Table of Contents
Here is a paper on surveying and sampling diamond-drill holes. The present paper gives a more thorough description of these methods, together with a few changes and notes suggested by further experience. Since, the methods, used constantly by the Cleveland-Cliffs Iron Co., have been developed to a higher degree of efficiency, and have given great satisfaction.
Until a few years ago but little attention was paid to the determination of the actual course of drill-holes, to the accurate sampling and analysis of the material, or to the scientific location of the drill-holes. It has now been common practice for several years in many districts to test the inclination of drill-holes by etching glass tubes with hydrofluoric acid, but the direction of deviation from the Vertical has not often been determined. Methods of determining the direction by means of a compass were first developed in connection with drilling in Victoria. The latest method used on the Rand in South Africa has been recently described by John I. Hoffman.
In the same paper Mr. Hoffman describes an ingenious method of wedging-off a drill-hole so as to start new holes at several successive points. By this means a hole can be deflected in any desired direction, and the curvature can be controlled to a considerable extent, provided the rock is soft enough to permit the hole to be wedged off. Unfortunately, in the Lake Superior district, the jasper iron-formation is so hard that it is very difficult, if not impossible, to do this. I tried to deflect one hole so as to avoid drill-rods stuck in the hole, by using a 12-ft. wedge of the hardest steel obtainable, but the bit cut the wedge and hardly touched the jasper. If there had been a dike of softer rock higher up in the hole I might have been able to deflect it at that point. Two years ago I described how the curvature of a diamond-drill hole can be controlled by other means, and gave an example where it had been done successfully. Although the hole in this instance kept to the desired inclination, it failed to intersect the ore encountered in a previous hole because the direction of deviation from the vertical of the previous hole had not been determined, although the amount of deviation had been determined by the usual hydrofluoric acid tests.
Since, I have experimented with two methods of determining the direction of deviation, one applicable to non-magnetic and one to magnetic rock-formations, with very good results. We now test all drill-holes except very shallow ones for both the amount and the direction of deviation, usually at 200-ft. intervals, and we have the satisfaction of knowing very closely the actual position of any ore or other formation encountered. In non-magnetic rock-formations the old method of a compass suspended in gelatine is successfully used, with improvements worked out by George Maas and myself, and patented by Mr. Maas (Patent No. 1,003,624). In magnetic formations a method of marking the drill-rods is used in connection with hydrofluoric acid tests. This method was, to the best of my knowledge, first applied by John Deacon, Superintendent of the Republic Iron & Steel Co.’s properties at Negaunee, Mich., who used it several years ago in testing diamond-drill holes at the Cambria mine. I found that with great care good results can be obtained by this method, but it is more difficult and expensive than the compass method. It is the only one I know of that is practicable in magnetic formations where the direction of strike underground is unknown. If the strike were known it would be easy to take an impression of the bottom of the hole with a piece of lead and determine the direction of deviation by comparing the direction shown by an acid etching with the strike shown by the piece of core broken from the bottom just before making the test, when made to coincide with the impression in the lead. Jean Florin describes a similar method, using a compass and camera instead of the acid.
Surveying in Non-Magnetic Formations
Figs. 1 and 2 show the cases used to test for inclination and course, using the latter when more than one test is to be made at one time, since it may be inserted at any point in the drill-
rods at the same time that the first case is used at the end of the rods. As it is desired to use as large a glass tube in the case as possible, and as the outside diameter is limited by the size of an E hole, a material was selected which combined the greatest possible toughness and tensile strength with non-magnetic properties. Phosphor-bronze was chosen, which is entirely non-magnetic, and which can be obtained with a tensile strength of 60,000 lb. and an elastic limit of 54,000 lb. per sq. in. By using a case of the dimensions given in Fig. 1, a glass tube 1 1/8 in. in outside diameter can be used, and according to Nystrom’s formula for the collapsing-strength of small tubes, P = 4Tt²/fd√L, using a factor of safety of 4, this case should be safe in a hole 2,800 ft. below water-level. A little wicking is used to make a perfectly tight joint.
The compass invented by Mr. Maas, shown in Fig. 3, has the advantage over the old forms of compass used for this purpose that it is pivoted in a cage which prevents it from coming in contact with the glass tube, and insures a free swing in the gelatine. The cage is below and rigidly attached to a cork float.
