Abstract: The present invention provides a method for determining magnetometer errors during wellbore survey operations. It is capable of determining errors on up to three axes, with or without the use of an external reference measurement of the local magnetic field, and is capable of providing an accurate result using data from a minimum number of surveys. A model is used to correct the observed data and the corrected data are transformed from the tool coordinate system to a different coordinate system referenced to the earth. The difference between the corrected transformed data and reference data in the earth coordinate system is minimized to determine the model parameters. The present invention also provides a method for determining residual uncertainty in the measurements and for quality control of the measurements.

Abstract: The present invention provides a method for determining magnetometer errors during wellbore survey operations. It is capable of determining errors on up to three axes, with or without the use of an external reference measurement of the local magnetic field, and is capable of providing an accurate result using data from a minimum number of surveys. A model is used to correct the observed data and the corrected data are transformed from the tool coordinate system to a different coordinate system referenced to the earth. The difference between the corrected transformed data and reference data in the earth coordinate system is minimized to determine the model parameters. The present invention also provides a method for determining residual uncertainty in the measurements and for quality control of the measurements.

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.

Abstract: A mud pulse telemetry system uses a downhole pulser to produce sequences of positive and/or negative pulses according to a selected pattern. Positive pulses, negative pulses, and combinations thereof may be produced. A flow rate sensor at the surface measures changes in the flow rate at the top of the wellbore instead of or in addition to changes in the pressure. The flow rate changes are detectable even though the pressure pulses themselves may have a poor signal to noise ratio. This enables the invention to function effectively in underbalanced drilling wherein the use of light muds with a high gas content is required. One embodiment of the invention uses a conventional downhole pulser with the main valve closed and the pilot valve operating in a direct pulse mode.