Introduction

A geohazard is defined as "A geological state, which represents or has the potential to develop further into a situation leading to damage or uncontrolled risk". Geohazards are found in all parts of the earth and are always related to geological conditions and geological processes, either recent or past. Important offshore geohazards (Figure 1) include (i) slope instability and mass wasting processes (including debris flows, gravity flows); (ii) pore pressure phenomena (e.g. shallow gas accumulations, gas hydrates, shallow water flows, mud diapirism and mud volcanism, fluid vents, pockmarks); (iii) seismicity. Excess pore pressure development appears a critical aspect in most of the offshore geohazards.

Figure 1: Schematic diagram showing main offshore geohazards

Submarine slope failure is the most serious threat on both local and regional scales. In addition to damaging offshore installations, slope failures may also cause devastating tsunamis (see ICG-10). ICG personnel have for a long period been involved in the studies of the Storegga Slide area, offshore Mid-Norway. These studies were triggered by the discovery of Europe's third largest gas reservoir Ormen Lange within the slide scar.

The Ormen Lange data base includes seismic data, well logs, pore pressure data, geological data and geotechnical test results, and as such represents one of the most comprehensive multidisciplinary data sets on submarine landsliding from any continental margin. This work has lead to a number of other slope stability R&D projects in recent years.

Pore pressure is a fundamental variable in the behaviour of soil. Despite this, our ability to accurately measure, monitor and predict pore pressures in offshore sediments is limited, and rarely done. Therefore, it is important to improve our understanding of excess pore pressure genesis (processes, migration), accurate measurement and its implications.

Further, it is hoped that tools can be developed allowing regional excess pore pressure fields to be mapped in detail, for example through geophysical methods, geological interpretation or observational or survey techniques. Once regional excess pore pressure fields are detected, then sensors and instrumentation systems designed for both short-term measurements and long-term monitoring may make specific measurements.

In summary, the three critical themes with ICG-6 are:

• Assessment of offshore geohazards in the upper tens to hundreds of metres below the seabed on high-latitude continental margins
• Geophysical methods for offshore geohazards
• Understanding and determining pore pressure in offshore sediments

Objectives
The overall scientific objectives within the Offshore Geohazards projects are:

• Multi-disciplinary approach: establish correlations between different data sets (i.e. geological, geotechnical and geophysical) in order to improve the assessment and prediction of geohazards
• Geophysical methods and tools to extract geotechnical information and pore pressure (e.g. S-waves and surface-waves)
• Gas and gas hydrates: increase our knowledge about their effects on slope instability
• Assess the implications of gravity flows on seabed infrastructure and prevention

Active Research Activities within ICG-6

Assessing Offshore Geohazards: Site surveying and sampling of shallow, submarine landslides in coastal and deepwater environments

High-resolution geophysical surveying led recently to the discovery of a myriad of smaller-scale mass movements in different geological environments along the Norwegian margin, from coastal areas (Finneidfjord) to open oceans in intermediate water depths (Vesterålen) and deepwater (Lofoten) (for locations, see Figure 2). The landslides are characterized by variable geomorphological complexity, and occur in either clusters of isolated landslides (Finneidfjord, Vesterålen) or as part of a larger, multi-phase slope failure (Lofoten).

The open-water landslides lie relatively close to larger-scale mass movements or canyon systems. The slip planes often lie at shallow depth, e.g. within the top 10 m of sediment. Geophysical data indicate that the landslides are likely recent phenomena, that failure happened repeatedly and possibly frequently in different sedimentary environments. Despite the smaller dimensions, their release may affect coastal communities (tsunamis, onshore retrogression) or have dramatic consequences for seabed infrastructure.

Little more is known about the sedimentary processes, pre-conditions, similarities and differences of these landslides, except that anthropogenic factors cannot be discarded for the Finneidfjord area. It is therefore important to further investigate these slope failures using an integrated approach. The three outstanding issues are to (1) identify and parameterize the slip planes in the different settings; (2) understand the processes involved in failure (sedimentary processes, stratigraphy, lithology and chronology); and (3) assess the present-day stability and geohazard risk.

