New discoveries at Woolsey Mound, MC118, northern Gulf of Mexico

Woolsey Mound, a 1km-diameter carbonate-gas hydrate complex in the northern Gulf of Mexico, is the site of the Gulf’s only seafloor monitoring station-observatory in its only research reserve, Mississippi Canyon 118. Active venting, outcropping hydrate, and a thriving chemosynthetic community recommend the site for study. Since 2005, the Gulf of Mexico Hydrates Research Consortium has been conducting multidisciplinary studies to 1. Characterize the site, 2. Establish a facility for real-time monitoring-observing of gas hydrates in a natural setting, 3. Study the effects of gas hydrates on seafloor stability, 4. Establish fluid migration routes and estimates of fluid-flux at the site, 5. Establish the interrelationships between the organisms at the vent site and the association-dissociation of hydrates. A variety of novel geological, geophysical, geochemical and biological studies has been designed and conducted, some in survey mode, others in monitoring mode. Geophysical studies involving merging multiple seismic data acquisition systems accompanied by the application of custom processing techniques verify communication of surface features with deep structures. Supporting geological data derive from innovative recovery techniques. Geochemical sensors, used experimentally in survey mode, including aboard an AUV, double as monitoring devices. A suite of pore-fluid sampling devices has returned data that capture change at the site in daily increments; using only noise as an energy source, hydrophones have returned daily fluctuations in physical properties. Ever-expanding capabilities of a custom-ROV have been determined by research needs. Processing of new as well as conventional data via unconventional means has resulted in the discovery of new features…..vents, faults, benthic fauna…..and modification of others including pockmarks, hydrate outcrops, vent activity, and water-column chemical plumes. Though real-time monitoring awaits communications and power link to land, periodic data-collection reveals a carbonate-hydrate mound, part of an immensely complex hydrocarbon system

La sismica ad alta risoluzione digitale a mare: vincoli teorici, elaborazione numerica, nuovi sviluppi (High-resolution marine digital seismic method: theoretical constrains, digital processing, new developments )

Marcello Bernabini, Rinaldo Nicolich, Luigi Sambuell

Improving 3D Water Column Seismic Imaging Using the Common Reflection Surface Method

© 2020 Elsevier B.V. Water column processing has gained attention in recent years since a seismic model of a water column could assist marine data processors to correctly image the sub-seafloor geology, which is the target of primary interest. In addition to seismic processing, water column imaging has gained interest in the physical oceanography community for improved study of oceanographic processes. However, seismic water column processing is challenging since the internal reflections of the ocean are inherently weak and are often masked by noise. In this work, we adopt the common reflection surface stack technique in order to improve the imaging of ocean water layers. The common reflection surface stack is a robust data preconditioning and stacking technique in seismic processing that relies on the kinematic wavefront attributes of seismic waves. The method is applied to a multichannel 3D data set collected for oil and gas exploration in the deep-water Gulf of Mexico. The method greatly improves inline sections but does not significantly enhance crosslines and horizontal slices, which are more sensitive to both the acquisition geometry and the temporal variability of ocean water masses

Temporal and Spatial Variations In Three-Dimensional Seismic Oceanography

Seismic oceanography is a new cross-discipline between geophysics and oceanography that uses seismic reflection data to image and study the oceanic water column. Previous work on seismic oceanography was largely limited to two-dimensional (2D) seismic data and methods. Here we explore and quantify temporal and spatial variations in three-dimensional (3D) seismic oceanography to address whether 3D seismic imaging is meaningful in all directions and how one can take advantage of the variations. From a 3D multichannel seismic survey acquired for oil and gas exploration in the Gulf of Mexico over a 6-month period, a 3D oceanic seismic volume was derived. The 3D seismic images exhibit both temporal and spatial variations of the ocean, and theoretical and data analyses were used to quantify their contribution. Our results suggest that temporal variation is more prominent in the crossline direction than in the inline direction, causing discontinuities in crossline images. However, a series of 3D inline images can be seen as snapshots of the water column at different times, capturing temporal variation of thermohaline structures induced by ocean dynamics. Our findings suggest the potential uses of marine 3D seismic data in studying time-evolving mesoscale ocean dynamics

Fluid emission affecting lowstand shelf deposits on the flank of a volcanic island (Zannone Island, Tyrrhenian Sea, Italy).

