Maarten Vanneste

Gas hydrates at the Storegga Slide: Constraints from an analysis of multicomponent, wide-angle seismic data

Geophysical evidence for gas hydrates is widespread along the northern flank of the Storegga Slide on the mid-Norwegian margin. Bottom-simulating reflectors (BSR) at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 , outside but also inside the Storegga Slide scar area. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P- and S-wave velocities of hydrate- and gas-bearing sediment layers. The relationship between the velocities constrains the background velocity model for a hydrate-free, gas-free case. The seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3% and 6% in a zone that is as much as 50 m thick overlying the BSR. Hydrates are most likely disseminated, neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably. Average free-gas concentrations beneath the hydrate stability zone are approximately 0.4% to 0.8% of the pore volume, assuming a homogeneous gas distribution. The free-gas zone underneath the BSR is about 80 m thick. Amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone (BGHS). This gives rise to enhanced reflections that terminate at the BGHS. The stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations. Gas-bearing layers can be as thin as 2 m.

Identification of Weak Layers and Their Role for the Stability of Slopes at Finneidfjord, Northern Norway

The 1996 Finneidfjord landslide, which took four human lives in northern Norway, initiated along a weak layer in the fjord-marine sediments before developing retrogressively across the shoreline. The integration of results from sediment cores, free-fall cone penetrometer tests and high-resolution 3D seismic data indicates that the slide-prone layer is a regional bed likely sourced from clay-slide activity in the catchment of the fjord. The sediments in this regional layer are softer and more sensitive than the typical bioturbated, fjord-marine deposits, which explains their role in slope instability. In addition, biogenic gas in the stratified event bed may further affect its geotechnical properties. Similar, fine-grained, stratified beds with comparable origin and properties occur in other Norwegian fjords. They are presumably also present along coastlines of other previously glaciated margins, where they could contribute to mass movements.

Origin of shallow submarine mass movements and their glide planes—Sedimentological and geotechnical analyses from the continental slope off northern Norway

Submarine landslides are often characterized by a basal surface of rupture parallel to the stratigraphy, in which downslope movement is initiated. However, little is known about the sedimentology and physical properties of the sediments within these surfaces. In this study, we present a multiproxy analysis of the sediments collected from a giant piston core penetrating a shallow submarine mass transport deposit, in combination with high-resolution seismoacoustic data to identify and characterize the basal glide plane and the weaker sediments in which movement was initiated. The initial phase of instability consists of a single fracture that formed due to the downslope movement of a mostly intact slab of sediments. The 16 m long core, comprising mostly undisturbed massive and laminated ice-rafted debris-rich clay penetrated this slab. The base of the slab is characterized by a high-amplitude semicontinuous reflection visible on the subbottom profiler data at about 12.5 m depth, interpreted to originate from the glide plane on top of a plumite deposit. This plumite has dilative behavior with pore pressure decrease with increasing shear strain and high undrained shear strength. Movement probably started within contouritic sediments immediately above the glide plane, characterized by higher sensitivities and higher water contents. The occurrence of the mass movements documented in this study are likely affected by the presence of a submarine landslide complex directly downslope. The slide scar of this landslide complex promoted retrogressive movement farther upslope and progressive spreading of strain softening along the slide base and in the slide mass. Numerical models (infinite slope, BING, and retrogressive slope models) illustrate that the present-day continental slope is essentially stable and allow reconstruction of the failure processes when initiated by an external trigger.

Arctic gas hydrate provinces along the western Svalbard continental margin

In 2001, a high-resolution seismic survey was conducted for the detailed study of the distribution, both spatially and vertically, of gas hydrate and free gas accumulations west of Svalbard, as part of the HYDRATECH and INGGAS projects. High-resolution single-channel seismic reflection and the 4-component ocean-bottom seismometer (OBS) data illustrate the widespread nature of gas hydrates and free gas accumulations north of the Knipovich Ridge off Western Svalbard, by the presence of a nearly-continuous polarity-reversed bottom-simulating reflection (BSR) on down-slope seismic profiles. In the absence of a distinct and/or a continuous BSR, it is the sudden change in reflection amplitude and frequency content that marks the base of the hydrate zone. The BSR coincides with the top of the free gas zone. Compressional wave velocity analyses and modelling reveal increased velocities above the BSR attributed to a gradual increase of partial hydrate saturation (6–10% of pore volume). A sharp drop in compressional-wave velocity across the BSR is due to free gas accumulation. The sub-bottom depth of the BSR closely matches the calculated stability limit for methane hydrates. To the east of the Knipovich Ridge, mud diapirism is observed in a deeper basin (∼2250 m water depths). The domes rise from an extensive chaotic source zone buried under a 200–400 ms thick sediment drape, and are more pronounced in the south. At some places, there is evidence of stratigraphically-controlled shallow gas accumulations (bright spots) and short cross-cutting BSR-like features that might point towards the presence of hydrate and/or free gas. The diapiric movement is believed to be a recent and still ongoing process of mass mobilisation. In both the cases, the nearby and tectonically-active slow-spreading, Knipovich Ridge is assumed to play an important role in the generation of elevated heat and methane fluxes as well as faulting and subsequent fluid migration. As a result, shallow subsurface hydrates (

Shallow Landslides and Their Dynamics in Coastal and Deepwater Environments, Norway

In this manuscript, we present the first results of integrated slope stability studies to investigate smaller-scale mass movement processes in different physiographic settings of Norway. These include coastal areas (Sorfjord, Finneidfjord), and pristine open ocean settings in intermediate (Vesteralen) and deep waters (Lofoten) on the Norwegian margin. Triggers, pre-conditioning factors and sedimentary processes associated with these landslides are currently not well constrained.

