Practical Application of a Multiple-Survey Magnetic Correction Algorithm

Abstract

Abstract Conventional methods for drillstring magnetization compensation of magnetic surveys process each survey independently. Limitations of these methods include the need for an accurate estimate of the local magnetic field, while they often result in excessive scatter in corrected azimuth values and poor accuracy in attitudes near horizontal east-west. Recent developments in survey processing include corrections which operate on several surveys simultaneously. The additional information thus obtained can be used to determine biases and other parameters affecting three axes, without the need for precise estimates of the local magnetic field strength and dip angle. Surveys processed in this way show more stable azimuth values and retain greater accuracy in the critical attitudes near horizontal east-west. The use of a multiple-survey correction technique together with in-field referencing to monitor changes in the local field addresses two major shortcomings in magnetic surveying. Multiple-survey analysis can be used to monitor performance of accelerometers as well as magnetometers. Accelerometer variance is measured but not routinely used to make corrections when there is no significant improvement. The better practice is to use calibration values obtained in the workshop, rather than replacing them with measurements taken in the down hole environment. The benefit of the technique is in closely monitoring sensor performance and identifying suspect surveys. Real-time verification that the tool is operating within a modeled error budget improves the value of positional uncertainty calculations. The improvement in quality control can obviate the need to run additional surveys for verification purposes. P. 379