To this end, a multi-disciplinary data set was collected, consisting of high-quality Calypso cores and several gravity/piston, a wealth of geophysical data (very-high-resolution 2D topas and 3D chirp seismic, ocean-bottom seismometer data, swath bathymetry, side-scan sonar), and cone penetration test data (including dissipation tests). Geological (non-destructive MST logging, XRF-scanning, dating, etc.) and geotechnical laboratory analyses (index tests, fall cone undisturbed and remoulded shear strength, oedometer, triaxial shear strength and direct simple shear tests) are being conducted to shed light on these issues.

Examples of the target areas are presented in Figures 3 to 6.

Figure 2: Overview map of the Norwegian margin, with the study areas for the CDog (SEABED) and LOslope (NFR) project marked.

Figure 3: Survey map illustrating the data acquisition accomplished onboard R/V Seisma (NGU) and G.O. Sars (Univ. Bergen) in the Summer of 2010 for the CDog project.

Figure 4: Example of VHR-3D chirp seismic profile (in-line) in Finneidfjord across the landslide deposit (central lobe) with unprecedented resolution. The VHR-3D system is courtesy of NOCS.

Figure 5: Swath bathymetry (MAREANO) and examples of HR-2D topas seismic profiles across landslides off the Vesterålen margin (CDog project).

Figure 6: Dip magnitude extracted from part of the swath bathymetry data (R/V Jan Mayen) collected off the Lofoten margin (LOslope project) with location of the Calypso core used for geological and geotechnical analysis.

Natural laboratories:

• Part of these activities also serve to further develop Finneidfjord into a natural laboratory. The underlying reasons are its accessibility and the fact that Finneidfjord hosts the three major geohazards identified by the offshore industry, being (1) (multi-phase) slope failures on a series of well-defined slip planes with block generations; (2) high sedimentation rates and thus likely excess pore pressure generation, and (3) the presence of a prominent gas front in the immediate vicinity of the landslides. Part of the basin infill shows well-stratified sequences which are ideal for testing a variety of high- to very-high-resolution geophysical mapping.
• Similarly, and even more accessible, is Trondheim bay. Also here, landsliding is a recurrent phenomenon, and it is known that part of the land reclaimed from the sea over the course of the last decades followed by urbanization (Trondheim harbour) subsides more than adjacent areas.

Support:
SEABED consortium* - CDog project; ICG + University of Bergen, National Oceanography Centre Southampton, MARUM University of Bremen
Norwegian Research Council - LOslope project; ICG + University of Tromsø
Joint ICG/NGU funding of Post-doc research position (Dr. Jean-Sebastien L'Heureux, NGU)
NFR/NRC funding for Ph.D. fellowship (Nicole Baeten, UiT) and Senior Researcher (Dr. Jan Sverre Laberg, UiT)
* Partnership of several oil companies with licenses offshore Norway, with the aim to improve the understanding of geomorphology and geology of the region, in particular the upper 500 m of sediment.

Geophysical Methods and Modelling for Geohazards characterization

Shear wave profiling
Conventional P-wave seismic data are paramount to obtain a detailed image of the shallow subsurface where most offshore geohazards occur. However, P-waves are not suited to extract geotechnical information, such as shear strength, in the absence of boreholes. P-waves sense both the solid matrix and the pore fluids. Shear waves on the other hand are primarily influenced by the matrix and to a lesser degree by pore fluids.

Despite the more complicated data acquisition and processing techniques, shear wave data have a number of advantages over P-wave data, e.g. the possibility to "see" through gas clouds, lithology and fluid discrimination, and overpressure quantification (shallow water flows). Even though S-wave velocity increases rapidly in the shallow subsurface, their values are still low compared to P-wave velocities, which suggest that S-wave data have higher vertical resolution, even though the typically lower quality factor for S-waves offsets this to some degree.

