CA1215743A - Method and apparatus for indicating the presence of conductive material in a medium - Google Patents

Abstract

ABSTRACT
The presence of electrically conductive grains, particularly those of precious metals in a medium, such as the ground (11 ), is indicated by detecting the "short--circuiting" by such grains across a narrow gap (25) between two mutually spaced and electrically insulated electrodes (17, 19) which are brought into investigating contact with the medium, e.g. in the form of a probe. "Short-circuiting"
is detected by measuring the resistance across the two elec-trodes.

(Fig. 1)

Description

'7~3 TITLE OF INVENTION
METHOD AND APPARATUS FOR INDICATING THE PRESENCE OF
CONDUCTIVE MATERIAL IN A MEDIUM

TECHNICAL FIELD
The present invention relates to a method and apparatus for indicating the presence of, or searching for, electrically conductive material in the form of grains, particularly of precious metals, in a medium, particularly in the ground or in a geological mass. By precious metals are typically intended gold, silver and metals from the platinum group, primarily platinum.

TECHNICAL BACKGROVND
It is known -to search for metals in the ground with the aid of electromagnetically operating metal detectors. However, such detectors are not practicably usable when the metal is present in the orm of spread-out small grains and they do not function at all other than in conjunction with large amounts of metal.
In my Canadian Patent 1,188,734 there is described a method of indicating the presence in the ground of grains of metals and metallic minerals while utilizing a probe which can be driven into the ground, and which is provided wi-th two electrodes of different materials, which are insulated from each other and have different contact potentials. When one of the electrodes comes into contact with a grain of metal, for instance, in the ground, the voltage which can be measured across the electrodes is changed.
This voltage change is detected and converted to an indication that an electrically conductive grain has been discovered. Since the ].a detection is based on contact potentials, the method does not give the possibility of distinguishing for instance between a pure metal and a mineral which has approximately the same contact potential, and there is al50 a risk of disturbances due to the exterior electromagnetic fields. Furthermore, the method requires some conductivity, i.e. moisture, in the ground and has been found to be less effective r~

2-for indicating the presence ~f the very small grains.
OBJECT OF THE INVENTION
The object of the present invention is to provide a new apparatus for in~ication of the kind mentioned in the introduction, whereby the problems and limitations mentioned above are circum-vented and whereby indication of even very small grains, par-ticularly of precious metals, can be performed extremely simply, efficientl~, flexibly and versatilely.
SUMMAR~ OF INVENTION
According to the invention, there is provided an apparatus for detecting the presence of electrically conductive material in the form of grains, comprising: a probe including a shaft having an axis, a bridging washer, an electrode assembly and a conical tip; said electrode assembly being connected to said tip and having a frusto-conical shape, said assembly including a pluralit~ of stacked, thin lamella electrodes separated by much thinner insulators, said eleatrodes and insulators being in the shape of annular washers; said bridging washer connectiny said shaft to said electrode assembly, said electrodes, bridging washer and tip being formed from a wear resistant carbide material;
means connected to said electrodes for measuring the resistance which can be measured between said electrodes; and an adjustable threshold detection circuit means connected to said resistance measuring means for providing an indication signal when measured resistance falls below a set threshold value.
The invention is based on the fact that grains or small ~ 6'~

79~3 -2a-particles, particularly of precious metal, have been found to have the ability of causing "a short c.ircuit" between two elec-trodes in close mutual proximit~, but which are both mechanically and electrically separated, this short~eircuiting being so low-resistive that the resistance which can be measured across the electrodes is lowered noticeably during the short-circuiting, and this lowering can be detected very simply, reliably and repro-ducibl,v. A similar noticeable lowering in resistance is normally not obtained when the "short-circuiting" is caused by a grain of ordinary metal naturally present, or an electrically conductive mineral. Grains of precious metals can therefore be easily dis-tinguished from other grains~
The utilized electrodes, which may be mounted in some kind of carrier and/or constitute a self-supporting unit, thus have exposed mutually contiguous eleetrode parts, between which there is a small spacing or a "gap" which can be bridged by a grain, which is simultaneously in electrieally conductive eontact with both electrode parts. The mutually contiguous, exposed elec-trode parts are suitably glven an extension such that there is formed one or more elongate ,~

