Dual tube drilling consists of sending compressed air down an outer diameter tube (for bit lubrication) and bringing to the surface geologic samples of excellent quality to allow for correlation with the area's geology. It generally utilizes a top drive rig. The hole diameter is only slightly larger than the outside diameter of the outer tube. The use of the dual tube drilling method allowed the collection of representative rock cutting samples from the subsurface without extraneous rock cutting from the rest of the borehole.