ICG activities on this level are two-fold.

First, NGI developed a prototype seabed-coupled shear wave vibrator with flexible polarization direction, which was used at specific locations in the North Sea in collaboration with Statoil (Figure 7). In combination with conventional airguns and coupled ocean-bottom cables, this would allow recording the full seismic wavefield. Part of the processing, in particularly the surface wave part (see underneath, under publications), were taken care of by ICG. Shear wave data analysis is complemented with NGI's in-house full-waveform simulations LaySac (Figure 8).

Second, in collaboration with LIAG (Hannover) and ICG-Theme 1, a dedicated land 2.5D shear wave survey was conducted in the harbour of Trondheim, an area claimed for the sea and undergoing active subsidence, close to landslide features documented in the fjord. A shear wave vibrator and specially-designed geophone cable was used for the acquisition, in SH mode, courtesy of LIAG. The multi-channel data are fully processed at ICG (Figure 9). The final processing step, pre-stack depth migration, is currently undertaken (Sauvin et al., in prep.).

Figure 7: Sketch and picture of NGI's prototype shear wave vibrator for seabed-coupled surveying

Figure 8: Full waveform synthetic seismic data (acceleration) simulating the seismic response of an overpressured zone (shallow water flow) at about 175 m below the surface, using a seabed-coupled shear wave source.

Figure 9: Example of shear wave shot gathers (top) and processed (post-stack depth migrated) section with colour-coded shear wave velocity. The sketch illustrates the relationship between the onshore and offshore geophysical data.

Surface wave data
Using the geometrical dispersion that characterise surface waves (i.e. phase velocity becomes frequency or wavelength dependent) allow to determine shear wave velocity of the shallow sub-surface in high detail. To this end, surface wave data are typically recorded by a linear array of closely spacing geophones/seismometers, using vibrator, impact source, sledgehammer or dropping a sandbag as a source.

Most of the energy generated will be emitted as surface waves that penetrated about half to a wavelength down in the sub-surface. Whereas all the wavelengths travel through the top part, only few wavelengths manage to penetrate to larger depth, leading to a mix-determined problem. The wave equations yield a discrete number of solutions for surface wave modes at given frequency: the fundamental mode and higher surface wave modes. Dependent on the source and receivers, either Rayleigh/Scholte waves or Love waves are be detected, with the former being a combination of P- and SV-waves and the latter solely SH-waves trapped.

The marine shear wave data revealed several higher surface wave modes of both Scholte and Love polarizations, a unique seismic data set. These were subsequently used to invert for a shear wave profile with depth, in collaboration with Polytecnico di Torino (Figure 10). The data were further used to fine-tune and validate NGI's internal multi-modal surface wave inversion code, which is based on an optimised curve-fitting routine and Marquardt-Levenberg algorithm, and which inverts for both shear wave velocity and layer thickenss (Figure 11). Other surface wave tools available at ICG are the swami code (Rix and Lai), Geopsy (Wathelet) and SurfSeis (KGS), as well as in-house MatLab/Octave routines.

Figure 10: (Top) Multi-modal Love surface waves obtained using a densely populated effective OBC and NGI's prototype shear wave vibrator at the Gjøa field, North Sea; (bottom) Resulting shear-wave velocity profile from inversion (Vanneste et al., Socco et al., in press).

Figure 11: Results from NGI's multi-modal surface wave inversion algorithm. (Top) Multi-modal Scholte/Rayleigh data (dots) and best-fit forward models (lines); (bottom) Resulting shear-wave velocity profile from inversion.

Can wavelet transforms be used to infer excess pore pressures?
High-resolution seismic reflection data from part of Finneidfjord are characterised by a distinct shallow gas front overlying a zone of intense amplitude attenuation (Figure 12). The quality factor of a medium expresses how well a medium propagates seismic energy, here P-wave, and is composed of intrinsic attenuation and scattering. The presence of free gas affects causes both intrinsic attenuation and scattering, and the frequency-dependence of partially-saturated soils can be analysed through continuous wavelet transforms (Li, et al., Geophysics, 2006), which yields improved time-frequency characterization and localization, compared to Fourier or Gabor transforms.

