GAS CHARGED SEDIMENTS AND ASSOCIATED SEABED MORPHOLOGICAL FEATURES IN THE AEGEAN AND IONIAN SEAS, GREECE

1
GAS CHARGED SEDIMENTS AND ASSOCIATED SEABED
MORPHOLOGICAL FEATURES IN THE AEGEAN AND IONIAN
SEAS, GREECE
Contribution 10, by Papatheodorou, G.,
Christodoulou, D. and Ferentinos, G.

• Laboratory of Marine Geology Physical
  Oceanography,
• Department of Geology, University of Patras,
  Greece ethagefo_at_upatras.gr

2
During the last fifteen years, marine seismic
surveys in the Aegean and Ionian Seas (Fig. 1)
have revealed numerous acoustic anomalies, i.e.
acoustic turbid zones, gas pockets, gas plumes,
enhanced reflectors, columnar disturbances, wipe
outs and meso- to micro-morphological features,
such as pockmarks, domes, mud volcanoes and
elongated depressions (Papatheodorou et al.,
1993). These are attributed to the presence of
gas in marine sediments interstices. The gas
charged sediments are found in Pleistocene and
present day fjord-like environments, Pleistocene
and present day deltaic environments, lakes and
pre-Quaternary open sea environments. The gas
found in the Quaternary and present day
fjord-like and deltaic environments is assumed to
be of biogenic origin. The gas found in the
pre-Quaternary open sea environments is
associated with faulting and salt doming and may,
therefore, be of thermogenic origin.
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Fig.1. Map showing the location of the surveyed
areas where gas-charged sediments have been
found. (A) Corfu shelf (B) Northwestern Aegean
Sea (C) Lake Trichonis (D) Amvrakikos Gulf (E)
Gulf of Patras (F) West Gulf of Corinth
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OPEN SEA ENVIRONMENT
In the eastern Corfu shelf (Ionian Sea) the
central part is covered by a thin veneer of
sediments underlain by deformed and upward
curving sedimentary strata. The deformation and
doming strata is caused by salt diapirism. The
acoustic character of the sediments in the 3.5kHz
and sparker records indicates that they are
gas-charged because of the acoustic turbid zones
and enhanced reflectors. Side scan sonar images
show that along the rims of the salt diapir there
are dark patches of high reflectivity.
Abbreviations ATZ Acoustic Turbid Zone ER
Enhanced Reflector GS Gas Seepage D Doming
HRP High Reflectivity Patches.
Fig. 2. Block diagram showing the dome structure
and related 3.5kHz, sparker profiles and side
scan sonar images.
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OPEN SEA ENVIRONMENT NORTHWESTERN AEGEAN SEA
In the Sporades shelf, in the Northwestern Aegean
Sea, gas is found in the upper 70-80m as is
indicated by the acoustic turbid zones and
columnar disturbances in the airgun records (Fig.
3) and water column reflections in the side scan
sonar images (Fig. 4), which probably represent
gas plumes (GS) rising from elongated mounds,
which appear to be mud volcanoes (Fig. 4).
Negative topographic features, probably pockmarks
near the hanging-wall of a faultline, indicate
that gas uses the fault plain as a migratory path
(Fig. 5).
Fig. 4. Side Scan Sonar image of the Sporades
shelf showing mud volcanoes (MV) and gas seeps
(GS) in the water column.
Fig. 3. Airgun seismic profile across the
Sporades shelf showing gas charged sediments.
ATZ Acoustic Turbid Zone, CD Columnar
Disturbance, F Fault, M Multiple
Fig. 5. Side Scan Sonar image of the Sporades
shelf showing fault line (F) on the seafloor
associated with small shallow pockmarks (PM).