The most accurate and satisfactory method of testing the course of a hole is to use the compass in a glass tube about 6 in. long, open at each end. A section of rubber stopple is forced into the tube, leaving about 1.5 in. space for acid at one end, and 4 in. space for gelatine at the other. The gelatine may be prepared beforehand, but usually a small weighed portion of dry gelatine is carried to the drill and dissolved on the ground in a given quantity of water, care being taken that the water has no chance to evaporate while dissolving the gelatine. The proportions are so chosen that when dissolved the solution will keep liquid as long as possible after being lowered in the drill-hole, and yet will become perfectly solid when cold. For instance, with Nelson’s Improved Brilliant Gelatine we use 5/6 g. and dissolve it in 50 cc. of water. In a hole where the rods can be lowered in 20 min. or less, a 1 1/8-in. tube is used with paper wrapping. When it takes from 20 to 30 min. to lower the rods, a 1-in. tube is used with several wrappings of paper. If deeper than this, a thermos bottle is used, and by wrapping with paper the gelatine may thus be kept liquid 50 min. By using 6 g. of pure common salt with the 50 cc. of water the gelatin may be kept liquid for 90 min. The time the gelatine remains liquid was determined by tests in water cooled by ice to 43° F., the approximate temperature of the underground water in the Lake Superior district.
In the first two cases, when the thermos bottle is not necessary, the dissolved gelatine is poured into the tube, which is then heated by immersing in water heated to boiling by live steam. When hot, the compass is dropped in and a stopple is placed in that end; then about 1 in. of dilute hydrofluoric acid is poured into the other end and that end closed. The tube is then wrapped in paper and placed with gelatine end up in the bronze case, which is attached to the bottom of 20 ft. of brass E rods and lowered into the hole, losing as little time as possible. The brass rods are screwed to the bottom of the regular drill-rods, using an A to E reducing-coupling if the hole is being drilled with A rods. The bronze case and brass rods are made for an E hole, so that they can be used in either case. If two tests are to be made at the same time, another tube and compass are placed in the case shown in Fig. 2 and inserted in the drill-rods at the proper point, using 20 ft. of brass rods on each side.
The tube is left stationary in the hole 50 min. after the rods are lowered, giving the gelatine time to cool and set, and the acid time to etch a good line. If salt is used in the gelatine the tube must be left stationary for 5 hr. after the gelatine starts to cool. Acid diluted with 12 parts of water gives the best results. This solution is prepared in the office and carried to the drill in hard-rubber bottles, which are much more convenient than the paraffin bottles in which the acid is usually furnished.
When the tube is brought to the surface, the positions of the north and south points of the needle are marked on the glass with a diamond point and the tube is washed out. This tube then forms a permanent record of the inclination and course of the hole at the depth at which the test was made.
The thermos bottle, 1 1/8 in. in outside diameter, consists of two clear glass walls, with a vacuum between. When it is necessary to use this, both the hot gelatine and the compass are placed in the bottle, which is then closed by a rubber stopple. The stopple also closes one end of a 1 1/8-in. tube 3 in. long, serving to connect the bottle and tube and preserve them in the same relative position shown in Fig. 4. Dilute acid is placed in the tube, the other end closed, and the tube and bottle placed in the bronze case and lowered into the drill-hole. It only takes the gelatine 90 min. to solidify in the thermos bottle, so that if it takes 40 min. or more to lower the rods, the test is usually left in the hole 50 min. after the rods reach the bottom, just long enough to get a good etching. It may be left
in the hole over night, but in that case the acid should be more dilute. When the tube is brought to the surface, the north and south points are marked on it, corresponding to the position of the compass-needle in the thermos bottle. The 3-in. tube then forms a permanent record of course and inclination, just as the 6-in. tube does.
In either case the inclination is read in a goniometer and is corrected for capillarity according to a curve which is prepared for each size of tube by testing tubes at known angles. Fig. 5 shows a curve for 1 1/8-in. tubes. It will be noted that for these tubes the correction is only 3.75° at 45°, which is the maximum. The angle can be read to 0.5°, and I feel certain that the results of tests for inclination can be relied upon to within 1°.
