On the value of multiphysics instrumentation on the seafloor

It is well known that seismic and electromagnetic experiments provide complementary information regarding lithology and fluid content. There are many published examples where marine seismic and electromagnetic experiments have been conducted on the same geological target and their results combined to provide new insights, and a few where such pairs of experiments have been conducted on coincident profiles. For both techniques, normally the most time-consuming and therefore most expensive component is seabed instrument deployment and recovery. In 2023 we conducted a novel experiment that involved acquisition of both seismic and electromagnetic data using seafloor instruments equipped with both types of sensor that therefore only needed to be deployed once. We deployed 35 Scripps instruments equipped with orthogonal horizontal electrodes, orthogonal horizontal coil magnetometers and 14 instruments from the UK Ocean Bottom Instrumentation Facility equipped with orthogonal horizontal electrodes, three-component fluxgate magnetometers, hydrophones and short-period three-component geophone packages. These instruments were deployed for c. two weeks at c. 4-km intervals along a transect across the continent-ocean transition southwest of the UK. During this time, we shot two different airgun sources and then conducted a controlled source electromagnetic (CSEM) experiment by towing Southampton’s deep-towed electromagnetic transmitter along the transect. Our transect coincided with a pre-existing seismic reflection profile collected with a 10-km streamer as part of an Irish government project (covering part of the transect) and a new profile with a 2-km streamer acquired during a test cruise by the National Oceanography Centre in 2024 (covering the remainder of the transect).

 

The resulting rich dataset allows a variety of analyses, some of which are only made possible by the multiphysics acquisition. Electromagnetic sensors are normally located by acoustic triangulation; these locations can be improved by using the airgun shots. The fluxgate magnetometers can be used as compasses to orient both the electromagnetic sensors and the horizontal geophones. At crustal level, the seismic and CSEM experiments allow us to obtain coincident P-wave velocity and resistivity models at similar resolution. Magnetotelluric data provide constraints on the mantle lithosphere and the lithosphere-asthenosphere boundary. Our seismic experiment is not designed to image at these depths, but there are some constraints from teleseismic events such as the 8^th September Morocco earthquake, which is well recorded across our array. Additional constraints may come from ambient noise cross-correlation of hydrophone data, which extend to 10-12 s period. Airgun shots and the Morocco earthquake are well recorded by magnetometer channels and our electromagnetic transmitter is well recorded by geophone channels; these coupled signals may yield further complementary information about Earth structure.