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

GASGAS CHARGED SEDIMENTS AND ASSOCIATED CHARGED SEDIMENTS AND ASSOCIATED SEABED MORPHOLOGICAL FEATURES IN THE SEABED MORPHOLOGICAL FEATURES IN THE

AEGEAN AND IONIAN SEAS, GREECEAEGEAN AND IONIAN SEAS, GREECE

Laboratory of Marine Geology & Physical Oceanography, Laboratory of Marine Geology & Physical Oceanography,

Department of Geology, University of Patras, Greece Department of Geology, University of Patras, Greece [email protected]

Contribution 10, by: Contribution 10, by:

Papatheodorou, G., Christodoulou, D. and Ferentinos, GPapatheodorou, G., Christodoulou, D. and Ferentinos, G.

During the last fifteen years, marine seismic surveys in the During the last fifteen years, marine seismic surveys in the Aegean and Ionian Seas (Fig. 1) have revealed numerous acoustic Aegean and Ionian Seas (Fig. 1) have revealed numerous acoustic anomalies, i.e. acoustic turbid zones, gas pockets, gas plumes, anomalies, i.e. acoustic turbid zones, gas pockets, gas plumes, enhanced reflectors, columnar disturbances, wipe outs and meso- enhanced reflectors, columnar disturbances, wipe outs and meso- to micro-morphological features, such as pockmarks, domes, mud to micro-morphological features, such as pockmarks, domes, mud volcanoes and elongated depressions (Papatheodorou et al., 1993). volcanoes and elongated depressions (Papatheodorou et al., 1993). These are attributed to the presence of gas in marine sediments These are attributed to the presence of gas in marine sediments interstices. The gas charged sediments are found in Pleistocene interstices. The gas charged sediments are found in Pleistocene and present day fjord-like environments, Pleistocene and present and present day fjord-like environments, Pleistocene and present day deltaic environments, lakes and pre-Quaternary open sea day deltaic environments, lakes and pre-Quaternary open sea environments. environments.

The gas found in the Quaternary and present day fjord-like and The gas found in the Quaternary and present day fjord-like and deltaic environments is assumed to be of biogenic origin. The gas deltaic environments is assumed to be of biogenic origin. The gas found in the pre-Quaternary open sea environments is associated found in the pre-Quaternary open sea environments is associated with faulting and salt doming and may, therefore, be of with faulting and salt doming and may, therefore, be of thermogenic origin.thermogenic origin.

Fig.1. Map showing the location of Fig.1. Map showing the location of the surveyed areas where gas-the surveyed areas where gas-charged sediments have been found.charged sediments have been found.

(A): Corfu shelf(A): Corfu shelf

(B): Northwestern Aegean Sea(B): Northwestern Aegean Sea

(C): Lake Trichonis(C): Lake Trichonis

(D): Amvrakikos Gulf(D): Amvrakikos Gulf

(E): Gulf of Patras(E): Gulf of Patras

(F): West Gulf of Corinth(F): West Gulf of Corinth

In the eastern Corfu shelf (Ionian Sea) the In the eastern Corfu shelf (Ionian Sea) the central part is covered by a thin veneer of central part is covered by a thin veneer of sediments underlain by deformed and sediments underlain by deformed and upward curving sedimentary strata. The upward curving sedimentary strata. The deformation and doming strata is caused by deformation and doming strata is caused by salt diapirism. The acoustic character of the salt diapirism. The acoustic character of the sediments in the 3.5kHz and sparker records sediments in the 3.5kHz and sparker records indicates that they are gas-charged because indicates that they are gas-charged because of the acoustic turbid zones and enhanced of the acoustic turbid zones and enhanced reflectors. Side scan sonar images show that reflectors. Side scan sonar images show that along the rims of the salt diapir there are along the rims of the salt diapir there are dark patches of high reflectivity.dark patches of high reflectivity.

Abbreviations: Abbreviations: ATZATZ: Acoustic Turbid Zone; : Acoustic Turbid Zone; ERER: Enhanced Reflector; : Enhanced Reflector; GSGS: Gas Seepage; : Gas Seepage; DD: Doming; : Doming; HRPHRP: High Reflectivity : High Reflectivity Patches.Patches.

OPEN SEA ENVIRONMENTOPEN SEA ENVIRONMENT

Fig. 2. Block diagram showing the dome structure and related 3.5kHz, Fig. 2. Block diagram showing the dome structure and related 3.5kHz, sparker profiles and side scan sonar images.sparker profiles and side scan sonar images.

