AU626115B2 - The use of resistivity in highly conductive layers to detect mineralisation - Google Patents

Description

2 6 115 FORM COMMONWEALTH OF AUSTRALIA PATE14TS ACT 1952-59 COMPLETE

SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Application Number: -PJ 1804 Lodged: -06.12.196,E Int. Class Complete Specification Lodget1: Accepted:' Published: Priority: Related Art: Name o1 Applicant: Address of Applicant: Actual Inventc7: Address tor Service: Au-A-cA ck 456 Kent Streezt, S--y jwSuh- ls Poauz-ralia Complete Specification for the Invention entitleo: T-TE OF FZSISTIT=. IN IaGHE x COUN 2TIVE TO DE!E2 ALT-AT26%

~~~NT

The iollovino statement is a lull oescription of -this invention, Including, the best metho ol periorminr known zo wE;-us.- The present invention relates to a method of producing geological data useful in aiding the detection of opal deposit sites. In particular, the present invention relates to a method for producing geological data using close grid spacing resistivity measurements.

Opal prospecting has, to date, proceeded largely on the basis of visual observation of an area's apparent geology, drilling test holes and guesswork. No viable mapping or other geophysical technique has been developed to assist the opal prospector.

Resistivity is a geophysical measurement which has been widely used in exploration for large ore bodies, for instance ores of copper, lead, zinc, gold, nickel and tin. However, it has previously not been applied to the search for minerals found in highly oconductive layers, as the conventional belief is that such methods :15 cannot be useful in these conditions. Further, conventional resistivity mapping techniques have used large grid sizes, generally to 100 m. Small-scale measurements have been disclosed ior particular applications, e.g. in borehole logging to detect oO0 permeable/non-permeable rocks, and to measure resistivity within an ore body as an aid to identifying mineral composition.

0 OoThe present invention relates to a method of locating from a o oground surface position through the overburden a potential opal-bearing region contained within a claystone layer in the 0 0 underlying ground wherein said claystone layer is of substantially uniform high electical conductivity and wherein said potential opal-bearing region is more electrically resistive than the claystone layer, the method comprising the steps of:setting up an electrode array on said ground surface position having two fixed spaced apart current electrodes to form a transmitting dipole and two spaced apart potential electrodes forming a receiving dipole movable at predetermined intervals along lines parallel to a line joining the current electrodes to provide a grid of potential measurement positions; introducing a current between the electrodes of the transmitting dipole; -L ii -L 1~ I I 2a measuring the potential difference between the electrode, of the receiving dipole at a plurality of positions on said grid and thereby the resistivity within the claystone layer at each said position; and determining from the measurements of resistivity the location of regions of anomalous resistivity within the claystone layer which regions correspond to potential opal bearing regions.

The present invention differs in one respect from prior resistivity techniques in that it attaches meaning to very low resistance measurements, and small variation therein, neither of which are features known in the prior art.

Further, as will be further explained below, the application S of the present method is quite different from those previously used.

0 0 The prior art uses resistivity either to assist analysis of rock :o0 oo15 type, differentiate 0 0 o i ts 3 different layers or locate large inclusions of altered material. The present invention operates through the overburden and top sandstone layers, effectively ignoring them. The clay layer of interest is relatively much more conductive and so is responsible for the bulk of current carried. The present invention seeks to find and interpret anomalous areas within a relatively uniform highly conductive clay layer.

The method of this invention is based on a particular theory of opal formation, and of the conditions necessary for the formation of silica salts in nature from which precious opal is formed, and in particular in the 0o"° absence of impurities of multivalent cations and anions.

o o The structure of precious opal, a mineral only rarely 1 0 encountered in nature, is a cubic face-centered packaging of O like-sized (100-400 nm) spherical particles of silica, the spaces between which are filled with amorphous silica of a second generation with a refractive index different from 0000 that of the spherical particles.

20 Precious opal forms in nature as a result of the rare combination of a number of conditions which are 0O difficult to fulfill. Its formation can be divided into the following stages: 1) nucleation and growth of spherical 0 09 one-sized sol particles of silica in solution, 2) precipitation of silica particles from solution, 3) packing of particles with the formation of a regular structure, and cementation of silica particles by filling the space 0between them with second-generation silica.

