Summary
The objectives of this study are (1) to process both previously released and recently collected bathymetric, sidescan sonar, ground truth data in the Alaminos Canyon area, Gulf of Mexico; (2) to qualitatively and quantitatively compare the effects of different spatial interpolations on discrete depth soundings; (3) to evaluate the geological factors that contribute to the sidescan sonar image; (4) to characterize and interpret the surface and near surface geomorphological features.
Seafloor morphology or bathymetry is the basic and most important property of deep water environments. For examples, detailed bathymetry is a necessity in laying out a telephone cable route, estimating potential geological hazards, and building sediment transport models. Bathymetry is especially important for the continental slope of the Gulf of Mexico where the seafloor is mainly a surfical representation of subsurface allochthonous salt. As hydrocarbon traps are often closely related to allochthonous salt structures, an accurate and detailed bathymetry will help refine the extent of the underlying salt sheets in the hydrocarbon evaluation stage and the construction of platforms and pipelines in the prospecting stage.
The most extensive seafloor survey in the Gulf of Mexico was conducted by the National Oceanic and Atmospheric Administration (NOAA) from 1988 to 1991. This survey was made using a multibeam echosounder, Sea Beam, in the U.S. Exclusive Economic Zone (EEZ), which extends 200 nautical miles offshore from the United States and the U.S. trust territories. Both gridded and raw data were released in 1992. However, the NOAA bathymetry mapping project was terminated after only part of the EEZ was surveyed. One of the unfinished areas is the Alaminos Canyon, in which the most detailed previous bathymetric map was based on seismic surveys with grid spacing of 8 to 14 km (Bryant et al., 1990). In 1990, William Sager and others surveyed a major portion of the canyon using the same multibeam survey system (Sager, 1997). The Sea Beam soundings, with a spatial resolution of about 74 m to 120 m in the Alaminos Canyon, have never been published. The study uses the 1990 WHOI Sea Beam survey data and reveals the fine features of the bathymetry.
Spatial interpolation derives values at unsampled locations form sampled locations. This process is especially important when data points are randomly sampled and a high spatial resolution raster is necessary for many analyses and visual presentations. Different interpolation algorithms can generate different results an affect the accuracy of results. However, a complex and computation-extensive algorithm does not necessary outperform a simple algorithm. The choice of algorithm is data dependent, and should be made empirically. In this study, to fully use the depth soundings collected by multibeam systems, different interpolation algorithms are tested and compared. These comparisons not only offer criteria for choosing the best estimator but also present the accuracy of the interpolated result.
The long range GLORIA II sidescan sonar imagery of the Gulf of Mexico was released by National Geophysical Data Center (NGDC) in 1987. In the approximately 12° longitude by 6° latitude gray-scaled mosaic backscattered image, detrital sediments transported from the Mississippi Canyon and basins surrounded by high slope gradients are evident. But for a scale of tens of minutes by tens of minutes, several factors make geological interpretation difficult. These factors include the noisy nadir below the ship track, ambiguity of true or artificial pixel values, and the uncertainty of georeferencing. This study reprocesses the NGDC released GLORIA II data to differentiate artifacts from true backscattering, to register geographic coordinates for each foot print, and to examine parameters affecting backscattering.
The ground truth samples are used to describe physical properties of the deep water sediment and compare their spatial variations. When the bathymetry data, sidescan sonar imagery, and physical properties of the sediment are combine, we can better understand the geomorphological driving processes and sediment movements in the Alamions Canyon.
Related Publications: Bryant, W.R., Liu, J.Y., Ponthier, J., “Engineering and Geological Constraints of Intraslope Basins and Submarine Canyons of the Northwestern Gulf of Mexico,” Gulf Coast Association of Geological Societies Transactions, Vol. XLV, pp. 95-101, 1995.
