ABSTRACT
When applied to mineral exploration the reverse circulation theory underwent several adaptations before attaining its current level of development. A subsequent diamond-drill programme failed to confirm the findings of the reverse circulation drilling and the reserves were downgraded from 4.5 million ounces to 0.9 million ounces. The design of the early reverse circulation bits also proved to be a problem and substantially escalated the cost per foot drilled. Subsequent experience and development accelerated the design of the downhole hammer reverse circulation hammer to a point where it is now accepted as a field standard and the previous problems of extremes in soft formations, hard formations, broken ground and water inflows can be readily accommodated. A series of hammer sizes has enabled diversity of range and in a period of just three years the exploration drilling industry has undergone a complete transformation in methodology.