This technique was successfully applied to synthetic data generated with NORSAR2D, and translated to the Finneidfjord case study, using a Mexican Hat Wavelet. The two horizons picked were the gas front and a peak interpreted to correspond with somewhat coarser material. The inversion scheme by Carcione & Picotti (Geophysics, 2006) is further used invert for pore pressure, and subsequently calibrated with in situ piezometer data acquired on site (Morgan et al., 2009). This ongoing work needs further fine-tuning and QC. This work furthermore heavily depends on accurate physical properties of the soils, i.e. another reason why high-quality sampling is paramount.

Figure 12: Excess pore pressure estimated at the top of the shallow gas accumulation in Finneidfjord, from inversion of seismic amplitudes after wavelet decomposition and rock physical models.

Basin modelling
Modelling the generation and migration of excess pore pressure provides important information on slope stability aspects and risk assessment. Basin2 (now obsolete) was used at NGI/ICG to investigate pore pressure development within the North Sea Fan (NSF), off south Norway (Figure 13). In this area, located off the mouth of the Norwegian Channel, an extensive glacial fan has accumulated. Loading of the soils by the infill of glacigenic debris flows during peak glacial times and subsequent lateral excess pore pressure migration is believed to have played a very important role in the Storegga Slide about 8200 years ago.

Modelling has indicated that, at present, excess overpressures still exist within the NSF, but the magnitude has decreased compared to peak glacial conditions. It must be stressed that site-specific soil profiles should be used in basin modelling, as default soil profiles do not necessarily represent the soil conditions adequately. The source code of Basin2 is still available at NGI, and will be extended to use improved relationships based on laboratory testing at the given site under investigation.

Figure 13: Modelling of pore pressure evolution for the NSF can study, using Basin2

A Selection of Past Research Activities

Euromargins - Slope stability on Europe's PAssive COntinental Margins (SPACOMA)
This project was organised under the European Science Foundation (ESF) and aims at an improved understanding of continental margin slope stability in different geological settings along the European margin, from the river-fed depositional systems along the south-European margins of the Atlantic and the Mediterranean, to the high Arctic north of Svalbard. Research on correlation between geological and geotechnical parameters as well as the effect of gas hydrates on geotechnical properties were parts of ICG activities. This also included tsunami modelling.

Ormen Lange - Significant effort was put into finalising the results from the four years of studies under the Ormen Lange project. The results are published in a special issue of Marine and Petroleum Geology (Solheim et al., 2005), and the parts directly related to this project cover mainly different geological aspects related to the Storegga Slide, as well as the pore pressure issue.

NGI's internal project SIP-8 (Offshore Geohazards) - This project was run in parallel with the ICG project, but with a close cooperation to ensure mutual benefit for the various activities. SIP-8 emphasis was placed on geotechnics and the development of methods and tools, such as an improved deepwater coring system. Another ICG related activity was to investigate the potential of seismic attributes to predict sedimentological and geotechnical properties, using Ormen Lange as case study.

ASSEM (Array of Sensors for long term Seabed Monitoring of geohazards) - This EU-funded project aimed at developing instruments for measurement and long-term monitoring of physico-chemical and geotechnical properties, such as pore pressures, gas content, etc.,  in the seafloor the upper geological layers. Pore pressure measurements conducted at Finneidfjord were part of this initiative.

