Mark E. Vardy

Multiple widespread landslides during the long-term evolution of a volcanic island: insights from high-resolution seismic data, Montserrat, Lesser Antilles

New high‐resolution multichannel seismic data (GWADASEIS‐2009 and JC45/46‐2010 cruises; 72 and 60 channels, respectively) combined with previous data (AGUADOMAR‐1999 and CARAVAL‐ 2002; 6 and 24 channels, respectively) allow a detailed investigation of mass‐wasting processes around the volcanic island of Montserrat in the Lesser Antilles. Seven submarine deposits have sources on the flanks of Montserrat, while three are related to the nearby Kahouanne submarine volcanoes. The most voluminous deposit (∼20 km 3) within the Bouillante‐Montserrat half‐graben has not been described previously and is probably related to a flank instability of the Centre Hills Volcano on Montserrat, while other events are related to the younger South Soufriere Hills‐Soufriere Hills volcanic complex. All deposits are located to the south or southeast of the island in an area delimited by faults of the Bouillante‐Montserrat half‐graben. They cover a large part of the southeast quarter of the surrounding seafloor (∼520 km 2), with a total volume of ∼40 km 3. Our observations suggest that the Bouillante‐Montserrat half‐graben exerts a control on the extent and propagation of the most voluminous deposits. We propose an interpretation for mass‐wasting processes around Montserrat similar to what has happened for the southern islands of the Lesser Antilles.

Morphodynamic evolution of Kongsfjorden¿Krossfjorden, Svalbard, during the Late Weichselian and Holocene

Abstract We present a combination of fjord bathymetry and shallow seismic data from Kongsfjorden and Krossfjorden, Svalbard, to characterize and analyse change in the fjord coastal environment physiography and the glaciosedimentary processes since the Last Glacial Maximum. Swath bathymetry reveals a series of several styles of landform, frequently superimposed upon each other, permitting the reconstruction of the relative timings of deposition of each landform with the oldest successively overlain and cross-cut by younger landforms and erosional processes. Large transverse ridges interpreted as recessional moraines are overlain by streamlined lineations formed subglacially during a subsequent ice advance. A complex of recessional morainal ridges occurring within the central fjord are incised by glacial lineations and meltwater channels from younger glacial events.

Deriving shallow-water sediment properties using post-stack acoustic impedance inversion

In contrast to the use of marine seismic reflection techniques for reservoir-scale applications, where seismic inversion for quantitative sediment analysis is common, shallow-water, very-high-resolution seismic reflection data are seldom used for more than qualitative reflection interpretation. Here, a quantitative analysis of very-high-resolution marine seismic reflection profiles from a shallowwater (<50 m water depth) fjord in northern Norway is presented. Acquired using Sparker, Boomer, and Chirp sources, the failure plane of multiple local landslides correlates with a composite reflection that reverses polarity to the south. Using a genetic algorithm, a 1D post-stack acoustic impedance inversion of all three profiles is performed, calibrating against multi-sensor core logger (MSCL) data from cores. Using empirical relationships the resulting impedance profiles are related to remote sediment properties, including: P-wave velocity; density; mean grain size; and porosity. The composite reflector is consistently identified by all three data sources as a finer-grained (by one φ), lower density (c. 0.2 g/cm3 less than background) thin bed, with an anomalous low velocity zone (at least 100 m/s lower than background) associated with the polarity reversal to the south. Such a velocity contrast is consistent with an accumulation of shallow free gas trapped within the finergrain, less permeable layer. This study represents the first application of acoustic impedance inversion to very-high-resolution seismic reflection data and demonstrates the potential for directly relating seismic reflection data with sediment properties using a variety of commonly used shallow seismic profiling sources.

