Abstract: New and improved methods and apparatus exemplifying the present invention are disclosed herein for measuring the changes in total elongation of a drill string due to variations in the tensional forces acting thereon during a typical well drilling operation on a floating platform, measuring the changes in the total length of the drill string as it is moved into and out of the borehole from the platform, and measuring the changes in the position of the platform in relation to the upper end of the drill string which are caused by wave motion. Thereafter, these measurments are uniquely combined for producing an output signal which is representative of the changes in the borehole depth. The combined measurements are also converted for producing another output signal which is representative of the actual rate of penetration of a drill bit coupled to the drill string.