An electromagnetic investigation of the continent-ocean transition southwest of the UK.

In ocean-continent transition zones at rifted continental margins, distinguishing between crustal rocks, hydrated mantle rocks, and the boundary between continental and oceanic mantle is crucial. These materials exhibit distinct resistivity characteristics, making them identifiable through geophysical techniques. Marine Controlled-Source Electromagnetic (CSEM) surveys are particularly effective in mapping subsurface structures both onshore and offshore due to their sensitivity to conductivity contrasts.

Our study focuses on using multiple geophysical techniques to investigate crustal and mantle rocks at magma-poor rifted margins. We focus on the continent-ocean transition at the Goban Spur, located southwest of the UK. Here, previous seismic work suggested the presence of a broad zone of exhumed serpentinised mantle, in between continental crust confirmed by drilling and oceanic crust represented by the prominent linear seafloor spreading magnetic anomaly 34. We deployed 49 seafloor instruments on a c. 200 km transect spanning these three basement types, coincident with a pre-existing high-quality seismic reflection profile, to collect seismic, magnetotelluric (MT), and controlled-source electromagnetic data.

For navigation, the CSEM system integrated USBL positioning, CTD measurements, and an altimeter. The transmitter utilized a compact waveform with a fundamental frequency of 0.25 Hz, enhanced by maximizing the amplitude of the 3rd and 7th harmonics. The transmitter dipole moment was 30,000 A·m, powered by a current of 100 A.

For data analysis, the compact waveform was processed in short 4-second time windows. We do stack the 4-second FFT up to 60 seconds or longer.  This approach retained essential information while enhancing the signal-to-noise ratio, enabling robust time-series analysis.

CSEM and MT methods have shown promise in resolving debates about lithospheric structure. While these techniques have previously imaged fluid-rich zones in subduction settings, this study is the first to apply them to continent-ocean transitions in rifted margins. We present results from preliminary analysis of both CSEM and MT datasets, focusing on lateral changes in resistivity at the seaward limit of continental crust.