In-field fast calibration of FOG-based MWD IMU for horizontal drilling

A fiber optic gyroscope (FOG)-based measuring-while-drilling (MWD) device for horizontal drilling is developed and an in-field fast calibration method of the MWD inertial measurement unit (IMU) is presented. The IMU consists of a FOG triad and a quartz accelerometer triad. An error model for the inertial sensors is established and 12 errors are confirmed as the main error sources for horizontal drilling after an analysis. A five-rotation-in-level-plane (5RILP) in-field fast calibration scheme for the main errors of the MWD IMU is illustrated. The scheme can be conveniently implemented on an almost level plane, such as a table plane, without any special external equipment. To solve the calibrated errors, a systematic approach based on specific force measurement observation is adopted. Observation equations of the IMU errors are derived according to the rotation sequence of the calibration scheme. Simulation results show that the proposed calibration method is effective even if there are 5 degrees errors in level and 10 degrees errors in heading. Calibration experiments using the developed FOG-based MWD equipment on a table plane confirm the validity of the proposed in-field calibration method. Inertial navigation tests show that the actual accuracy of the MWD IMU is greatly improved by 60.6% after in-field calibration.