Surveying Techniques With a Solid-State Magnetic Multishot Device

Abstract

Summary An electronic magnetic multishot instrument was introduced in 1985 to overcome accuracy and reliability limitations associated with conventional photomechanical multishot systems. This paper describes the survey system and the instrument's performance capabilities. Our aim is to develop a tool-performance model for general application to the management of surveying operations. Accuracy is considered in two parts: sensor errors are considered in detail, and external effects on accuracy—e.g., axial misalignment, bottom-hole-assembly (BHA) deflection, geomagnetic influences, and drillstring-induced interference—are analyzed and quantified. We found that geomagnetic influences and drillstring-induced interference dominate the ultimate performance of all magnetic tools, particularly this electronic multishot instrument. This paper also describes "in-hole referencing, " a method of eliminating these errors that involves surveying the first part of an openhole section with a high-accuracy gyroscope and aligning data derived from subsequent magnetic surveys with it. Using this method, we can achieve gyroscope-survey levels of accuracy with magnetic instruments in highly deviated and extended-reach wells without incurring the costs and penalties of running wireline surveys.