THE ROLE OF FREE GAS IN THE ACTIVATION OF SUBMARINE SLIDES IN
FINNEIDFJORD
A. I. BEST
Challenger Division for Seafloor Processes, Southampton Oceanography Centre,
European Way, Southampton, SO14 3ZH, United Kingdom
C. R. I. CLAYTON
School of Civil Engineering & Environmental Technology, University of Southampton,
Highfield, Southampton, SO17 1BJ, United Kingdom
O. LONGVA
NGU, N-7491, Trondheim, Norway
M. SZUMAN
Challenger Division for Seafloor Processes, Southampton Oceanography Centre,
European Way, Southampton, SO14 3ZH, United Kingdom
Abstract
Free gas is evident on high resolution seismic profiles near the 1996 Finneidfjord submarine
landslide, Norway. A bright reflector at about 6 m sub-seabed depth on intact terrain
adjacent to the slide coincides with the initial failure surface. We hypothesise that the
bright reflector comprises free gas collecting in relatively sandy layers, and that the free gas
could have contributed to the generation of excess pore pressures and the initiation of the
submarine landslide. Preliminary measurement of the bright layer reflection coefficient from
seismic sections, and interpretation of the available geotechnical data, support this
hypothesis.
Keywords: Submarine slide, Finneidfjord, seismic reflectivity, geotechnics, free gas.
1. Introduction
Finneidfjord in Norway (see location map in Fig. 1 of Longva et al., 2003, this volume) was
the site of a large submarine landslide, believed to have started underwater late on 19 June
1996. It retrogressed rapidly, so that after a few hours, by about 00h25 on 20 June it
reached the E6 highway that skirts the fjord over a length of some 250 – 300 m. In its final
extent the slide involved some 1 million cubic metres of ground, of which about 90% was
below sea level. Four people died, and several houses, a major section of highway, and a
beach were destroyed. A number of studies were subsequently commissioned, including a
geotechnical assessment of the causes of the slide (Janbu, 1996).
As noted by Longva et al. (2003), the 1996 Finneidfjord slide seems to have failed initially
on a so-called weak layer that can be traced to adjacent intact sediments at about 6 m subseabed depth. This weak layer is of particular interest because it appears as a bright reflector
on high resolution seismic sections, and the brightness and polarity of the reflector indicates
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free gas. Our hypothesis is that the free gas is somehow linked to elevated pore pressures in
the weak layer that may predispose the terrain to slope failure. At present, there is
insufficient data to test rigorously this statement, but we can nevertheless gain some insight
into the likely role of free gas from the available high resolution seismic profiles and
geotechnical data.
Figure 1. a) High resolution seismic survey lines in Finneidfjord, Norway and location of Figs. 2 & 3.
b) Bathymetry, 1996 slide area, and location of gas blankets as interpreted from the seismic profiles in Fig. 1a.
The Role of Free Gas in Finneidfjord
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2. Seismic evidence for free gas
2.1 GASSY ZONES
In September 1998, a high-resolution seismic, sub-seabed survey (see Fig. 1) was carried
out over the slide area using NGU’s survey vessel, the RV Seisma, fitted with a hullmounted parametric sonar (Topas PS 40). The sonar uses a broadband (1 – 10 kHz) pulse
that can give up to 10 cm vertical resolution. Fig. 1b shows the distribution of sub-seabed
gassy zones in relation to the 1996 slide area as identified on the seismic lines indicated in
Fig. 1a. Gassy zones occur to the south, and immediately to the west of the slide, although
there is no evidence for gas blankets beneath the 1996 slide area. The main input of
sediments to the fjord comes from the River Røssåga to the south. The river drains through
an area of raised glaciomarine sediments with severe riverbank erosion. The main zone of
gassy sediments to the south of the slide in Fig. 1b coincides with the pro-delta of the River
Røssåga. The gas is probably of biogenic origin as the mean annual sediment accumulation
rate through the Holocene may locally approach 3 mm.
2.2 SEISMIC CHARACTER OF GASSY ZONES & BRIGHT REFLECTOR
The main gassy zone to the south of the 1996 slide, shown in Fig. 2, is recognised by
chaotic reflectors truncating otherwise continuous reflectors from the northwest. The bright
reflector identified as the slide failure surface further upslope extends from the gassy zone
to under the slide and beyond.
Figure 2. Seismic profile showing 1996 slide deposit, bright reflector, and gassy zone (gas blanket). There is some
evidence for gas chimneys beneath the slide (although difficult to see at this scale). Note vertical exaggeration. See
Fig. 1a for location of seismic line.
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The bright reflector is laterally continuous over the slide and gas blanket areas and seems to
act as a barrier to upwards fluid migration, except in a few locations where fluid (gas?)
appears to have ruptured the cap and percolated through (see Fig. 2). The nature of the
bright reflector can be viewed more clearly in Fig.3. Its seismic character suggests it may
comprise multiple (possibly interfering) reflections from several discrete sedimentary
layers.
