Comparison of Electromagnetic Geophysical Prospecting Methods Over Known Sulphide Zones in the Flin Flon Area, Saskatchewan

Transcription

1 REPORT No. 28 Comparison of Electromagnetic Geophysical Prospecting Methods Over Known Sulphide Zones in the Flin Flon Area, Saskatchewan by A. R. Byers DEPARTMENT OF MINERAL RESOURCES Metallic and Industrial Minerals Branch Geology Division HON. J. H. BROCK ELBAN K MINISTER C. A. L. HOGG DEPUTY MINISTER PROVINCE OF SASKATCHEWAN 1957 ~ 10

2 SASKATCHEWAN DEPARTMENT OF MINERAL RESOURCES REPORT No. 28, 1957 A Comparison of Geophysical Methods over Known Conductors in the Flin Flon Area A.R. BYERS Geology Department, University of Saskatchewan ABSTRACT Under the auspices of the Saskatchewan Department of Mineral Resources the following geophysical prospecting methods and instruments were tested over known conductive zones of sulphide mineralization: (1) Electromagnetic-galvanic using point electrodes. (2) Electromagnetic-induction with horizontal coils-boliden Survey Unit. (3) Electromagnetic-induction using vertical coils (a) Sharpe Model SE-100 survey unit, and (b) Doolan prospecting instrument. The instruments, surveying t echniques and principles of interpretation are briefly described, and the results of the tests are presented and compared. The t est data indicate that all four units are capable of locating conductive zones of disseminated to massive sulphides, and under the conditions of t est they may be compared as follows: (1) On the basis of sensitivity or capability of detecting a conductor the methods rank, in decreasing order of sensitivity: (i) electromagnetic-galvanic, (ii) Sharpe and Doolan vertical coil induction, (iii) Boliden horizontal coil induction. (2) For geophysical prospecting of large areas the electromagnetic-galvanic and Boliden horizontal loop techniques are superior. (3) For prospecting smaller areas in the order of a few claims the Doolan Instrument is quite satisfactory.

3 CONTENTS Page I NTRODUCT!ON 3 G1,:0LoG1CAl. CoKDITIONs IN THE VICINITY OF THE INvESTIGATIO::V.. 4 FUNDAMENTALS OF ELECTROMAGNETIC!{ETHODS THE ELECTROMAGNETI C-GALVAJ\IC :METHOD Introduction Inst t Surveying Technique Principles of Interpretation Test of Ground-Survey Method R esults of T est Summary and Conclusions THE BoLrnEN OR HoRrzoNTAL L oor METHOD Introduction Instrument Snrveying Technique Principles of Interpretation Results of Survey Comparison with Other Methods Summary and Conclusions VEn.T1CAL Loor!fETHoDs Introduction General Principles THE DOOLAN PROSPECTlNG UN1T Int.rodnction Ins tnnnen t Operation Snrveying Technique Results of Tests Snmmary and Conclusions SHARPE MonEL SE-100 ELECTIWMAGNETJC Srnin:Y UNIT Survey Unit Introduction Instru1nent Surveying T echnique Results of Test Summary and Conclusions GENERAL CONCLUSIONS

4 FIGURE ILLUSTRATIONS P age 1 Electromagnetic-galvanic geophysical survey unit. Upper picture shows generator unit and lower picture the search or receiving instru1ncnt P rofiles 2,400, 3,200, 3,800, and 4,200N, 150W base line. Electromagnetic-galvanic survey Profiles 3,800, 4,200, 6,600, 7,000, and 7, GOON, 122W base line. E lectromagnetic-galvanic survey The Boliden electromagnetic geophysical survey unit Pro les 2,400, 3,200, 3,800, and 4,200N, 150W base line. Boliden horizontal loop survey Profiles 3, 00, 4,200, 6,600, 7,000, and 7,600N, 122W base liue. Bolidcn horizontal loop survey Doolan electromagnetic survey unit Doolan and Sharpe electromagnetic survey, ".A" zone, 150W base line Doolan elccti omagnetic snrvey, "B" zone, 122W base line P LAN Plan of E lectromagnetic Survey, Scale 1"=400'... I n zjoclcet 2

5 INTRODUCTION The principal objective of geophysical exploration is to facilitate the discovery of mineral or oil deposits, especially in areas where widespread overburden prevents or hinders the use of ordinary methods of prospecting. Within the past decade the Canadian mining industry has employed several electromagnetic methods in the search for new mineral deposits with some measure of success. Mr. C. A. L. Hogg, Deputy Minister of Mineral Resources for the Province of Saskatchewan, being fully -aware of the importance of such methods, arranged to have studies made of certain electromagnetic methods under conditions to lbe met with in northern Saskatchewan. This report is an attempt to meet the Department's demand for unbiased information concerning the usefulness of prospecting by the following methods: 1. An electromagnetic-galvanic method as developed by B. S. Bjarnason, consulting geophysicist, Toronto. 2. Electromagnetic horizontal loop method as employed by the Boliden instrument built by the Boliden Mining Company of Sweden. 3. Electromagnetic vertical loop method. (a) Sharpe Model SE-100 Electromagnetic Survey Unit manufactured 'by Sharpe Instruments Limited, Toronto. ('b) The Doolan electromagnetic instrument made by F. Doolan of Flin Flon, Manitoba. The area selected to test the methods and instruments lies immediately east of Mosher Lake, Saskatchewan, or about seven miles southwest of Flin Flon. This area was.picked for the following reasons: 1. Accessibility- the Flin Flon-Denare Beach highway skirts the south end of Mosher Lake from which the area is readily accessible by boat. 2. Known geologic conditions-the area had been mapped insome detail by a geological survey party of the Department of Mineral Resources. Also, previous geophysical prospecting and exploration by diamond drilling by Hudson Bay Exploration and Development Company, Limited, had outlined four conductive sulphide zones. 3. Topography- the topography of the area is fairly typical of a large part of northern Saskatchewan and includes areas of muskeg as well as thinly drift-covered, hilly topography. The field studies were carried out between June 26 and July 7,

