Instruments for monitoring the direction of a borehole

A method of surveying a borehole comprising positioning at the mouth of the borehole a survey instrument comprising an elongate casing having its longitudinal axis coincident with the axis of the borehole, an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis thereof, and a rate gyroscope unit mounted in the outer gimbal and adapted to provide outputs indicative of the rates of rotation about the outer gimbal axis and an axis transverse to the outer gimbal axis;

• sensing the rate of rotation about said transverse axis by means of the gyroscope unit and applying a torque to the outer gimbal so as to pivot the outer gimbal about its axis, whereby the alignment of said transverse axis relative to an East/West direction is determined from the sensed rate;
• moving the survey instrument along the borehole;
• continually sensing the rate of rotation about the outer gimbal axis by means of the gyroscope unit as the instrument moves along the borehole and applying a torque to the outer gimbal in dependence on the sensed rate so as to stabilize the outer gimbal about its axis;
• continually sensing two components of gravity in two transverse directions relative to the outer gimbal or the casing;
• determining at least the inclination and azimuth of the borehole at a plurality of points along its length from the sensed components of gravity, and at high inclination angles of the borehole, the rate of rotation about said transverse axis is continually sensed by means of the gyroscope unit as the instrument moves along the borehole and a torque is applied to the outer gimbal in dependence on the sensed rate so as to maintain the high-side angle as determined from the sensed components of gravity zero.