Figure 3. Evidence for fluid (gas?) percolation through the bright reflector, giving rise to laterally discrete bright
reflectors in the sediments above.
In some locations the fluid (gas?) seems to have accumulated in overlying sedimentary
layers that also appear as bright reflectors. Knowledge of the physical properties of this
bright reflector would be helpful in assessing its role in the 1996 slide; unfortunately, there
are no cores through this layer at present.
2.3 REFLECTION COEFFICIENTS
It is relatively straightforward to assess whether the bright layer is in fact anomalously
bright for typical sediments found in this area, and whether it has negative polarity, which
would indicate gassy pore fluids. However, bright reflectors and polarity reversals can also
be caused by interference of closely spaced reflectors (Warner, 1990); this has not yet been
tested. So far, we have simply modelled the expected reflection coefficients of the seabed
reflector and the bright reflector based on estimates of velocity, density and attenuation, and
then compared the results to the observed reflection coefficients.
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Assuming normal incidence reflections (e.g. Telford et al., 1986), linear attenuation
mechanisms and spherical spreading, the amplitudes of the seabed reflector (A1), first
seabed multiple (Am), and bright reflector (A2) can be expressed as:
A1 =
A0 exp{−α 1 v1 t1 }
R12
v 1 t1
Am = −
A2 =
A0 R122
exp{−2α 1 v1t1 }
2 v1 t 1
A0 exp{−α 1 v1 t1 − α 2 v 2 ∆t}
(
1 − R122 )R23
(v1t1 + v 2 ∆t )
(1)
(2)
(3)
where: A0 is the unknown source amplitude; W W2 – t1; t1 and t2 are the two-way-travel
times of the seabed and bright reflectors (in seconds), respectively; v1 and v2 are the average
compressional wave velocities of the water column and sediments above the bright reflector
(in metres/second); and α1 and α2 are the average compressional wave attenuation
coefficients (in Nepers/m) of the water column and sediments above the bright reflector
(that include the effects of absorption and scattering). The reflection coefficients R12 and R23
correspond to the seabed and bright reflector, and can be calculated according to wellknown formulae (e.g. Telford et al., 1986).
Manipulation of equations 1 - 3 gives expressions for R12 and R23 in terms of the parameters
A1, Am, A2, t1 and t2 measured on the seismic profiles, given suitable values for the remaining
input parameters (see below). Spence et al. (1995) and Bull et al. (1998) used a similar
method, but did not account for different velocities above and below the reflector of
interest, nor the attenuation of the water and sediment layers. In practice, it is quite difficult
to find parts of the seismic profiles where all three reflectors can be recognised with
sufficient clarity to enable comparison of the same pulse peak/trough. Some processing was
applied to the raw data to assist with this: median filtering to improve the lateral polarity
stability of the reflectors; and flattening and phase reversal of the multiple and bright
reflectors to improve identification of the correct peak/trough.
A three-layer forward model was used of seawater (half space, layer 1) over silty clay (layer
2) over silty sand (half space, layer 3); typical input parameters were taken from the
literature (Bowles, 1997; Hamilton, 1971; Kaye and Laby, 1995): v1 = 1471 m/s (seawater
at 5 °C); v2 = 1519 m/s (silty clay, 76% porosity); v3 = 1677 m/s (silty sand, 53% porosity);
α1 = 0.00037 dB/m (seawater at 5 °C and 5 kHz); α2 = 1.0 dB/m (maximum likely value for
non-gassy muds at 5 kHz); ρ1 = 1024 kg/m3; ρ2 = 1420 kg/m3; ρ3 = 1830 kg/m3. The values
of A0 = 1.0 x 106, t1 = 46.6 milliseconds and t2 = 56.7 milliseconds were chosen to match
the seismic profile observations (note: different location to Fig. 2).
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An average of four amplitude readings (± 1 standard deviation) was used for each reflector
to calculate the observed reflection coefficients (A1 = 12886 ± 3088; A2 = -17969 ± 1166;
Am = -336 ± 66). In Table 1, the model overestimates the observed seabed and bright
reflector reflectivities, the latter showing a clear phase reversal in the seismic section (not
shown). In fact, it is impossible to attain the observed bright reflector’s reflectivity (both
magnitude and polarity) by adjusting the input parameters within reasonable ranges for nongassy silty clay and silty sand. Future work will attempt to model the effect of gas on the
reflector.
Figure 4. Example traces used to measure the reflection coefficient of the bright reflector. First breaks are marked
with solid horizontal lines; amplitudes used in the calculations are indicated. The bright reflector is immediately
preceded by a “non-bright” reflector causing some interference.
Table 1. Comparison of modelled and observed reflection coefficients.