S~ 3 gaps. These earl run parallel to each other and/or in diffe-rent ~irections for increasing ~hc probability that a gr~in will be able to bridge a gap.
A gap between two electrode parts or edges does not need to be open or un~illed, but can be filled with a suit-able electrically insula~ing material, which possibly also keeps the electrodes together.
An unfilled "gap opening" can however facilitate brid-ging over the gap in question by a grain in certain cases.
The distance between the electrodes, i.e. the gap width, is small, typically less than about 1 mm, preferably within the interval from about 0.01 mm to about 0.5 mm, and particularl~ within the interval of about 0.02 mm to 0.05 mm.
To advantage, a plurality of electrodes may be arranged in an electrode pack or an electrode unit, so that a large number of exposed gaps is obtained. The electrodes can suit-ably be given a leafed or laminated configuration. In con-junction with a plurality of electrodes, these may be alter-natingly coupled together into two electrode sets, within which the electrodes are mutually connected and between which the resistance is measured. The electrodes can also be arrtanged or connected together in different groups with diffe-widt~s within the different groups. Of the respective group the electrode pairs across which the resistance is measured, can be coupled in separately, further to which they can be coupled in simultaneously in parallel, which gives the possibility of examining within which size range the dis-! covered grain probably lies. It is namely the case that a least or probable grain size corresponds to each gap width.
Special arrangements may be made for allowing adjustment,at least within given limits~ of the gap width between con-tiguous electrodes. In such a case these can be separated by compressible insulating material, for example.
In conjunction with utilizing the invention there are two preferred aspects. The one implies that the electrodes are caused to pene~rate into the medium which is to be exa-mined. In this case the electrodes are, as mentioned, ~'' ,3 ~

~L~it7~

suitably arranged on a carrier allowing their suitable handliny.
The carrier with the electrodes may constitute a probe, a blade, a share or the like~ which can be easily driven into ground, for instance, and/or brought to move in and along a surface layer of ground. In this case the electrodes are to advantage arranged in connection to a tip, an edge or the like, which goes down into the ground first. It is advantageous to allow an electrode pack or an electrode unit to comprise at least the main portion of such a tip or edge, which may be tapered in a generally conical or wedge-shaped way. The previously mentioned gaps can then to advantagebe distributed over practically the whole oE the tip or edge sur-face. The pointed or wedge-like shape or the like is advantageous in that, in relation to the driving-in direction in the ground, the sloping or inclined surfaces obtained, contribute to grains which are to be detected in conjunction with driving-in being pressed, or scrape against the exposed gap-defining electrode parts, whereby the contact resistances will be lower. With the same object, and for facilitating driving-in, a tip region may be given a drill, milling cutter or helical configuration. With the aim of providing gaps running in a spiral formation on a tip arrangement, the tip may also be made up from two or more conical spirals, which are screwed into, and electrically insulated ~rom each other.
The other aspect on the utilization of the invention implies that the medium which is to be examined is transferred to, and is brought into contac~ with, an electrode arrangement which can then be arranged in a vessel, a conduit or the like, e.g. in a washing pan or washing plant, e.g. in conjunction with a wash chute. The medium is accordingly caused suitably to move over or .,~ .

impinge against the electrode arrangement, e.g. in conjunction with a change in direction.
It should be emphasized that the present invention can to advantage be combined with the invention according to my Canadian Patent 1,188,734 so that utilized electrodes can be combined and/or be arranged on the same carrier or probe.
Further distinguishing features and advantages of the invention will be apparent from the following detailed description of exemplifying embodiments made with reference to the accompanying drawing.

SHORT DESCRIPTION OF DRAWINGS
Fig. 1 is a schematical vertical view, partially in section, of a first embodiment of an apparatus in accordance with the invention with the tip portion of the apparatus driven into the ground. Fig. 2 is a vertical sectional view to a larger scale of the tip portion of the apparatus according to Fig. 1. Fig. 3 is an enlarged partial view of an area in the arrangement according to Figs. 1 and 2 where two electrodes are mutually contiguous while forming a gap, this gap being illustrated as bridged over b~v a grain, eOg. of gold. Fig. 4 is a schematic perspective view of a second embodiment of the tip portion of the apparatus. Fig. 5 is a schematic, partial, perspective view of a further embodiment of a tip portion for an apparatus in accordance with th~ invention.
Fig. 6 is a schematic view from above of a second embodiment of an apparatus in accordance with the invention in the form of a washing pan with electrodes arranged in the bottom of the pan.

