Introduction: Satellite Imagery, Visualization and Geological Interpretation of the Exumas, Great Bahama Bank
Paul M. (Mitch) Harris, James Ellis, 2013. "Introduction: Satellite Imagery, Visualization and Geological Interpretation of the Exumas, Great Bahama Bank", Satellite Imagery, Visualization and Geological Interpretation of the Exumas, Great Bahama Bank: An Analog for Carbonate Sand Reservoirs, Paul (Mitch) Harris, James Ellis
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The Exuma Islands and surrounding carbonate sand bodies of Great Bahama Bank are an important training venue, an area of interest to researchers of modern carbonates, and a valuable modern analog for understanding carbonate sand bodies in the subsurface. We hope to promote this interest by making readily available a set of processed satellite images, offshore/onshore digital elevation model (DEM), and interpretation maps organized into a GIS, along with examples of how this data can be visualized and used for geological interpretation.
The clearest satellite images of the Exumas 1972 to 2005 were acquired and processed. Image processing was undertaken to maximize apparent water penetration and delineate submerged features. The primary images are 28.5-m Landsat TM, 15-m ASTER, and 0.6 m Quickbird. The processed scenes were georeferenced to a base image in GIS, creating a stack of co-registered images that can be effectively used for change detection. Masks were created for water and land to enable integration of different images and maps. A digital bathymetric map was created from water depths estimated from the spectral characteristics of a Landsat TM image. This offshore depth map was integrated with an onshore DEM derived from NASA Space Shuttle elevation data (SRTM) for the islands.
The overall distribution of carbonate sands, i.e., total sand body, was interpreted primarily with the offshore DEM and satellite imagery. By selecting different water depth intervals, different portions of the total sand body were highlighted for visual analysis and extracted for morphometric measurements. A water depth interval termed the “shallowest sands” (0–1.8 m water depth) revealed geologically-reasonable patterns of deposition and accounted for 37% of the total sand body in the subarea studied. Forty-eight out of the 617 shallowest sand bodies have areas larger than 100,000 m2. An evaluation of the shape of these shallow sand bodies using the Form Factor shape parameter of Russ (1999) suggests larger sand bodies are relatively more irregular compared with smaller sand bodies.
The total sand body was also divided into flood tidal delta lobes, ebb tidal delta lobes, and sand flats/island-attached sands - flood tidal delta lobes comprise 85% of the total sand body. The average distance from any portion of the flood tidal delta to the channel opening along an ebb-flood dividing line was 2.7 km ± 1.5 km (with a range from 0 to 7.3 km). Fifty-two active tidal channels, either terminating within the tidal delta lobes or open to the east toward the open ocean, were interpreted from the Landsat image and offshore DEM. Channel lengths average 2.9 km and range between ∼ 0.5 and 8.2 km, whereas their depth averages 5.4 ± 2.7 m and ranges from 0 to 13 m. Sediment was deposited in the flood delta lobes up to ∼ 4400 m from the tidal channels and averages 900 m ± 650 m from them.
Interpreting Pleistocene and Holocene ridges from enhanced satellite imagery of one key island (Hawksbill Cay) indicated 38% of the island is Pleistocene at the surface, and the Holocene ridges were deposited around the Pleistocene topography in a complex fashion. Integrating elevations with the interpretation map indicated that Holocene ridges have elevations from near sea level to 12 m (mean elevation of 7.7 ± 2.5 m) while Pleistocene landforms have elevations from near sea level to 19 m (mean elevation of 9.3 ± 3 m). Spectral classification of a key island (Shroud Cay) dominated by tidal flats reveals a consistent pattern of depositional environments related to subtle elevation differences. Changes between 1967 and 2005 in channels and emergent sand bars/beaches between Shroud Cay and Hawksbill Cay show the area was relatively stable throughout the 40-year period, but one portion showed a significant increase of emergent sand bars/beaches.
Our application of spatial analysis techniques to the Exumas and its carbonate sand bodies provides new insights into this modern carbonate setting and potentially provides data that can have value as an analog in reservoir characterization and modeling. To increase accessibility, improve learning, and promote spatially accurate feedback, the stack of images, color-coded DEM, and geologic maps were exported out of GIS into easier and more flexible viewing programs such as GeoPDFs, GoogleEarth, and Animations.