Electricity: measuring and testing – Of geophysical surface or subsurface in situ – For small object detection or location
Reexamination Certificate
1998-03-18
2001-03-20
Snow, Walter E (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
For small object detection or location
C324S239000, C324S243000
Reexamination Certificate
active
06204667
ABSTRACT:
FIELD OF INVENTION
The present invention relates to methods for measuring electromagnetic fields, and more particularly to a method of measuring electromagnetic field gradients.
BACKGROUND OF THE INVENTION
Electromagnetic induction has long been used for detection of hidden and buried metal objects such as mines and buried ordinance and for security checks, treasure hunting, and earth resource exploration. The method involves generating a primary electromagnetic field from a transmitter coil that induces a current in nearby conductive objects. This induced current produces a relatively weak secondary electromagnetic field that is detected by a receiver coil. The measured secondary electromagnetic field is then analyzed to detect, and sometimes characterize, buried or hidden objects.
Existing metal detectors employ many well-known transmitter and receiver coil configurations. One consideration in coil configuration is that the primary electromagnetic field generated by the transmitter coil not only induces a secondary electromagnetic field in the hidden object but also induces an electromagnetic field in the receiver coils. Because the object's induced secondary electromagnetic field is relatively weak compared to the primary electromagnetic field, the primary electromagnetic field will tend to mask the secondary electromagnetic field unless compensated for. This compensation is referred to as nulling or balancing. Ideally, nulling the primary electromagnetic field results in the metal detector indicating a field strength of zero, or null, in free space.
One approach to nulling is to employ multiple bucking coils. As shown in the patent to Won, U.S. Pat. No. 5,557,206, the disclosure of which is expressly incorporated herein by reference, another approach is to concentrically locate the receiver coil within the transmitter coils such that the direct electromagnetic field is automatically nulled.
While the approaches above present methods for measuring electromagnetic field strength, none measure electromagnetic field gradients. The measurement of electromagnetic field gradients is useful in characterizing hidden objects and provides another tool for determining the size, shape, or composition of the objects. Thus, there remains a need for method of measuring electromagnetic field gradients.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for measuring electromagnetic field gradients. The electromagnetic gradiometer of the present invention generates a time varying primary electromagnetic field, such as by use of a transmitter coil attached to a waveform generator. The primary electromagnetic field induces a responsive, relatively weak, time varying secondary electromagnetic field about a hidden object. The combined primary and secondary electromagnetic field is measured simultaneously at two spaced apart detectors, such as at two distinct receiver coils. The detectors are configured such that they detect the primary field in equal magnitudes but in opposite sense and connected in series such that the primary electromagnetic field is automatically nulled. The difference in the secondary electromagnetic field measured at the two points is divided by the distance between the detectors to determine the electromagnetic field gradient therebetween. In one embodiment, there are two detectors which are axially aligned with the transmitter coil and symmetrically located thereabout. Alternatively, an array of detectors may be used to measure a plurality of electromagnetic field gradients by switching or otherwise pairing the detectors.
REFERENCES:
patent: 3826973 (1974-07-01), Pflaum
patent: 5065099 (1991-11-01), Sinclair et al.
patent: 5668475 (1997-09-01), Orban et al.
Coats & Bennett PLLC
Geophex Ltd.
Snow Walter E
LandOfFree
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