OPEN SEA ENVIRONMENTOPEN SEA ENVIRONMENTNORTHWESTERNNORTHWESTERN

AEGEAN SEAAEGEAN SEA

Fig. 3. Airgun seismic profile across the Sporades shelf showing Fig. 3. Airgun seismic profile across the Sporades shelf showing gas charged sediments. gas charged sediments. ATZATZ: Acoustic Turbid Zone, : Acoustic Turbid Zone, CDCD: Columnar : Columnar Disturbance,Disturbance, F F: Fault, : Fault, MM: Multiple: Multiple

Fig. 4. Side Scan Sonar image of the Sporades shelf Fig. 4. Side Scan Sonar image of the Sporades shelf showing mud volcanoes (MV) and gas seeps (GS) in the showing mud volcanoes (MV) and gas seeps (GS) in the water column.water column.

Fig. 5. Side Scan Sonar image of the Sporades shelf Fig. 5. Side Scan Sonar image of the Sporades shelf showing fault line (F) on the seafloor associated with showing fault line (F) on the seafloor associated with small shallow pockmarks (PM).small shallow pockmarks (PM).

In the Sporades shelf, in the Northwestern Aegean Sea, gas is In the Sporades shelf, in the Northwestern Aegean Sea, gas is found in the upper 70-80m as is indicated by the acoustic turbid found in the upper 70-80m as is indicated by the acoustic turbid zones and columnar disturbances in the airgun records (Fig. 3) zones and columnar disturbances in the airgun records (Fig. 3) and water column reflections in the side scan sonar images (Fig. and water column reflections in the side scan sonar images (Fig. 4), which probably represent gas plumes (GS) rising from 4), which probably represent gas plumes (GS) rising from elongated mounds, which appear to be mud volcanoes (Fig. 4). elongated mounds, which appear to be mud volcanoes (Fig. 4). Negative topographic features, probably pockmarks near the Negative topographic features, probably pockmarks near the hanging-wall of a faultline, indicate that gas uses the fault plain as hanging-wall of a faultline, indicate that gas uses the fault plain as a migratory path (Fig. 5). a migratory path (Fig. 5).

PRESENT DAY AND PLEISTOCENE FJORD-LIKE PRESENT DAY AND PLEISTOCENE FJORD-LIKE ENVIRONMENTSENVIRONMENTS

AMVRAKIKOS GULFAMVRAKIKOS GULF

The Pleistocene / Holocene boundary in Amvrakikos Gulf seems to be a gas accumulative horizon as is 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 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 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 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 (Fig. 6b). Low relief intra-sedimentary and seabed doming (Fig. 6b) is apparently due to the expansion of the sediment-trapped gases.of the sediment-trapped gases.

Fig. 6. 3.5kHz profiles of the Amvrakikos Gulf showing buried pockmarks (BPM) developed at the Pleistocene/Holocene 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).interface; gas plumes (GPL), enhanced reflectors (ER), doming (D) and faults (F).

b

a

Fig. 7. 3.5kHZ and corresponding airgun seismic profiles of the Gulf of Fig. 7. 3.5kHZ and corresponding airgun seismic profiles of the Gulf of Patras showing: Pleistocene gas charged sediments indicated by the Patras showing: Pleistocene gas charged sediments indicated by the acoustic turbid zones (acoustic turbid zones (ATZATZ); Gas pockets (); Gas pockets (GPGP) in the Holocene ) in the Holocene sedimentary cover associated with faulting (sedimentary cover associated with faulting (FF) and doming () and doming (DD) of the ) of the seabed. Notice how the acoustic character of the gas charged sediments seabed. Notice how the acoustic character of the gas charged sediments changes due to the difference in the frequency and resolution of the changes due to the difference in the frequency and resolution of the profiling systems used. Holocene / Pleistocene boundary (profiling systems used. Holocene / Pleistocene boundary (HOL/PLHOL/PL); ); Enhanced Reflector (Enhanced Reflector (ERER); Gas Plume (); Gas Plume (GPGP); Multiple (); Multiple (MM).).

