Electricity: measuring and testing – Of geophysical surface or subsurface in situ – For small object detection or location
Patent
1987-05-19
1990-01-16
Strecker, Gerard R.
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
For small object detection or location
324233, 324239, G01V 311, G01R 3312
Patent
active
048946184
DESCRIPTION:
BRIEF SUMMARY
This invention relates to metal detection apparatus and in particular to maintaining substantial discriminatory sensitivity in mineralized environments.
In this specification the term "metal detector" is used to refer to detectors of a type used for the purpose of discriminating metal within the ground but it can also be used to refer to a single detector or an array of detectors where used to detect metal objects in soil on a moving conveyor system. Such a system may be used, for example, to locate gold nuggets in soil moving on the conveyor system.
The invention is directed to some of the difficulties encountered when using a detector in the presence of ground containing varying proportions of minerals magnetic characteristics of which may vary from one location to another.
These minerals, such as ferrous oxides may produce strong signals in the detector which make identification of a target signal (for example, a signal from a gold nugget) difficult to identify, that is these strong variable ground signals can "mask out" the signal from a target object. The extent of this difficulty is very dependent on ground conditions.
There are a number of types of metal detectors each having a different method of operation. There is however a common principle of operation common to all these types of detectors. This involves the production of a magnetic field by a coil.
A conducting material within an effective range will interact with this field and change characteristics of the field which characteristics can then be analysed.
This invention is directed to a type of metal detector known as an alternating magnetic field metal detector where the field variations are substantially sinusoidal.
This will include detection electronic circuitry which compares the phase and magnitude of an emf signal induced in a receiver coil with that of a transmitted signal.
The object of this invention is to provide means by which there can be more effective detecting in a discriminatory way in such conditions ground containing mineralisation, particularly where the magnetic characteristics are variable spatially.
It is well known that an induced signal in any material may have a phase relationship that is useful in identifying it's magnetic characteristics; this phase relationship can be referred to in terms of quadrature components of the signal. These can be characterised by a purely reactive component and at 90 degrees to this, a purely resistive component which is some times refered to as the loss component.
In general, mineralised ground will provide a large reactive component and a small loss component whereas conducting objects, because of eddy currents generated within them have large loss components by comparison.
It is this difference which allows for the possibility of locating conducting objects even when mineralisation in the ground produces a large return signal.
The best available conducting metal target detectors use two synchronous demodulators known as the "X" and "R" channel, such that the "X" channel has it's sensitive component axis aligned within a small angle of the purely reactive retransmitted components, and the "R" channel has it's sensitive component axis aligned within a small angle of the purely resistive retransmitted components. The sensitive axis of the "R" channel can be manually varied in order to align it so that it is at quadrature with the ground vector. The detector is said to be gound balanced for the areas of ground whose vector is aligned at quadrature with the sensitive axis of the "R" channel. For these detectors the "R" channel is known as the "object channel". Ground balancing can also be achieved by adding a proportion of the "X" channel to the "R" channel by control of the proportion added. For detectors using this latter ground balancing method the composite signal is called the "object channel". Many detectors do not provide for either of these controls.
U.S. Pat. No. 4,128,803 uses an "X" and "R" signals channel and a third demodulator between these two channels to detect and identi
REFERENCES:
patent: 3621380 (1971-11-01), Barlow
patent: 3975674 (1976-08-01), McEuen
patent: 4628265 (1986-12-01), Johnson et al.
patent: 4677384 (1987-06-01), Payne
patent: 4700139 (1987-10-01), Podhrasky
patent: 4719421 (1988-01-01), Kerr
Minelab Electronic Industries Ltd.
Strecker Gerard R.
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