The main cost influencing factor of deep geothermal drilling is the low penetration speed in the hard and abrasive crystalline rock formations encountered. Traditional rotary drilling techniques using roller cones have proven inefficient, with very quick wear and low drilling speeds. Even though significant progress has been made on cutters and design of PDC bits, the constraints exerted on the rock and its hardness at very high depth is such that unconventional drilling techniques must be envisioned. ...