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Abstract

Directional drilling in oil fields relies heavily on in situ measurements of the natural magnetic field (measurements while drilling, or MWD), which are used to construct the well path. However, these measurements are complicated by a combination of internal (core and crustal) and external (ionospheric and magnetospheric) signals and noise from magnetic elements in the borehole assembly. While internal signals are accounted for through Earth's internal field models, external signals create diurnal and spatio-temporal variations that must be mitigated to ensure accurate readings. One common method involves correcting MWD readings with data from an adjacent land-based magnetic observatory, assuming the land-based signals are similar to those at the seabed drilling site. However, we demonstrate in this paper that sea level and seabed horizontal magnetic fields can differ significantly (up to 30% of sea level values in some oceanic regions), as shown through global forward modeling of the magnetic field using realistic models of conducting Earth and time-varying sources. To accurately account for these differences, we developed a numerical approach to efficiently calculate the spatio-temporal evolution of the magnetic field, which we used to propose and validate a formalism for obtaining trustworthy seabed signals through measurements at adjacent land-based sites and modeling results, eliminating the need for additional measurements at the seabed site.