Core break-off mechanism

A core break-off mechanism comprising:

• an inner, core break-off tube extending along a tube axis which is sleeved within and angularly rotatable relative to an outer drill tube, wherein the core break-off tube has a tubular wall of varying thickness such that its inside diameter (C-ID) is offset with respect to its outside diameter (C-OD) by a pre-determined amount in a radial direction transverse to the tube axis, and the drill tube has a tubular wall of varying thickness such that its inside diameter (D-ID) is offset with respect to its outside diameter (D-OD) by the same pre-determined amount in a radial direction, and wherein the drill tube has a drill end and a stepped indented shoulder on one lateral side of the tube axis proximate said drill end by which an end of the core break-off tube is retained within the drill tube, whereby when the break-off tube and the drill tube are held in one is relative rotational alignment, the two offsets cancel each other such that the D-OD of the drill tube and the C-ID of the break-off tube are centered together on the tube axis and are in positional alignment for drilling a core from base rock to a desired depth, and when the break-off tube is rotated 180 degrees with respect to the drill tube, the offsets add together, causing the rock core within the inner tube to become radially displaced by a distance equal to the sum of the two offsets so as to generate sufficient force to break off the rock core from the base rock.