Fiber optic magnetic field sensing system based on lorentz force method for downhole applications

Abstract

A method and apparatus for magnetic sensing is described. The apparatus includes a strain-sensing fiber coupled to a conducting strip. The strain-sensing fiber may be, for example, a distributed feedback fiber with Bragg gratings. A current may be induced to flow on the conducting strip by electrically coupling a photodiode to the conducting strip and then activating a laser optically coupled to the photodiode. In the presence of a magnetic field, a Lorentz force will be exerted on the conducting strip, causing a displacement of the conducting strip that will induce strain on the strain-sensing fiber. The strain on the strain-sensing fiber may be measured by laser-pumping the strain-sensing fiber and measuring the reflected waves. The measured strain may be used to calculate the magnitude of the magnetic field. Multiple strain-sensing fibers may be optically coupled in series and deployed into a borehole for distributed magnetic field measurements. The magnetic field measurements may be used to determine the resistivity of formations surrounding the borehole and, for example, to monitor the movement of relatively-low resistivity water fronts.