2D characterization of near-surface V P/V S: surface-wave dispersion inversion versus refraction tomography

International audienceThe joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling

Revealing the Extent of Submarine Permafrost and Gas Hydrates in the Canadian Arctic Beaufort Sea Using Seismic Reflection Indicators

AbstractThe Canadian Arctic Southern Beaufort Sea is characterized by prominent relict submarine permafrost and gas hydrate occurrences formed by subaerial exposure during extensive glaciations in Pliocene and Pleistocene. Submarine permafrost is still responding to the thermal change as a consequence of the marine transgression that followed the last glaciation. Submarine permafrost is still underexplored and is currently the focus of several research projects as its degradation releases greenhouse gases that contribute to climate change. In this study, seismic reflection indicators are used to investigate the presence of submarine permafrost and gas hydrates on the outer continental shelf where the base of permafrost is expected to cross‐cut geological layers. To address the challenges of marine seismic data collected in shallow water environments, we utilize a representative synthetic model to assess the data processing and the detection of submarine permafrost and gas hydrate by seismic data. The synthetic model allows us to minimize the misinterpretation of acquisition and processing artifacts. In the field data, we identify features along with characteristics arising from the top and base of submarine permafrost and the base of the gas hydrate stability zone. This work shows the distribution of the present submarine permafrost along the southern Canadian Beaufort Sea region and confirms its extension to the outer continental shelf. It supports the general shape suggested by previous works and previously published numerical models.Plain Language Summary: Submarine permafrost, ground beneath the seafloor that perennially remains below 0°C, is present on the continental shelf of the Canadian Beaufort Sea. During the Late Pleistocene (∼1 Million years ago), the continental shelf was subaerially exposed to the cold Arctic air causing the formation of ice in the ground. This period was followed by a sea level rise that flooded the continental shelf with warmer waters, resulting in an intensive change of the thermal regime. The relict permafrost still reacts to this thermal change and is continuously thawing. Associated with the presence of relict permafrost, extensive gas hydrates exist to >1,000 m below the seafloor. Climate warming threatens both the stability of permafrost and associated gas hydrates. Their thawing and decomposition can cause a release of greenhouse gases which in turn amplifies climate warming. This study uses marine seismic reflection data to identify permafrost and gas hydrate in the southern Canadian Beaufort Sea. We find indicators of the top and base of permafrost and the base of the gas hydrate stability zone in the outer continental shelf area. Our work shows that the permafrost and gas hydrates still extend to the outer continental shelf and thereby supports previously published numerical models.Key Points: Seismic reflection data reveal occurrences and extent of submarine permafrost and associated gas hydrates at the Canadian Beaufort Shelf Synthetic modeling of permafrost and gas hydrate is required to assess seismic processing minimizing the potential for misinterpretation Indicators of top and base of permafrost and the base of gas hydrate stability support previously published numerical models Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Korean Ministry of Oceans and FisheriesEnvironmental Geoscience Program of the Geological Survey of Canadahttps://dx.doi.org/doi:10.22663/KOPRI-KPDC-00001958.

Revealing the Extent of Submarine Permafrost and Gas Hydrates in the Canadian Arctic Beaufort Sea Using Seismic Reflection Indicators

The Canadian Arctic Southern Beaufort Sea is characterized by prominent relict submarine permafrost and gas hydrate occurrences formed by subaerial exposure during extensive glaciations in Pliocene and Pleistocene. Submarine permafrost is still responding to the thermal change as a consequence of the marine transgression that followed the last glaciation. Submarine permafrost is still underexplored and is currently the focus of several research projects as its degradation releases greenhouse gases that contribute to climate change. In this study, seismic reflection indicators are used to investigate the presence of submarine permafrost and gas hydrates on the outer continental shelf where the base of permafrost is expected to cross-cut geological layers. To address the challenges of marine seismic data collected in shallow water environments, we utilize a representative synthetic model to assess the data processing and the detection of submarine permafrost and gas hydrate by seismic data. The synthetic model allows us to minimize the misinterpretation of acquisition and processing artifacts. In the field data, we identify features along with characteristics arising from the top and base of submarine permafrost and the base of the gas hydrate stability zone. This work shows the distribution of the present submarine permafrost along the southern Canadian Beaufort Sea region and confirms its extension to the outer continental shelf. It supports the general shape suggested by previous works and previously published numerical models. Key Points Seismic reflection data reveal occurrences and extent of submarine permafrost and associated gas hydrates at the Canadian Beaufort Shelf Synthetic modeling of permafrost and gas hydrate is required to assess seismic processing minimizing the potential for misinterpretation Indicators of top and base of permafrost and the base of gas hydrate stability support previously published numerical model

Reflection seismic indicators for submarine permafrost and gas hydrate distributions on the Canadian Arctic Beaufort Shelf

Since the last glaciation the Canadian Arctic Beaufort Shelf is subjected to marine transgression. From subaerial mean annual temperatures during terrestrial exposure of ≤ -20°C, thermal conditions changed up to present submarine bottom water temperatures near -1°C. While conditions during the Pliocene favoured extensive formation of permafrost and gas hydrates, present occurrences are exposed to degradation due to the warmer climate. Today, submerged offshore permafrost is still responding to this thermal change. Ongoing degradation creates the potential of methane release of previously trapped biogenic gas within the relic permafrost and from gas hydrate dissociation. The mobilisation of methane and its possible release to the atmosphere plays a significant role in climate change. Yet, both the extent of permafrost and underlying gas hydrates is still poorly known. Here, we present seismic indicators for offshore permafrost and gas hydrates in 2D multichannel reflection seismic data acquired in the Canadian Beaufort Sea. Seismic lines that run from the shallow shelf towards deeper water show layer-crossing reflections that become gradually shallower towards the north-west into deeper water. These reflections show an amplitude-varying characteristic and are phase-reversed. We first use shot gathers from a synthetic model based on the field seismic acquisition characteristics and borehole geophysical data to verify our general ability to detect permafrost-and gas hydrate-related reflections. The synthetic data were processed using the same data processing applied to the field data and reveal clear top and base of permafrost and gas hydrate reflections. With this encouraging result, we can exclude any potentially misleading processing artefacts in the field seismic data. We interpret the amplitude-varying, phase-reversed and layer-crossing reflections seen in the field data as seismic indicators for the base of permafrost and base of gas hydrates. In contrast to the synthetic data, top of permafrost and top of gas hydrates are not clearly identified in the field data. However, additional seismic indicators support the interpretation of the presence of permafrost including attenuation of acoustic penetration and velocity pull-up effects at presumably horizontal strata. Furthermore, strong amplitude variations beneath the current base of gas hydrates and bright spots indicate trapped free gas accumulations from possible previously dissociated gas hydrates