Maria Filomena Loreto

Regional versus detailed velocity analysis to quantify hydrate and free gas in marine sediments: the South Shetland Margin case study

Abstract The presence of gas hydrate and free gas within marine sediments deposited along the South Shetland margin, offshore the Antarctic Peninsula, was confirmed by low and high resolution geophysical data, acquired during three research cruises. Seismic data analysis has revealed the presence of a bottom-simulating reflector that is very strong and continuous in the eastern part of the margin. This area can be considered as a useful site to study the seismic characteristics of sediments containing gas hydrate, with a particular focus on the estimation of gas hydrate and free gas amounts in the pore space. Pre-stack depth migration and tomographic inversion were performed to produce a regional velocity field of gas-phase bearing sediments and to obtain information about the average thickness of gas hydrate and free gas layers. Using these data and theoretical models, the gas hydrate and free gas concentrations can be estimated. Moreover, the common image gather semblance analysis revealed the presence of detailed features, such as layers with small thickness characterized by low velocity alternating with high velocity layers, below and above the bottom-simulating reflector. These layers are associated with free gas trapped within the hydrate stability zone and deeper sediments. Thus, the use of the detailed and the regional velocity field analysis is important to give a more reliable estimate of gas content in the marine sediments.

Evidence of extensional and strike-slip deformation in the offshore Gökova-Kos area affected by the July 2017 Mw6.6 Bodrum-Kos earthquake, eastern Aegean Sea

The interpretation of new multichannel seismic profiles and previously published high-resolution swath and seismic reflection data from the Gökova Gulf and southeast of Kos Island in the eastern Aegean Sea revealed new morphotectonic features related to the July 20, 2017 Mw6.6 Bodrum-Kos earthquake offshore between Kos Island and the Bodrum Peninsula. The seafloor morphology in the northern part of the gulf is characterized by south-dipping E–W-oriented listric normal faults. These faults bend to a ENE–WSW direction towards Kos Island, and then extend parallel to the southern coastline. A left-lateral SW–NE strike-slip fault zone is mapped with segments crossing the Gökova Gulf from its northern part to south of Kos Island. This fault zone intersects and displaces the deep basins in the gulf. The basins are thus interpreted as the youngest deformed features in the study area. The strike-slip faults also produce E–W-oriented ridges between the basin segments, and the ridge-related vertical faults are interpreted as reverse faults. This offshore study reveals that the normal and strike-slip faults are well correlated with the focal mechanism solutions of the recent earthquake and general seismicity of the Gökova Gulf. Although the complex morphotectonic features could suggest that the area is under a transtensional regime, kinematic elements normally associated with a transtensional system are missing. At present, the Gökova Gulf is experiencing strike-slip motion with dominant extensional deformation, rather than transtensional deformation.

Tectonic Processes and BSR Features along Chilean Margin

From seismic interpretation we can argue that basal and frontal accretions are the main processes that model this part of the Chilean margin. Frontal accretion areas are characterized by a smoothed oceanic crust top, anticline reliefs, high offset thrusts and positive flowers structures; while, basal accretion areas, are characterized by an irregular oceanic crust top, uplifted slope, low offset thrusts, normal faults affecting the upper prism sediments, negative flower structures and re-activated thrusts. In particular the BSR is strong and continuous, within prism sediments, where the frontal accretion processes are acting, while it is weak and discontinuous where the active process is the basal accretion. We can suppose that the uplifting by basal accretion generates extensional tectonic movements. This can favour fluid escape and, consequently, a change of temperature. A consequence of this change is that the BSR disappears or becomes weaker. It is possible to extrapolate that strong and continuous BSRs can be developed in presence of frontal accretion processes, because it is most probable that high-fluid flux can occur in compressive conditions, while weak and discontinuous BSR can be associated to basal accretion, because off-crapped sediments can uplift the entire accretionary prism favouring fluid escapes.

Characteristic of Gas Hydrates Reservoir from Seismic Analysis of Multichannel Seismic Profiles Offshore South Chile

We used a procedure already tested in order to characterize the gas hydrate and free gas reservoir along South Chile. We used the multichannel seismic data to obtain an accurate velocity model by using the Pre stack Depth Migration. The velocity model can