Electricity: measuring and testing – Of geophysical surface or subsurface in situ – For small object detection or location
Reexamination Certificate
1999-05-19
2001-12-04
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
For small object detection or location
C324S336000
Reexamination Certificate
active
06326791
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the detection of metallic targets such as unexploded ordnance, mines and buried treasure by detecting the electro magnetic response of such targets when exposed to electromagnetic fields.
BACKGROUND OF THE INVENTION
In many parts of the world contamination of the electromagnetic response by effect of magnetic soil, produces signals that can be interpreted as a response from a wanted target. The problem is most often encountered by users of electromagnetic detectors in search for unexploded ordnance, mines and buried treasures.
It is known in electromagnetic surveying techniques to differentiate responses from different types of targets by processing data obtained with respect to time or displacement so as to obtain additional data which can be used to provide the desired differentiation.
For example, in U.S. Pat. No. 5,654,637 (McNeill), responses from receiver coils at different distances above terrain are scaled and summed to eliminate responses from a particular depth, such as a susceptible surface layer. This technique depends on wanted and unwanted responses occurring at different depths.
SUMMARY OF THE INVENTION
It has been observed, by laboratory and in situ measurements, that the response from magnetically susceptible soil, after being exposed to an electromagnetic pulse, as received by an induction coil, has a characteristic power law time decay response, linear when plotted on a log-log scale.
On the other hand response from a confined metallic target, such as bombs, mines or coins, will produce more complex non-linear log-log scale response, characterized by a relatively more slowly decaying initial response, followed by a more rapidly decaying late response.
According to the invention, there is provided a method for discriminating electromagnetic responses to application of an electromagnetic pulse of metallic objects in soil from superposed responses due to magnetic susceptibility of the soil, comprising substantially cancelling, from at least a portion of the response, any component having power law decay characteristic having an exponent which is characteristic of the soil. This cancellation can be performed in various ways, as further described below. The invention also extends to apparatus for performing the method.
REFERENCES:
patent: 4600356 (1986-07-01), Bridges et al.
patent: 5537041 (1996-07-01), Candy
patent: 5576624 (1996-11-01), Candy
patent: 5654637 (1997-08-01), McNeill
patent: 2041532 (1980-09-01), None
Geonics Limited
Ridout & Maybee
Strecker Gerard R.
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