One of the conditions needed for the growth of spherical particles and stability of the silica sols is high solution purity, the absence of impurities of multivalent cations and anions. Colloidal particles of SiO 2 in the solutions during the formation of precious opal reach dimensions of 200 to 450 nm. Experiments show that the sols with such particles form during very slow concentrations of colloidal or true aqueous solutions of silica of a high degree of purity by means of evaporation.

L -4- Silica solutions change into a sol during evaporation and contain numerous impurities. During its evolution the solutions are purified by sorption or filtration through rocks.

Geological research suggests that the formation of precious opal is connected with the activity of ground water. Ground water in the aric climate of Australia is confined to clay horizons and is found there in a capillary or film state, which is expressed in the slight moisture level of the clays. The fairly intense dynamics of the ground water is determined mainly by evaporation. Therefore migration of chemical elements as a result of the dissolving oo" action of ground water occurs in an upward direction. Since o country rocks were formed in shallow marine basins, the o°°o 15 ground water was initially rich in K Na Ca 2 Mg 2 SO2 15 2 3+ S2- °o o and Cl-. Sources of Fe 2 A13 and SiO 2 may be the clay oa o° minerals, feldspar, micas, chlorites and ash material.

Organic materials which break down liberate K Ca Mg 2 NH, PO-, CO SO4 and I-I 2 PO4 and form humic acids.

There are many sorption processes adsorption, chemosorption, chromatographic separation and ion exchange reactions) in nature. Purification occurs S. because of the absorption of ions on hard developed surfaces, and in the solutions of electrolytes as a consequence of the ion exchange reactions change at the interface of the solid and liquid phases. If the dimensions of the solid phase are very small, absorption and ion exchange occur at the same time, which is typical for minerals capable of dispering to a colloidal state, such as clays. In sedimentary deposits of precious opal, the purification of silica solutions occurs as the sorption by 'lay minerals, montmorillonite, which break down during acid weathering, and also by the newly formed colloidal phases, kaolinite, iron hydroxides, ordinary opal, and humic acids.

Accordingly, precious opal is typically found in locations associated with the depletion of multivalent anions and cations. It is the latter depletion which gives rise to relatively high resistivity anomalies.

1 'N t *e O *r 4 0 0 04 0 0 0 000 0 5 It is particularly noted that even in a known opal field, the present invention will not definately identify opal deposits and areas with no opal. The data or maps produced suggest only areas of higher or lower resistivity, where opal formation may have taken place if other factors have been conducive to opal formation.

A preferred method of prospecting using the present invention involves a number of steps. Firstly, visual inspection is conducted to determine whether the required geological strata are present. Generally, opal forms in a clay layer below a sandstone layer. However, this layer may have eroded, so that the presence of the top sandstone layer in nearby areas may be a useful indication.

Secondly, test drilling in areas which seem favourable on the basis of visual inspection.

Thirdly, if opal or minerals associated with opal or suggestive of its presence are located, the method according to the present invention is undertaken to assist in determining the extent of the deposits and the appropriate mining procedure.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram showing an exemplary grid layout; Figure 2 is a schematic diagram of a Gradient Electrode configuration; Figure 3 is a graph showing the time domain waveforms used in a preferred embodiment; Figure 4 shows a comparision of varying electrode position and separation; and Figure 5 is an example of a contour map produced according to the present invention.

Preferably, measurements are performed as follows.

A Current I is injected into the ground at two electrodes, distance x apart, forming a transmitting dipole, and the voltage V due to the applied current is measured between two points distance x apart forming a receiving dipole.

0000 0 0 00 0 0~ 0 0 0c 0000 c-r -6- The transmitting and receiving dipoles lie on a straight line, and the distance between the nearest transmitting electrode and receiving point is nx where n is an integer. The distance x is called the spread length and n the separation.

Resisitivity is the reciprocal of conductivity.

The better a material conducts, the lower its resistivity.

The resistivity of a material in ohm m is the resistance in ohms between opposite faces of a one cubic metre cube of the 10 material. The resistance in ohms of the cube is given by i0 Ohm's Law as the ratio of the voltage in volts to the current in amps.

Solutions contained in rocks usually have a lower o resistivity than the minerals comprising the rock, so the 1 more porous a rock the better it conducts and the lower its o o resistivity.

