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Project 6: Offshore Geohazards
Project Manager: Maarten Vanneste (NGI)
Introduction
The project has placed
main focus on the following themes
-
Assessment of offshore
geohazards
-
Geophysical methods for
identification and mapping of offshore geohazards
-
Pore pressures in
offshore sediments
A geohazard can be
defined as “a geological state that 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.
The focus on offshore
geohazards has increased greatly over the last decade, as the offshore
petroleum industry has moved into increasingly deeper areas and an
increasing proportion of the field installations are placed directly on
the seafloor.
Important offshore
geohazards and research topics include (see also figure 1):
-
Slope instability
-
Shallow gas
-
Natural gas hydrates
and their climate-controlled dissociation
-
Shallow water flows
-
Mud diapirism and mud
volcanism
-
Active fluid seepage
and seafloor pockmark formation
-
Seismicity – which may
trigger slides – may cause tsunamis and disturb subsurface geological
conditions, e.g. pore pressures
-
Excess pore pressure
development, in relation to fluid migration and sediment accumulation
-
Assessment of
geotechnical properties of seabed sediments
-
Mitigation and
prevention of geohazards
Figure 1: Schematic
diagram showing main offshore geohazards (© Tore J Kvalstad)
Slope instability is
considered the most serious offshore threat on both local and regional
scales. Large submarine landslides may – in addition to damaging
offshore installations – cause tsunamis with devastating consequences to
third parties and socio-economic activities in coastal areas. ICG
personnel have for a long period been involved in the studies of the
Holocene Storegga Slide, offshore Mid-Norway. A huge data base has been
acquired and analysed in order to ensure the safe development of the
large “Ormen Lange” gas field, Norway’s second largest gas reservoir
which is located within the slide scar. The examination of a large
geophysical data set has furthermore revealed that mega-landslides have
occurred regularly over the last 500 ka in this area. As a consequence,
research activities performed under the “Offshore Geohazards” project
have been - and will still be - related to investigating the stability
and consequences within the Storegga Slide and North Sea Fan areas. The
Ormen Lange data base include seismic data, well-logs, pore pressure
data, geological data and geotechnical test results, and as such
represents one of the best and most comprehensive data sets from any
continental margin which can to some extent be used in similar studies
on other glaciated margins. Hence, with regards to physiographic
setting, the main focus has been placed on high-latitude, (formerly)
glaciated continental margins, in particular the Norwegian-Barents Sea-Svalbard
continental margins on which submarine landslides appear to be an order
of magnitude larger compared to other landslides in other (e.g.
river-fed) settings.
Figure 2: Sketch showing important factors
for instability in the Storegga Slide area.
Pore pressure is one of
the fundamental variables in the geomechanical behaviour of soil and
soil structures (e.g. slopes). This is related to the reduction in
effective stress and thus shear resistance when excess pore pressure
develops in sedimentary basins. Despite the importance of excess pore
pressure in geohazards, the ability to measure, monitor and predict
these pressures in offshore sediments is limited. Therefore, we aim at a
better understanding of pore pressures, both in regards to sources or
processes leading to the development and migration of excess pressures
and the effect that excess pressures (positive and negative) have on the
engineering properties of soils. Furthermore, it is hoped that tools can
be developed allowing regional excess pore pressure fields to be mapped
or modelled, 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. It is important to
stress that subsurface fluid migration as the result of pressure
gradients is a complex 3D process which is closely related to the
sedimentation history, structural setting and lithology, and its proper
assessment will require basin modelling. A key example is the North Sea
Fan at the outlet of the prominent Norwegian Channel which has acted as
the dominant sediment supply route during the Last Glacial Maximum. The
massive and rapid accumulation of glacial sediments on the upper
continental slope has resulted in excess overpressures which have
subsequently migrated laterally and were trapped within the Storegga
Slide complex, where failure then occurred well after glacial retreat.
This clearly indicates the need of regional and integrated studies for
improving our understanding of slope instability processes and pore
pressure development in particular.
Objectives
The overall scientific
objectives within the Offshore Geohazards project are:
-
To identify important
gaps in our knowledge.
-
To establish improved
correlations between the different data types (geological,
geotechnical and geophysical) in geohazard investigations, and to
improve the assessment and prediction of geohazards from knowledge of
geological history of an area.
-
Improve our ability to
perform robust submarine slope stability evaluations and understanding
potential trigger mechanisms related to submarine slides in a
high-latitude continental margin setting.