To determine the course of the hole, the tube is placed in a special two-circle goniometer, Fig. 6, with both circles set at 0° so that the tube is vertical, and the vertical circle parallel to the crosspiece over the top of the instrument. If the inclination of the hole is steep, the tube is twisted until the etching shows the dip to be either directly towards or away from the eye; that is, until the cross-thread bisects the ellipse etched on the glass. If the inclination is shallow, it is more accurate to twist the tube so that the dip is to the right or left of the observer and in the plane of the vertical circle. The vertical circle, holding the glass tube, is then turned until the north and south marks on the tube are in line with the crosspiece, when a pointer on the horizontal circle reads the course of the drill-hole. Figs. 3 and 4 show tubes with acid, gelatine, compass, and north and south points marked, just as they are taken from the drill-hole.
We have found the method described above very successful, and two tests at the same point almost always agree to within a few degrees. When this is not the case, more tests are made, and so far we have always been able to ascertain which are correct. We have made tests at a depth of 2,000 ft., and by using salt in the gelatine, tests can be made at even greater depths.
The precautions to be taken are : 1, that the compass swings perfectly freely, and does not catch on the cage; 2, that the gelatine keeps liquid long enough; 3, that the compass is not used when there is much local magnetic attraction in the rock-formation ; and 4, that the compass is not affected by the steel drill-rods or casing, or by other iron in the hole. The first and second precautions are easily taken; the third can be judged only by a knowledge of the formation and by taking tests at different depths, which, if concordant, would indicate that there is no appreciable magnetic attraction. The fourth precaution is important. We use two 10-ft. lengths of brass rods, and so have no iron within 20 ft. of the compass. Tests with 10, 20, 30, and 40 ft. of brass rods at the same depth gave the same reading in a hole dipping 50° N 45° E., so that a length of 20 ft. is conservative. The results of a second test with shorter lengths of brass rods at another point gave :
These data show that the true course is between N. 47° E. and N. 50° E., and that 20 ft. of brass rods is conservative. It is impossible to obtain results accurate to closer than 2° or 3°, because of the small size of compass necessary and the personal error in marking the glass tube and reading the goniometer.
Surveying in Magnetic Formations
When the rock-formation is known to be magnetic, or when several tests with the compass do not agree, there seems to be no way of determining the course of a hole but by lowering the rods in such a way that the test-tube can be oriented at any point in the hole. We have done this by the method used by Mr. Deacon. The rods are first screwed together in one or two long lines on the surface, just as they will be lowered into the hole, with a bronze case at the end, and at intermediate points in the rods if desired, all the joints being made as tight as usual. Great care is necessary that no twist be left in the rods when screwing them together on the ground. This trouble is not experienced when there is snow, as the rods slip easily, and no torsion is introduced. When the ground is bare, difficulty may be avoided by placing level planks at short intervals for the line of rods to rest upon, and to prevent their touching the ground at any point. A straight stretch of railroad-track, with ties exposed, is very convenient for this purpose. If not more than 500 ft. long, the rods will turn on grass without leaving any twist in them.
When all connected, each joint, including the bronze case, is marked with a chisel, so that it can be screwed up again to exactly the same place. The joints are marked exactly on top as they lie on the ground, so that when the rods are in the hole the marks will point in the same direction. The rods are then broken into 20-ft. lengths, convenient for handling, being careful to keep one wrench on the couplings so as not to disturb any joints not to be broken.
Dilute hydrofluoric acid is poured into a glass tube, the tube marked with a diamond, and placed in the bronze case so that the mark on the tube corresponds with that on the case. A convenient way of doing this is to cut a mark across the top of the stopple, set this to agree with the mark on the tube, and then place the tube in the case so that the mark on the stopple agrees with that on the case. The tube is then lowered into the hole, being careful exactly to match the marks at every joint, and again keeping the wrench on the couplings. The mark on the last rod is placed directly in front of the drill and this direction determined, which is the direction of the mark on the glass tube. The tube is left stationary at the bottom of the hole for about 50 min. when acid diluted 12 to 1 is used, and then withdrawn and washed. To determine the course of the hole, another mark, exactly opposite, is made on the tube, and the course is found by using the goniometer as described in connection with the gelatine test.
