Wireline Drilling Method
Most drilling today is done using the wireline method, which was introduced in the 1960s. Using this method, an inner tube containing the core is detached from the core barrel assembly when the core barrel is full or a blockage occurs. The tube and core contained in it are pulled to the surface by wire dropped down the string of drill rods. A latch or “overshot assembly”, which snaps on to the top of the inner tube, is used for this purpose. The inner tube is then rapidly hoisted to surface within the string of drill rods.
After the core is removed, the inner tube is dropped down into the outer core barrel and drilling resumes. Thus, the core is retrieved without having to pull all of the rods.
Large drills have recovered cores of up to 100 mm in diameter from depths of more than 4,500 metres (15,000 feet).
Core is carefully placed in sequential order in boxes or trays and taken to a geological field camp or office for examination by geologists. Interesting sections are split along their length by a core-splitter, with half of the section being returned to its place in the core box and the other sent to the assayer for analysis. The trays or boxes are usually stored on racks in a core shack.
Sometimes it is desirable to obtain a second intersection of a particular geological structure or ore zone from a single hole. This is done by placing a wedge at some point above the intersection to deflect the bit in another direction. This wedging procedure is used frequently in deeper holes, where it can save considerable time and money.