Towards geophysical and geotechnical integration for quick-clay mapping in Norway

Quick clay is a known hazard in formerly-glaciated coastal areas in e.g., Norway, Sweden and Canada. In this paper, we review the physical properties of quick clays in order to find a suitable, integrated and multi-disciplinary approach to improve our possibilities to accurately identify the occurrence of quick clay and map its extent both vertically and laterally. As no single geophysical method yields optimal information, one should combine a variety of geophysical methods with geotechnical data (in situ measurements using Cone Penetration Testing (CPTU), Seismic CPTU (SCPTU) and Resistivity CPTU (RCPTU); laboratory tests) for an in-depth quick-clay assessment at a given site. In this respect, geophysical data are used to fill the gaps between geotechnical boreholes providing ground-truth. Such an integrated and multi-disciplinary approach brings us closer to 2D or pseudo-3D site characterization for quick clays and as such, an improved assessment of the potential hazard they pose. The integrated approach is applied in practice on two Norwegian quick-clay sites. The first site, Hvittingfoss, was remediated against potential landslides in 2008 whereas the second one, Rissa, was the scene of a major quick-clay landslide in 1978, quick clays being still present over a large area. The collected data and preliminary site characterizations illustrate the high diversity as well as the complexity and clearly emphasize the need for higher resolution, careful imaging and calibration of the data in order to accomplish the assessment of a quickclay hazard.

An In-Situ Free-Fall Piezocone Penetrometer for Characterizing Soft and Sensitive Clays at Finneidfjord (Northern Norway)

The identification and characterization of weak layers, potentially acting as detachment planes, are key elements in submarine landslide research. In this study, the MARUM Free-Fall Piezocone Penetrometer (FF-CPTU) was used to characterize soft and sensitive clays at Finneidfjord, a site with historical landslide events. The FF-CPTU measurements are compared with standard, industry Cone Penetration Testing (pushed CPTU) data in order to verify and validate the penetration rate effect by using an empirical closed-form solution to convert dynamic properties to quasi-static ones. The quasi-static properties and sedimentological/laboratory results across the weak layers show significantly lower values for the CPTU parameters (qt and fs) and undrained shear-strength (su), build-up of excess pore-water pressure (∆u) as well as a normally-consolidated behavior in comparison with the surrounding sediment. These findings allow us to develop a 2D model of the sub-surface in which the weak horizon is mapped, and to demonstrate that the light-weight FF-CPTU instrument is a powerful, versatile device for the geotechnical evaluation of submarine mass movements and their consequences.

Investigations of Slides at the Upper Continental Slope Off Vesterålen, North Norway

Multibeam bathymetry, high-resolution seismic profiles and sediment cores were collected from the upper continental slope outside Andoya, the northernmost island in Vesteralen, northern Norway (69°N). Eight small slides are identified at water depths between 500 and 800 m. These are linked to a larger slide related to the development of the Andoya Canyon by high resolution seismic. Slope angles adjacent to the headwalls of the small slides are 3–4° while slide deposits have accumulated where slope angles are 2–3°. The slides occurred in parallel-stratified glacial marine sediments, and three seismic horizons are interpreted. One of these horizons coincides with failure planes in four of the slides. A 12 m long core terminates above this level, but penetrates another horizon representing a slip plane in two of the slides. The core comprises silty clay with varying content of ice rafted debris. The upper slope shallower than 450 m appears stable although it may be up to 8° steep. Tension cracks up to 2 m deep on the lower slopes may suggest that deformation is active near the canyon systems.

Gas hydrate dissociation and sea-floor collapse in the wake of the Storegga Slide, Norway

Two-dimensional seismic data from the Mid-Norwegian margin provide evidence for sediment liquefaction and fluid mobilisation within the sediments that were located at the base of the hydrate stability zone before the Storegga Slide occurred. The disturbed subsurface sediments are overlain by a prominent roll-over structure and sea-floor collapse. This indicates fluid escape from the formerly hydrated sediment and suggests that the landslide caused a pressure drop strong enough to dissociate, the gas hydrates. We calculate that this fluid escape must have taken place within less than 250 years after the slide, as the effect of pressure decrease on hydrate stability was later compensated by a temperature decrease, related to the slumping process. The volume of expelled fluids from the collapse structure exceeds the volume of the gas hydrate dissociation products, implying that gas hydrate dissociation significantly affected the surrounding sediments.

The 1930 Landslide in Orkdalsfjorden: Morphology and Failure Mechanism

The 1930 landslide and associated 15-m high tsunami along the shore of Orkdalsfjorden, mid Norway, killed one person and caused major damage to port facilities. The combination of witness testimony with swath bathymetry data, high resolution seismic reflection data and gravity cores show that the failure propagated rapidly (up to 25 m/s) and progressively over a clay layer in a retrogressive manner. The volume of sediment evacuated downslope of the 8–12 m high and 3 km long headwall amounts 18.5 · 106 m3 during this event. The transformation of the failed mass into a sediment gravity flow caused subsequent slope failures on the opposite side of the fjord and the breakage of submarine cables at distances of 3 and 18 km away from the initial landslide.