-
To improve the use of
geophysical data to better characterise and image the subsurface and
to extract high-quality geotechnical information (e.g. soil
shear-strength), particularly with regards to the application of
S-waves and surface-waves.
-
To improve spatial
geophysical mapping both at acquisition (survey design planning),
processing and imaging level, in which a key word is controlled
illumination for better vertical and lateral resolution. Modelling is
important for this task.
-
To increase our
knowledge about the effects and detection of shallow gas and gas
hydrate occurrences, which in addition to slope instability form the
other serious offshore geohazard along the NW European continental
margins.
-
Development of
theoretical and practical tools for identification of pore pressures
and understanding their influence on slope stability and other
geohazards problems.
-
To improve risk
assessment and mitigation processes.
-
Rather than performing
case studies, we must focus on integrated investigations of specific
landslides in which geology and geomorphology, slide dynamics and
tsunami simulations form key stepstones, and which will bring together
different ICG activities.
It is furthermore
important that the results obtained within this project – in combination
with the other activities of ICG – receive the necessary public
outreach. Therefore, increased efforts are undertaken to report ICG
activities and results to the international scientific community by
conference presentations (e.g. Offshore Technology Conference, European
Geophysical Union, American Geophysical Union, etc.) and workshops (e.g.
IGCP511) as well as publications in highly-rated peer-reviewed
scientific journals.
Research Activities,
Cooperation and Links within the project’s lifetime
ICG aims at establishing
cooperation with relevant partners and projects. Additionally,
cross-disciplinary work activities and integration between the different
Research Projects at ICG has gained momentum. At present, the following
activities and projects have direct links to the ICG Offshore Geohazards
project:
The Seabed Project
This Seabed project is run by a partnership of initially 11 (and now 9)
oil companies, all with license interests offshore mid-Norway. The aim
is an improved understanding of the geomorphology as well as the geology
of the region, in particular with regards to the shallow (upper ca.500
m) geology and geohazards. A new research activity for developing the
Finneidfjord as a natural laboratory for offshore geohazard
investigations is currently under review.
Ormen Lange studies
Significant effort has been put into the finalising of the results from
the four years of in-depth 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. The data as well as the results are
available for new research activities within ICG.
Euromargins – Slope
stability on Europe’s PAssive COntinental Margins (SPACOMA)
This project is organised under the European Science Foundation (ESF)
and funded through the Norwegian Research Council. A key aspect of the
research activities within this project aim 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 Ocean and the Mediterranean
Sea, to the high-latitude glacier-fed margins of the Arctic Ocean north
of Svalbard. Research included swath-bathymetry mapping and
geomorphological studies, the correlation between geological and
geotechnical parameters and to a lesser degree the effect of gas hydrate
on geotechnical properties.
Within this project we
have created the possibility for in-depth integrated research activities
by bringing together a dynamic team specialised in geology,
geomorphology, slide dynamics and tsunami simulations. We have devoted
special attention to two specific offshore geohazard areas, both with
their own distinct specification. The first integrated study dealt with
the spectacular Hinlopen Slide exposed on the northern Svalbard margin.
This slide is characterised by enormous headwalls exceeding 1400 m high,
and rafted blocks which are an order of magnitude larger than in other
landslide areas, and thus pose a challenge to understand their mobility
and slide mechanisms. The second integrated study looks into the North
Sea Fan area off the Norwegian Channel, adjacent to the Storegga Slide.
In this area, excess pore pressure due to rapid sediment accumulation
during the last peak glaciation has been documented which could
facilitate failure. Since this area is of great importance in Norway’s
offshore activities and society, incipient landsliding and the
possibility for tsunami generation should be scrutinized, as it may
present a serious socio-economic threat if failure were to occur.
This project will be
finished in 2007, though the compilation of the research articles is
ongoing.
Figure 3: (A) Colour-coded swath bathymetry image (c. 4795 km2)
overlain with 10 m depth contours of the Hinlopen Slide scar area
located on the northern Svalbard margin off the mouth of the Hinlopen
Trough (HT, blue arrow). The numbered tracks nicely illustrate the
different characteristics of the escarpments, typically hundreds to an
unprecedented >1,400 m high. Numbers along the tracks represent slope
angles. The western (B) and eastern (C) headwall areas are displayed
with a 1:1 vertical exaggeration. Giant rafted blocks have been moved to
about 60 km downslope the headwall area (D). The largest rafted block
contains 1.89 km3 of volume and rises 452 m about the
surrounding seabed. The swath-bathymetry data were acquired within the
EUROMARGINS project by the University of Tromsø. (© Vanneste et al.,
Earth and Planetary Science Letters, 2006). This data set and its
geomorphological interpretation has been used as input into slide
dynamic and tsunami models.
