A Proper Use of the Adjacent Land-Based Observatory 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, and 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 spatio-temporal 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. In this paper, we show that the sea level and seabed horizontal magnetic fields differ significantly, reaching up to 30% of sea level values in many oceanic regions. We made this inference from the global forward modeling of the magnetic field using realistic models of conducting Earth and time-varying sources. To perform such modeling, we elaborated a numerical approach to efficiently calculate the spatio-temporal 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 modeling results, thus without needing additional measurements at the seabed site. Plain Language Summary Knowing the position of existing and new oil wells is vital for economic and safe sub-horizontal drilling operations. A lower uncertainty in well positions allows for hitting smaller targets and avoiding the risk of well collisions. Determining the well position relies particularly on magnetic sensors installed close to the drill bit. For this, the models of spatially variable but constant in time magnetic field in the region of interest are routinely used. However, spatially and temporally varying space-weather-related geomagnetic field disturbances may affect the accuracy of the well position. One of the common ways to mitigate this problem is to correct magnetic field readings at a drill bit for the magnetic field measurements from an adjacent land-based magnetic observatory. However, this method assumes that the land-based signals are similar to those at the seabed drilling site. In this paper, we show that this is not the case, that is, the sea level and seabed magnetic fields differ significantly due to the electrical conductivity of the seawater column above the seabed site. Moreover, we propose and justify a numerical scheme which allows researchers to obtain trustworthy seabed signals by still using land-based data but exploiting the results of dedicated modeling.