Direct monitoring of active geohazards: emerging geophysical tools for deep-water assessments

Seafloor networks of cables, pipelines, and other infrastructure underpin our daily lives, providing communication links, information, and energy supplies. Despite their global importance, these networks are vulnerable to damage by a number of natural seafloor hazards, including landslides, turbidity currents, fluid flow, and scour. Conventional geophysical techniques, such as high-resolution reflection seismic and side-scan sonar, are commonly employed in geohazard assessments. These conventional tools provide essential information for route planning and design; however, such surveys provide only indirect evidence of past processes and do not observe or measure the geohazard itself. As such, many numerical-based impact models lack field-scale calibration, and much uncertainty exists about the triggers, nature, and frequency of deep-water geohazards. Recent advances in technology now enable a step change in their understanding through direct monitoring. We outline some emerging monitoring tools and how they can quantify key parameters for deepwater geohazard assessment. Repeat seafloor surveys in dynamic areas show that solely relying on evidence from past deposits can lead to an under-representation of the geohazard events. Acoustic Doppler current profiling provides new insights into the structure of turbidity currents, whereas instrumented mobile sensors record the nature of movement at the base of those flows for the first time. Existing and bespoke cabled networks enable high bandwidth, low power, and distributed measurements of parameters such as strain across large areas of seafloor. These techniques provide valuable new measurements that will improve geohazard assessments and should be deployed in a complementary manner alongside conventional geophysical tools.

How to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record: Insights from active submarine channels

Submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea. Modern observations show that submarine channels can be sculpted by supercritical turbidity currents (seafloor sediment flows) that can generate upstream-migrating bedforms with a crescentic planform. In order to accurately interpret supercritical flows and depositional environments in the geologic record, it is important to be able to recognize the depositional signature of crescentic bedforms. Field geologists commonly link scour fills containing massive sands to crescentic bedforms, whereas models of turbidity currents produce deposits dominated by back-stepping beds. Here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations, time-lapse seabed mapping, and sediment cores, thus providing the link from flow process to depositional product. These data were collected within the proximal part of a submarine channel on the Squamish Delta, Canada. We demonstrate that bedform migration initially produces back-stepping beds of sand. However, these back-stepping beds are partially eroded by further bedform migration during subsequent flows, resulting in scour fills containing massive sand. As a result, our observations better match the depositional architecture of upstream-migrating bedforms produced by fluvial models, despite the fact that they formed beneath turbidity currents.

Quantification of near-bed dense layers and implications for seafloor structures: new insights into the most hazardous aspects of turbidity currents

Turbidity currents pose a serious hazard to expensive oil and gas seafloor installations, especially in deep-water where mitigation, re-routing or repair is costly and logistically challenging. These sediment-laden flows are hazardous because they can be exceptionally powerful (up to 20 m/s), and can flow for long distances (>100s km) over several days duration, causing damage over vast areas of seafloor. Even less powerful flows (~1-2 m/s) can damage seafloor equipment, or break strategically important submarine telecommunication cables. The consequences of turbidity currents impacting seafloor structures depends on the velocity, duration, direction of impact and, perhaps most crucially, the sediment concentration (or density) of the flow. While some recent studies have successfully monitored turbidity currents in deep-water, imaging flow properties close to the seafloor has proven problematic. We present innovative approaches to the quantification of the velocity and sediment concentration of dense near-bed layers that provide new insights into this important aspect of turbidity current flow. Firstly, we describe a novel experimental setup that is capable of measuring near-bed sediment concentration in dense (>10% volume by concentration) flows. Density contrasts are measured using Electrical Resistivity Tomography – a technique initially developed for geophysical characterisation of subsurface reservoirs. Velocity is measured using Ultrasonic Doppler Velocity Profiling and concentration is characterized using an Ultra High Concentration Meter. Secondly, we outline some recently developed geophysical approaches for the quantification of sediment concentration and velocity for real-world flows based on recent work in fjords, estuaries and deep-sea canyons. This includes integrated moored deployments of Acoustic Doppler Current Profilers, Multibeam Sonars, and a novel Chirp array. We outline some limitations and advantages of these methods. Finally, we underline the value and importance of establishing multiple field-scale test sites in a variety of settings, including deep-water, that will enhance the industrys understanding of turbidity current hazards. Our results demonstrate the importance of near-bed dense layers for turbidity current interaction with seafloor structures. Density contrasts and pressure build up at the base of a flow may lead to uplift, undermining and loss of support, dragging, or pipeline rupture; hence quantification of this layer is crucial for hazard assessment. Measurements of sediment concentration within turbidity currents are incredibly rare, and yet are a vital input for any numerical model that aims to predict sediment transport by turbidity currents in deep-water settings. Currently it is necessary to infer densities and velocities; however, such inferences are poorly calibrated against experimental or real world data. Our measurements underline the importance of understanding near-bed dense layers.

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 interpretations 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.