Several Gas Related Features (GRFs) have been detected offshore the Western Pontine Archipelago by means of very high resolution multibeam bathymetry, high resolution seismic profiles, ROV video observations, water and sediment sampling. Importance of GRFs is related to their association with hydrocarbon occurrence in the subsurface, settlement of endemic ecosystems and the possible direct or indirect linkage with marine geohazards (submarine slides, earthquakes, damage to seafloor infrastructures). In particular, at a broader spatial and temporal scale, geological emissions of methane may be take into consideration as geological factors controlling Quaternary atmospheric and climate changes. Pontine Archipelago is located 30 kilometers from the Italian peninsula (Eastern Tyrrhenian Sea) and is composed of five Plio-Pleistocene volcanic islands: Ponza, Palmarola, Zannone (western sector) and Ventotene and S. Stefano (eastern sector). The research is focused on specific GRFs located offshore the eastern sector of Zannone Island, at water depth ranging between 105-130 m (outer continental shelf). GRFs comprise: giant pockmark, several pockmarks and dome topographic features. The giant pockmark has elongated shape and is 900 m long and 500 m wide; pockmarks are characterized by circular, sub-circular and elongated shapes with dimensions ranging between 2-80 m; whereas dome topographic features are mostly cone-shaped structures with dimensions between 8-36 m. ROV observations have revealed the occurrence of active fluid emissions escaping from the seafloor, characterized by different discharge modalities (continuous and intermittent) and presence of widespread bacterial mats, possible chemosynthetic bivalve aggregations, small scales cone structures and several burrows linked to bioturbation and/or fluid escaped. Moreover, water column backscatter data acquired by multibeam have revealed the occurrence of plumes extended to at least 70 m into the water column. Analysis of very high seismic profiles show the occurrence of several flares at the same location of plumes individuated by water column backscatter data and the occurrence of lowstand prograding deposits, covered by a few meters thick of Holocene deposits. Lowstand prograding deposits have a max thickness of about 35 m and across the giant pockmark show a chaotic seismic facies, indicating intense deformation. To date, rare cases of active shallow-water cold seeps have been described in the Mediterranean Sea. Analysis of the morphological, sedimentological and stratigraphic characteristics of the study area provide the first evidence of an active shallow-water cold emission site in the Eastern Tyrrhenian Sea

The Zannone Giant Pockmark. First evidence of a giant complex seeping structure in shallow-water, central Mediterranean Sea, Italy

An active giant pockmark located offshore Zannone Island (central Tyrrhenian Sea, Italy), is analyzed by very high resolution multibeam bathymetry, high resolution seismic profiles, ROV video observations, sediment and water sampling. The active fluid emission area is located on the outer shelf, between 110 and 130 m water depth, and affects the Late Quaternary lowstand and highstand deposits resting on rocky bedrock. A variety of fluid-escape features characterizes the area, including the Zannone giant pockmark, several smaller pockmarks, hummocky terrains and areas of positive relief. Ground-truth video data show active seepages, bacterial communities, widespread lithified pavements, mounds, and cone-shaped structures. Evidence of active seepage includes both continuous and intermittent bubble release from the seafloor and a well-defined plume rising 70 m above the seafloor. The Zannone giant pockmark is about 900 × 500 m (surface of some 0.5 km2). It represents the first evidence of an active shallow-water seepage area in the central Tyrrhenian Sea (Italy) and the first record of a morphologically complex giant pockmark in the entire Mediterranean Sea. Some speculations on processes originating the observed features are explored, including possible occurrence of multiple eruption events, processes of fluidization–liquefaction and minor slides that may have modified the original morphology. Factors peculiar to the study area – shallow depth, thin sedimentary cover resting on a faulted rocky basement, seeping occurring through non-cohesive sandy sediments appear to have been key to the formation and morphology of the Zannone giant pockmark