This method, of course, gives accurate results only when the rods turn easily in the hole, so that there is no twist in them after they are lowered, which is usually the case except in very deep holes, or in holes where the inclination is low, or where the curvature is excessive. In these latter cases, unless the hole is rifled, the twist may probably be removed after the rods are in the hole by raising and lowering them several feet a few times. Precaution should be taken that the tube cannot turn in the bronze case, by forcing it in with a little waste alongside. On withdrawing the test it is well to examine each joint in the rods as they come out of the hole to see that none have moved while lowering or hoisting, and to be sure that the tube has not moved in the case.
This method requires great care and patience, but is capable of very good results. My experience has been that it cannot safely be left to assistants, whereas the compass method will give good results in the hands of any ordinarily careful and intelligent assistant, or even in the hands of the drillmen.
I have tested the same drill-hole by both methods and found the results to agree very closely. In one case the first tests in an old hole, by compass, did not agree very well because of casing left in the hole. Two or three tests were taken at each point, however, and the averages gave a smooth curve for the drill-hole. Later tests were made with marked rods, and the results agreed so well with the average of the compass-readings that when the two were plotted there was a difference of only 12 ft. at the bottom of the hole, although it was 1,697 ft. below and 262 ft. horizontally from the collar. The averages given below show that the hole described a spiral of considerable curvature. These results would have agreed even better if the compass had not been considerably affected by iron casing in the hole to a depth of 1,500 feet.
Deflections of Drill-Holes
J. S. Curtis gives an interesting theory of the cause of bore-hole deflections, with results of experiments which he made to substantiate his theory. He endeavors to show that the influence of terrestrial magnetism should cause vertical drill-holes to deviate to the north in the southern hemisphere, and affirms that this is the case in the great majority of holes, although the direction may be changed by the character of the country-rock.
In our experience the latter feature is much the more important, and from results of our drilling I would not say that the great majority of drill-holes deviate either north or south in all districts. If the strata are flat and uniform the holes may do so, but if the strata dip steeply this is not the case. In one district, where the dip is steep, we are certain of the course of 14 holes which deviated from the vertical. Of these holes, one went approximately north, one approximately south, one NE., five NW., one SE., and five SW. Putting it in another way, seven deviated to the north and seven to the south, while two deviated to the east and ten to the west. If these results show anything, they only show that the majority of the holes deviated to the west, but equally to the NW. and SW.
Fig. 7 shows a plan of holes which deviated considerably from the vertical. These holes were drilled on several ranges in the Lake Superior district. It will be seen that there is no general course that can be predicted.
It is very difficult to keep vertical diamond-drill holes straight, and I believe that a hole can be located with more assurance of striking a certain point in depth if it is given an inclination of 85° against a steeply-dipping formation than if it is started in a vertical position. We have drilled only two holes with this inclination, the results being:
These two holes are not enough for a generalization, but they kept straighter than vertical holes in the same district. In addition, we knew in what general direction the holes would deviate, which we do not know when we start a vertical hole. In 1910 I gave a series of curves showing the curvature to be expected in an inclined hole when dipping against a steep jasper formation. I would change the curve for a hole started at 85°, since, under these conditions, that angle seems to be a critical one, and the hole does not flatten as would be expected.
In view of the sometimes surprising curvature of drill-holes, I feel that all holes should be tested both for course and for inclination at 100- or 200-ft. intervals, whether started vertical or at an angle, otherwise there is no certainty as to the exact place at which the ore or other strata is actually cut. We recently started a vertical hole which, at a depth of 800 ft., was found to have an inclination of only 51° from the horizontal. Another hole, started at an angle N. 54° W., was found to be running N. 64° E. at the bottom. A third hole, started vertical, deviated 377 ft. in a total depth of 1,290 ft., and was only 47° from the horizontal at the bottom.
Fig. 8 shows a report form filled out by the drillmen every shift for the drill-foreman and the head office, and Fig. 9 a sample-tag which is placed in every bag of core or sludge. The record of time of drilling and footage of each diamond-bit is kept, to obtain data on the several stones in the bit with the idea of determining which are the most economical. These tests have shown that the wearing quality of the stone depends considerably upon the specific gravity and upon the structure.
Although it is important to have the drillmen report the amount of core saved from each material, yet they rarely measure it accurately, and if the analyses of core and sludge are to be combined, as described below, the core is remeasured when it reaches the office.
Samples from Drill-Holes
The following directions for saving samples are posted in the drill-shanties and enforced by the inspector: