Imaging the time dependence of water boundaries in the Faroe-Shetland Channel

The Faroe-Shetland channel is a major pathway for the thermohaline circulation in the North Atlantic. Cold deep water from the Nordic Seas flows to the southwest whilst above warm North Atlantic water flows to the northeast. The boundary between these two water bodies creates a strong impedance contrast that is imaged on seismic reflection data, and can be traced across the channel to where it interacts with the continental slope. In the contexts of mixing and representativeness of hydrographic observations, an important issue is how internal waves populating a water mass boundary like this one evolve over short time periods. The seismic method acquires a sequence of snapshots of the boundary as the acquisition system is towed through the water layer. These snapshots can be reconstructed to show how the shape of the boundary changes and, for the Faroe-Shetland channel, these reconstructions show that the internal wavefield is ergodic. That is, the statistical properties of a series of measurements at one location are the same as the properties of a single measurement at many locations. Further, their spectra show a reduction in amplitude with decreasing wavelength as predicted by the Garrett-Munk spectrum. These reconstructions also show that the longer wavelength features are stationary or very slow moving with respect to the seismic acquisition vessel, so that stacking does not damage the large-scale geometry. In these data a second boundary can be mapped at the bottom of the channel. We interpret this as being caused by the strong currents in the channel mobilising sediments in the bottom nepheloid layers. Although the sediment concentrations are small the fact that these bodies form reflections reinforces the interpretation based on forward models that, typically, the bottom has little reflectivity rather than a signal-to-noise imaging problem.