Bruno Marsset

Hydrate dissolution as a potential mechanism for pockmark formation in the Niger delta

Received 17 February 2010; accepted 9 March 2010; published 11 August 2010. [1] Based on acquired geophysical, geological and geotechnical data and modeling, we suggest hydrate dissolution to cause sediment collapse and pockmark formation in the Niger delta. Very high‐resolution bathymetry data acquired from the Niger delta reveal the morphology of pockmarks with different shapes and sizes going from a small ring depression surrounding an irregular floor to more typical pockmarks with uniform depression. Geophysical data, in situ piezocone measurements, piezometer measurements and sediment cores demonstrate the presence of a common internal architecture of the studied pockmarks: inner sediments rich in gas hydrates surrounded by overpressured sediments. The temperature, pressure and salinity conditions of the studied area have allowed us to exclude the process of gas‐hydrate dissociation (gas hydrate turns into free gas/water mixture) as a trigger of the observed pockmarks. Based on numerical modeling, we demonstrate that gas‐hydrate dissolution (gas hydrate becomes mixture of water and dissolved gas) under a local decrease of the gas concentration at the base of the gas‐hydrate occurrence zone (GHOZ) can explain the excess pore pressure and fluid flow surrounding the central hydrated area and the sediment collapse at the border of the GHOZ. The different deformation (or development) stages of the detected pockmarks confirm that a local process such as the amount of gas flow through faults rather than a regional one is at the origin of those depressions.

PASISAR: a new tool for near-bottom very high-resolution profiling in deep water

Detailed sea-bed geology (stratigraphy, faulting, lateral facies variation) of deep water areas (over 300 m water depth) is nowadays being explored, both for scientific purposes such as a better insight into marine sedimentary processes and tectonics, and for future offshore industry developments. This requires us to address new technical challenges.

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.

Identification of Shear Zones and Their Causal Mechanisms Using a Combination of Cone Penetration Tests and Seismic Data in the Eastern Niger Delta

In a site investigation of the eastern part of the offshore Niger delta, cone penetration tests (CPTU) showed significant drops in tip resistance, associated with decreases in sleeve friction and induced excess pore pressures at the interface between superficial sediments and the underlying deposits of a mass-transport complex (MTC) called NG1. Such signature characteristics of weakened zones are clearly expressed at three sites where the drop in tip resistance reaches more than 40% over 2–3 m-thick intervals. Correlations between CPTU profiles and both 3D and ultrahigh-resolution 2D seismic data suggest that the weakened zones surround syndepositional the within the frontal part of NG1. Hence, weakening appears associated with the remobilization of thrust faults, inducing localized plastic shear. Relatively recent, deep-seated structural movements affecting NG1 are suspected to have remobilized these thrusts faults. When considering the sole influence of gravity, the fact that shear strength is mobilized within scattered, limited zones along steeply dipping syndepositional faults is not favorable for the further development of a continuous slope-parallel failure surface above NG1.

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.

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.

Very High Resolution Multichannel Recording For Shallow Seismic

This paper describes the use of multichannel techniques for very high resolution seismic data recording which improves the signal/noise ration, spatial accuracy and signal penetration. The practical use of this technique is as simple as the one used for single channel purposes.

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.

PASISAR: processing of very high-resolution near-bottom seismic data

The PASISAR very high resolution seismic data are recorded through a single channel streamer attached to a deep towed side-scan sonar, using conventional seismic sources located at the sea surface. This device provides an outstanding improvement of both horizontal and vertical resolutions of seismic profiles in deep waters. However, the motions of the towed fish and the dissymmetry of the acquisition device dramatically alter the quality of data. Therefore a dedicated processing is mandatory in order to provide the interpreters with legible seismic sections.<<ETX>>