6 GEOLOGICAL CONDITIONS IN THE VICINITY OF THE INVESTIGATION The general geology of the area a'bout Mosher Lake is shown on Map No. 14F, Denare Beach Sheet (northeast quarter), which accompanies Report No. 14 of the Geology Branch, Department of Mineral Resources, Province of Saskatchewan. The bedrock consists of fine-grained, massive to pillowed flows of andesite or basalt with interlayered bands of tuff and agglomerate. In places these rocks are intruded by sills and dikes of granodiorite, andesite, rhyolite, and quartz-feldspar porphyry. One-half to three-quarters of a mile east of Mosher Lake the volcanic rocks are cut off by a major intrusion of granodiorite and related off-shoots of diorite and gabbro. The formational trend is north and the dip is vertical to steeply west or east. Sulphide bodies of disseminated to massive pyrite and pyrrhotite occur as replacement-type deposits within pyroclastic bands, or shear and fracture zones cutting the massive lavas. Surface prospecting, geophysical surveying, and diamond drilling have indicated the presence <Yf eighteen deposits within a belt nine miles long and parallel to the main granodiorite-volcanic contact. The majority of these deposits lie close to or within 2,000 feet of the intrusion of granodiorite. Two of the deposits, located at Birch Lake, contain sufficient quantities of copper and gold to malke them of probable economic importance. At the present time they are being actively developed by Hudson Bay Mining and Smelting Company, Limited. The sulphide bodies which were used for the present tests all lie east of the sout.ih half of Mosher Lake, and are indicated lby the letters A, B, C, and D on the Plan of Electromagnetic Survey which accompanies this report. These conductive bodies were originally detected 'by Hudson Bay Exploration and Development Company, Limited, using a Boliden horizontal loop, electromagnetic survey unit. 'Subsequent diamond drilling by the company tested the conductive zones, and the following descriptions of their geological settings were obtained from an examination of the core by the writer and logs of the holes kindly supplied by the Hudson Bay Exploration and Development Company, Limited. The "A" deposit is a mineralized shear zone up to 120 feet wide in massive andesite. The mineralization consists of pyrrhotite and pyrite in massive bands from one foot to three feet thick and sections of disseminated sulphides from three to forty feet thick separated by narrower bands of unmineralized rock. The dip of the zone is 75 to 80 degrees west. The depth of overburden ranges from two to ten feet. The "B" sulphide zone consists of massive to disseminated pyrite and pyrrhotite in a ratio of about two to one. The sulphides 4

7 replace the fine-grained matrix of a tuffaceous agglomerate, leaving the more acidic fragments largely unreplaced. The volcanic rocks are cut by sills and dikes of rhyolite, quartz-feldspar porphyry, and granodiorite. The maximum thickness of the zone as indicated by the drill hole 70 feet south of line 4,200N is approximately 180 feet, made up of three bands of massive to nearly massive pyrrhotite and pyrite: (1) a band 70 feet thick on tjhe west or hanging wall side, (2) a central 'band 25 feet thick, (3) a footwall band 15 feet wide. The intervening rock is mineralized with disseminated sulphides. The dip of the deposit is 60 to 70 degrees west. The thickness of overburden is from four to 20 feet. The "C" deposit, as indicated by two drill h oles, is a zone of disseminated to massive pyrrhotite and a little pyrite in bands from two to ten feet thick within a zone 20 to 30 feet wide. The wal'lrock is a tuffaceous agglomerate similar to that of deposit "B", and: the mineralization appears to occur at about the same horizon in the volcanic sequence of flows and pyroclastic bands. The deposit also dips t o the west. Depth of overburden is about five feet. The "D" zone, as shown by two drill holes, occurs under similar conditions as d~scribed for "B" and "C" deposits, and is prdba:bly the northern extension of the same zone. The deposit is five to twenty feet thick and consists of disseminated pyrrhotite and pyrite. The dip is about 70 degrees west. Overburden is one foot to five feet thick. Tests on the electrical conductivity of a typical specimen of the mineralized rock containing ajbout 70 per cent very finely disseminated pyrrhotite and pyrite indicate a resistivity in the orde r of 10 to 25 ohm. cm. FUNDAMENTALS OF ELECTROMAGNETIC METHODS All electromagnetic methods ar e based on the well known principle that a current flowing through a conductor produces lines of magnetic force in the form of rings a;bout it. These lines of force have the same properties as those found associated with an ordinary magnet and will affect the position of a magnetic needle in the same manner. If the current (Primary current) in the conductor is alternating, the magnetic field (Primary field) associated with it will also alternate in intensity and change in direction. Now lby the theory of electromagnetic induction, if a second conductor is brought within this alternating magnetic field, there will be induced in it an alternating current (Secondary current) which will have its own magnetic field (Secondary field) a~ociated with it. This secondary electromagnetic field combines with and modifies either the (1) direction, (2) intensity, or (3) quality of the primary electromagnetic field. This modification 5

8 of the primary field may be detected and measured in a number of ways depending on the method employed. The electromagnetic methods of geophysical prospecting may be divided into two groups: (1) electromagnetic-galvanic methods in which a current of electricity is passed through the ground between either point electrodes or line electrodes, (2) electromagnetic-inductive methods in which the primary energy is supplied by insulated loops or coils and inductive coupling as outlined in tjhe first paragraph. In the electromagnetic-galvanic methods the two electrodes are connected to a source of alternating current which flows through the ground 'between and around them in a uniform pattern providing the ground is homogeneous. A corresponding alternating magnetic field is associated with this current and will give rise to a uniform-horizontal magnetic field component under ideal conditions. Now, if a subsurface 'body is a much better conductor than the surrounding medium, the current will be concentrated along it and the horizontal component will be a maximum wbove the current concentration. The vertical component will ;be zero at a location immediately above this concentration and will have a maximum and minimum, respectively, on either side of it. The electromagnetic field may be detected on the surface with special types of receiving coils and registering instruments. The simplest methods involve the determination of the direction of the resultant magnetic field or of the absolute values of the horizontal and/ or vertical components of the field. The field produced' by the cable connecting the electrodes to the generator may complicate the picture. This may be reduced by carrying the ca:ble around the area in a square or the field due to the ca:ble may be calculated and corrected for. The electromagnetic-inductive methods may be divided into two groups depending on how the electromagnetic field is generated. The field may be produced by either (1) a horizontal coil or a large loop, or (2) a vertical coil. An alternating current is circulated through the coil or loop, producing an alternating electromagnetic field. This field will stimulate currents by induction in any subsurface conductor which in turn will generate a secondary electromagnetic field and alter the primary field. The frequency of the primary current will depend on the depth of penetration required, the nature of the conductor sought, whether massive or disseminated ore, and on the presence of conducting layers near the surface. Frequencies above 5,000 cycles per second lack depth penetration and may produce excessive interference from topography and from near-surface noncommercial conductors. At low frequencies, under 100 cycles per second, energy transmission 'becomes inefficient. At the present time most electromagnetic methods employ a current with a 6

