The Geometry of the Two Orogens Convergence and Collision Zones in Central Georgia: New Data from Seismic Reflection Profiles

Sub-seabed gas is commonly associated with seabed depressions known as pockmarks—the main venting sites for hydrocarbon gases to enter the water column. Sub-seabed gas accumulations are characterized by acoustically turbid or opaque zones in seismic reflection profiles, taking the form of gas blankets, curtains or plumes. How the migration of sub-seabed gas relates to the origin and distribution of pockmarks in nearshore and fjordic settings is not well understood. Using marine geophysical data from Loch Linnhe, a Scottish fjord, we show that shallow sub-seabed gas occurs predominantly within glaciomarine facies either as widespread blankets in basins or as isolated pockets. We use geospatial ‘hot-spot’ analysis conducted in ArcGIS to identify clusters of pockmarks and acoustic (sub-seabed) profile interpretation to identify the depth to gas front across the fjord. By combining these analyses, we find that the gas below most pockmarks in Loch Linnhe is between 1.4 m and 20 m deep. We anticipate that this work will help to understand the fate and mobility of sedimentary carbon in fjordic (marine) settings and advise offshore industry on the potential hazards posed by pockmarked seafloor regions even in nearshore settings.

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Inverted Basins by Africa–Eurasia Convergence at the Southern Back-Arc Tyrrhenian Basin

Geosciences ◽

10.3390/geosciences11030117 ◽

2021 ◽

Vol 11 (3) ◽

pp. 117

Author(s):

Maria Filomena Loreto ◽

Camilla Palmiotto ◽

Filippo Muccini ◽

Valentina Ferrante ◽

Nevio Zitellini

Keyword(s):

Seismic Reflection ◽

Magnetic Data ◽

Normal Faults ◽

Sedimentary Sequence ◽

Initiation Zone ◽

Arc Setting ◽

Seismic Reflection Profiles ◽

Back Arc ◽

Flower Structures ◽

Tyrrhenian Basin

The southern part of Tyrrhenian back-arc basin (NW Sicily), formed due to the rifting and spreading processes in back-arc setting, is currently undergoing contractional tectonics. The analysis of seismic reflection profiles integrated with bathymetry, magnetic data and seismicity allowed us to map a widespread contractional tectonics structures, such as positive flower structures, anticlines and inverted normal faults, which deform the sedimentary sequence of the intra-slope basins. Two main tectonic phases have been recognised: (i) a Pliocene extensional phase, active during the opening of the Vavilov Basin, which was responsible for the formation of elongated basins bounded by faulted continental blocks and controlled by the tear of subducting lithosphere; (ii) a contractional phase related to the Africa-Eurasia convergence coeval with the opening of the Marsili Basin during the Quaternary time. The lithospheric tear occurred along the Drepano paleo-STEP (Subduction-Transform-Edge-Propagator) fault, where the upwelling of mantle, intruding the continental crust, formed a ridge. Since Pliocene, most of the contractional deformation has been focused along this ridge, becoming a good candidate for a future subduction initiation zone.

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Marine deep seismic reflection profiles off central California

Journal of Geophysical Research Atmospheres ◽

10.1029/90jb00493 ◽

1991 ◽

Vol 96 (B4) ◽

pp. 6423-6433 ◽

Cited By ~ 13

Author(s):

John Ewing ◽

Manik Talwani

Keyword(s):

Seismic Reflection ◽

Central California ◽

Deep Seismic Reflection ◽

Seismic Reflection Profiles

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Marine substrate response from the analysis of seismic attributes in CHIRP sub-bottom profiles

Brazilian Journal of Oceanography ◽

10.1590/s1679-87592017124306503 ◽

2017 ◽

Vol 65 (3) ◽

pp. 332-345 ◽

Cited By ~ 1

Author(s):

Larissa Felicidade Werkhauser Demarco ◽

Antonio Henrique da Fontoura Klein ◽

Jorge Antonio Guimarães de Souza

Keyword(s):

Grain Size ◽

Seismic Reflection ◽

Ground Truth ◽

Fine Sand ◽

Seismic Attributes ◽

Coarse Sand ◽

Unconsolidated Sediments ◽

Shallow Gas ◽

Size Classes ◽

Seismic Reflection Profiles

Abstract This paper presents an evaluation of the response of seismic reflection attributes in different types of marine substrate (rock, shallow gas, sediments) using seafloor samples for ground-truth statistical comparisons. The data analyzed include seismic reflection profiles collected using two CHIRP subbottom profilers (Edgetech Model 3100 SB-216S), with frequency ranging between 2 and 16 kHz, and a number (38) of sediment samples collected from the seafloor. The statistical method used to discriminate between different substratum responses was the non-parametric Kruskal-Wallis analysis, carried out in two steps: 1) comparison of Seismic Attributes between different marine substrates (unconsolidated sediments, rock and shallow gas); 2) comparison of Seismic Attributes between different sediment classes in seafloors characterized by unconsolidated sediments (subdivided according to sorting). These analyses suggest that amplitude-related attributes were effective in discriminating between sediment and gassy/rocky substratum, but did not differentiate between rocks and shallow gas. On the other hand, the Instantaneous Frequency attribute was effective in differentiating sediments, rocks and shallow gas, with sediment showing higher frequency range, rock an intermediate range, and shallow gas the lowest response. Regarding grain-size classes and sorting, statistical analysis discriminated between two distinct groups of samples, the SVFS (silt and very fine sand) and the SFMC (fine, medium and coarse sand) groups. Using a Spearman coefficient, it was found that the Instantaneous Amplitude was more efficient in distinguishing between the two groups. None of the attributes was able to distinguish between the closest grain size classes such as those of silt and very fine sand.

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