Cruise report: RRS James Clark Ross 42. A Seismic Tomographic and Hydroacoustic Study of Ascension Island, 2nd-18th May 1999, Montevideo-Ascension

The structure of the Earth’s crust at oceanic volcanic islands is of interest for the information it provides both about the processes involved in the formation of these islands and about the rheology of the underlying oceanic lithosphere, which deforms in response to the island load. Since some of the acoustic energy from large underwater explosions is commonly coupled into the oceanic sound channel as socalled “T-phases”, hydrophones around several such islands and broad-band seismic stations on the islands are being used for monitoring of the Comprehensive Test Ban Treaty. Cruise JR42 involved a four-day seismic experiment around the island of Ascension, a 4-km-high volcano some 90 km west of the Mid-Atlantic Ridge. During the experiment, a 6186 cu. in. airgun array was fired at one-minute intervals for about two days along a series of lines extending up to 45 km from the coast of the island. These shots were received by hydrophones and sonobuoys offshore and seismometers onshore. The shots were used to locate and calibrate three permanently installed hydrophones, to study the coupling of seismic energy into the island slope, and to study the structure of the crust beneath the island. The experiment was funded by the US Department of Energy (through Lawrence Livermore National Laboratory (LLNL)), the US Office of Naval Research, the UK Natural Environment Research Council, the United Nations (through the Comprehensive Test Ban Treaty Office in Vienna), IFREMER’s Centre de Brest, and funds held at the University of Southampton by T. A. Minshull. On the passage from Montevideo to Ascension Island, an unrelated scientific party conducted oceanographic measurements as part of the “Atlantic Meridional Transect”.