9 frequency between 500 and 4,000 cycles per second, with 1,000 cycles per second being the most commonly used. With the electromagnetic-inductive methods as with electromagnetic-galvanic techniques the instruments used for measuring the electromagnetic field depend on the quantities to be measured and the data required. The simplest methods determine only the direction of the r esultant magnetic field, or the horizontal and/ or vertical components of the field. Other methods determine the in plhase and out of phase components of the field generally in terms of the current and phase of the primary current. Still other methods find the ratios of field intensities and their phase differences between different points in the area investigated. Methods which measure the in phase and out of phase components have greater interpretative value. The in phase component is largely caused by the primary field, whereas the out of phase components are due to the induced secondary currents. The electromagnetic-galvanic methods lend themselves to areas where adequate contact of the electrodes with the ground is r eadily made and where near surfaced, non-commercial conductors would produce too much screening effect on electrical inductive methods. Inductive methods would have to be employed when the surface formations are poor conductors. Theoretically vertical coils would be more suita'ble for the detection of narrow, steeply dipping conductive bodies, whereas horizontal coils or loops give better effective coupling with horizontal subsurface conductors. THE ELECTROMAGNETIC-GALVANIC METHOD Introduction Electromagnetic-galvanic methods of prospecting differ from other electromagnetic methods in that the electrical energy is supplied directly to the ground by line or point electrodes instead of by induction. Much of the earlier work involving these methods was done under the a uspices of the Geological Survey of South Africa during its investigation into t he practical application of electromagnetic methods. At the present time in Canada several exploration geophysical companies employ electromagnetic-galvanic methods. The apparatus which was t ested during the present investigation was designed and developed by B. S. Bjarnason, consulting geophysicist of Toronto. Instrument The power or energ1zmg source is a gasoline engine-driven generator which supplies an alternating current of 1,000 cycles per second to two point electrodes by means of an insulated copper 7

10 Figure I Electrom agnetic-galvanic geophysical survey unit. Upper picture shows generator unit and lower picture the search or receiving instrument. 8

11 wire or cable, see Figure 1. The line joining the electrodes must be paraflel to the structural or formational strike of the area so that profiles can be measured at right angles thereto. The distance between the electrodes may vary depending on the size of the area to be explored. The current flowing in the ground between the electrodes will spread out through a large volume of ground in a pattern determined by the nature of the formations through which it flows. A corresponding alternating magnetic field is associated with this current and will give rise to a horizontal magnetic field component under ideal conditions of level topography and uniformity of rocktype. If a conductive body, more conductive than the surrounding formations, is situated within the area, the alternating electric current will converge towards the body with the result that the current density will,be greater within it than in the surrounding rocks. Also, the alternating magnetic field associated with this concentrated current flow will be correspondingly more intense over the conductive body than over the surrounding less conductive formations. The primary alternating magnetic field caused by the current flowing throug1h the wire connecting the electrodes to the generator may also set up alternating currents in the conductive body by induction. The magnetic field due to the induced currents in combination with the magnetic field due to the current flowing through the body will distort the normal magnetic field pattern or distribution. By determining the extent of this distortion it is possible to ascertain the location of the conductive body. The receiving equipment varies from the simplest to the most complex, depending on what information ~bout the electromagnetic field is sought. The equipment as designed by B. S. Bjarnason consists of two types of detecting apparatus: one for surveys carried out in the normal manner on the ground, and an instrument for use in airborne surveys. The ground survey unit consists of a small diameter receiving coil mounted in a non-metallic rectangular case fitted with handles at both ends for carrying and holding the coil in any desired position, see Figure 1. Leveling bubbles and a clinometer are also attached to the box so that the position of the coil may be accurately determined. The coil is connected to an electronic circuit consisting of an amplifier, vacuum tube voltmeter and a ten-turn micrometer rheostat (helipot), and! a pair of headphones. The airborne apparatus consists of a receiving coil mounted in a small bomb attached to the underside of an airplane and a recording instrument mounted in the airplane. The recording instrument may be manually 9

12 operated and readings recorded by the operator, or it may be automatic. The entire unit can 1 be easily carried in a light airplane, such as a Cessna 180. Surveying Technique An insulfl,ted copper wire is laid out on the ground parallel to the 'known structural trend or formational strike. The length of the wire is governed by the size of the area to be surveyed. One end of the wire is grounded by a suitable electrode and the other end is attached to one terminal of the generating unit, the other terminal of which is grounded to a second electrode. With the airborne detecting apparatus mounted in a suitable aircraft, profiles are flown perpendicular to the wire-electrode system laid out on the ground. The distance between the flightlines would depend on the amount of detail required, but for prospecting for base metal deposits the interval probably should not exceed one-eighth of a mile. The "barren" parts of the area are thus easily and' quickly eliminated, and only the "favorable" zones or conductors indicated by the airborne survey need be investigated in more detail on the ground. Mr. Bjarnason estimates that an area two by five miles, equivalent to about 130 claims, could be surveyed in five and' onehalf hours with flight lines one-eighth of a mile apart. This estimate includes four hours for laying out the ground electrode system and' one and a half hours of flying time. Unfortunately through lack of the necessary equipment no aivborne tests were made during the investigation. For a detailed survey on the ground, picket lines two hundred feet apart and perpendicular to the line joining the electrodes would have to be cut and chained at 100-foot intervals. The initial air reconnaissance survey would eliminate much line cutting and chaining as lines would only have to be run over the known conductive zones. Using the ground receiving unit the following data could be recorded and plotted at each 50- or 100-foot interval along the profile lines: (1) The intensity or strength of the horizontal and/ or vertical components of the electromagnetic field. (2) The dip and strike of the ellipse of polarization Which is a function of the normal ground field produced by the regular current distribution between the two electrodes, the field due to the subsurface current concentrations "in the conductive body, and the primary field of the wire or generator leads. 10

