Pre-stack waveform inversion of VHF marine seismic data. A case study in Norway

A quantitative physical model of the shallow marine sediments is of crucial importance in broad range of environmental and engineering contexts, from the assessment of tsunamigenic landslides hazard and off-shore structure stability, to the identification and monitoring of carbon capture and storage sites. However, in contrast to reservoir geophysics, where quantitative seismic interpretation and inversion are extensively employed tools, sub-seabed characterisation is still heavily reliant on direct samplings, using cores, boreholes and CPTUs. In this context, the role of seismic reflection is often limited to a mostly qualitative interpretation of the reflection architecture.Here we will present the first published application of pre-stack elastic full waveform inversion to a Very-High-Frequency (VHF, 0.4 – 2.5 kHz) multi-channel seismic reflection dataset. A custom-built local multi-parameter sequential waveform inversion method has been developed and tested on two common-shot gathers from a multi-channel Boomer seismic profile acquired in northern Norway. The decimetre-resolution impedance and Poisson’s ratio obtained models strongly agree with the a-priori geotechnical and geological information available in the area, proving the potential of the method in obtaining an accurate remote characterisation of the shallow sediments within a reasonable computational cost and using a traditional multi-channel sub-bottom profiler

Characterisation of shallow overpressure in consolidating submarine slopes via seismic full waveform inversion

Pore pressures higher than hydrostatic correspond to localized reductions of the level of shear stress required to induce lateral mass movement in a slope, and therefore play a key role in preconditioning submarine landsliding. In this paper, we investigate whether multi-channel seismic reflection data can be used to infer potentially destabilizing pore-pressure levels at a resolution and sensitivity useful for in-situ slope stability characterization. We simulate the continuous deposition of sediment on consolidating slopes in two scenarios, with combinations of sedimentation rate and permeability distribution leading to disequilibrium compaction. Ultra-high-frequency (UHF; 0.2–2.5 kHz) seismic reflection data are computed for each model and a stochastic full waveform inversion (FWI) method is used to retrieve the sub-seabed properties from the computed seismograms. These are then interpreted as time–depth variations in the effective stress (σ′) regime, and therefore local overpressure ratio and factor of safety, using a combination of p-wave velocity to σ′ transforms. The results demonstrate that multi-channel UHF seismic data can provide valuable constraints on the distribution of physical properties in the top 50 m below seabed at a sub-metric scale, and with a sensitivity useful to infer destabilizing excess pore pressure levels.</p

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State-of-the-art remote characterization of shallow marine sediments: the road to a fully integrated solution

Current methods for characterizing near-surface marine sediments rely on extensive coring/penetrometer testing and correlation to seismic facies. Little quantitative information is regularly derived from geophysical data beyond qualitative inferences of sediment characteristics based on seismic facies architecture. Even these fundamental seismostratigraphic nterpretations can be difficult to correlate with lithostratigraphic data due to inaccuracies in the time-to-depth conversion of geophysical data and potential loss and/or compression of high-porosity and under-consolidated seafloor material during direct sampling. To complicate matters further, when quantitative information is derived from marine geophysical data, it often describes the sediments using terminology (e.g., acoustic impedance and seismic quality factor) that is impenetrable to geologists and engineers. In contrast, for hydrocarbon prospecting, reservoir characterization using quantitative inversion of geophysical data has developed enormously over the past 20 years or more. Impedance and amplitude-versus-angle inversion techniques are now commonplace, whereas computationally expensive waveform inversions are gaining traction, and there is a well-developed interface between these geophysical and reservoir engineering fields via rock physics. In this paper, we collate and review the different published inversion methods for high-resolution geophysical data. Using several case study examples spanning a broad range of depositional environments, we assess the current state of the art in remote characterization of shallow sediments from a multidisciplinary viewpoint, encompassing geophysical, geological, and geotechnical angles. By identifying the key parameters used to characterize the subsurface, a framework is developed whereby geological, geotechnical, and geophysical characterizations of the subsurface can be related in a less subjective manner. As part of this, we examine the sensitivity of commonly derived acoustic properties (e.g., acoustic impedance and seismic quality factor) to more fundamentally important soil properties (e.g., lithology, pore pressure, gas saturation, and undrained shear strength), thereby facilitating better integration between geological, geotechnical, and geophysical data for improved mapping of sediment properties. Ultimately, we present a number of ideas for future research activities in this field

State-of-the-art remote characterization of shallow marine sediments: the road to a fully integrated solution

