Method and Device for Determining a Drill Bit's Position in a Borehole

A method for determining the position of a lower portion of a drill string in a bore hole, the drill string comprising a number of drill string components, each drill string component being interconnected by means of at least one joint, and wherein several ultrasonic sensors are arranged after each other along a drill string, and wherein the method comprises localizing the position of a chamfer of the joint relative to the ultrasonic sensors, wherein the method further comprises:

• arranging ultrasonic sensors over a length along the drill string, the length being larger than maximum expected distance between neighbouring joints of drill string components;
• continuously or in short intervals determining the position of the chamfer of the joint along the ultrasonic sensors;
• following the chamfer in a joint at least until the chamfer in neighbouring joint comes within the measuring range of an ultrasonic sensor;
• recording the position of the chamfer of a joint in a period wherein the joint is selected as primary joint;
• and
• determining the position of the lower portion of the dull string in the bore hole by summing up measured moved distance travelled by joints while they are primary joints.