13 Principles of Interpretation If the conductor is a tabular-shaped, vertical body in which the current is concentrated and flows along its strike, then the horizontal anomalous component of the magnetic field will have a maximum rubove the current concentration. The vertical anomalous component will be zero over the same point and will have a maximum and minimum, r espectively, on either side of the conductor. If the conductive body is wide, the current ("due to induction") may be concentrated along its margins, and' the horizontal component ("due to induction") will show not a single maximum but a maximum and minimum, respectively, over the edges. The slopes of the curves, however, are never as simple as the foregoing examples because of the superposition of currents prodluced by induction as well as by galvanic action. Also, as the cable is approached along any line, the intensity of the vertical component increases very rapidly, see profiles 3,200N, Figure 2 and 3,800N, Figure 3. The strength of the horizontal component will also increase as the cable is approached but to a much lesser extent than the vertical component. The plane containing the ellipse of polarization is easily measured 'by orientating the receiving coil until a minimum of sound is heard in the headphones. The dip of this plane will tend to 'be a:way from the line-electrode system with the angle of dip slowly increasing as the cajble is approached. If a vertical, tabularshaped conductor is present beneath the surface, the angle of dip should decrease to zero over the body and then increase again. The clinometer was attached to the receiving unit in such a way that it read zero degrees when the unit was held in a vertical plane and 90 degrees in the horizontal position. Within the area between the electrodes and over homogeneous and level ground the strike of the ellipse of polarization should paral'lel the cruble. If, however, the line of traverse passes over a conductive body, tihe strike will first increase in one direction as the 1 body is approached, return to normal over the body, then increase in the opposite direction, reach a maximum, and finally decrease again to normal. This, of course, could vary depending on the size and attitude of the conducting body. Test of Ground-survey Method A light weight, lb. per 100 feet, single strand, Formel insulated copper magnet wire was laid out on the ground parallel to the formation strike for a distance of 4,720 feet, see Plan of Electromagnetic Survey. The wire was grounded at its south end by attaching it to three iron rods driven 30 inches into wet ground. The nortjh end of the wire was attached to one terminal of the generating unit, the other terminal being grounded to a second electrode near the shore of Mosher Lake. About two hours were required to lay out and connect the wire-electrode system. 11

14 The resistance between each electrode and the ground should be under 100 ohms. For this test, the measured resistance was 60 ohms for the south electrode and 80 ohms for the north electrode. The generator supplied an alternating current of 1,000 cycles per second at 190 volts, and 'between 1.0 and 1.25 amperes. A current reading was made every fifteen minutes during the period of operation. The generator was stopped only to refuel the gasoline motor, i.e., 3;bout every one and a half hours. Picket lines 200 feet apart and approximately at right angles to the wire-electrode system and two base lines originally laid out by Hiudson Bay Exploration and Development Company, Ltd., were used to control the location of the receiving apparatus. The picket lines used in the test and the length of each traverse are also shown on the Plan of Electromagnetic Survey. Readings were t aken at 50- or 100-foot intervals along each of the lines, and the following data on the electromagnetic field were observed and' recorded: (1) The strength or intensity of the horizontal component. (2) The strength of the vertical component. (3) The amount and approximate direction of dip of the plane containing the ellipse of polarization. The horizontal component was obtained by rotating the coil when in a vertical plane about a vertical axis until a position of minimum sound was obtained. The coil was then turned through 90 degrees, and the intensity of the horizontal component was measured by r ecording the number of scale divisions on the micromet er rheostat after it was adjusted to produce a zero reading on the dial of the vacuum voltmeter. One hundred scale divisions on the micrometer rheostat or helipot used are equivalent to a magnetic field intensity of about 0.46 micro-gauss in terms of the exciting current in the insulated cable. The strength of the vertical component was obtained by holding the coil in a horizontal position, and the number of scale divisions of the micrometer rheostat recorded after adjusting the instrument as described for measuring the horizontal component. To find the plane of the polarization ellipse, the coil is held in a vertical plane and turned to a position of minimum sound. It is then rotated through 90 degrees and tilted until another null point is found. This may r equire slight adjustments of azimuth as well as tilt. The tilt or dip and the direction of dip were then recorded. The time required to take a set of readings and move to the next station 50 feet away was about one minute. 12

15 Figure N.. fl)..,.. dl 60 z "' -,(>() '... zoo,oo,o.x,c rur 0 < II: 0?OO W..,... IX N z UOOf-- ~ ,<---; y.1.j I '-----i I zoow zoo oooc Z _,/ 3200 N,ooo r~---~/ ~,() / ; i,, c::7''----~,t'-\ ,~171~r-_ _ _'-' cc :I~,oo.,... " zo Cl o 0 " < Cl z 0. I I O ,;..., oo N L ~,...,...,,,, oow,00, oo zoo, ,,, zoo ~ ELECTROMAGNETIC GALVANIC TESTS MOSI-IER LAKE AREA, SASK. 13 Profiles olong lines 2400, 3200, 3800 e,. 4200N Reference : I 50 Bose Line HORIZONTAL COMPONENT VERTICAL COMPONENT DIP COM PONE NT RATIO w---.sulphide ZONE I I

16 7600 N w 400 '. :~~~ ==::::::::: l,vo ct..1:. -..._ ,... - C> W O O GOO BOO FEE T N 0 z ~= ~ ~ <I) Q [ FEET oo 7000 N ~ [ tt (Indicated } 10 FEE T EM.-GALVANIC TEST S MOSHER LAKE AREA, SASK. Profiles along lines , 7000, a 7600N. Reference W HOR I ZONTAL COMPONENT Bose Line. VERTICAL COMPONENT - - OIP CO MP ON E NT RATIO ~ SULPHIDE ZONE 14

17 Results of Test The results of the t est are displayed graphically,by the profiles shown in Figures 2 and 3. Each profile corresponds to a traverse along a picket line and shows: (1) changes in the intensity of the horizontal and vertical components of the magnetic field in terms of scale dri.visions of the micrometer rheostat, (2) the compliment of the angle of dip of the polarization ellips e, (3) the intensity ratio obtained 'by dividing the horizontal component by the vertical component. Mr. Bjarnason kindly supplied the writer with corrected horizontal component curves for profiles 4200N andl 3800N, 150 W. base Iine. The correction was made by subtracting the calculated normal horizontal electromagnetic field component from the horizontal component actually observed. The two sets of curves were so similar that the use of corrected curves would apparently make little or no difference in the interpretation of the observed data. Therefore, they have not 1been plotted on the profiles. On the first traverse along line 4200N, reference 150 W. base line only the intensities of the horizontal component of the electromagnetic field were observed and recorded, and, therefore, only the one curve is shown in the profile, see Figure 2. Summary and Conclusions A study of the profiles clearly shows that the method is capable of indicating the location of the conductive zones as outlined lby the diamond drill holes. Furthermore it indicates the position of the conductive zones in places wher e the Boliden method yielded negative results, see profiles 7000N, 122W. base line, Figures 3 and 6. The most diagnostic component appears to be the horizontal component of the electromagnetic field. The ratio between the horizontal and vertical components. also clearly indicates the locations of the conductive zones except in profile 3800N, see Figure 2. However, the ratio curves do not add very much to the inter,pretation. The very high maximum between 1,400 and 1,500 feet east of 'base line 122W., see profile 6600N, Figure 3, cannot be accounted for at the present time, unless a conductive zone of disseminated sulphid'es which was not detected by the Boliden survey, see profiles 6600N, Figure 6, lies beneath the surface. Therefore, from the test-data, it appears safe to conclude that uncorrected values of the horizontal component of the magnetic field are sufficient to indicate the position of a conductive body. 'Dhe approximate area covered by one cable-electrode set-up would 1 be a rectangle whose length would be consider rubly in excess of the distance between the electrodes and whose width would be at least equivalent to the electrode spacing. However, the area would be bisected parallel to the cable by a zone in which readings 15

