An Installation in the Northern Gulf of Mexico for Monitoring Interactions Between the Water Column and Sea-floor Sediments Containing Gas Hydrates
Authors: McGee Thomas M. , Woolsey J. Robert 
Published: October 2008

A program has been initiated to monitor outcrops of gas hydrates on the continental slope of the northern Gulf of Mexico. This will be done by means of a multisensor station deployed on the sea floor for significant periods of time. Sensors will be seismic, acoustic, electromagnetic and spectroscopic. Data will be digitized on site and transmitted by optic fiber cable to an offshore platform from whence they will be relayed to a shore facility via satellite.

Preliminary tests of seismic techniques over gas hydrate outcrops began in June, 1998, as part of a research cruise to areas of Mississippi Canyon where hydrates are known to occur at or near the sea floor. Further preliminary work is being done during fiscal year 1999-2000. It includes very-high-resolution seismic profiling over possible station sites (this time in the Viosca Knoll area), tests of three-component seismic detectors, both seismometers and accelerometers, and deployment of a vertical hydrophone array.

Introduction

The Center for Marine Resources and Environmental Technology(CMRET, formerly the Marine Minerals Technology Center, MMTC) of the University of Mississippi, the U.S. Naval Research Laboratory at Stennis Space Center, Mississippi, and the U. S. Geological Survey (USGS) at Woods Hole, Massachusetts, have initiated a program to install a multisensor monitoring station on the continental slope of the northern Gulf of Mexico. The station will monitor physical and chemical parameters of the water column and sea floor for the purpose of remotely observing transient changes to the water column and sea floor in the vicinity of outcrops of gas hydrates. Rationale for the program stems from a growing realization that the stability of the sea floor in the region may be influenced by the presence of gas hydrates and instances of their sporadic disassociation.

The need for such observations was discussed at a meeting of research scientists held at the Stennis Space Center on March 17,1998. Harry Roberts of the Coastal Studies Institute, Louisiana State University, reported observations from repeated manned submersible visits to hydrate outcrops that "support a pattern of episodic venting. Short-term episodes of venting are probably regulated by fault movement, perhaps controlled by local salt adjustment. Destabilization of gas hydrates by oceanographic processes also causes short-term episodic gas expulsion. These events occur with inter-annual to intra-annual frequencies." Ian MacDonald of the Geochemical and Environmental Group, Texas A&M, also reported physical changes to hydrate outcrops that had been documented by repeatedly diving on the same sites.

Water depth at the outcrops to be monitored is about 1000 meters. It would be difficult and costly to make direct observations of transient events at such depths. It is expected to be more cost effective to design and deploy a remotely operated station that will measure a number of physical and chemical parameters, more-orless continuously, over an extended period of time. Studies preliminary to choosing a site for the station began in June, 1998, during a research cruise in the Mississippi Canyon area which was sponsored jointly by CMRET and USGS. Comparison of several techniques of seismic data acquisition were made over areas of known hydrate occurrence. The use of a broadband surface source and a deep-tow receiver seemed to be the most promising. During the 1999-2000 fiscal year, that technique will be applied to obtain very-high-resolution seismic profiling over potential station sites in the Viosca Knoll area.