PLANNING A DRILLING CAMPAIGN
Choice of a Drilling Method
Whether drilling is designed to test the grade and width of an orebody, to find new ore, or merely to secure geological information, the choice of a method usually lies between churn drilling and diamond drilling. The principal factors which influence the choice are the nature of the information required, the shape and attitude of the target, and the physical nature of the ground. The respective advantages of churn drills and diamond drills are indicated in Table 4.
The chief advantage of the churn drill apart from possible economy is that it cuts a hole of relatively broad diameter and thereby yields a large and representative sample. The best results and costs are obtained in rocks that are soft to medium hard, provided they do not slump or cave readily into the hole. But, with heavy equipment, rather hard rocks can be penetrated without great difficulty. The churn drill is used to best advantage in drilling prospect holes through flat and moderately inclined mineralized zones of considerable horizontal extent, in which the valuable metal content is rather evenly distributed. Therefore, churn drills, with various modifications, are widely used for prospecting placer deposits, for blast-hole drilling, for prospecting mineral deposits in bedrock, and to determine stratigraphy. The so-called "porphyry copper" deposits have been explored largely by churn-drill operations.
Much of the preliminary prospecting for ore in the Tri-State zinc region since 1928 has been done by boring churn-drill holes about onequarter mile apart in order to determine the stratigraphy, mineralization, and other pertinent data, and scores of square miles have been prospected by this method. More than 50,000 drill holes nave been completed in the district to date. The drilling depth ranges from a few feet to 500 feet, the average depth being about 300 feet.
Diamond drilling competes with churn drilling under conditions where reasonably good core recovery can be expected, and under these conditions, for example, in the "porphyry" copper deposits, the present trend appears to be in favor of diamond drilling, partly because of the increased speed and economy of the modern type of bit, partly because the sample, although smaller than that from a churn drill, is more uniform in cross section and therefore, under favorable conditions, more accurate, and partly because of the advantage of recovering a sample intact in the form of a solid core. But if the rock is decomposed or friable or badly fractured, it does not yield a good core, and if the ground is very broken and cavernous, the bit gets wedged in the hole or the water escapes and fails to "return." In these unfavorable condi¬tions, diamond drilling is slow and expensive. In really hard rock, the diamond drill makes faster progress than the churn drill, provided the rock is uniform, but if it consists of hard veinlets or nodules in a soft matrix, the effect on bits is disastrous. In the Tri-State district, where the limestone contains chert and jasperoid in nodules and irregular masses and the rock is shattered in and around the orebodies, the diamond drill cannot compete with the churn drill.
Shot (calyx) drilling can compete with diamond drilling in rocks which are uniform and not too hard. In Northern Rhodesia, one of the few places where it has been used extensively in exploration for metals, it proved faster and cheaper than diamond drilling. However, the holes are less uniform in diameter, tend to deflect more, and can¬not be drilled satisfactorily at angles flatter than 35 degrees from the vertical. Shot drills would be unsuitable for drilling iron ore because of the contamination of the sludge sample by particles of shot.
Combinations of diamond and churn drilling are sometimes advantageous. Thus on the Mesabi Range, a churn drill may be used to dig a hole to bedrock, after which a diamond drill bores through the cherty taconite, and churn drilling is resumed when friable iron-formation is reached. For this purpose a string of rods similar to diamond-drill rods and equipped with a chopping bit is used instead of the usual churn-drilling cable and tools; the sludge is recovered by a stream of water as in diamond drilling.65 In the modification known as "structure drilling," the water is pumped down between the rods and the casing and rises through the inside of the rods. This method recovers cuttings in the form of relatively large fragments, which are desired for testing the susceptibility of iron ore to concentration by "washing."
The diamond drill meets with no competition from the churn drill, of course, where the target can be reached most advantageously by flat or inclined holes. Therefore, diamond drilling is virtually the only method used in testing steeply dipping veins.
Hammer drilling may compete successfully with diamond drilling both in speed and cost, where a large number of closely spaced short holes is needed. It is inconvenient for steep holes, especially upward, and its greatest disadvantage is that it does not yield a core sample.