidden object. Shoemaker discloses using typically two, but up to four, pre-determined discrete frequencies to generate a primary magnetic field. The induced secondary magnetic field is then detected and used to identify the hidden object. Like Candy, the Shoemaker approach relies on differences in magnetic decay properties to distinguish ferrous from non-ferrous objects. The Shoemaker device is incapable of identifying the size or shape of the hidden object or of determining other material properties of the hidden object.
Basic electromagnetic theory suggests that different hidden objects will exhibit different responses across a low-frequency broadband spectrum and that these different responses will be indicative of various object characteristics such as size and shape and magnetic permeability and conductivity. Thus, without the capability of generating time-varying multi-frequencies, conventional metal detectors cannot be expected to induce multi-frequency responses in objects that could be used to evaluate features and characteristics of the object.
Thus, there is a need for a method or process that not only detects hidden objects but also is capable of identifying certain features or characteristics of an object such as material composition, geometry and size, conductivity, magnetic permeability, etc.