5~4~3 5a DESCRIPTION OF EMBODIMENTS
.
The apparatus in accordance with the invention, illustrated in Figs. 1 - 3, has -the form of a probe intended for being able to be driven into ground, for e~ample, where the pres-ence of grains of gold or the like is to be investigated. The apparatus includes an electrode or measuring head 1 in the form of a tip portion, and adapted on one end of a carrier 3 in the form of a cylindrical shaft. The other end of the carrier or shaft 3 is provided with a handle 5 facilitating handling the apparatus.
cable 7 leads to an electronic unit 9 from the handle 5, the unit 9 including a plurality of circuits, which will be described more closely later. It will be understood that the electronic unit 9, e.g.
in suitable miniatyrized form, could be built into the handle portion 5.
The electrode or measuring head 1, which is formed for being easily driven into the ground 11, has a generally pointed configuration. The head comprises a conical tip port-tion 13, rigidly connected to a central rod or pin 15 extending up through the tubular shaft 3, a great plurality of annular electrode washers 17, 19, and a similarly annular bridging washer 21. The washers 17, 19, are flat and have a typical thickness of about 0.5 mm (there only being illu-strated a small number for the sake of clarity~, and are oriented with their planes at right angles to the longitu-dinal direction of the apparatus, i.e-. to the direction of the shaft. The washers 179 19 are electrically insulated fTom each other, from the tip portion 13 and from the brindging washer 21 with the aid of intermediate insulation-material 23, e.g. having the form of thin washeTs OT being molded in situ. The washers 17, 19 have the form of thin truncated cones. Their outer diameter increases in the direction away from the tip portion 13 to the bridging washer 21. The low-I es~ washer 17 ma~es with the tip portion 13 and ~he uppermost ¦ washer 19 mates with the bridging washer 21, so that the 25 washers 17, 19 together form a truncated cone with a least diameter mating with the greatest and upper diameter of the tip portion 13, and with a greatest diameter mating with the greatest, and lower diameter of the bridging washer 21. The cone angle for the tip portion 13 is greater than the cone angle for the electrode array itself.
In pairs, the electrodes 179 19 define gaps 25, which thus go all the way round and extend in plan at right angles to the extension of the shaft 3. These gaps may be bridged over by conductive grains 27, as is indicated in Fig. 3, thus causing short-circuiting between the two contiguous electrodes or washers 179 19. This overbridging and short--circuiting can be facilitated if the insulation 23 does not ~Q

~5;`7ql3 extend all the way out, i.e. if there is an unfilled or free gap mouth or opening 29. The sharp edges of the electrodc washers 17, 19 will thus be free for contact with a grain 27.
The washers 17, 19 are alternatingly electrically connected to each other and to two connecting lines 31, 32 extending through the hollow shaft 3 and to the electronic unit via the cable 7. Every two washers, i.e. the washers 17, are connected to the line 31, and every two other washers, i.e. the washers 19, are connected to the line 32. This is provided by utilizing the arrangement that the washers have eccentric equal middle holes, the washers 17 having their hole displaced to the right in Fig. 2 and the washers 19 their hole displaced to the left. It will be understood that in assembling the electrode pack it is possible to start with similar washers which are assembled non-axially and which, after uniting with the tip portion 13 and the bridg-ing washer 21, are ground or otherwise machined to the desired final exterior form. Alternatively, all the washers could be given an equally great central, axial hole with an inwardly thrus~ing connection pin or the like, said pin being alterna~ingly placed to the left and to the right.
The anllular bridging washer 21 is adapted for giving an oblique downwardly and outwardly directed juncture bet-ween the shaft 3 and the rear portion of the electrode pack 17, 19 having the larger cross section. The lesser cross section or diameter of the shaft 3 gives several advantages.
Driving-in is facilitated, as well as withdrawal. Further-more, on withdrawal,Metal particles possibly torn off from the metal shaft,(and not naturally present~3are deflected, so that they are not pressed against the gaps 25 $o give un-desired short-circuiting. In this connection it should be pointed out that although the tip portion 13 and washers 17, 19, 21 are suitably formed from very wear-resistant ma~rial such as carbide, this does not need to be the case with the shaft 3.
The central pin 15 is connected in some su~able way~
not more closely illustrated, but obvious to one skilled _ I--in the art, to the handl~ portion 5. It is possible to make this connection adjustable, so ~hat ~he pin is pulled more or less heavily upwards. If the insulation 23 is compress-ible to some extent, it will thus be possible to vary the width of the gaps 25 within given limits.
The electronic unit 9 includes a resistance-measuring circuit 35, In operation this circui~ uninterruptedly mea-;
sures the resistance between the parallel-connected electro-des 17 and the parallel-connected electrodes 19 and leaves a measurement signal of a magnitude in response to the measu-red resistance. The response may be linear or logarithmic or in accordance with a desired function. The magnitude of the measuring signal is detected by an adjustable threshold detection circuit 31 supplying an indication signal to an indication circuit 37 as soon as the detected signal magni-tude, even if it is of very short duration, falls below the set threshold value. The indication circuit 37 may be of an optional kind and may, for example, include one or more of the functions digital display, light signal and acoustic signal. It will be understood that all of the circuits given above are of a conventional kind and may be realized without difficulty by one skilled in the art, for which reason they da not need to be described in deta~ here. It should be emphasized that the nature and frequency of obtained indicat-~5 ions can give atrained operator a very good impression ofwhat kind of discovery he has made.
An alternative embodiment of an electrode head with similarly laminated configuration is illustrated in Fig. 4, the head being wedge-shaped, however, and having gaps with an extension substantially in the direction of the probe.
The electrode head 41 thus comprises a great plurality of substantially triangular electrode lamellae 47, 49 assemb-led in parallel in~o a pack, although with intermediate electrical insulation. Between two contiguous electrode lamellae 47, 49 there is thus formed a gap 45 with a sub-stantially V-shaped configuration. The gaps thus extend round the tip itself. The electrode head 41 is attached to ~I