0 O0 o For this dipole-dipole geometry, the resistivity in ohm m of a uniform earth is given by: o= V GX 271 I 2 where I is the transmitted current in amps V is the received voltage in volts 0 X is the spread length in feet G is a geometrical constant depending on the A 0o 25 separation n, and is given by: S 2 n(n l)(n 2) 5= 2 oo For n 1, 2, 3 and 4, G 3, 12, 30 and respectively. The multiplication by G compensates for the fact that in a uniform earth the received voltage V 30 resulting from a given transmitted current I decreases as 3O the separation n increases. The presence of an electronic conductor is indicated by a resistivity lower than that for barren soil.

A grid is usually established for making the measurements. This consists of a baseline and a series of parallel lines normal to it. The direction and location of the baseline, and the spacing and length of th- parallel lines are determined by the nature of the particular JT 0 V ~il -7-r 7 investigation being undertaken. Resistivity measurements are made along the parallel lines. This enables a correlation of anomalies from line to line to give an indication of their strike of an anomaly.

An example of the method according to the present invention is as follows.

A series of measurements were made using a Huntec kw IP Transmitter and Motor Generator, and a Huntec Mk IV IP Receiver. It has been found that IP equipment is more applicable to the present application than conventional for conductivity measuring equipment.

It should be noted that while a grid size of o00. metres and 5 metre increments was used, smaller or larger 0 4 grid sizes are possible. However, a grid of greater than O0 O 00 metres spacings would probably not be useful in showing the 15 O required small detail.

o 09 o0 0 Two adjoining areas were surveyed using three 000 0 0°0 separate gradient array configurations (see Figure A 0000 total of 10 claims were investigated.

20 In the gradient array (see Figure 2) thb two current electrodes (A and B) are placed a large distance ~apart and kept fixed. The potential dipole (MN) is moved o along lines parallel to the line joining the current °0 electrodes. The separation between M and N should be as 000 25 small as possible so that the readings more closely I 0 0 represent point measurement to enhance resolution).

o0 This distance will be determined by the necessity to have 1 adequate primary voltages. Readings are plotted below the mid point of the potential dipole (MN).

The length of lines to be profiled is normally restricted to 2AB/3 as is the total width of the survey block (see Figure 2) so as to minimize distortion of the apparent resistivity, due to varying depths of penetration of the electrical field. An electrode separation (AB) of 350 metres was employed on both gradient arrays, with survey lines of 200 metre length. Two additional current electrode arrays were used at the completion of the survey to test the amplitude of the distortion of the apparent resistivities, with distance from a current electrode.

-8- This survey was conducted in the time domain mode (see Figure 3) using a 7.5 Huntec transmitter powered by a three phase, 11OV, 400 Hz alternator driven by a 20 HP Onan petrol engine. A frequency of 0.125 Hz was employed throughout the survey and current electrodes were prepared using aluminium foil. A Huntec Mark IV IP receiver was used to measure the primary voltage that exists between the potential electrodes while the current is flowing in the ground. This measurement is then used to calculate apparent resistivity apparent resistivity geometric constant primary voltage/applied current). Non-polarisable porous pots, ten metres apart, were used for potential electrodes o o o and readings were taken every five metres over each area.

The primary advantage of the gradient array over 8 15 other Resistivity configurations is rapid coverage. This array is also the least susceptible to topographic 0 variations whilst providing good overburden penetration.

The method provides excellent lateral resolution.

000o The apparent resistivity (Pa) in gradient arrays is 20 a function of the primary voltage measured between potential electrodes the input current and a constant (K) dependant on the location of the potential electrodes with C C respect to the current electrodes.

Pa K Vp/I o° 25 The current electrodes for each of the gradient 25, CC C arrays used were located at the following coordinates.

These are shown and labelled in Figure 1.

C CI ao C

I.

•i i_*ll( ~~lili~ I/ llilil_;;_/ I~ ii ~ij ii I_ GRADIENT ARAY 1 1A -9 ELECTRODE CO-ORDINATES (725N, 1250E) (1070N, 1250E) (650N, 1250E) 995N, 1250E) (650N, 1250E) (1150N, 1250E) 2 (1225E, 800N) (1575E, 800N) The measurements were conducted over a period of two weeks using currents ranging from 5.1 to 8.0 amperes.

Any variation in the current over the course of each day due to changing conditions at the electrodes, was monitored and accounted for in the data reduction.

The clay level is extremely conductive (approx oo ohm while the surrounding sandstone unit is relatively o resistive (20 ohm Therefore the lines of current 15 1 0 between the transmitting electrodes are concentrated within I o the conductive clay level. Hence the clay level (the region of interest) dominates the signal response measured at the potential electrodes. However this places a number of limitations on the gradient array configuration: (For the following points refer to Figure 2) The primary voltage (Vp) measured at the potential 4o electrodes (MN) is small, and must be kept at a readable level.

