An aeromagnetic survey over the volcanic island of Surtsey off the south coast of Iceland

Submarine volcanic activity was observed in the Vestmannaeyjar archipelago off the south coast of Iceland in November 1963, at a location where the pre-eruption oceanic depth was 130 m.  The eruption continued until July 1967. As a result of the eruption, a volcanic island, Surtsey, and its short-lived satellite islands (Surtla, Syrtlingur, and Jólnir) were created.  The progression of the eruption was very well documented at the time.  However, data on structures below sea level has been limited to drillholes on the rim of the Surtur crater on the main Surtsey island.  In order to study the existence and possible location of pillow lava from the initial phases of the eruption and shallow intrusions within and below the edifices formed in 1963-1967, a six-hour-long aeromagnetic survey was completed in October 2021 over the Surtsey area. The survey is done using a Geometrics MagArrow drone magnetometer, here adapted for operation while fixed to an aircraft. The survey covered 60 km².  The spacing between profiles was 200 m and the flight elevation 100 m a.s.l. The MagArrow has a sampling frequency of 1000 Hz, which for an aircraft flying at 50 m/s gives a reading every 5 cm. To remove noise and perturbations from the aircraft, the data is low-pass filtered in two steps, firstly by averaging 50 measurements providing 20 Hz data, then by applying low pass filter with a cutoff frequency of 0.225 Hz, removing wavelengths smaller than 200-250m.  Initial data processing indicates some variations in the sources to the anomalies observed.  Major anomalies arise from the subaerial lavas on Surtsey itself, while the submarine remnants of the island Syrtlingur, active in 1965, show no anomalies.  This suggests that it is exclusively made of tuffs with no significant intrusions, similar to the structure of Surtsey itself below sea floor according to the drill cores obtained in 1979 and 2017.  In contrast, a clear anomaly is observed over the submarine remnants of the satellite island Jólnir, which was formed over several months in 1966. Apparently, this anomaly can only be explained by a magnetic body located no deeper than at 100 m depth below the seafloor at the eastern part of Jólnir, the same location as the vent active in 1966.