18 would have no significance due to interference 'by the electromagnetic field generated by the cable. Thus to make a complete survey of an area a second set-up of the cable-electrode system would be necessary. The distance between the two set-ups would have to,be such that the central zone of the first rectangular area would be overlapped by one side of the second rectangle. Under the conditions of the present test with an electrode spacing of 4,700 feet, the size of the area covered is a1bout 8,000 feet by 5,000 feet, and the central, unusable zone is 1,600 feet wide. With one additional set-up of the ca1ble-electrode system the total area would 1 be 8,000 feet by 7,000 feet or a'bout 25 claims. A two-man field party could survey the above area in approximately 100 hours taking readings at 50-foot intervals along lines 200 feet apart. The above estimate includes the time required for laying out and ta 1 king up the cable and electrodes. Thus two claims per 8-hour day could be surveyed. With an additional man operating a second receiving or search unit the number of claims surveyed could be about doubled. The fact that as many receiving units as there are operators can be employed gives this method a d'ecided advantage over other electromagnetic methods as far as speed of surveying is concerned. It is suggested that the operation of the receiving unit could be simplified lby replacing the t en-turn micromet er rheostat with a series of fixed resistances in multiples of 10 or 100 coupled to the vacuum tu 1 be voltmeter by switches. The intensity of the horizontal field strength could then be obtained by noting the scale reading on the voltmeter and multiplying it,by the resistance factor. THE BOLIDEN OR HORIZONTAL LOOP METHOD Introduction This electromagnetic-induction method measures the in phase and out of phase components of the magnetic field in t erms of per cent of a normal or uniform field by means of a special compensator in the r eceiving circuit. The instrument was designed and developed by S. Werner of Sweden in The Boliden Mining Company of Sweden obtained the pat ent rights in 1943 and commenced manufacturing the instrument. In Canada the Boliden instrument has been employed by Hudson Bay Exploration and Development Company Limited for large scale geophysical prospecting in northern Manitoba and Saskatchewan with con - siderable success. Moreau, Woodard and Company, Limited also use a Boliden instrument for their electromagnetic geophysical surveys. The Boliden Mining Company has discontinued making this instrument. However, Midwest Mining Supplies, Limited of Winnipeg, who have been the Canadian agents, may obtain the manufacturing rights for North America. 16

19 Instrument The transmitter consists of a 45-inch diameter, circular coil which is carried in a horizontal position by means of a special shoulder 'harness. The power unit is a vacuum tube oscillator fed by four 45-volt "B" batteries and a 1.5-volt "A" battery. The current is supplied to the coil at 3,600 cycles per second. The receiving and recording unit consists of a 26-inch diameter, circular coil connected to an electronic circuit containing two potentiometers. Power is supplied by two 45-volt "B" batteries and one 1.5-volt "A" battery, see Figure 4. The receiving coil is carried in a horizontal position at a fixed distance from the transmitting coil, usually 200 feet. The current generated in the receiving coil is caused by the secondary alternating magnetic field re-transmitted from the ground and by the primary alternating magnetic field produced by the transmitting coil. The current is conducted to the recording instrument where it is compared to a constant alternating voltage. This constant voltage is obtained from a small coil mounted on 1!he transmitting coil and connected to the recording instrument by a cable. This coil is not affected by the secondary magnetic field re-transmitted from the ground 'because of the small size of the coil and its proximity to the transmitting coil. Therefore, the current induced in the small coil by the primary alternating magnetic field may be considered to remain constant and to represent the primary field. The two potentiometers of the recording instrument are termed the in phase and out of phase potentiometers. The voltage as measured lby the in phase potentiometer is that of a current in phase with the secondary currents induced in the compensating and receiving coils by the primary field and the current induced in the receiving coil by the in phase component of the secondary field re-transmitted from the ground. The voltage as recorded by the out of phase potentiometer is due to the secondary current in the receiving coil produced by the out of phase component of the secondary magnetic lield. A 4-stage amplifier is connected to the circuit and its output is connected to a pair of earphones. When a reading is made, the potentiometers are adjusted until a minimum of sound is heard in the earphones. The potentiometer scales are calibrated to read in per cent of a normal field, that is the instrument is adjusted to read zero in an area known to be underlain by no conductive bodies. 17

20 Figure 4 The Boliden electromagnetic geophysical surv ey unit. 18

21 Surveying Technique A base line is laid out parallel to the regional structural trend and picket lines two hundred feet apart are run perpendicular to the base line. Traverses are made along each picket line and readings taken every hundred feet or at closer intervals if the results warrant. The party required to operate the apparatus consists of four men, see Figure 4. The leading man carries the receiving coil. He is followed by the second man who carries the recording instrument on his back and also keeps a record of the readings. The third man or operator adjusts the instrument and makes the readings. The fourth and last man in the party carries the transmitting unit consisting of the transmitting coil and energizing unit. The coils are separated at a normal spacing of 200 feet by means of the electric cable. This spacing has proved to give best results for prospecting under conditions, met with in northern Manitoba and SaS'katchewan. It is possi 1 ble to cover four to five line-miles per day witlh readings at 100-foot intervals. This represents an area of 100 acres or two claims when the traverse lines at e 200 feet apart. Principles of Interpretation As a t rubular-shaped vertical conductor is approached both the in phase and out of phase readings should rise and then fall to low values over the conductor and again rise on the other side. If a graph is prepared, it should show a zone of low values above the conductor, and two zones of high values symmetrically located on either side, see Figure 5. The width of the anomaly will depend on the thic'kness of the conductor and the spacing of the two coils. It is always much wider than the conductor. Both the in phase and out of phase readings show the same general curve. However, the ratio between the two readings is ta!ken as a fair indication of the conductivity of the body producing the secondary field. A good conductor should produce a greater deviation of the in phase component than the out of phase component, see profiles 3,200N and 3,800N, Figure 5. Ratios of in phase to out of phase components greater than two are considered to indicate conductors which should be investigated by drilling. 19

22 4200 N ~it===~ ct 200W O tt IOOOE FE ET :::. (I ndicat e d ) 0:: 0 z 3800 N IN PHAS E COM PO NEN T - OUT OF PHAS E COMPONENT - SUL PHIDE Z ONE t (!) z 0 ct w o l:=:..,.,...,;;::.;,=--.l:.~ 0:: 0:: w 16 f w :::. 0 f- z ~:~ts!:: ~ 2400 N E FEET BOLIDEN SURVEY MOSHER LAKE AREA, SASK. Profiles along lines 2400, 3200, 3800, N. Reference- 150 W Bose Line. 2doE FEET Figure 5 20

