Print this pagePSGas Hydrate Geohazard Assessment in the Northern Gulf of Mexico Using a Vertical Line Array*
By
Erika Géresi 1, N.R. Chapman 1, T.M. McGee 2, and J.R. Woolsey 2
Search and Discovery Article #40159 (2005)
Posted July 25, 2005
*Poster presentation at AAPG Annual Convention, with SEPM, Calgary, Alberta, June 19-22, 2005.
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1School of Earth and Ocean Sciences, University of Victoria, Victoria, BC Canada ( [email protected] )
2MMRI/CMRET, University of Mississippi, Oxford, MS USA
Abstract
In order to understand the consequences and causes of sea-floor instability in the presence of gas hydrate, it is imperative to understand the geological setting and the physical properties of the hydrates, and to be able to estimate the distribution and concentration of the gas hydrate deposits. Conventional seismic techniques often fail to image the complex geological features, especially around and under salt domes or gas hydrates, which have high propagation velocities for seismic waves. Therefore, new techniques in data acquisition and 
processing
are sought to improve the image of complex areas, especially in the presence of gas hydrate.
This paper reports the progress in processing and development of a remote, multi-sensor sea-floor station planned for the continental slope of the northern Gulf of Mexico to monitor changes in the shallow sub-bottom over an extended period of time. A set of experiments with a prototype Vertical Line Array (VLA) were carried out as part of the development. VLA, multi- and single-channel reflection seismic data were collected from Mississippi Canyon (MC798) (800m) and Atwater Valley Area (AT14) (1300m). This research project integrated these datasets in order to assess potential geohazards such as gas hydrates using propagation velocities, physical properties and the acoustic character of the sub-bottom.
An inversion approach was developed using (1) travel-times obtained from the tau-p transform of Vertical Cable common receiver gathers, and (2) amplitude versus offset data to determine velocities, dips and depths. The processing result of VLA data showed improved vertical resolution and better reflectivity contrast. The physical properties of the sub-bottom estimated from the inversions and interpretation made possible a more advanced assessment of the nature and distribution of potential geohazards such as gas hydrate at the study areas.
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uMigration Velocity Analysis & PSDM
uMigration Velocity Analysis & PSDM
uMigration Velocity Analysis & PSDM
uMigration Velocity Analysis & PSDM
uMigration Velocity Analysis & PSDM
uMigration Velocity Analysis & PSDM
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IntroductionThe conventional seismic techniques often fail to image the complex geological features, especially around and under salt domes and sills or in the presence of gas hydrates that have high propagation velocities for seismic waves. The conventional 3-D marine seismic survey generally acquires several 2-D lines over the target area. The source vessel pulls the streamer carrying receivers positioned on the same line where the source is activated. Thus, source-receiver azimuths follow the direction of those 2-D lines. This means that all the energy spread away from the 2-D profile is missed. Missing energy is responsible for the lack of illumination in most targets under complex geology. Therefore, new techniques in data acquisition and One of the new seismic data acquisition techniques is the vertical cable (VLA). In comparison with the conventional method, the VLA provides advantages such as flexibility during operation to cover the surveyed area, recording the signal in a quieter environment, and an opportunity for direct separation of up-coming and down-going wave fields. However, with the unconventional acquisition geometry, the VLA data can not be treated directly by the conventional procedures of seismic data The
Gas HydrateGas hydrate is a solid, ice-like substance containing natural gas -mostly methane and water. Gas hydrates occur offshore on continental margins where nutrient-rich waters deliver organic detritus for bacteria to convert to methane and, to a lesser extent, in the permafrost sediments of Polar Regions. Most marine hydrates are stable at the relatively low temperatures and high pressures found in sediments at water depths about 500 meters. The sediment interval in which gas hydrates occur is known as the gas hydrate stability zone (GHSZ). Below and above this zone free natural gas may exist. Large accumulations of gas hydrates have been identified the US eastern seaboard, the Pacific Northwest, Japan etc. The presence of gas hydrate can be inferred from seismic evidence such as bottom simulating reflectors (BSRs) or changes in seismic velocity (e.g., Hovland and Judd, 1988. The petroliferous northern Gulf of Mexico is noted for its obvious absence of BSR’s, a characteristic it shares with other ”active” passive margins with mobile salt and/or shale. HazardGas hydrates may pose a geohazard in coastal areas because the dissociation, or melting, of solid gas hydrate to water and gas result in slope instability that can trigger large submarine landslides and even tsunamis. Similarly, as natural gas and oil exploration moves into deeper water where gas hydrate deposits are more likely, such as the deepwater Gulf of Mexico, drilling operations may cause gas hydrate melting, sediment instability, borehole collapse and gas blowouts. ClimateMethane, an important green house gas, released from gas hydrate in marine sediments may contribute to global warming.
Gulf of Mexico Sea-Floor StabilityIn the Gulf of Mexico, gas hydrate mounds form along the intersections of faults with the sea floor. They are edifices largely constructed of water from the sea and hydrocarbon gases that have migrated up the faults from buried reservoirs. In addition to gas hydrates, they also contain various minerals deposited by bacteria feeding on the hydrocarbons. The mounds are ephemeral, capable of changing greatly within a matter of days. Many geoscientists familiar with recent geologic processes in the Gulf of Mexico think that events which cause changes to the hydrate mounds also trigger episodes of sea- floor instability. Hydrates contained in sediments are stable as long as the sediments are within the hydrate stability zone (HSZ) as defined by pressure, temperature and chemical composition. If hydrocarbon gases migrating up faults encounter sediments of sufficient permeability that lie within the HSZ, hydrates can form within the pore spaces and support the sediment frame. This increases the sediment’s shear modulus and thereby its bearing capacity. Common indicators of bearing capacity are the speeds at which compressional (P) and shear (S) waves propagate below the sea floor and the efficiency of P-to-S conversion (PS) at reflecting horizons. A comprehensive monitoring station would be capable of measuring these and many other parameters that are relevant to the formation and dissociation of gas hydrate. The general configuration of the hydrate stability zone in MC798 has been observed seismically and, in light of the heat-flow data, its thickness estimated to be about 400m (Trevor Lewis, pers.com.). The negative reflection about 500ms below the sea floor is interpreted as the base of the hydrate stability zone.
Vertical Line Array (VLA) Acquisition TechniqueA prototype VLA has been designed and constructed as part of the development leading to deploying a remote, multi-sensor sea-floor station planned for the continental slope of the northern Gulf of Mexico. The station will use one VLA and four horizontal arrays of hydrophones to monitor changes in the shallow sub-sea-bottom over an extended period of time. The prototype Vertical Line Array (VLA) was deployed and recovered successfully in 2002 and 2003 by the CMRET. During the 2002 and 2003 VLA deployments, survey tracks were carries out while firing a surface-towed 80 in3 watergun. In 2002, surface source deep receiver single channel reflection data were also recorded.
Travel-Time Inversion for Vertical Cable GeometryThe basic unit of An analytic proof for the 2-D travel-time equation was developed using horizontal and vertical slowness for models with dipping interfaces and homogeneous layers. Travel-time curves in both τ - p and T - X domains were computed as well as the ray path for vertical cable geometry.
VLA
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