0u 4 4 4t 44 4 04.4 As the current electrode separation (AB) is increased to lengthen the survey lines, Vp decreases rapidly.

To increase Vp it is necessary to increase the potential electrode separation (MN) causing a loss of 3 lateral resolution.

The potential electrode separation (MN) is contrained by the size of the anomalous zone.

Therefore a compromise is required between the area of data coverage and the desired 1. ,al resolution.

It has been observed when the current electrode separation is i ed, the resultant resistivities obtained show a pr, tionate increase. This is due to the increased influence of the relatively 1 r L1--^I~Uili _i CC' 00 oe 0 o o.

o ro o 060 0 000 10 resistive sandstone unit underlying the clay level. As can be envisaged from the lower diagram of Figure 2, the electric field will penetrate to greater depths, particularly in the central region, as the electrode separation is increased.

The Gradient Array surveys undertaken to date have used AB/2 greater than 100 metres. It appears that any increase in AB/2 should result in an increase in apparent resistivity. However, variations of the depths and 10 thicknesses of the units, as the electrode separation i0 increases will effect the apparent resistivities. The contribution of the clay level to the observed value is diminished and the signal strength is reduced. This is very noticeable due to the scale of the survey, i.e. mapping 15 extremely small changes, and is enhanced by the fact that the conductive clay level is relatively thin.

The results of the work carried out to test the amplitude of the 'distortion' due to variations in depth of penetration of the electrical field, are shown as profiles in Figure 4. Line 1250E is the centre line between the current electrodes, while line 1300E is the outermost of the survey. Grid 1 is the initial survey configuration. Grid 1A is the result of shifting the current electrodes metres to the south using the same separation of 350 metres.

25 Grid lB is centred about the initial survey Grid 1 with a 25 current electrode separation of 500 metres (as shown in Figure These comparisons indicate the following: the increase in apparent resistivities from stations 825N to 900N (the midpoints of Grid 1A and Grid 1) is approximately 0.3 ohm m.

this effect decreases slightly from Line 1250E to 1300E. This is expected if the depth of penetration of the electric field is more constant along the line.

the smaller geological features are evident using each of the gradient array configurations.

comparison of the results of Grid 1 (350m) and Grid lB (500m) indicates a constant shift of approximately 0.4 ohm m along the survey lines, i.e. no advantage is gained by using a larger separation.

0000 0 00a 00 0 0 00 0 0s 00 0 0000 -L -9 r? i 11 The data produced may be interpreted using a variety of techniques to highlight anomalous areas. These may include an appropriate computer program. Figure 5 shows an exemplary hand-plotted contour map, with anomalous areas highlighted.

It will be appreciated by one skilled in the art that numerous variations are possible within the spirit and scope of the invention described.

0 0 a 0 0 0 0 0 0 0 0 0000 i I

Claims (6)

1. A method of locating from a ground surface position through the overburden a potential opal-bearing region contained within a claystone layer in the underlying ground wherein said claystone layer is of substantially uniform high electical conductivity and wherein said potential opal-bearing region is more electrically resistive than the claystone layer, the method comprising the steps of:- setting up an electrode array on said ground surface position having two fixed spaced apart current electrodes to form a transmitting dipole and two spaced apart potential electrodes forming a receiving dipole movable at predetermined intervals along lines parallel to a line joining the current electrodes to provide a grid of potential measurement positions; introducing a current between the electrodes of the transmitting dipole; S- measuring the potential difference between the electrodes of the receiving dipole at a plurality of positions on said grid and thereby 1 o« determining from the measurements of resistivity the location of regions of anomalous resistivity within the claystone layer which regions correspond to potential opal bearing regions. B

2. A method according to claim wherein the two potential o a o electrodes are spaced apart by no nore than 20 metres.

3. A method according to claim i, wherein the two potential electrodes are spaced apart by 10 metres and resistivity measurements are determined every 5 metres.

4. A method recording to any preceding claim wherein resistivity measurements from positions measured on the grid are plotted to produce a contour map showing lines of equal potential.

A countour map when produced by the method of claim 4.

6. A method of producing a resistivity data substantially as hereinbefore described with reference to the drawings. I DATED this ist day of May 1992. u :BARRY FREDERICK CLARK