NGI’s internal project
SIP-8 (Offshore Geohazards)
This project is run in parallel with the ICG project, but with a very
close cooperation to ensure mutual benefit for the various activities.
SIP-8 emphasis is placed on geotechnics and the development of methods
and tools, such as an improved deepwater coring system. The results of
this internal project – even though completed by now – are highly
important in other research activities. This Strategic Institute
Programme has contributed greatly to our understanding and assessment of
offshore geohazards, with the main benefits in seabed instability
mechanisms, modelling of mass flow and fluid transport, tools and
methods for assessment of material properties and design parameters,
quantification of uncertainties and frequencies of geohazards, as well
as their consequences.
ASSEM (Array of
Sensors for long term Seabed Monitoring of geohazards)
This EU-funded project, which is now completed, aimed at developing
instruments for measurement and long-term monitoring of properties, such
as pore pressures, gas content, etc., in the seafloor and the uppermost
geological layers. NGI contributed significantly to this project by
their responsibility for the development of the sensor technology,
seabed instrument systems and full-scale field testing. One of the ICG
field activities (in Finneidfjord, see below) was run in direct
cooperation with ASSEM.
GANS (NFR research
project): Does hydrate dissociation affect the seabed’s stability ?
Geophysical data from numerous continental margins worldwide have
indirectly indicated the presence of vast amounts of natural gas
hydrates within the uppermost hundreds of m of seabed sediments. These
remarkable ice-like structures are composed of water molecules encaging
individual gas molecules (most often methane). Naturally-occurring gas
hydrate accumulations are believed to play a significant role in (1) the
global carbon cycle, (2) climate change since methane – an active
greenhouse gas – is the dominant host in hydrates, (3) as a potential
future energy resource, and (4) as a potential contributor to slope
failure.
Hydrate stability
requires high pressures and low temperatures (fig. 3), but also depends
on the availability of sufficient amounts of gas and water to stabilise
the crystal structure. Along continental margins, hydrates are often
found in water depths typically exceeding 300 m, and their stability
limit lies at depth where the sub-surface temperature intersects with
the 3-phase thermodynamic gas-water-hydrate stability curve. Hydrate
stability modelling has illustrated that hydrate accumulations are
meta-stable, i.e. they depend on the temperature and pressure regime and
their variations through time. The highest sensitivity of hydrates
occurs (1) at their basal limit of occurrence and (2) in the shallower
water areas where the effect of climate-controlled sealevel and
bottom-water temperature changes are most pronounced. In addition, when
hydrate dissociates, its structure releases an excess volume of gas
which – entrapped within the sediments – will create overpressures. This
excess overpressure is highest in shallow waters. As a consequence, the
biggest threat for hydrate dissociation to facilitate slope failure is
found in the shallower water areas. In some areas, this critical zone
fits well with the location of landslides escarpments, for example on
the northern flank of the Storegga Slide area, albeit that the Storegga
Slide most probably developed retrogressively, which would give less
importance to hydrate dissociation as a potential trigger for slope
failure.
Despite the fact that
excess pore pressure results in the reduction in effective stress – and
thus weakens the soil’s resistance to failure – we lack quantitative
models of the role hydrate dissociation plays. The difficulty of forming
laboratory samples with hydrates in clayey material is one of the key
obstacles. Within the GANS project, a 3-years research project
(2007-2009) within the framework of PETROMAKS, Norwegian Research
Council and Seabed Phase III, NGI/ICG’s main focus is to investigate and
quantify the effect of gas hydrate and its dissociation on the
geomechanical stability of the sediments. More specifically, we will
work on developing material methods for the behaviour of clay during and
following hydrate dissociation, guidelines for evaluation of seabed
stability and foundation design, and simulation of possible impacts of
hydrate dissociation on soil properties and stability.