PRESENT DAY AND PLEISTOCENE FJORD-LIKE ENVIRONMENTSPRESENT DAY AND PLEISTOCENE FJORD-LIKE ENVIRONMENTS GULF OF PATRASGULF OF PATRAS

GAS-CHARGED SEDIMENTSGAS-CHARGED SEDIMENTS

Seismic data collected from the Gulf of Seismic data collected from the Gulf of Patras has shown the presence of a gas Patras has shown the presence of a gas accumulative horizon at about 15-20m accumulative horizon at about 15-20m under the seafloor (Fig. 7). This under the seafloor (Fig. 7). This accumulative horizon may be the boundary accumulative horizon may be the boundary between the Holocene and Pleistocene between the Holocene and Pleistocene sediments The accumulation of gas in the sediments The accumulation of gas in the Holocene / Pleistocene interface may be Holocene / Pleistocene interface may be due to differences in the gas-bearing due to differences in the gas-bearing properties of these sediments (Fig. 7). This properties of these sediments (Fig. 7). This horizon can not always restrain the vertical horizon can not always restrain the vertical migration of gas into the Holocene migration of gas into the Holocene sediments, as is indicated by the gas sediments, as is indicated by the gas plumes, gas pockets, enhanced reflectors, plumes, gas pockets, enhanced reflectors, intra-sedimentary and seabed dome-like intra-sedimentary and seabed dome-like features, which have been frequently features, which have been frequently detected on the seismic records (Fig. 7, 8). detected on the seismic records (Fig. 7, 8). Some of the pockmarks are associated with Some of the pockmarks are associated with seabed displacements due to faulting, seabed displacements due to faulting, suggesting that the gas uses fault planes as suggesting that the gas uses fault planes as a migration path (Fig. a migration path (Fig. 8).).

Fig. 8. 3.5kHz profile of the Gulf of Patras showing: Pleistocene gas charged sediments indicated by the acoustic 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).turbid zones (ATZ) and pockmarks (PM) formed along faults (F). (GPl: gas plumes).

In the Southeastern part of the Gulf of Patras, an active pockmark field was found (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 1.7kmThe pockmark field has an aerial extent of 1.7km² and was formed in soft layered Holocene ² and was formed in soft layered Holocene silts. The larger pockmarks were up to 250m in diameter and deep up to 15m. Detailed 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 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 (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 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 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. 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 (,) and small holes on the seafloor, which may be associated with gas escaping vents (Fig 13).).

POCKMARK ACTIVATION BY EARTHQUAKEPOCKMARK ACTIVATION BY EARTHQUAKE

On July 14th, 1993, a major earthquake of 5.4R, whose epicentre was located near the 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 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 in the pockmark field, which was located 10m above the seabed, the temperature increased anomalously on three occasions prior to the earthquake (anomalously on three occasions prior to the earthquake (Fig. 14). Furthermore, it was noted ). Furthermore, it was noted that few pockmarks three to four days after the earthquake were still venting gas bubbles as is 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 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 three sudden temperature increases were probably due to bubbles of hot gas rising from the pockmark field.the pockmark field.

Similar events related to bubbling in the seawater and increased temperature have been 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 described in the past in the nearby area of Aegion, 40km to the east, in 1817 and in Messolonghi lagoon in 1882.Messolonghi lagoon in 1882.

POCKMARK FIELD OFF PATRAS HARBOURPOCKMARK FIELD OFF PATRAS HARBOUR

Fig. 9. Investigated area in the Gulf of Patras. Fig. 9. Investigated area in the Gulf of Patras.

Fig. 10. 3D view of the seabed in part of the pockmark field.Fig. 10. 3D view of the seabed in part of the pockmark field.

Fault lineFault line

PATRAIKOSPATRAIKOSPOK 4POK 4

Fig. 11. 3.5kHz profile showing a composite pockmark Fig. 11. 3.5kHz profile showing a composite pockmark (POK 4) of the Gulf of Patras pockmark field. Methane (POK 4) of the Gulf of Patras pockmark field. Methane concentration was measured at the centre of the pockmark concentration was measured at the centre of the pockmark using a methane sensor (METS).using a methane sensor (METS).

Fig. 12. Methane concentration Fig. 12. Methane concentration variation versus water depth in the variation versus water depth in the POK 4 pockmark. Maximum POK 4 pockmark. Maximum concentration (700-800 nmol/l) was concentration (700-800 nmol/l) was observed at the water – sediment observed at the water – sediment interface.interface.

0,3 0,35 0,4 0,45 0,5 0,55 0,6 0,65 0,7 0,75

Methane Concentration (μmol/lt)

0

40

Dep

th (

m)

Fig. 13. ROV photo of the seafloor in the centre of pockmark POK 4. The red arrows show small holes probably 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.related to gas venting.

5.4R EARTHQUAKE5.4R EARTHQUAKE

Fig. 14. Temperature variation versus time in the hydrographic station, 10m above the seabed, from July 13th (Fig. 14. Temperature variation versus time in the hydrographic station, 10m above the seabed, from July 13th ( ~30h before ~30h before the earthquake) to July 17th (60h after the earthquake). The vertical line after the three temperature peaks indicates the time the earthquake) to July 17th (60h after the earthquake). The vertical line after the three temperature peaks indicates the time of the earthquake.of the earthquake.