23 0 0 ~ 16...J <l ~ a: 0 Z 8 ~ 0 I- ~ 16 U 24 ~ z N _ BOOE FEET ~(Indicated) 7600 N it::::y ZOO E E FEET C> z 0 8 <l w O a: a: 16 w I- 24 w ::;; z w N BOOE FEET BOLIDEN SURVEY MOSHE R LAKE AREA, SASK. Profiles olong lines , 6600, 7000, a 7600 N. 6,~ P-''"""'-122 W '"' ''"' ~ ~ ~ IN PHASE COMPONENT _ 1 OUT Of PHASE COMPONENT - l6 \.I SULPHIDE ZONE ZOOW O * [ FEET Figure 6 21

24 Topographic features such as hilly topography, swamps and shore-lines may produce appreciable anomalies. However, such effects can generally be distinguished since they usually produce a large out of phase component with very little or no change of the in phase component. Results of Survey No field t ests of the Boliden instrument wer e made by the writer as the entire ar ea had previously been surveyed by the Hudson Bay E xploration and Development Company, Limited. Mr. A. A. Koffman kindly made available the r esults of the survey for the areas in the immediate vicinity of the known conductive bodies. These are shown on the Plan of Electromagnetic Survey accompanying t his report. The general practice of the company is to plot only the in phase readings as long as they remain more or less uniform. Where a marked variation from plus to minus takes place the out of phase r eadings are also plotted so that the two may 'be compared. To facilitate comparing the r esults of the Boliden survey with the r esults of the electromagnetic-galvanic survey, profiles of the in phase and' out of phase components have been made for the lines covered by the latter survey, see Figures 5 and 6. Comparison with Other Methods In the area of "A" zone the Boliden method indicated a conductor from 1,800N to 3,800N. Traverses north of 3,800N did not indicate a conductive body. The electromagnetic-galvanic survey along 4,200 north gives a good indication of a conductor between 500 and 600 feet east of the 150W tbase line, see Figure 2. A profile of the in phase component as det ermined with the Boliden unit shows an anomalous condition between 450 and 650 feet east of the base line, see Figure 5. However, this was not interpreted as indicating a conductor at the time the survey was made by Hudson Bay Exploration and Development Company. Both the Sharpe and Doolan instruments were able to give good cross-overs as far nor th as line 4,500 north. The "B" conductive zone was originally outlined with the Boliden instr ument between lines 3,800N and 4,600N. Figure 6 shows profiles for lines 3,800N and 4,200N. On line 3,800N there is a greater change in the out of phase than the in phase component indicating that the conductive body is probably fading out towards 1 the south. The r esults of the electromagnetic-galvanic survey, see Figure 3, would indicate the same. However, the latter survey indicates a possible conductor between 100 and 200 feet east of the base line which is not apparent on the Boliden profile. The "C" conductor was outlined,between 6,400N and 6,800N by the Boliden method. The electromagnetic-galvanic survey also indicates the conductor along profile 6,600N, compare profiles in Figures 4 and 7. 22

25 The "D" conductor as outlined by the Boliden survey lies between 7,400N and 8,000N, see Plan of Electromagnetic Survey. A comparison with the galvanic survey can be made by comparing the profiles along 7,600N in Figures 3 and 6. From 6,800N to 7,600N the Boliden survey gives no indication of a subsurface conductor, see Plan of Electromagnetic Survey and profile 7,000N, Figure 6. The electromagnetic-galvanic survey along the same line indicates a subsurface conductor 550 feet east of the base line, see Figure 3. Summary and Conclusions The Boliden geophysical survey unit has been used with marked success for the past eight years in Northern Saskatchewan and Manitoba by Hudson Bay Exploration and Development Company, Limited. The net value of the sulphide ore deposits discovered to date is many hundreds of times the cost of the surveys. The equipment is quite mobile and surveys can be carried out rapidly. Since the distance between transmitting and receiving coils is fixed,by the electric ca:ble, it is unnecessary to chain the picket lines thus reducing the cost of the survey. The high initial cost of the equipment more or less prohibits its use 1 by the individual prospector or smau prospecting syndicate. Also four men are required to operate the unit as compared to two or three men for most other electromagnetic methods. The present survey indicates that the instrument is not as sensitive to subsurface conductors, especially disseminated sulphides, as the electromagnetic-galvanic and vertical loop machines. This has also proved true in surveys of other areas known to the writer. VERTICAL LOOP METHODS Introduction Severa! methods have been developed in which a vertical coil or loop is used for creating an electromagnetic field which excites the conductive body beneath the surface of the ground. The vertical coil is especially suitable for energizing steeply dipping or vertical conductive bodies. It also provides less interference from highly conductive surface layers. The energizing loop may be triangular, square, or circular in shape depending on the type of equipment 'being used. The loop is supplied with an electrical current of constant frequency which may vary from about 50 to as high as 80,000 23

26 cycles per second, depending upon the method used. At the present time in Canada the majority of the instruments use a current of 1,000 cycles. The power supply may be either a battery-powered vacuum tube oscillator or a portruble gasoline engine-driven generator. The search coil or receiving unit consists of a small coil which may he orientated with respect to tilt and azimuth. It is mounted on a tripod, a staff, or simply carried in the hands of the operator. The coil is connected through an amplifier to headphones. General Principles If an area is underlain by homogenous material, the electromagnetic field! produced at the surface by the vertical energizing coil will lbe polarized in a substantially horizontal direction. Hence, when a search coil is placed in such a position that (1) the plane of the vertica:l energizing coil passes through the axis of rotation of the receiving coil, and (2) the axis of rotation of the receiving coil passes through the centre of the vertical loop, a minimum signal or null point will be obtained when the tilt of the receiving coil a:bout its axis is zero. The axis of rotation lies in the plane of the receiving coil. If the two coils were exactly at the same elevation, the search coil would be horizontal. When a subsurface conductive body is present the magnetic field of the vertical energizing loop induces currents along the edge of the conductor. These currents, in turn, produce an electromagnetic field. This second- field combines with the field of the loop into a resultant component, whose direction may be determined by tilting the search coil about its axis which passes through the center of the energizing loop until a null or point of minimum sound is obtained. The coil now lies' parallel to the resultant component and a normal to the plane of the coil should point in the general direction of the conductive body. The current concentration and hence the body may thus be located by measuring dip angles along a profile at right angles to the strike of the body. To illustrate the principle underlying the method, suppose the energizing loop is set up over a tabular-shaped', vertical conductive zone and observations are made with the receiving coil at equispaced stations along a line perpendicular to the strike of the zone. On both sides and at some distance away from the conductor and also directly over the conductor the resultant vectors are essentially horizontal. As the conductive zone is approached the inclinations of the resultant vector will increase to a maximum and then fall to the horizontal over the conductor. Also the resultant vectors slope in opposite directions on opposite sides of the conductor. Hence, as the search coil is moved along the line toward the conductive zone, the dip angle becomes increasingly larger until a maximum dip is reached, after which it decreases until a zero dip angle is obtained when directly over the zone. As the 24