Fig. 4: Gas hydrate stability along the continental margin and its
variation since the Last Glacial Maximum (LGM, orange curves) to the
Present-Day (PD, dark blue curves). Panel A: bottom-water temperatures
along a slope transect for both glacial (LGM) and interglacial (PD)
times used in the model, taken from estimates and present-day
measurements along the mid-Norwegian margin. Panel B: Methane hydrate
stability limits for the LGM and PD, using the bottom temperature
functions shown in panel A and a steady-state geothermal gradient of
0.05°C/m, typical for the mid-Norwegian margin. Sealevel for the LGM is
120 m lower than today. Panel C: Sub-bottom depths of the methane
hydrate stability for PD and LGM. Panel D: Difference plot of the
methane hydrate stability limit between the LGM and PD, with positive
values for increasing stability conditions (subsidence of the base of
the hydrate stability zone) and negative values representing a shoaling
stability limit. This graphic illustrates that climate-induced changes
on the hydrate stability zone results in more stable hydrate conditions
in the deeper areas (water depths exceeding about 800 m) due to the
pressure effect (rising sea levels with limited bottom-temperature
change), whereas the shallower areas are less stable due to the inflow
of warmer water masses offsetting the positive effect of sea level rise.
Panel E: When methane hydrate dissociates, the solid matrix is replaced
by an incompressible volume of water and a compressible volume of
methane, which is compressed due to hydrostatic pressure. The total
volume of fluids released upon dissociation relative to the volume of
initial hydrate is highest in the shallower water areas and decreases
downslope, adding to the sensitivity of hydrate due to climate change in
the shallower water areas (modelling © Maarten Vanneste).
Gas seeps mechanisms
and monitoring (NGI internal research project)
It is not uncommon for gas leakage to occur around wells and platform
foundations upon exploration or recovery. In some cases these gases leak
to the seabed along the casings or as gas blow-outs, whereas in other
cases they fill up permeable layers in the shallow sub-surface. Such
blow-outs have often resulted in the formation of large seabed craters
and can well create stability issues for offshore installations in the
area even long after the gas expulsion has come to an end. In order to
limit and avoid damage due to gas leakage, it is important to survey and
monitor leakage as well as install the downhole targets away from weak
zones which can be used as migration pathways for the fluids. It is also
worth noting that fluids reaching the seabed could affect the
geochemical environment through bacterial breakdown of the gas.
Subsurface fluid
migration is basically a three-phase process, in which both gas and
liquid (pore water) play a role through the solid sediment grains of
different – often layered – lithologies, porosities and permeabilities.
Gas pockets could hinder drainage from underlying strata and thus slow
down compaction, which could affect the sediment’s strength.
Additionally, gas pockets themselves may exhibit excess pore pressure,
higher than the pressure in the pore water phase. What actually happens
when gas is injected into the shallow sub-surface is however not well
known.
It is therefore important
to monitor and warn for subsurface fluid leakage around installations as
well as survey the sites in detail afterwards, which requires an
assessment of the right equipment as well as strategy. One option would
be the monitoring of pore pressure at specific target locations and
depths, whereas time-lapse seismic experiments could provide both
qualitative and quantitative information about subsurface fluid
migration over time. Another key parameter could be the inversion of
high-resolution P- and S-wave data for both geotechnical parameters and
sub-surface velocities and convert these into pore pressure estimates.
The objectives within
this internal project are thus (1) to improve our understanding of
upward fluid migration following e.g. gas blow-outs along boreholes or
within sediments (fractures or faults, weak zones, etc.); (2)
investigate the influence of gas release on pore pressure, either
following leakage or as permeability barriers; and (3) analyse and
propose strategies and possibly specific measurement or imaging tools
which could be used for surveying and mitigation about possible
sub-surface leakage.
Basin modelling (NGI
internal research project)
As mentioned above, pore pressure stands out as one of the key
parameters governing slope stability, and understanding its evolution
through time in a particular geological setting should be considered a
priority for offshore geohazards. Since sedimentation patterns and
geological features are not uniaxial or symmetrical, fluid migration and
pore pressures are essentially multi-phase 3D processes involving both
liquids (pore water) and gas. Therefore, basin modelling becomes
paramount in offshore risk assessment, and requires that accurate
knowledge about seismic stratigraphy of the study area is integrated
into the model design.
As present, NGI/ICG has
limited basin modelling tools which need to be upgraded to 2D and
ultimately 3D models, incorporating sedimentation, compaction and pore
pressures. Initially, existing software and source code will be
analyses, after which the source code will be extended to full 3D mode.
