Cruise Objectives

The five primary objectives of the Gyre cruise were:

1. Characterize the shallow seismic stratigraphic framework of the two site survey areas:
   This objective addresses understanding the geologic framework of each site, i.e. to understand the stratigraphic and structural relations and how they might affect or alter hydrate occurrence. Success in meeting this objective requires collecting seismic reflection data sufficient to image the subbottom environment in which gas hydrate might occur and to relate local features to the broader understanding of the geology of the basins and structural highs in the Gulf of Mexico.
   
   
2. Acquire data to map the distribution of acoustic indicators of gas hydrate:
   Several seismic indicators exist for identifying hydrate in the subsurface: the bottom simulating reflection (BSR), zones of amplitude blanking, and zones of enhanced reflections that may indicate the presence of free-gas trapped beneath the hydrate stability zone. Understanding the spatial distribution of these indicators can help determine the likely presence of hydrate in the sediments, as well as the places where hydrate may be most concentrated (and therefore a target for a drilling experiment). Good spatial coverage of high-quality, high-resolution data are needed to meet this objective.
   
   
3. Tie to pre-existing public-domain seismic data and available well information:
   Part of interpreting the geologic framework of the sites involves integrating the cruise data with existing seismic data and their interpretations as well as calibrating the seismic data with existing sample information, preferably well data. MMS provided the nearest well ties for the two sites. Ties to public-domain seismic data were determined from pre-existing USGS data sets and knowledge of the deep-seismic reflection LSU-B line.
   
   
4. Identify transects to reoccupy with near-bottom instrumentation:
   Geophysical characterization of the sites for potential drilling requires integrating the seismic reflection data from this cruise with near-bottom instrumentation measurements that will be collected in August, 2003 (DTAGS, heat flow, electrical resistivity, and possible shallow coring). Because the instruments for making near-bottom measurements are not regional mapping tools, identifying the best locations to collect these specialized data needs to be carefully considered. Therefore, the data from this Gyre cruise are particularly important in identifying the best sites at which to collect the more expensive and more specialized near-bottom data.
   
   
5. Contribute to selecting potential targets for gas hydrate drilling:
   Integrating the information from objectives A through D should lead to narrowing the geographic boundaries for hydrate drilling targets, e.g., identify key locations in target lease block areas. While the purpose of this cruise is not to explicitly pick the drill sites, the seismic data are expected to contribute to prioritizing sites for drilling. Data will also be used to formulate models of hydrate-free and hydrate-bearing sediments. These models can be directly tested by the JIP drilling. The seismic data collected during G1-03-GM are near-zero offset and therefore do not yield seismic velocities that can be used to convert reflection travel times to reflector depths. However, the higher vertical resolution of the data will enable a more detailed interpretation of the gas hydrate stability zone than is possible with most standard industry data.