27 observations are continued beyond the zero point, the same r esults are obtained, except that the dips are in the opposite direction. Two instruments employing the vertical loop principle were tested in the field; namely, a Doolan prospecting unit and a Sharpe Model S-100 electromagnetic survey unit. The Doolan Prospecting Unit Introduction Through the courtesy of Mr. A. A. Koffman, Chief Geologist for Hudson Bay Exploration and Development Company, Limited, it was possible to test a light weight electromagnetic instrument made by Mr. Frank Doolan of the Electrical Department of Hudson Bay Mining and Smelting Company, Limited. The manufacturing rights for this instrument have recently been acquired by Midwest Mining Supplies Limited, Winnipeg. The tests were carried out over the "A" and "B" zones east of Mosher Lake, see Plan of Electromagnetic Survey. Instrument The transmitting or energizing a nd the rece1vmg or search coils are wound on circular frames having a diameter of 15 inches and weighing about seven pounds each, see Figure 7. The source of power for the transmitting coil consists of four 90-volt B batteries, a 1.5-vo'lt A-battery, a nd a 4-tube power oscillator which produces a signal with a frequency of 1,000 cycles per second. The power output is eight watts. The entire transmitting unit weighs a'bout 19 pounds and can be easily carried by one person. The receiving coil as used in the test was mounted in a square frame attac'hed to the top of a staff whose lower end is placed on the ground. The inclination or tilt of the coil is obtained by noting the angle indicated by a pointer on a semi-circular protractor with the frame held in a horizontal position as shown by a level bubble. The protractor was subdivided into left and right quadrants of 90 degrees each. Readings may be recorded as so many degrees right or left with the operator facing the transmitting coil. The receiving coil plus amplifier and headphones weighs about nine pounds. The receiving unit has since been modified to eliminate the frame and staff. The coil is now held by the operator and angles of tilt are indicated by a special liquid clinometer attached directly to the frame of the coil. Thus the instrument is easily carried even through thick underbrush. 25

28 Figure 7 Doolan Electromagnetic Survey Unit. 26

29 Operation In operation the transmitting coil is held in a vertical position by the operator and directed towards the person holding the receiving unit. The transmitter is operated by a switch-button located on the coil, and the current is kept on only during the time required to take a reading at the receiving station, usually 15 to 20 seconds. The operator of the rece1vmg coil stands facing the transmitter with the coil so orientated that its axis of rotation points toward the center of the transmitting coil. With the transmitter in operation usually by pre-arranged signal, the receiving coil is tilted hack and forth until the operator determines the position of minimum soun.d or null point. The angle of tilt is then recorded. Surveying Technique The instrument may be used (1) to trace out and delimit a known conductive zone in areas where it becomes buried beneath overburden, and (2) to 'locate a concealed or buried conductive body or zone. In the case where a conductive zone has been located at some point, the transmitter is set up directly over the zone, and the energizing coil in a vertical position is aimed at the receiving coil set up 200 to 400 feet away. The transmitter is turned on and a record is made of the angle of tilt of the receiving coil when in the plane of minimum sound. The receiver is then moved 50 feet along a line at right angles to the presumed strike of the conductive zone and another reading is t aken. When the angles of tilt at two adjacent stations are away from one another the conductive zone may be assumed to lie somewhere between. By moving the receiving unit 'back along the line to some intermediate point, a place may,be found where the angle of tilt is zero, and the conductive zone may be assumed to lie directly beneath this point. The transmitter is then moved up to this point and the operation is repeated. In the second case, where a conductive body is being sought, a 'base line is run through the area parallel to the formational or structural strike. Picket lines are then cut perpendicular to the base line at distances of 200 to 300 feet apart. With picket lines 200 feet apart, the transmitter is set up on one line 200 feet from the base line and the receiving coil is stationed on the next picket line and on the base line. The distance between the transmitter and receiver is thus approximately 285 feet, and the line joining them makes an angle of 45 degrees with the direction of the picket lines. It is then a simple matter to orient the two coils in their correct positions. The transmitter is turned on and a record made of the angle of tilt of the receiving coil. Then both transmitter 27

30 and receiver are moved 50 feet in the same direction along their respective picket lines, thus keeping their distance and angular relationship constant, and a second observation of the angle of tilt is made and recorded. As long as the readings remain about the same it may 'be assumed that no conductor has been crossed. However, as soon as a change in the angle of tilt occurs, it implies that a conductor is nearby. The transmitter is then moved back along the picket line until opposite the point where the change in tilt occurred and is then kept at this point. The receiver is now moved at 50-foot intervals until a cross-over or change in direction of tilt has lbeen obtained thus indicating the approximate position of the conductive zone. The transmitter may now be set up at this point and the r eceiver moved to the place where the transmitter was located. Readings are again ta!ken at 50-foot intervals until a cross-over is obtained and the position of the conductive zone located. A more accurate fix of the position may be made by determining the location where the angle of tilt is zero. With the position of the conducting body located on the two picket lines its strike is readily obtained and its continuation along strike may be followed by the method previously described. Results o! Tests To test the instrument over a known conductor the "A" conductive zone was selected. Mr. Doolan was shown where to set up the transmitter on picket line 3,600N at a point approximately over the zone, see Figure 8. 28

