Yannick Thomas

Control of Quaternary sea‐level changes on gas seeps

Gas seeping to the seafloor through structures such as pockmarks may contribute significantly to the enrichment of atmospheric greenhouse gases and global warming. Gas seeps in the Gulf of Lions, Western Mediterranean, are cyclical, and pockmark “life” is governed both by sediment accumulation on the continental margin and Quaternary climate changes. Three-dimensional seismic data, correlated to multi-proxy analysis of a deep borehole, have shown that these pockmarks are associated with oblique chimneys. The prograding chimney geometry demonstrates the syn-sedimentary and long-lasting functioning of the gas seeps. Gas chimneys have reworked chronologically constrained stratigraphic units and have functioned episodically, with maximum activity around sea level lowstands. Therefore, we argue that one of the main driving mechanisms responsible for their formation is the variation in hydrostatic pressure driven by relative sea level changes.

Pasisar: Performances of a High and Very High Resolution Hybrid Deep-Towed Seismic Device

The Pasisar seismic acquisition system combines a source at the sea surface and a deep-towed single channel streamer. This unconventional device geometry reduces the width of the first Fresnel zone which increases the lateral resolution. However, the device acquisition geometry generates artifacts on seismic profiles and induces large incidence angles of the seismic signal. A specific processing sequence must be applied to the data to obtain a readable seismic section. Penetration of the seismic signal depends on the energy of the signal reaching the seafloor and on its incidence angle. Because of smaller source energy, 800 Joules Sparker data cannot be acquired in water depth larger than 1500 m for example, whereas there is no depth limit for the use of this system with air gun sources. Differential acoustic absorption of seismic frequencies (below 1000 Hz) in the water column is negligible when compared with wave fronts expansion. Thus, the horizontal resolution of any seismic system strongly depends on the frequency spectrum of the seismic source and on the travel distances. Pasisar and conventional seismic profiles being usually simultaneously recorded, we illustrate the interest of using a hybrid seismic device by comparing horizontal resolutions as well as signal-to-noise ratio obtained with both the Pasisar and conventional systems. In addition, by carefully picking time arrivals of a reflection on simultaneously recorded surface and deep-towed seismic records, it is possible to estimate the average interval seismic velocity. We present the simplified example of a horizontal reflector.

Detailed Anatomy of Late-Holocene Deposits on the Adriatic Shelf From 3D Very High Resolution Seismic Data (Triad Survey)

Very High Resolution 3D seismic data collected on the Adriatic shelf define the smallscale geometric complexity of late Holocene deposits. Three groups of units have been distinguished from deeper to shallower: the Transgressive System Tract (TST), a condensed interval at the base of the Highstand System Tract (HST) and the progradational units of the HST. The HST deposits show large-scale shore-parallel undulations and shore-normal reliefs. The genesis of these features seems to be related to a combination of two mechanisms: 1) sediment deformation in relation with the condensed section and 2) depositional process related to shore parallel currents that increasingly interact with topography.

Multiscale seismic reflectivity of shallow thermoclines

Seismic Oceanography is coming of age as an established technique of observation of the thermohaline structure of the ocean. The present paper deals with the seismic reflectivity of the Armorican Shelf seasonal thermocline, west of France, based on two seismic experiments performed with a sparker source. The peak frequency was 500 Hz for the ASPEX experiment, where the thermocline was located at 27 m water depth, and reduced to 400 Hz associated to a higher source level for the IFOSISMO experiment, where the thermocline was 12 m deeper. Despite this settings, only the first experiment could clearly highlight the thermocline reflector, providing the first seismic observation of a shallow oceanic structure. To better understand the limitation of high-resolution seismic devices in detecting weak oceanic features, we develop a wavelet-based seismic analysis and consider, as a first approximation, a simple thermocline modelled by a Gauss Error function, allowing an analytical expression for the associated seismic reflectivity. We show that the acoustic impedance profile of the thermocline is mainly controlled by a sound velocity proportional to the temperature. We show that the seismic reflectivity is controlled by the reflection coefficient of the large-scale structure of the thermocline and by an attenuation factor which depends on the ratio between the seismic wavelength and the characteristic size of the thermocline. Depending on this ratio, the strength of the thermocline-related reflection may be too weak to be detected by seismic measurement.

