MODELING FIBER OPTIC DISTRIBUTED MAGNETIC SENSING FOR SUBSURFACE MONITORING

Distributed magnetic sensing (DMS) is an experimental geophysical sensing method combining distributed fiber-optic sensing and magnetostrictive materials to measure static and dynamic magnetic fields in the surface and subsurface. DMS is being developed to collect data that can be used to invert for resistivity. A particular goal is to create a high-density fiber optic array that can be left in place for time-lapse or continuous monitoring of CO2 sequestration sites, geothermal sites, groundwater aquifers, and hydrocarbon reservoirs. This presentation will provide information on the development and quantitative understanding of DMS as a new method to image and monitor subsurface structures and fluids.

Magnetostrictive materials expand or contract in the presence of a magnetic field, and when embedded in a fiber optic cable, the resulting fiber will also undergo strain that can be measured similarly to distributed acoustic sensing. We are modeling the magnetostrictive phenomena to quantify the amplitude and time scales of responses observed in laboratory and field settings. Our theoretical magnetostriction model assumes a multiple domain response in three dimensions. The induced mechanical strain is measured in one direction, similar to distributed acoustic sensing. We use the Landau-Lifshitz-Gilbert (LLG) equation to describe the angular dynamics of the magnetic moment of the material, then use this angular solution to solve for the magnetostrictive strain response of the material.

Results of the simulations show that the amplitude spectrum is sensitive to changes in the initial angle of the magnetic moment with the magnetic field, the magnetic field amplitude, and the magnetic field frequency. The amplitude spectrum typically shows peaks at the magnetic field frequency, double the magnetic field frequency, and harmonics at integer multiples of the magnetic field frequency. Small changes in the initial magnetic moment angle (~0.01 radians) change relative amplitudes of frequency peaks in the response. In addition, changes of ~0.1kA/m in the magnetic field amplitude alter the amplitude and presence of harmonic frequency peaks. The initial comparisons of laboratory tests to the model are promising, and it is likely that this model will be useful in predicting the DMS response.