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PRESENT DAY AND PLEISTOCENE FJORD-LIKE
ENVIRONMENTS AMVRAKIKOS GULF
The Pleistocene / Holocene boundary in Amvrakikos
Gulf seems to be a gas accumulative horizon as is
indicated by its strong reflectivity in the
3.5kHz records. Buried or fossil pockmarks
developed on the gas accumulative horizon, range
from 35 to 70m in diameter and are 5 - 6.5m deep
(Fig. 6a). Gas pockets in the Holocene and gas
plume in the water column indicate gas seeping
through the seabed (Fig. 6b). Low relief
intra-sedimentary and seabed doming (Fig. 6b) is
apparently due to the expansion of the
sediment-trapped gases.
a
b
Fig. 6. 3.5kHz profiles of the Amvrakikos Gulf
showing buried pockmarks (BPM) developed at the
Pleistocene/Holocene interface gas plumes (GPL),
enhanced reflectors (ER), doming (D) and faults
(F).
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PRESENT DAY AND PLEISTOCENE FJORD-LIKE
ENVIRONMENTS GULF OF PATRAS GAS-CHARGED SEDIMENTS
Seismic data collected from the Gulf of Patras
has shown the presence of a gas accumulative
horizon at about 15-20m under the seafloor (Fig.
7). This accumulative horizon may be the boundary
between the Holocene and Pleistocene sediments
The accumulation of gas in the Holocene /
Pleistocene interface may be due to differences
in the gas-bearing properties of these sediments
(Fig. 7). This horizon can not always restrain
the vertical migration of gas into the Holocene
sediments, as is indicated by the gas plumes, gas
pockets, enhanced reflectors, intra-sedimentary
and seabed dome-like features, which have been
frequently detected on the seismic records (Fig.
7, 8). Some of the pockmarks are associated with
seabed displacements due to faulting, suggesting
that the gas uses fault planes as a migration
path (Fig. 8).
Fig. 7. 3.5kHZ and corresponding airgun seismic
profiles of the Gulf of Patras showing
Pleistocene gas charged sediments indicated by
the acoustic turbid zones (ATZ) Gas pockets (GP)
in the Holocene sedimentary cover associated with
faulting (F) and doming (D) of the seabed. Notice
how the acoustic character of the gas charged
sediments changes due to the difference in the
frequency and resolution of the profiling systems
used. Holocene / Pleistocene boundary (HOL/PL)
Enhanced Reflector (ER) Gas Plume (GP) Multiple
(M).
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Fig. 8. 3.5kHz profile of the Gulf of Patras
showing Pleistocene gas charged sediments
indicated by the acoustic turbid zones (ATZ) and
pockmarks (PM) formed along faults (F). (GPl gas
plumes).
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POCKMARK FIELD OFF PATRAS HARBOUR
In the Southeastern part of the Gulf of Patras,
an active pockmark field was found (Fig. 9). The
pockmark field has an aerial extent of 1.7km² and
was formed in soft layered Holocene silts. The
larger pockmarks were up to 250m in diameter and
deep up to 15m. Detailed bathymetric and seismic
surveys have shown that the field consists of
single and composite pockmarks. The composite
pockmarks are formed by the amalgamation of
single ones (Fig. 10). Recent surveys over this
pockmark field using an ROV and a methane sensor
(METS) have revealed the presence of
high-concentration dissolved methane (700-800
nmol/lt) in the water column just above the
pockmark, indicating methane seepage from the
sediments (Fig. 11,12,) and small holes on the
seafloor, which may be associated with gas
escaping vents (Fig 13). POCKMARK ACTIVATION BY
EARTHQUAKE On July 14th, 1993, a major earthquake
of 5.4R, whose epicentre was located near the
pockmark field, appears that has activated the
pockmarks. In a temperature recording station in
the pockmark field, which was located 10m above
the seabed, the temperature increased anomalously
on three occasions prior to the earthquake (Fig.
14). Furthermore, it was noted that few pockmarks
three to four days after the earthquake were
still venting gas bubbles as is indicated by the
extremely high reflectivity plumes shown on the
sonographs (Fig. 15). The three sudden
temperature increases were probably due to
bubbles of hot gas rising from the pockmark
field. Similar events related to bubbling in the
seawater and increased temperature have been
described in the past in the nearby area of
Aegion, 40km to the east, in 1817 and in
Messolonghi lagoon in 1882.
10
Fault line
?
