A Proper Use of the Adjacent Land-Based Magnetic Field Data to Account for the Geomagnetic Disturbances During Offshore Directional Drilling

Directional drilling in the oil fields relies particularly on the "on-fly" measurements of the natural magnetic field (measurements while drilling; MWD); the MWD are eventually used to construct the well path. These measurements are the superposition of the signals from the internal (core and crustal),  external (ionospheric and magnetospheric) sources, and the noise from magnetic elements in the borehole assembly. The internal signals are mostly constant in time and accounted for through the Earth's internal field models. The signals of external origin give rise to diurnal and irregular spatiotemporal magnetic field variations observable in the MWD. One of the common ways to mitigate the effects of these variations in the MWD is to correct readings for the data from an adjacent land-based magnetic observatory/site. This method assumes that the land-based signals are similar to those at the seabed drilling site. This study shows that the sea level and seabed horizontal magnetic fields differ significantly in many oceanic regions. We made this inference from the global electromagnetic induction modelling of the magnetic field using realistic models of conducting Earth and time-varying external sources. To perform such modelling, we elaborated a numerical approach to calculate efficiently the spatiotemporal evolution of the magnetic field. Finally, we propose and validate a formalism allowing researchers to obtain trustworthy seabed signals using measurements at the adjacent land-based site and exploiting the modelling results, thus without needing additional measurements at the seabed site.