Educational components

• ICG is a research and educational organization, open to other international research institutes and facilities. At present, the following M.Sc., Ph.D and post-doc researchers are affiliated to ICG-6
• Nicole Baeten - Loslope project (Ph.D. student, Department of Geology, University of Tromsø, Norway)
• Eugene Morgan - Inversion of seismic data to determine pore pressure (Ph.D. student, Geohazards Engineering Research, Tufts University, Medford, USA)
• Guillaume Sauvin - Geophysics for Geohazards (Ph.D. student, NORSAR)

Selected Theses, Publications and Presentations
Kvalstad, T. (2007). What is the current "best practice" in offshore Geohazard Investigations? A State-of-the-Art Review. Offshore Technology Conferene, OTC-18545.

L'Heureux, J.-S. (2009). A multidisciplinary study of shoreline landslides: from geological development to geohazard assessment in the bay of Trondheim, Norway. Ph.D. thesis, NTNU.

Morgan, E., Vanneste, M., Longva, O., Lecomte, I., McAdoo, B., Baise, L. (2009). Evaluating gas-generated pore pressure with seismic reflection data in a landslide-prone area - an example from Finneidfjord, Norway. Proceedings, 4th International Symposium on Submarine Mass Movements and Their Consequences, Austin (USA), November 2009, pp. 387-398.

Polom, U., Hansen, L., Sauvin, G., L¿Heureux, J.-S., Lecomte, I., Krawczyk, C., Vanneste, M., Longva, O. (in press). High-resolution SH-wave reflection seismic for characterization of onshore ground conditions in the Trondheim harbour, central Norway. Near-Surface Geophysics.

Strout, J.M., Tjeltja, T.I. (2007). Excess pore pressure measurement and monitoring for offshore instability problems. Offshore Technology Conference, OTC-18706.

Socco, L.V., Maraschini, M., Boiero, D., Vanneste, M., Madshus, C., Westerdahl, H., Duffaut, K., Skomedal, E. (revised). On the use of NGI's prototype seabed-coupled shear wave vibrator for shallow soil characterization - Part II: Joint inversion of Love and Scholte waves. Geophysical Journal International.

Vanneste, M., Harbitz, C.B., De Blasio, F.V., Glimsdal, S., Mienert, J., Elverhøi, A. (in press). The Hinlopen-Yermak landslide, Arctic Ocean - Geomorphology, Landslide Dynamics and Tsunami Simulations. In The Importance of Mass-Transport Deposits in Deepwater Settings, Eds. Shipp, C., Weimer, P., Posamentier, H., Society of Sedimentary Geology, Special Publication.

Vanneste, M., Madshus, C., Socco, L.V., Maraschini, M., Sparrevik, P.M., Westerdahl, H., Duffaut, K., Skomedal, E. (revised). On the use of NGI's prototype seabed-coupled shear wave vibrator for shallow soil characterization - Part I: Acquisition and processing of multi-modal surface waves. Geophysical Journal International.

Zakeri, A. (2008). Submarine debris flow impact on pipelines. Ph.D. thesis, University of Oslo.

ICG Scientists - Alphabetic order
Shyam Chand, marine geophysics (NGU)
Carl Fredrik Forsberg, marine geology (NGI)
Tore Jan Kvalstad, geotechnics, technical expert (NGI)
Isabelle Lecomte, geophysics (NORSAR)
Jean-Sebastien L'Heureux , marine geology and geotechnics (NGU)
Oddvar Longva, marine geology (NGU)
Guillaume Sauvin, Geophysics (NORSAR)
Anders Solheim, marine geology (NGI)
James M. Strout, geotechnics, instruments, pore pressures (NGI)
Maarten Vanneste, P.I., marine geophysics and geology (NGI)

Affiliated Scientists/Institutes
Eugene Morgan, Tufts University, Medford, USA
Mark E. Vardy, National Oceanography Centre Southampton, UK
Ulrich Polom, LIAG, Hannover, Germany
Achim Kopf, MARUM University of Bremen, Germany
Jan Sverre Laberg, Nicole Baeten, University of Tromsø, Norway
Haflidi Haflidason, Berit O. Hjelstuen, University of Bergen, Norway
Valentina L. Socco, Polytecnico di Torino, Italy
Arash Zakeri, C-Core, Canada