Ultimately, we aim at coupling basin modelling with geographical
information systems (GIS) and integrated with 3D seismic data.
Integrated studies of
landslides on the Norwegian-Barents Sea-Svalbard continental margin
Based on seismic data and
geotechnical and borehole data acquired under the Seabed and Ormen Lange
Projects, submarine landslides other than the Storegga Slide are being
studied and compared to the more well-known Storegga Slide, as well as
the younger (4000 ka) Trænadjupet Slide. The Storegga Region has
experienced repeated sliding through the last 0.5 million years, and the
activity aims at understanding the causes for this and its implications.
It is however not the only remarkable feature on the Norwegian-Barents
Sea-Svalbard continental margin. Special attention is devoted on
integrated studies, in which the geological-geomorphological-geophysical
data are used to model slide dynamics, which in turn provide the
necessary input into tsunami simulations. Ultimately, such studies may
reveal socio-economic important results by addressing risk assessment.
As such, these integrated studies shed light on the entire slope failure
process, from failure to deposits, dynamics and consequences. Results of
such multi-disciplinary integrated studies which cross the borders
between the individual ICG projects are emerging, for example on the
remarkable Hinlopen landslide north of Svalbard (manuscript submitted,
review manuscript in preparation) and the North Sea Fan area, off the
Norwegian Channel (manuscript in preparation).
Studying
geo-parameters on high-latitude continental margins
This study is done in cooperation with the Euromargins – SPACOMA
project and learned from the outcome of another EU-research project
COSTA (Continental Slope Stability), and involves building up a data
base of geological, geotechnical and geophysical parameters from
continental margins with submarine slides, and to perform statistical
analyses of the data. So far, mainly data from the mid-Norwegian margin
have been analysed, but this will also be expanded with data from other
areas, including the high Arctic, and the Mediterranean. As part of this
activity, ICG is carrying out geotechnical analyses of sediment cores
from a large submarine slide to the north of Svalbard. In addition,
tsunami modelling is being performed for the Arctic Ocean (under a
sister ICG project) to evaluate the effects of such events in an
ice-covered Ocean. Also here, manuscripts are published, submitted or in
preparation.
Figure 5: Parameter correlation from the
Storegga area
Improving seismic
resolution, imaging and seismic attributes
In the past, a key focus of the Offshore Geohazards was placed
on geophysical processing techniques to enhance imaging in the upper few
hundred meters of the offshore deposits, in particular on geohazard-related
features. Submarine slopes often fail along "weak" layers, which can be
very thin. The identification of such layers is therefore important, it
requires high-resolution and unobscured data. Extraction of quantitative
information for geotechnical purposes from the seismic data is also an
aim of the geophysical research at ICG. Seismic wave propagation close
to the seafloor and at shallow depths is complex. Understanding wave
propagation at shallow depths below the seafloor is a necessity before
acquiring and processing data. The project looked into various modelling
tools to improve seismic acquisition, processing and imaging for
identification of shallow, thin and weak layers. Attenuation, which is
more important at shallow depths, decreases the amplitude of the waves
and reduces their frequency content, hence degrading the resolution.
Additionally, the occurrence of even small amounts of gas severely
distorts the data. Most of the standard techniques in seismic processing
assume a rather simple earth model. Improved velocity models at shallow
depths as well as model-based, amplitude-preserving processing are
needed. A project was initiated with the aim of improving the results of
high resolution acquisition for shallow depths with a model-based
processing and imaging approach. Seismic resolution is a critical issue
for interpreters. Point-Spread Functions (PSF), which is the response of
a point scatterer after depth imaging, can be an aid in seismic imaging.
With an estimate of the PSF at each location of a heterogeneous
structure, the seismic response of that structure can be estimated
assuming that reflectors can be represented as a set of point scatterers
(exploding reflector concept). Based on work conducted for petroleum at
NORSAR, a new method simulating the results of seismic imaging will be
used to check various offshore geohazards models. A method to compensate
for resolution effects in a 2D deconvolution process has also been
developed, partly at ICG and in cooperation with NORSAR and UiO, and is
now ready for ICG applications. This activity – in combination with
previous activities on seismic attributes – should be integrated and
extended with our efforts on shear wave data, and how these can
contribute to a better characterization of the subsurface (especially
when shallow gas is present), but also to allow improved focusing on
deeper targets and reservoirs.
