SAR & GOM

The GOM is a dynamic marine basin; it encompasses important energy and fisheries resources; it hosts intense population centers and real state development, it is a crucial transportation corridor and it has been a nexus of hurricane activity. SAR satellite data contributes to scientific study and effective management in the region. The utility of SAR data is enhanced by high spatial resolution, independences from cloud cover and its ability to distinguish fine-scale processes.
In 2007, more than 3000 new image collections were requested by users within the GOM. A preliminary review of this data archive shows that there is good potential for new types of research that can take advantage of the comprehensive coverage. In the following sections we outline some of the more promising research directions. Here we present three possible areas of investigation: a) hurricane studies; b) investigation of eddies, and c) study of natural hydrocarbon seeps.

Hurricane studies with SAR in the GOM.
The GOM area is well known for its intense hurricane activity. Consequently, one SAR application is to analyze wind data from hurricanes and monitor the atmospheric expressions of storms. There are two main applications of SAR data for hurricane studies. One is the qualitative analysis of hurricane structures, which focuses on the accompanying processes of hurricanes such as rain bands and, atmospheric rolls. The second application estimates ocean wind retrieval by using the capability of SAR to detect the modulation of the sea surface wind field. Hurricanes and other mesoscale storms are easily detected with existing observations, but SAR can contribute fine-resolution low-level information that cannot be seen below the cloud deck with other sensors. An example of this capability is the strong evidence for the existence of roll vortex secondary boundary layer circulation in hurricanes.

The Radarsat-1, Envisat and ERS-1 and ERS-2 spacecraft have passed over several storms in the Gulf of Mexico. We used NOAA post-season track data, extracted from the Historical North Atlantic Tropical Cyclone Tracks dataset, from NOAA (available at the Coastal Services Centers website: http://maps.csc.noaa.gov/hurricanes/) to match hurricane tracks with our GA. This review shows that, of the 213 hurricanes and storms developed on the Atlantic Ocean since 1992, 73 of them entered the Gulf of Mexico area. Intersection of hurricane tracks in the corresponding time frame with Radarsat, Envisat and ERS footprints, indicate that at least 30 tropical depressions and hurricanes have been captured in more than 100 SAR images in the Gulf of Mexico.

Marine applications of SAR images also include utilization of algorithms that extract wind vector information as well as ocean waves feature analysis. Wind stress is one of the main forces driving ocean dynamic process, retrieval of wind information from SAR imagery provides a useful complement to support traditional wind observations and numerical modeling. Therefore the GOM SAR data set may be useful for future studies of tropical storm processes.

Estimation of wind intensity during hurricane is problematic due to the difficulty of instrument ing extreme conditions. SAR data can provide an alternative source for this information. The significance of these observations and their possible major effect on air-sea fluxes also improve the prediction of hurricane intensity change. It has been found that SAR images could provide valuable details about a storms’ structure when it is out of range of coastal radars. SAR images has been used to analyze roll vortices in regions between the rain bands of hurricanes where no other sensor has capability to monitor this phenomena.

Eddies, Oceanic Fronts and The Loop Current
Circulation through the GOM is restricted by the Yucatan Channel and the Florida Straights . A major current, the Loop Current, flows through the Gulf transporting approximately 27 Sv. of water and occasionally meandering and shedding eddies. These eddies can play an important role in hurricane intensification due their heat transfer potential. In the GOM, eddies detached from the Loop Current are also important for weather and environmental effects. These phenomena can restrict oil exploration and production planning in deep water due to generation of very strong currents.

Depending on the wind stress, SAR satellites can detect eddies in several beam modes. Evaluation of the potential of SAR for eddy detection and vertical motion in ocean has been tested with high accuracy. We examined records showing the extent of the loop current and detached eddies and compared these with our G.A. The results indicate that at least 6400 SAR images cover both the Loop Current and eddy activity areas.

Natural seepage of hydrocarbons.
Oil and gas production from the GOM generates substantial economic benefits for the United States of America and Mexico and the GOM is recognized as one of the most important hydrocarbon systems in the world. Two important oil fields are located in the southern GOM: Reforma-Tabasco and Sonda de Campeche. This complex is also commonly called “The Cantarell Field”. More than 80% of crude oil and 90% of the national gas production is derived from these areas and its coastal plain. In the United States the production of oil and gas is highly concentrated in the offshore areas of Louisiana and eastern Texas, and to a lesser extent, along the southern half of the Texas coastal gulf and east of Louisiana as far as Mobile, Alabama. Hydrocarbons are released into the surface waters of the GOM as a result of accidental spills associated with production and transport of energy products within the GOM.

SAR data can be used to detect layers of floating oil released by natural or accidental events. Interactions between the sea surface and microwaves are very sensitive to variations in sea surface roughness. Rough surfaces scatter large amounts of energy back to the antenna and have bright signatures, while smooth surfaces reflect the energy away from the antenna and have dark signatures. The high sea reflectivity is due to Bragg scattering from capillary and short gravity waves; therefore, the backscatter intensity in sea water is stronger than in surfactants. The detection of oil slicks with single-polarization signals from SAR sensors has been found to be optimal at small incidence angles and under moderate wind speeds, ranging from 3-10 m/s.

The distribution and relative density of SAR collection appears to be strongly correlated with areas of energy production and exploration. In the Southern GOM, the Cantarel area has been the focus of over 3000 SAR image collections. In the northern GOM SAR images cover the continental slope off Lousiana with a collection density of between 660 and 900 images per unit area. Therefore, although the applications intended by users of SAR data are not well documented, one can speculate that energy exploration and pollution monitoring are among the major reasons for the historical collection of SAR data in this region.
Natural oil seeps offer additional possibilities for research. These geologic features contain faults and other conduits through which oil and gas migrate from sub-surface reservoirs and escape into the water column. Seeps are of interest for energy exploration. In addition, seep hydrocarbons have a profund impact on deep-sea ecology and may be a source of green house gas to the atmosphere.