Sismique très haute résolution 3D : une nouvelle méthode d'imagerie des sols superficiels

This note presents the first results of the development of a 3D Very High Resolution seismic method. Particular attention was paid to the design of an operational system to be in agreement with the expected goals in term of acquisition and processing. A set of three examples, collected in various geological environments, is presented.

Mass Transport Deposits Periodicity Related to Glacial Cycles and Marine-Lacustrine Transitions on a Ponded Basin of the Sea of Marmara (Turkey) Over the Last 500 ka

The Sea of Marmara (SoM) is affected by large earthquakes occurring on the North Anatolian Fault. Numerous submarine mass movements have occurred and the most recent turbidites in the basins of the SoM have been related to historical earthquakes. Within the SoM, the occurrence of submarine mass movements and their size appears modulated by eustatic changes that can be accompanied by transitions between a salty marine environment and a brackish lake environment. Detailed analysis, using a 3D high-resolution seismic dataset, of stratigraphy over the last 500 ka, within a ponded basin of the Western High, shows that intervals of draped sedimentary reflectors alternate with onlap sequences that followed episodes of rapid sea-level rise, with a periodicity of approximately 100,000 years (corresponding to glacial cycles). Mass Transport Deposits (MTDs) occur within the onlapping sequences. Detail analysis of the youngest large slide, which probably followed the lacustrine transition to during Marine Isotopic Stage 4 is presented; and the possible triggering processes are discussed. The potential triggers of MTDs during this transition, in the context of the SoM are: (i) gas hydrate dissociation by pressure drop; (ii) changes in sediments supply and transport dynamics; (iii) variations in pressure and/or ionic strength in pores. The latter case appears the most suitable hypothesis, as salt diffuses out of the pores of the marine clay-rich sediment dominated by smectite at the beginning of low stand/lacustrine stages. The pore water freshening induces clay swelling, which can potentially drive sediment slope failure.

Very High Resolution 3D Seismic Imaging of a Complex Shelf Structure in the Adriatic Sea

Very High Resolution 3D seismic data acquired on the Adriatic shelf image the internal geometry of the late-Holocene mud wedge in a transitional zone between shore-parallel undulated features and small-scale mud relief elongated perpendicular to the bathymetry. The 3D seismic approach applied to Very High Resolution studies allowed to image the internal structure of these features possibly related to either sediment failure or transport by current and to enhance the comprehension of geological problems of key importance.

Freshwater lake to salt-water sea causing widespread hydrate dissociation in the Black Sea

Gas hydrates, a solid established by water and gas molecules, are widespread along the continental margins of the world. Their dynamics have mainly been regarded through the lens of temperature-pressure conditions. A fluctuation in one of these parameters may cause destabilization of gas hydrate-bearing sediments below the seafloor with implications in ocean acidification and eventually in global warming. Here we show throughout an example of the Black Sea, the world’s most isolated sea, evidence that extensive gas hydrate dissociation may occur in the future due to recent salinity changes of the sea water. Recent and forthcoming salt diffusion within the sediment will destabilize gas hydrates by reducing the extension and thickness of their thermodynamic stability zone in a region covering at least 2800 square kilometers which focus seepages at the observed sites. We suspect this process to occur in other world regions (e.g., Caspian Sea, Sea of Marmara).Gas hydrates are maintained via a balance of temperature and pressure, if this changes then destabilization may occur. Here, the authors show instead that due to recent changes in the salinity of the sea water of the Black Sea, gas hydrates may become destabilized with widespread methane seepage.

Gas and seismicity within the Istanbul seismic gap

Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain.

Correction to: Gas occurrence and shallow conduit systems in the Western Sea of Marmara: a review and new acoustic evidence

The original version of this article unfortunately contained a mistake. The affiliation of Pierre Henry should have been the following: Aix Marseille Univ, CNRS, IRD, INRA, Coll France, CEREGE, Aix-en-Provence, France. This correction was requested by Aix-Marseille University.