Seismic Inversion for Site Characterization: When, Where and Why Should We Use It?

The application of seismic inversion techniques to the foundation and drilling top hole zones has garnered significant interest in recent years. The shift towards more geologically complex and deeper water sites, combined with the global economic climate, has driven a requirement for more cost-effective site characterisation. More often used by the exploration industry, seismic inversion has been touted as a potentially valuable tool for quantifying the spatial and depth variability in sediment properties. In doing so, this approach can reduce the risk of encountering unforeseen ground conditions and the need for excessive over-design. Despite its potential, the inversion of high-resolution seismic data has yet to see widespread use, leaving unanswered questions regarding how and where this tool can best fit into the site characterization work flow. We test the potential usefulness of seismic inversion using a range of existing site investigation data sets. We apply several different inversion methods, including acoustic impedance and seismic quality factor inversion, as well as artificial neural network multi-attribute regression, to tackle end-member potential uses. First, explore early-phase potential uses, showing how seismic quality factor and acoustic impedance inversion can be used to capture the spatial variability in facies architecture and bulk sediment properties that could be used in appraisal and pre-FEED studies to optimize borehole and penetrometer (CPT) depths/locations and to ensure effective site-wide characterization. Second, we apply a combined acoustic impedance and artificial neural network workflow to link seismic properties with CPT profiles. These results demonstrate the potential late-phase use of seismic inversion for short-range interpolation/extrapolation of more complex geotechnical properties through the generation of synthetic CPT profiles useful for infrastructure design and micro-siting late in the development cycle. While not a comprehensive list of applications, together these examples illustrate how seismic inversion can be utilized throughout the development cycle. If the required objectives are clearly defined and an appropriate inversion workflow developed, seismic inversion can help to reduce uncertainty in site-wide characterization and drive efficiencies in layout and design studies throughout a project lifetime.

Characterization of the slope-destabilizing effects of gas-charged sediment via seismic surveys

Finneidfjord, Norway has a history of submarine slope failure and hosts areas of buried, gas-charged sediment. Using shipboard seismic survey data from Finneidfjord, we illustrate how novel seismic data processing techniques can yield estimates of geotechnical properties of this gas-charged sediment. This data processing involves two steps: 1) estimate the amount of seismic attenuation that a gas-bearing layer creates, and 2) fit a wave attenuation model to the observed attenuation. We accomplish the first step using a wavelet-based spectral ratio approach, and find the seismic quality factor (inversely related to attenuation) from the change in amplitude spectra across the gas layer. The novelty of this paper really comes in the second step, where we create a Bayesian hierarchical model that combines a wave attenuation model with a spatial random (Gaussian) process model. The advantage of this combination is that our estimates of the gas properties not only must conform to the observations of attenuation (quality factor), but must also smoothly vary over space, as one would expect of any natural process. Ultimately, we end up with posterior distributions of gas properties at points throughout our gas-bearing layer. We use these posterior distributions to probabilistically evaluate the role that gas plays in slope failures in Finneidfjord. This method has other applications in natural gas resource exploration and carbon sequestration.

Geomorphology and remote physical properties of late Quaternary slide structures using decimetre-resolution 3D seismic volumes: insights for deep water gteohazard assessment

Traditional exploration methods, involving a combination of two-dimensional seismic profiles with cores and/or swath bathymetry/side-scan sonar, do not adequately sample all three spatial dimensions for true morphological mapping of submarine mass movement deposits. It is only with the acquisition of three-dimensional (3D) seismic volumes that the complex 3D nature of these features can be correctly imaged. Over the last 5-10 years, the interpretation of industry-scale 3D seismic volumes from numerous continental shelf locations has allowed effective mapping of the deposit morphology. Features such as head scarps, side walls, extensional/compressional ridges, striations on the basal surface, translated blocks, and preservation/deformation of internal reflectors have been shown to constrain the direction and method of material transport. Here we demonstrate the application of these techniques to the shallow-water environment using decimeter-resolution 3D seismic volumes, with case studies from Trondheimsfjorden (Norway) and Lake Windermere (UK Lake District). Through the mapping of top/base reflector morphologies, internal structure, translated blocks, and head scarps/side walls we demonstrate the same techniques can be used to differentiate flow mechanics (coherent slide blocks, slumped material, debris flows, and a mass flow) and quantify direction of motion at this radically different scale.