Problems in underground deep diamond drilling

Abstract

An unorthodox method of deep hole diamond drilling from underground positions is presented. In this method the starting inclination angle of the drill hole is placed on the side of the vertical, opposite to that commonly used in diamond drilling. When thus the starting angle is steeper than the mean schistosity inclination, the drill hole will bend backwards around the vertical through the drilling place, tending to reach the direction normal to the schistosity, or bedding, of the formations. In the paper are given examples showing how this effect is utilized to reach greater depths from the same drilling position, thereby also reducing the otherwise necessary expensive driving of crosscuts. In addition the wear on the rods and couplings is minimized. It is noted that the holes drilled by this method will at no point become vertical, but will be turning around the vertical through the drilling position. Therefore the method requires special surveying practice. The deepest hole drilled by this method is 690 m, finished at a depth of about 1200 m under the mean surface. This hole cut the vertical plane through the drilling position, normal to the drill hole azimuth, at a level 500 m below the drilling position, and at this point had an inclination of 80°.