Surveying a well borehole by means of a rate gyro using an error nulling system

摘要

A positional survey of a well borehole 12 which is subject to deviation from the vertical is made by means of a survey tool 10 having a housing in the form of a sonde 16 containing a rate gyro 24, having an axis of rotation coincident with the longitudinal axis of the sonde, and means 30 for measuring the direction of gravity relative to the sonde, i.e. the angle of inclination of the sonde. The method comprises moving the sonde in the borehole e.g. by means of a slickline 20, taking measurements at spaced locations, and combining the measurements to determine the borehole azimuth and inclination at the spaced locations. The sonde determines a reference North direction at each location by means of a number of measurements made with the rate gyro housing at each of two opposite azimuthal positions relative to its axis of rotation, the housing being rotatable by means of a drive 26. Each set of measurements is averaged, and the standard deviation determined as an indication of the quality of the measurements. The two averages should sum to zero; any residual value is a measure of error, e.g. as a result of drift, and can be used to provide the necessary compensation.