a shaft 43, not more closely illtlstrated, which ~r~ has thc same cross section as the r~ar part of thc electro~e head The cross section of the shaft ~3 could however be reduced in dimension similar to the shaft 3 in the apparatus accord-ing to Figs. 1-3.
An electrode array is illustrated in Fig. 5, this array being particularly suitable for use in examining narrow and deep rock cracks and the like. The array consists quite simply of two like, thin, strip elec~rodes 51, 53 joined to-gether in parallel with the aid of a very thin electricallyinsulating intermediate layer 55. This defines a U-shaped gap, which thus extends both vertically along the two narrow side surfaces and horisontally along the narrow bottom sur-face. The electrodes 51, 53 may have optional hight and be attached to a suitable carrier or in a suitable handle port-ion, and be connected ~o an electronic unit as described previously.
In Fig. 6 there is schematically illustrated the arrangement of an electrode array 61 in accordance with the invention, at the bottom 63 of a washing pan 65. The elec-trode array 61 comprises two comb-like electrode sets 67, 69, the individual electrodes of which, i.e. the comb teeth, are insulated from each other and placed between each other with a spacing such tha~ there is formed a plurality of gaps running substantially parallel and radially. The electrode arrangement 61 is placed at the bottom 63 adjacent the wall 71 joining onto the bottom of the washing pan and at the place where grains of gold normally collect in conjunction with the washing or panning process. The electrode sets 67, 69 are coupled to an electronic unit of a kind described previously, by means of a cable, not shown, suitably con-nected to the outside of the bottom 63 or wall 71.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS.

1. An apparatus for detecting the presence of electrically conductive material in the form of grains, comprising: a probe including a shaft having an axis, a bridging washer, an elec-trode assembly and a conical tip; said electrode assembly being connected to said tip and having a frusto-conical shape, said assembly including a plurality of stacked, thin lamella electrodes separated by much thinner insulators, said electrodes and insulators being in the shape of annular washers; said bridging washer connecting said shaft to said electrode assembly;
said electrodes, bridging washer and tip being formed from a wear resistant carbide material; means connected to said elec-trodes for measuring the resistance which can be measured between said electrodes; and an adjustable threshold detection circuit means connected to said resistance measuring means for providing an indication signal when measured resistance falls below a set threshold value.

2. Apparatus as claimed in Claim 1, wherein said bridging washer has a frusto-conical configuration which tapers in a direction reverse to the taper of the frusto-conical electrode assembly, the diameter of the shaft being less than the greatest diameter of the electrode assembly.

3. Apparatus as claimed in Claim 1, wherein said tip has a greater cone angle than said electrode assembly.

4. Apparatus as claimed in Claim 1, wherein the annular edge of each insulator between adjacent electrodes stops short of the perimeter of adjacent electrodes so that a gap is formed between adjacent electrodes.

5. Apparatus as claimed in Claim 1, wherein each electrode has a thickness of about 0.5 mm.

6. Apparatus as claimed in Claim 1, wherein each of said insulators has a thickness of from about .01 mm to about .05 mm.