Apparatus to measure the earth's local gravity and magnetic field in conjunction with global positioning attitude determining

Abstract

A processor uses signals from at least two GPS receivers and a 3-component accelerometer located on a stable non-magnetic platform to determine the roll, yaw and pitch angles of the platform, thus providing a measurement of geographic north. The processor also uses signals from a 3-component magnetometer on the platform to determine the magnetic north and thence the magnetic declination. A proton magnetometer is used to provide a check on the measurements of the 3-component magnetometer. Additionally, three single component magnetometers provide a measure of the local magnetic field gradient and serve to monitor artificially induced magnetic fields. An alternate embodiment of the invention uses at least three GPS receivers and may dispense with the 3-component of accelerometer to determine geographic north. Temporal local variations of the earth's magnetic field are tracked and used to correct conventional magnetic well surveys. For use in offshore drilling applications where the rig is on a drillship, the platform floats on water away from the drillship and tethered to a buoy. Signals from an inertial motion sensor are used by the processor to correct the observations of the GPS sensors, the magnetometers and the accelerometers for the motion of the platform in water.