Fig. 15. Side Scan Sonar mosaic of the pockmark field of the Gulf of Patras, showing gas plumes (white 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.arrows) rising in the water column from the pockmarks.

Seismic data collected in the southern coastline of the gulf of Corinth has shown the 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. presence of a small pockmark field in Eleona Bay (Fig. 16). These pockmarks are ). These pockmarks are

circular to subcircular in plan view and are less than 100m in diameter (Fig. circular to subcircular in plan view and are less than 100m in diameter (Fig. 17). They ). They

appear to be flat floored, about 10m deep and are developed in silty sands (Fig. 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 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). 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.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) 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 (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) 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 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 temperature, salinity, methane, hydrosulfide and carbon dioxide in association with the

earthquake activity in the region. earthquake activity in the region.

PRESENT DAY DELTAIC ENVIRONMENTSPRESENT DAY DELTAIC ENVIRONMENTS

WEST GULF OF CORINTHWEST GULF OF CORINTH

Fig. 16. Investigated area in the Western Gulf of Corinth.Fig. 16. Investigated area in the Western Gulf of Corinth.

Fig. 17. Side Scan Sonar mosaic showing the pockmark field in Eleona Bay.Fig. 17. Side Scan Sonar mosaic showing the pockmark field in Eleona Bay.

Fig. 18. 3.5kHz profile showing a flat-floored pockmark (POK 4) in Eleona Bay.Fig. 18. 3.5kHz profile showing a flat-floored pockmark (POK 4) in Eleona Bay.

0.02

0.04

0.06

0.08

15:15:50 15:18:43 15:21:36 15:24:29 15:27:22 15:30:14 15:33:07 15:36:00

Meth

ane c

oncentr

atio

n (

μm

ol/l

t)

DateTime09/23/02 09/23/02 09/23/02 09/23/02 09/23/02 09/23/02

15:16 15:20 15:24 15:28 15:33 15:3720.10

21.02

21.94

22.86

23.78

24.70 T

emp

(C)

15:16 15:20 15:24 15:28 15:33 15:3712.0

18.0

24.0

30.0

36.0

42.0

Sal

init

y(p

pt)

15:16 15:20 15:24 15:28 15:33 15:3752.0

62.6

73.2

83.8

94.4

105.0

DO

(%)

ELEONASELEONAS

POK 4POK 4

23/09/0223/09/02

Fig. 19. Methane concentration Fig. 19. Methane concentration ((μμmol/l), water temperature (mol/l), water temperature (ºC), ºC), salinity (‰) and DO (%) versus time salinity (‰) and DO (%) versus time just above the seabed in Eleonas just above the seabed in Eleonas pockmark “POK 4”. The yellow pockmark “POK 4”. The yellow dotted lines define the measurements dotted lines define the measurements just above the freshwater seepage.just above the freshwater seepage.

Fig. 20. ROV photos in the Eleonas pockmark POK 4 showing the sidewalls of the pockmark.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 Fig. 21. ROV photo in the Eleonas pockmark POK 4 showing the freshwater seepage.showing the freshwater seepage.

Fig. 22. ROV photos in the center of the Eleonas Fig. 22. ROV photos in the center of the Eleonas pockmark POK 4 showing flat-floored area pockmark POK 4 showing flat-floored area densely populated with small polychates.densely populated with small polychates.

Fig. 23. Side Scan Sonar mosaic of the pockmark field north of Valimitika. The east-west chain of pockmarks is 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).the trace of the Aegion fault (Soter, 1999).

REFERENCESREFERENCES

• Papatheodorou, G., Hasiotis, T. and Ferentinos, G., 1993. Gas-charged sediments in the Aegean Papatheodorou, G., Hasiotis, T. and Ferentinos, G., 1993. Gas-charged sediments in the Aegean and Ionian Seas, Greece. and Ionian Seas, Greece. Marine GeologyMarine Geology, , 112112, 171-184., 171-184.

• Hasiotis, T., Papatheodorou, G., Kastanos, N. and Ferentinos, G., 1996. A pockmark field in the 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. Patras Gulf (Greece) and its activation during the 14/7/93 seismic event. Marine GeologyMarine Geology, , 130130, , 333-344.333-344.

• SoterSoter, S., 1999. , S., 1999. Macroscopic seismic anomalies and submarine pockmarks in theMacroscopic seismic anomalies and submarine pockmarks in the Corinth–Patras Corinth–Patras rift, Greecerift, Greece. TectonophysicsTectonophysics,, 308308, , 275–290275–290..