Current methods for characterizing near-surface marine sediments rely on extensive coring/penetrometer testing and correlation to seismic facies. Little quantitative information is regularly derived from geophysical data beyond qualitative inferences of sediment characteristics based on seismicfacies architecture. Even these fundamental seismostratigraphic nterpretations can be difficult to correlate with lithostratigraphic data due to inaccuracies in the time-to-depth conversion of geophysical data and potential loss and/or compression of high-porosity and under-consolidated seafloor material during direct sampling. To complicate matters further, when quantitative information is derived from marine geophysical data, it often describes the sediments using terminology (e.g., acoustic impedance and seismic quality factor) that is impenetrable to geologists and engineers. Incontrast, for hydrocarbon prospecting, reservoir characterization using quantitative inversion of geophysical data has developed enormously over the past 20 years or more. Impedance and amplitude-versus-angle inversion techniques are now commonplace, whereas computationally expensive waveform inversions are gaining traction, and there is a well-developed interface between these geophysical and reservoir engineering fields via rock physics.In this paper, we collate and review the different published inversion methods for high-resolution geophysical data. Using several case study examples spanning a broad range of depositional environments, we assess the current state of the art in remote characterization of shallow sediments from a multidisciplinary viewpoint, encompassing geophysical, geological, and geotechnical angles. By identifying the key parameters used to characterize the subsurface, a framework is developed whereby geological, geotechnical, and geophysical characterizations of the subsurface can berelated in a less subjective manner. As part of this, we examine the sensitivity of commonly derived acoustic properties (e.g., acoustic impedance and seismic quality factor) to more fundamentally important soil properties (e.g., lithology, pore pressure, gas saturation, and undrained shear strength), thereby facilitating better integration between geological, geotechnical, and geophysical data for improved mapping of sediment properties. Ultimately, we present a number of ideas for future research activities in this field.<br/

Widespread and progressive seafloor-sediment failure following volcanic debris avalanche emplacement: Landslide dynamics and timing offshore Montserrat, Lesser Antilles

Landslides associated with flank collapse are volumetrically the most significant sediment transport process around volcanic islands. Around Montserrat, in the Lesser Antilles, individual landslide deposits have volumes (1 to 20 km3) that are up to two orders of magnitude larger than recent volcanic dome collapses (up to 0.2 km3). The largest landslide deposits were emplaced in at least two stages, initiated by the emplacement of volcanic debris avalanches which then triggered larger-scale failure of seafloor sediment, with deformation propagating progressively downslope for up to 30 km on gradients of <1°. An unusually detailed seismic, side-scan sonar and bathymetric dataset shows that the largest landslide off Montserrat (forming Deposit 8) incorporated ~ 70 m of in-situ sediment stratigraphy, and comprises ~ 80% seafloor sediment by volume. Well-preserved internal bedding and a lack of shortening at the frontally-confined toe of the landslide, shows that sediment failure involved only limited downslope transport. We discuss a range of models for progressively-driven failure of in-situ bedded seafloor sediment. For Deposit 8 and for comparable deposits elsewhere in the Lesser Antilles, we suggest that failure was driven by an over-running surface load that generated excess pore pressures in a weak and deforming undrained package of underlying stratigraphy. A propagating basal shear rupture may have also enhanced the downslope extent of sediment failure. Extensive seafloor-sediment failure may commonly follow debris avalanche emplacement around volcanic islands if the avalanche is emplaced onto a fine-grained parallel-bedded substrate. The timing of landslides off Montserrat is clustered, and associated with the deposition of thick submarine pyroclastic fans. These episodes of enhanced marine volcaniclastic input are separated by relatively quiescent periods of several 100 ka, and correspond to periods of volcanic edifice maturity when destructive processes dominate over constructive processes

Combinations of volcanic-flank and seafloor-sediment failure offshore Montserrat, and their implications for tsunami generation

Recent seafloor mapping around volcanic islands shows that submarine landslide deposits are common and widespread. Such landslides may cause devastating tsunamis, but accurate assessment of tsunami hazard relies on understanding failure processes and sources. Here we use high-resolution geophysical data offshore from Montserrat, in the Lesser Antilles, to show that landslides around volcanic islands may involve two fundamentally different sources of sediment (island-flank and larger seafloor-sediment failures), and can occur in multiple stages. A combination of these processes produces elongate deposits, with a blocky centre (associated with island-flank collapse), surrounded by a smoother-surfaced deposit that is dominated by failed seafloor sediment. The failure of seafloor sediment is associated with little marginal accumulation, and involves only limited downslope motion. Submarine landslide deposits with similar blocky and smoothsurfaced associations are observed in several locations worldwide, but the complex emplacement processes implied by this morphological relationship can only be revealed by high-resolution geophysical data. Such complexity shows that the volume of landslide deposits offshore of volcanic islands cannot simply be used in tsunami models to reflect a single-stage collapse of primary volcanic material. By applying predictive equations for tsunami amplitude to investigate general scenarios of volcanic island landslide generation, we show that the tsunami hazard associated with volcanic island collapse remains highly significant. Volcanic flank failures, even if relatively small, may generate large local tsunamis, but associated seafloor sediment failures, even if they have a much greater volume, have a substantially lower potential for tsunami generation