31 / / SHARPE SURVEY CD 0 II) ~ A ZONE ~ MOSHER LAKE AREA ~ DOOLAN SURVEY Figure 8 29

32 The receiver was first moved along picket line 3,400N at 50-foot intervals until a cross-over had been established, and then along picket line 3,200N until a second cross-over was obtained. The position of zero tilt was obtained on both lines, and the line joining them agrees closely with the location of the zone as previously determined by diamond drilling. The transmitter signal was clearly audible with practically no background noise and null points could be easily read even at the most distant station or a:bout 500 feet from the transmitter. To test the method for locating an unknown conductive zone the "B" sulphide 'body was selected, see Plan of Electromagnetic Survey. The operators of the transmitter and receiver had no knowledge of the location of the zone, and there was nothing on surface to indicate its presence. Mr. Doolan was told to set the transmitter up on line 4,200N and 200 feet east of base line 122W and the receiver on line 4,000N at the 'base line, see Step 1 of Figure 9. He was told that the conductive zone was somewhere to the east. Observations of the angle of tilt were made at 50-foot intervals moving both transmitter and receiver along their respective picket lines. The r esults are shown in Step 1, Figure 9. A marked change in the angle of tilt occurred between 350 and 400 feet east of the base line, i.e., from 10 minus to 5 minus. The transmitter was then brought back along line 4,200N until approximately opposite the change in readings, see Step 2 of Figure 9. With the transmitter remaining at this point, the receiver was moved along line 4,000N until a cross-over was obtained between 550 and 600 feet east of the base line. The r eceiver was then moved 1 between these two points until a tilt of zero degrees was 0 btained. 1 30

33 ~ - -,,A200N N STEP C1> c..j C1> II) 0 (0 3: C\I. C\I STEP N 4 N 4200N B MOSHER DOOLAN STEP 3 ZONE LAKE AREA SURVEY 4000N SULPHIDE ZONE - TRANSMITTING COIL-TM Figure 9 31

34 The transmitter was then moved to this position on line 4,000N and readings taken by moving the receiving coil along line 4,200N until a cross-over was obtained between 300 and 400 feet east of the base line, see Step 3, Figure 9. The receiver was then moved back until a point was located where the angle of tilt was zero. The line joining the two zero positions was considered to represent the location and' strike of the conductive zone. This agrees fairly closely with the location of the zone as determined by the previous diamond drilling. As in the first test the transmitted signal was clearly audible at all times and the null point could be easily o'btained. Summary and Conclusions The Doolan vertical loop unit is a light weight, easily portruble instrument. The design is simple and the unit can be readily operated 'by two men after a short period of instruction. Under the conditions of the test the instrument proved to be capable of (1) outlining a known conductive zone, and (2) locating an unknown surbsurface conductor. The transmitting signal of 1,000 cycles per second was clearly aud'i!ble up to distances of 600 feet, and with practically no background noise from the amplifier the nul'l point was easily obtained. Because of its relatively low cost, about one quarter of the cost of other electromagnetic-induction instruments, and its simplicity of operation, the Doolan survey unit appears to be particularly well suited to the requirements of the individual prospector or small prospecting syndicate who wish to (1) carry out an electromagnetic survey of a small group of claims or (2) to outline the extent of a known sulphide occurrence preliminary to diamond drilling. Sharpe Model SE-100 Electromagnetic Survey Unit Introduction This unit, which operates on the vertical loop principle, was tested under similar conditions as set up for the Doolan instrument, mainly for comparative purposes. The instrument is manufactured and sold by Sharpe Instruments Limited, Toronto, Ontario. The instrument and its method of operation are described in Booklet "C", a brochure issued by the company. Instrument The transmitter consists of a four-foot square coil suspended on two spreader- bars which are bolted to a short mast which in turn rotates on a tripod. A pointer and scale allow the coil to be 32

35 oriented in any predetermined direction. The power supply consists of a 1,000 cycles per second generator driven by an air-cooled gasoline engine, both mounted on a packframe for purposes of transportation. A control circuit, housed in a box mounted on a second packboard, tunes the transmitting coil and indicates the generator output current and the circulating coil current. The complete transmitting unit can be easily carried by two men and can be assembled within three to four minutes. The receiving unit consists of a small circular coil, an amplifier, and clinometer which are rigidly mounted on a staff. The coil lies in the horizontal plane when the staff is held in a vertical position. A pair of headphones which are plugged into the amplifier completes the unit. Surveying Technique The surveying procedure for tracing the extension of a known conductive zone is similar to that already described for the Doolan instrument. To survey an area for unknown conductors, a base line is run through the area parallel to the formational or structural trend. Picket lines 100 to 400 feet apart are then run at right angles to the base line. The transmitting unit is set up at a point which is the centre of an area 2,000 feet square. The receiving coil is moved along a picket line not closer than 200 feet to the transmitter. Readings are taken at 50- to 100-foot intervals. Before each reading is taken the transmitting coil must be oriented so that its plane passes through the axis of rotation of the receiving coil, and the receiving coil aligned to have its axis of rotation passing through the center of the transmitting coil. This r equires a considerable amount of pre-survey calculating in order to obtain the correct azimuths for the two coils for each point of observation. The orientation of the two coils also involves some delay before a reading can be taken. Results of Test The instrument was used to trace out the "A" conductive zone in the same manner as described for testing the Doolan unit and the results are shown in Figure 8. These results are comparable with those obtained with the Doolan instrument. However, the Sharpe r eceiver gave a lot of background noise and at some stations it was difficult to determine the position of the null point with any degree of accuracy. A reading was also taken at the same time and at the same station with the Doolan r eceiver. At all times it gave a sharper null point with less background noise, and, consequently, the r eading could be made more quickly than with the Sharpe receiver. In general 33

36 the angles of tilt as recorded by the two receivers agreed fairly well. The instrument is more sensitive than the Boliden unit as it was capable of tracing out the northerly continuation of the "A" zone beyond 3,800 N, see Figure 8. Comparisons of surveys in other areas have also confirmed its greater sensitivity. The effective range of the instrument under the conditions of test was a:bout 1,400 feet and not 2,000 feet as stated in Booklet "C", Sharpe Instruments Limited. Summary and Conclusions The Sharpe Model SE-100 electromagnetic survey unit is fuuy capable of detecting and locating subsurface conductive zones and can be readily transported and operated by three persons. However, in wooded areas time is lost in orienting the transmitting loop, and the speed of the survey is slower than either the electromagnetic-galvanic or Boliden methods. GENERAL CONCLUSIONS The electromagnetic survey techniques described in this report were capa:ble of locating conductive zones under the conditions of test. The writer is not prepared to ma:ke a detailed comparison of the methods without further tests being made and data collected regarding costs of operation. In an approximate way, however, the four units may be compared as follows: (1) On the basis of sensitivity or capability of detecting a conductor the methods may be ranked (i) electromagnetic-galvanic, (ii) Sharpe and Doolan vertical coil induction, (iii) Boliden horizontal coil induction. (2) For the geophysical prospecting of large blocks of ground the electromagnetic-galvanic and Boliden horizontal loop techniques are undoubtedly superior, especially, the former if two or more receiving units are employed. (3) For the geophysical prospecting of smaller areas in the order of a few claims the Doolan instrument is quite satisfactory, especially, in view of its simplicity of operation, portability and relatively low initial cost. 34