Fig. 10. 3D view of the seabed in part of the
pockmark field.
Fig. 9. Investigated area in the Gulf of Patras.
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PATRAIKOS POK 4
Fig. 11. 3.5kHz profile showing a composite
pockmark (POK 4) of the Gulf of Patras pockmark
field. Methane concentration was measured at the
centre of the pockmark using a methane sensor
(METS).
Methane Concentration (µmol/lt)
0
Depth (m)
Fig. 12. Methane concentration variation versus
water depth in the POK 4 pockmark. Maximum
concentration (700-800 nmol/l) was observed at
the water sediment interface.
40
12
Fig. 13. ROV photo of the seafloor in the centre
of pockmark POK 4. The red arrows show small
holes probably related to gas venting.
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5.4R EARTHQUAKE
Fig. 14. Temperature variation versus time in the
hydrographic station, 10m above the seabed, from
July 13th (30h before the earthquake) to July
17th (60h after the earthquake). The vertical
line after the three temperature peaks indicates
the time of the earthquake.
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Fig. 15. Side Scan Sonar mosaic of the pockmark
field of the Gulf of Patras, showing gas plumes
(white arrows) rising in the water column from
the pockmarks.
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PRESENT DAY DELTAIC ENVIRONMENTS WEST GULF OF
CORINTH
Seismic data collected in the southern coastline
of the gulf of Corinth has shown the presence of
a small pockmark field in Eleona Bay (Fig. 16).
These pockmarks are circular to subcircular in
plan view and are less than 100m in diameter
(Fig. 17). They appear to be flat floored, about
10m deep and are developed in silty sands (Fig.
18). During a preliminary survey, using an ROV, a
methane sensor (METS) and a CTD, it was found
that fresh groundwater was seeping from the
pockmarks (Fig. 19, 20, 21, 22). However, the
methane probe revealed low methane concentration
in the seawater. Another pockmark field was
discovered in the same area north of Eleona Bay
(Fig. 23) (Soter, 1999). A detailed survey is at
present going on within the framework of the
ASSEM program (Array of Sensors for long term
SEabed Monitoring of geohazards) financed by the
European Union. The aim of the project is the
long term monitoring of temperature, salinity,
methane, hydrosulfide and carbon dioxide in
association with the earthquake activity in the
region.
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Fig. 16. Investigated area in the Western Gulf of
Corinth.
17
Fig. 17. Side Scan Sonar mosaic showing the
pockmark field in Eleona Bay.
18
Fig. 18. 3.5kHz profile showing a flat-floored
pockmark (POK 4) in Eleona Bay.
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ELEONAS POK 4 23/09/02
Fig. 19. Methane concentration (µmol/l), water
temperature (ºC), salinity () and DO () versus
time just above the seabed in Eleonas pockmark
POK 4. The yellow dotted lines define the
measurements just above the freshwater seepage.
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Fig. 20. ROV photos in the Eleonas pockmark POK 4
showing the sidewalls of the pockmark.
Fig. 21. ROV photo in the Eleonas pockmark POK 4
showing the freshwater seepage.
21
Fig. 22. ROV photos in the center of the Eleonas
pockmark POK 4 showing flat-floored area densely
populated with small polychates.
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Fig. 23. Side Scan Sonar mosaic of the pockmark
field north of Valimitika. The east-west chain of
pockmarks is the trace of the Aegion fault
(Soter, 1999).
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REFERENCES

• Papatheodorou, G., Hasiotis, T. and Ferentinos,
  G., 1993. Gas-charged sediments in the Aegean
  and Ionian Seas, Greece. Marine Geology, 112,
  171-184.
• Hasiotis, T., Papatheodorou, G., Kastanos, N.
  and Ferentinos, G., 1996. A pockmark field in the
  Patras Gulf (Greece) and its activation during
  the 14/7/93 seismic event. Marine Geology, 130,
  333-344.
• Soter, S., 1999. Macroscopic seismic anomalies
  and submarine pockmarks in the CorinthPatras
  rift, Greece. Tectonophysics, 308, 275290.