Search

Ifremer has recently developed a deep-towed seismic system designed for High to Very High resolution near-bottom marine seismic surveys. This paper presents this new tool which is dedicated to the study of deepwater geological objects especially geohazards. After a technical description of the system, we expound an overview of the complete workflow from survey design to amplitude preserved processing illustrated by data acquired during sea trials.

We report on a reconnaissance analysis of the geochemical composition of authigenic carbonates and sediment samples collected from various seepage sites on the Niger deep-sea fan. Our aim has been to investigate whether evidence for the presence of authigenic carbonates and gas hydrates within sediments is discernible from solid-phase sediment geochemistry. We show that sedimentary Sr/Ca and Mg/Ca ratios can be used to infer the presence of authigenic aragonite (Sr-rich) and Mg-rich carbonate phases (high-Mg calcite, dolomite) in cold seep settings. Using Sr/Ca and Mg/Ca ratios, the proportion (wt.%) of authigenic carbonates in Niger Fan sediments can be calculated from a mixing model between sediment fractions of terrigenous material, biogenic calcite, aragonite and high-Mg calcite. This approach was applied to high-resolution geochemical profiles along sediment cores recovered from various cold seep settings (mud volcano, diapirs, pockmarks, gas-hydrate bearing sediments). Our data reveal that authigenic carbonates occur as discrete phases in sediments from gas-hydrate-bearing areas, suggesting that such carbonate-rich sediment layers may represent paleo-indicators for ancient methane seepage in marine sediments, possibly associated to gas-hydrate reservoirs. (C) 2007 Elsevier B.V. All rights reserved.

We present a new method to characterize free gas, gas hydrates and carbonate concretions occurrence which are considered as high-risk factors for sub-sea developments in the Niger delta. This method is based on the combination of 3D seismic data to the geotechnical site characterizations using piezocone CPTU tests (Cone Penetration Test with additional measurement of the pore water pressure). A special processing of the 3D seismic data has enabled the determination of the interval compressional velocity. Using the effective-medium theory, velocity anomalies (negative and positive) within the first 15 m were translated in gas hydrate and free gas distribution. The calibration of the P wave velocity anomalies was done thanks to in-situ geotechnical testing carried out during two oceanographic surveys (2003 and 2004). Comparison between in-situ testing, recovered cores and the prediction of the gas and the gas hydrate distribution based on the compressional wave velocity have shown that 3D seismic data is a valuable tool to identify heterogeneous areas but the use of the piezoeone was essential to discriminate between gas hydrate occurrences and carbonate concretions' presence. Furthermore, in-situ compressional wave velocity (V-p) measurements have clearly demonstrated what it was suspected from the 3D seismic data, the co-existence in the study area between gas hydrate and free gas. (c) 2007 Elsevier B.V. All rights reserved.

The study area, offshore Nigeria, is located in one of the compressional zones within the Niger Delta, which is characterized by imbricate thrust structures. Although the low mean slope angle (around 2°), bathymetry data from the study area have shown the existence of several submarine landslides which coincide with known subsurface faulted compressive features. In this paper, we have focused on a submarine slide occurring in water depths ranging between 1690 and 1750 m. Headwall scars, internal architecture and associated deposits have been characterized using a combination of 3D seismic data, near-bottom echosounder seismic profiles, Kullenberg cores and in-situ geotechnical measurements. The slide shows horseshoe shaped headwall scars and depositional lobes with positive relief. Monitoring of excess pore pressure for 12 months indicates the presence of negative hydraulic gradients, which is either an indication of a local present-day mechanical activity of subsurface faults or related to the regional extension of the Niger Delta and the possible creation of a regional depression in the hydraulic regime. In order to identify the triggering mechanism of the observed landslide, a three-dimensional slope stability model (SAMU-3D) based on the upper bound theorem of plasticity was used. Calculation results have shown that the gravity loading generated by the sediment weight alone is not sufficient to explain the observed submarine slide. Cylindrical cavity expansion theory was used to locally simulate the compressional structure movements and to evaluate the strength generated within the upper sediment layers. Slope stability assessment carried out by considering this additional structural strength has shown that regional compressional gravity driven deformation can explain the observed submarine failures.

Very High Resolution (VHR) 3D seismic data were collected on the Adriatic shelf offshore the city of Ortona in order to define the small-scale geometric complexity of late Holocene deposits. Three groups of units are distinguished from the bottom up: (1) units U1 to U3 forming the Transgressive System Tract (TST); (2) units U4 and U5 representing the condensed interval deposited on the Maximum flooding surface (MFS) and the basal unit of the High stand System Tract (HST); and (3) U6 to U 10 forming the progradational units of the HST. The muddy HST deposits show seafloor and subsurface sedimentary features over up to 40% of its extent on the Adriatic shelf. In the area offshore Ortona, the HST shows shore-parallel undulations in water depth of ca. 30-70 m and mud reliefs in water depth of ca. 70-110 m. The mud reliefs are subcircular in plan view and occur in swarms, perpendicular to the regional contour, that are connected to each other by shore-parallel junctions. The genesis of these features is interpreted as a combination of two different mechanisms: (1) sediment deformation in relation with the condensed section; and (2) depositional process related to shore-parallel currents interacting with topographic irregularities. The evolution of the HST sedimentary pattern is closely related to a "key unit" (U5) that influences both the seaward transition from linear to complex features and the stacking pattern.

Very high-resolution seismic data from the Kaiser-I-Hind sand bank (southern Celtic Sea) recently highlighted the internal structure of the enigmatic Celtic Banks, which are among the deepest and largest shelf sand ridges. The main body of the bank is made up of 4 seismic/depositional units which reflect a transgressive evolution. New data on the detailed architecture of two of these units allow discussion of bank growth in terms of either (1) a channel–levee system preserved both by lateral migration and aggradation of the channels, or (2) a package of large offshore tidal sediment bodies (bar chains and/or giant dunes). Careful geometrical observations of seismic discontinuities make the second hypothesis more likely. The unit architecture is analysed in terms of long- to short-term processes of build-up. Long-term processes are evinced by the landward stacking of erosive sub-units in response of the last post-glacial sea-level rise, whereas short-term processes control the seaward progradation of sand bodies and fills due to the ebb predominance of the Western Channel Approaches.

The Bohai Sea is a shallow sea at the east coast of China. It communicates with the Yellow Sea through a narrow strait. During and since the Late Pleistocene, the Bohai Sea has been a filled subsiding basin submitted to successive regressive-transgressive cycles. In the East Bohai Sea, a number of finger-shaped ridges are present near the strait, where tidal currents are dominant. Very-high resolution seismic reflection data have been interpreted relative to sedimentological data provided by a formerly drilled borehole. Seven seismic units are identified and correlated with the following events: units U7-U6 with the Bohai relative sea level rise (65,000–53,000 yr B.P.) and a relative sea level fall (53,000–39,000 yr B.P.); unit U5 with the Xianxian relative sea level rise (39,000–22,000 yr B.P.); unit U4 with the period of the late Würm glaciation (22,000–15,000 yr B.P.); units U3-U2 with the transitional period from the end of the Pleistocene to the beginning of the Holocene (15,000–9000 yr B.P.); and unit U1 with the Holocene (Huanghua) marine intrusion in the Bohai Sea (since 9000 yr B.P.). In the Bohai Sea, formation of the tidal ridges is probably due to a substantial increase in tidal-current velocities during the late Holocene rise of sea level. Constriction due to the strait amplifies the tidal process, so that the sediments of the late Holocene transgressive sequence have been remodeled as tidal ridge.