Figure 6: Ormen Lange. a) Geotechnical
wells data. b) 1D density model inferred from a). c) PFSs at different
depths for 0km offset. d) PSFs at different depths for 1km offset.
Figure 7: Gullfaks. a) Ideal synthetic
reflectivity section. b) Corresponding PSDM (see text) section at far
offset without pulse deconvolution. c) PSDM section at far offset with
pulse deconvolution. d) PSDM section at far offset with complete
resolution compensation (new method).
The use of S-waves in
geohazard studies
Whereas conventional
high-resolution P-wave seismic data are absolutely necessary to obtain a
detailed image of the shallow subsurface where most offshore geohazards
occur, P-wave data only represent a part of the acoustic wavefield and
they are not sufficient to extract important geotechnical information,
such as shear strength. This calls for the use of shear wave data in
combination with P-wave data in the offshore realm. Whereas P-waves
sense both the solid matrix and the pore fluids, shear waves 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
of which the most important one are the possibility to “see” through gas
clouds, lithology and fluid discrimination, and overpressure
quantification. 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.
Within ICG we intend to
use such integrated data set for a more detailed investigation and
analysis of shallower subsurface, with emphasis on potential geohazards
(e.g. shallow gas, gas hydrates, shallow water flows, etc.). The
underlying idea is to adopt S-wave as well as surface wave
characteristics, forward modelling as well as inversion for obtaining
detailed wave velocity distribution, dispersion and shear modulus
values, thereby enabling the indirect estimation of important
geotechnical parameters, such as shear strength. Also
parametric/numerical studies will be performed, in order to investigate
the sensitivity of P- and S-wave data for the detection of e.g. gas
zones and shallow water flows as prime geohazards.
Whereas shear wave
information can theoretically be derived from wide-angle seismic
experiments with a surface towed source-receiver system (despite it
being complicated by the sometimes subtle changes in reflectivity
encountered) and P-to-S converted seismic experiments with surface-towed
source and ocean-bottom geophone registration (non-symmetrical,
velocity-dependent common-conversion point mode), pure shear waves
generated and recorded at the seabed will add another dimension and
allow for flexible source orientation, which is necessary for
investigating anisotropy analysis. Illumination and synthetic seismic
studies have also revealed that PP, SVSV and SHSH give comparable
subsurface coverage and illumination footprints, better than converted
waves. To this end, NGI has developed a large shear wave prototype
vibrator with flexible polarization direction to be used for offshore
hydrocarbon exploration and subsurface characterization. The fact that a
significant portion of the emitted energy of the seabed coupled vibrator
will generate surface waves, the source offers the potential for
geotechnical characterization as well, which is unique for offshore
investigations. Another added value of such a shear wave vibrator source
is the possibility to use it for shear wave VSP (vertical seismic
profiling), a class of seismic measurement for obtaining higher-resolution
images with geophones installed within the borehole.
This pilot project is now
well underway and substantial progress has been made. Geophysical data
were first acquired from the Gullfaks Field in the North Sea, and
recently (May 2007) a highly successful survey was conducted on the Gjøa
field, also in the North Sea, in collaboration with Statoil. These data
are currently being processed and modelling/inversion procedures are
being developed. Subsequent to the results of the Gjøa data set, we
intend to down-scale the presently large and heavy S-wave vibrator to a
smaller and therefore more flexible version for operation from smaller
research vessels, including academic vessels, for geohazard studies.
This commercial activity will be lined up with ICG’s research
activities. We will look into accurate shear-wave data processing
routines in combination with P and P-to-S converted data, optimised
survey planning (including NORSAR’s 2D/3D ray tracing software) and its
use in the interpretation of geohazards. We have presented and published
our background study at OTC2007, and intent to present the first results
in peer-reviewed journals and international conferences. These
activities should culminate into a renewed effort to attract funding for
the development of the more flexible, smaller, shear wave source for the
Fall of 2007, under the umbrella of ICG.
Our research activities
on shear waves and surface waves also led to the establishment of a
Memory of Understanding with U. Polom of the Leibniz Institute in
Hannover, Germany, guided by NORSAR.
Figure
7: Overview of potential applications using S-waves (either by shear
wave generation or conversion from incident P-waves) and P-wave data for
imaging, subsurface characterization and quantification.