R12
R23
Forward model
0.18
0.17
Observations
0.05
-0.52
3. Geotechnical perspective
3.1 GROUND & SEABED CONDITIONS
A considerable amount of ground investigation was done in this area prior to the slide, in
support of the re-alignment of the E6 highway that skirts the fjord. The areas both to the
north and to the south of the slide were investigated, both onshore and near to the shore, but
the geotechnical properties of the sea shore and the steepest sea bed in the vicinity of the
slide remained unknown. These earlier investigations showed that the beach zone consisted
of up to 5 m of sandy material, overlying soft to medium, sometimes sensitive and
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sometimes slightly laminated, silty clay with traces of organic material and layers of silt
over rock. Rockhead sloped towards the fjord, with the clay layer therefore thickening
downslope towards the shore. At the shoreline rock was encountered at a level of about –15 m.
The undrained shear strength of the clay, as determined by two previous ground
investigations, varied from 10 – 50 kPa. Where less sensitive it had higher strength,
between 25 – 40 kPa. In areas of sensitivity its strength was generally lower, between 10
kPa - 20 kPa. Sensitivity generally varied from 5 to 35, although quick clay (sensitivity 40 60) was encountered in an area to the south of the slide. The salinity measured in a number
of samples was less than 1o/oo (cf. sea water ca. 30o/oo), showing this low plasticity recent
marine clay to have been completely leached. Typical effective angles of friction can be
expected to be of the order of 25° or more.
3.2 LANDSLIDE BEHAVIOUR
As noted by Longva et al. (2003), the beach area at the top of the slide was relatively flat
before failure, with the steepest slopes lying some 5 – 25 m below sea level, and perhaps 30
m offshore. It is on these steeper submarine slopes that it is postulated that failure started,
retrogressing up and onto the beach and the road behind it. Based on profiles made in 1984,
the steepest parts of the slope before failure probably stood at about 18° to the horizontal.
Back analysis using parameters commonly adopted for sensitive Norwegian clays confirmed
that whilst a monolithic deep failure of the clay in this area could not occur, the shallower
steepest parts of the slope had much lower margins of safety.
Longva et al. (2003) proposed several possible reasons for the failure. The location of the
initial failure suggests that over-steepening of the seabed, or the development of excess pore
pressures at shallow depth under the steepest part of the slope may have been the most
likely causes. It has been estimated (Janbu, 1996) that an excess pore pressure of as little as
5 kPa in the top 10 m or so of the slope would be sufficient to bring the slope to failure.
3.3 EFFECT OF GAS
Two possible mechanisms exist to suggest that the presence of gas could reduce the stability
of the underwater slopes. It has been noted above that the sensitive clays on the seabed at
this location are not uniform, but vary from location to location, and sometimes contain
‘fabric’ (the non-uniform distribution of different particle sizes within the deposit, for
example: varving; silt pockets within clay; silt and sand particles along fissure planes).
Fabric is extremely important for the drainage of clays, as noted by Rowe (1972), since silt
and sand sized layers and laminations provide a fast preferential drainage path for excess
pore pressures within the deposit.
The presence of gas bubbles within the coarser fractions of soil, or in clays, containing
fabric can significantly reduce the overall permeability of the ground, since they block the
larger pores that carry the majority of the flow. An excess pore pressure, either equal to the
air entry value of the finer fraction or sufficient to cause hydraulic fracture of the ground
above, would be required before the gas could migrate upwards to the seabed. Thus it seems
reasonable to expect small amounts of overpressuring in the gas at shallow depths below the
seabed at Finneidfjord.
In the context of the already limited stability of this slope, variations in this overpressure
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could have been sufficient to trigger instability. There appears to be a general outward flow
from groundwater at higher head under land areas towards the sea. A decrease in
permeability immediately under the seabed would reduce flow, and raise excess pore
pressures at this level, thus weakening the sediment, and perhaps triggering failure.
The second mechanism is based upon the possible weakening effect of gas bubbles within
the clay. Wheeler (1988) and Sills et al. (1991) have shown that large gas bubbles (i.e.
considerably larger than the mean particle size) form within marine clays and that,
depending upon their initial volume fraction, the total stress, and the gas pressure, undrained
shear strength may be either increased or decreased as a result of their presence. Their
results suggest that reductions in undrained strength are associated with bubble flooding,
which occurs in the laboratory at higher back pressures, and might therefore be expected to
be more significant at greater depths below sea level.
4. Conclusions
Seismic data reveal the presence of a bright reflector adjacent to the 1996 slide and
coinciding with the depth of the initial failure surface. The available seismic and
geotechnical evidence suggests this bright reflector has a high-sand/low-clay content and
acts as a gas reservoir. Even if the role of gas in the actual slide event is unknown, the bright
reflector strongly indicates the possibility of high pore pressures.
5. Acknowledgements
We are grateful to N. Janbu for advice and access to unpublished work. This work forms
part of the EC COSTA project (EVK3-CT-1999-00006).
6. References
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Hamilton, E.L., 1971. Prediction of in-situ acoustic and elastic properties of marine sediments. Geophysics,
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