Statistical analyzes of the shapes of marine sand dunes off the Opal Coast (Eastern English Channel)

Abstract

Numerous very high-resolution seismic lines (Sparker/single-channel seismic reflection) were acquired in the Eastern Channel, along the the Opal Coast, between the Bay of Somme and Cape Gris-Nez, during the oceanographic missions GEOBAS (2016-2020), TREMOR 1 (2014), TREMOR 2 (2017) and MARCOPALE (2023). In this sector, numerous sandy banks are observed, especially the Bassure of Baas. These geophysical data were analyzed as part of the TURBODUNES project, by comparing two reflectors, respectively the sea floor and the base of the dunes (corresponding to the top of the deformed Cretaceous and Eocene bedrocks). The difference between the two signals makes it possible to identify areas with dunes and areas where dunes are absent. Assuming a constant boat speed, the extraction of signals provides spatial information on the height of the dunes. We carry out analyzes of these signals using different methods, including Fourier spectral analysis, empirical mode decomposition and structure functions. Empirical mode decomposition is a method which allows a one-dimensional series to be decomposed into a sum of several series, called “modes”, each having a characteristic wavelength. This makes it possible to quantitatively characterize the shape of the dunes via different modes, each having a wavelength ranging between 25 and 400 m. The lines for which dunes are absent nevertheless have profiles with strong multi-scale variability, with scale-invariant Fourier spectra with a slope of -2, for scales between 2.5 m and approximately 1 km.