Figure 8: S-wave source. a) Sketch of the shear wave vibrator source
design developed at NGI. b) Oil exploration prototype (NGI)
Surface seismic waves
in geohazard studies
Whereas surface waves are widely used for land applications, they are
hardly utilised in the offshore domain. The development of the NGI shear
wave vibrators from which an important part of the energy generates
surface waves however will make it now possible to apply a similar
methodology for geotechnical investigations offshore as on land. The
underlying idea is (1) to acquire multi-channel shot gathers, (2) to
perform detailed frequency, amplitudes and phase analysis, (3) to
extract dispersion curves (phase velocity vs. frequency) and (4)
ultimately invert the dispersion curve data and back-calculate for shear
wave velocity below the surveyed area, (5) which can then be translated
– often through (semi-)empirical relationships into shear strength of
the sediments.
The Spectral Analysis of
Surface Wave (SASW) technique has been studied to check its feasibility
and applications to offshore geohazards, with a particular focus on
detection of (thin) weak layers below stiff layers, or sandwitched
between two stiff layers in the seafloor (1D layering only). So far it
has been found out that the SASW technique is not a perfect choice to
apply, but the technique with a simple inversion has a good potential to
get a first (or initial) prediction for shear wave velocity profiles of
the seafloor, which may be used for more advanced inversion algorithms
where the initial guess values are a dominant factor.
Impact studies:
Submarine debris flow impact on seafloor installations (pipelines and
piles)
This
research project (with partial involvement of Statoil) is run by ICG
PhD-student Arash Zakeri and takes on the challenging problem of
submarine debris flow impact on a variety of seafloor installations
through both a series of experimental analysis as well as computation
fluid dynamic (CFD) modelling. The experimental work has been programmed
in two stages. The first stage involves the determination of drag forces
on submerged pipelines upon the impact of underwater slurry flows which
was carried out between March and May 2007 at St. Anthony Falls
Laboratory at Minnesota University in Minneapolis. The second stage is
planned to start in March 2008, and will look into a pile study with
similar slurry experiments. Arash has contributed significantly within
this field with a number of ICG reports on the impact of debris flows on
structures (literature review), a review of computational fluid dynamic
theory behind such impacts and he is also preparing a note on
rheological properties. Arash is currently in the process of compiling a
number of scientific papers on these issues. This impact study is a
close collaboration with ICG’s Slide Dynamics programme. This research
is expected to culminate by Arash’s dissertation by December 2008.
The integrated
picture: From Geology to Rock Physics in geohazard studies
In geohazard assessments, geo-technicians need to know a few important
parameters, such as the shear strength, to be able to estimate the
stability of the structure (thin and weak layers, over-pressured layers,
gas hydrate and free gas distribution and accumulation mode, etc). If
well data exist, correlations can be made between seismics and some of
these geo-technical parameters. However, seismic data are the only one
left in the larger part of the zone of interest as few wells are
available. It is therefore important to have a good understanding of the
regional geological processes making up the sequences as well as how
rock physical parameters affect the elastic properties of the soil,
especially at shallow depth where unconsolidated sediments are found.
The elastic properties of shallow sediments
depend on
-
Constituent (sand-clay)
properties.
-
Mixing process of
constituents, i.e. load-bearing matrix, grain sorting, clay at grain
contacts or in pore space, etc.
-
Degree of compaction
-
Degree of cementation
and cement properties
-
Pore fluid properties,
saturation and distribution
Specific studies are
needed for shallow depths as oil exploration has been concentrating on
much deeper structures. Rock physics models for unconsolidated sediments
have been implemented and tested, based on the Hertz-Mindlin model for
granular materials with the effect of clays taken into account by the
Hashin-Shtrikman averaging method. The fluid effect is finally predicted
by the Gassmann equations. Cementation due to e.g. gas hydrates, may
also be included using the Dvorkin approach. Using log data from Ormen
Lange, investigations are on-going to check if standard trends in
porosity, pressure and temperature can be used to characterize the data.
The calibrated rock physics model can be used to predict geophysical
properties associated to overpressure, gas hydrates and shallow gas, and
investigate effects in seismic data.
This implies that an
in-depth integration of geology, geophysics, rock physics and soil
mechanics should be envisioned, which will lead to quantitative seismic
interpretation and improved geotechnical and subsurface
characterization.
