A magnetic survey correction method (MSCM) has been developed and used to process MWD surveys. The local magnetic field strength and dip angle required for the method were obtained from a commercially available computer model of the Earth's magnetic field. The method corrects for both radial and axial magnetic drillstring interference and has led to improved quality control for magnetic surveys. The only nonmagnetic tubular incorporated in the drillstring is the MWD's own nonmagnetic housing. Comparison of the MWD results with north-seeking gyro surveys confirmed that the theoretically predicted accuracy of the MWD surveys processed with the correction method is generally better than conventional surveys taken in a full length of nonmagnetic drillcollars. No additional rig time was incurred and operational benefits include reduced rental, inspection and risk of lost-in-hole costs.
Magnetic survey accuracy can be enhanced by using locally measured geomagnetic parameters for the correction method. Field examples of enhanced magnetic surveying with electronic magnetic multishot tools are given. The achievable accuracy of magnetic surveying onshore in the Netherlands is comparable to the accuracy of current north-seeking gyro tools and a reduction in the number of gyro surveys is therefore planned.