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We estimate the P- and S-wave seismic attenuation in polar firn from spectral analysis of diving-wave first-breaks of three-component active-source seismic observations carried out in 2010 on the Whillans Ice Stream (WIS), a fast-flowing ice stream in West Antarctica. The resulting quality factors are then successfully modeled using a rock-physics theory of wave propagation which combines White's mesoscopic attenuation theory of interlayer flow with that of Biot/squirt flow. The first theory describes an equivalent viscoelastic medium of a stack of two thin alternating porous layers of thickness much larger than the pore size but smaller than the wavelength. On the other hand, in the so-called Biot/squirt-flow model, there are two loss mechanisms, namely the Biot global-flow one and local flow from fluid-filled micro-cracks (or grain contacts) to the pore space and back, where the former is dominant over the latter. The fluid saturating the pores is assumed to be fluidized snow, which is defined as a mixture of snow particles and air, like powder, having zero rigidity modulus.

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 page, Submarine mass movements may have profound effects on the morphology and stratigraphic architecture of continental margins. Furthermore, they can represent a threat to human life and infrastructures, and also have implications for hydrocarbon exploration/production. In polar regions, they are one of the predominant sedimentary processes on the continental slope. Exploration seismology has been widely employed to study masstransport deposits (landslides, glacigenic debris flows, mass flows, etc.), which are usually characterized by chaotic reflections. In this study, we analyse a seismic profile acquired in the southern part of the Storfjorden trough mouth fan (NW Barents Sea margin), showing the presence of two submarine mass-transport deposits (MTDs). A giant MTD (PLS-1) is located on the lower continental slope at 2.6–3 km depth, while a more recent MTD (PLS-2) occurs at 1.9–2.4 km depth. Velocity and attenuation seismic tomography, seismic attributes analyses and rock-physics models reveal distinct petrophysical properties for PLS-1 and PLS-2. Despite the known influence of burial depth(s),fluid flow content, and compaction on the internal character of MTDs, the two deposits studied here, in fact, show distinct petrophysical characteristics that reflect lithological variations - more than to any other control. These results suggest different source areas for the two MTDs. The inferred coarser sediment in PLS-1 indicates provenance from areas with abundant glacigenic debris flows, (such as the Bjørnøya Trough-Mouth Fan). Conversely, the finer, relatively fluid-rich sediments of PLS-2 that underwent little translation could have an origin in the area between trough-mouth fans. Here, slow-moving ice resulted in a relatively scarce release of subglacial debris at the shelf edge and the continental slope was subject to enhanced erosion and degradation with a comparatively higher production of relatively fine-grained turbidite flows

International audience; In many cases, the seismic response of bottom‐simulating reflectors is characterised by low frequencies called “low‐frequency shadow”. Generally, this phenomenon is interpreted as attenuation due to partial saturation with free gas. Actually, this frequency loss may have multiple causes, with a normal moveout stretch as a possible candidate. To analyse this phenomenon, we compute synthetic seismograms by assuming a lossy bottom‐simulating layer, with varying quality factor and thickness, bounded by the upper hydrate‐brine/gas‐brine and lower gas‐brine/brine interfaces. First, we estimate the shift of the centroid frequency of the power spectrum as a function of the travelled distance of the seismic pulse. Then, we perform one‐dimensional numerical experiments to quantify the loss of frequency of the seismic event below the bottom‐simulating reflector as a function of the quality factor of the bottom‐simulating layer and its thickness (due to wave interference). Then, we compute shot gathers to obtain the stacked section, with and without the normal moveout stretch correction and with and without the presence of wave attenuation in the bottom‐simulating layer. The results indicate that the low‐frequency shadow due to the normal moveout stretch is stronger than that due to attenuation and may constitute a false indicator of the presence of gas. In fact, often, the low‐frequency shadow overlies events with higher frequencies, in contradiction with the physics of wave propagation. This is particularly evident when the low‐frequency shadow is so extensive that the presence of high frequencies below cannot be justified by the acquisition geometry.