Fig. 9. Flowcharts illustrating the integration of geology, geophysics,
rock physical models and soil mechanics to move from subject specific
analysis towards a quantitative seismic interpretation and subsurface
characterization (including geotechnical aspects). Blue labels refer to
individual disciplines, green labels to parameters, red labels annotate
improvements when incorporating shear wave data into the analysis and
interpretation. Red lines indicate where the use of shear wave
information is vital. (© Vanneste et al., OTC2007)
Field experiment in
Finneidfjord, Northern Norway
This activity will be run in close cooperation with and a follow-up of
the ASSEM project. Stability problems in the seabed sediments are known
to exist at this location, and a landslide occurred from land to sea in
1996, which killed two people. Conditions at the site are representative
of 'offshore' sites with stability problems; however, the Finneidfjord
site has two important advantages: first, its accessibility and second,
the existence of a rich database of geological and geophysical data. The
latter includes swath bathymetry seafloor images, high-resolution
seismic profiling, sediment coring. Additionally, seabed monitoring
instruments have been deployed, and data analyses are underway. As such,
this location was defined as the ideal field laboratory for continued
studies as well as testing of equipment.
It is ICG’s wish to
further develop the geotechnical and geological understanding of this
key location for research in subsea geohazards, in particular related to
soil stability and the behaviour of gaseous soils. The area may well
serve as a field laboratory for further studies and testing of
equipment. This project will – most hopefully – start in 2008, after the
initial starting phase has been delayed. This site would be perfectly
suited for testing for example a new shear wave source.
Figure 11: Map, swath
bathymetry (upper right) and seismic profile (lower) from the Finneidfjord field experiment. The monitoring instrument packages were
deployed at the two yellow stars in the map (upper left)
GIS analysis tools
applied to offshore areas
Present swath bathymetry data have improved to the level where GIS
tools, commonly used on land, can be utilised in analyses and
classification of submarine slides. The techniques may be tested in a
part of the Storegga Slide, and developed further. The aim is to
establish this as a technique which subsequently can be used for
analyses of potentially slide-prone areas in planning of seafloor
facilities, such as pipelines, etc. It will be important to create a
bridge between often used GMT (Generic Mapping Tools) and GIS and work
towards a common strategy with integrated solutions.
Looking ahead:
Potential deep drilling and new research proposals on submarine
landslides
Preliminary activities and contacts have been established to prepare a
new EU proposal on continental margin stability investigations, in which
ICG/NGI could play a crucial role considering our expertise in for
example pore pressure measurements, soil modelling and geotechnical
competence, as well as the advanced in-house laboratory testing
possibilities.
iODP, or the integrated
Ocean Drilling Program, is organising a workshop on drilling in
geohazard areas, in Portland (USA), August 2007. The Offshore Geohazards
project of ICG has expressed its interest and engagement in this
activity, and contributes with a white paper and several presentations
for this workshop. This workshop will provide an ideal platform to
introduce ICG and strengthen its international reputation on submarine
geohazards. It is expected that this workshop, in combination with the
“Deep Seafloor Frontier” foresight paper will set the stage for new
proposals and potential drilling targets, in which special attention
will probably be devoted to new monitoring and measuring technologies.
At the same time, efforts
are made for deeper involvement of NGI/ICG in deep drilling proposals
and support for other geohazard-related drilling proposals, and extend
our further expertise to non-glaciated margins.
Educational
components
ICG is essentially a
research and educational organization. This implies that we are always
keen of establishing contacts with other research institutes and
facilities on the international scene where exchange of students and
guest researchers is important. A Memory of Understanding between ICG
and the University of Tromsø, National Oceanography Centre
(Southampton), Liebniz Institute (Hannover) and other institutes
furthermore should enhance collaboration and involvement in geophysical
surveying methods and offshore slope stability issues. All topics
considered are suitable for MSc and Ph.D students. At present PhD
student Arash Zakeri is affiliated to this project. One French MSc. is
completed in seismic imaging, (Bulteau, 2004). The project is open for
proposals from teachers and students interested in offshore geohazards.
Personnel
Åsmund Drottning
- rock physics
Leiv Gelius
– geophysics
Carl Fredrik Forsberg
– marine geology
Tore Jan Kvalstad
– geotechnics
Isabelle Lecomte
– geophysics
Oddvar Longva
– marine geology
Joonsang Park
- geophysics
Leif Rise
– marine geology
Anders Solheim
– marine geology
James M. Strout
– geotechnics, instruments, pore pressures
Maarten Vanneste
- marine geophysics
and geology
Harald Westerdahl
- geophysics
Shaoli Yang
– geotechnics, geo-parameter database |
