A Synergistic Combination of Technologies: Multibeam Bathymetry, Acoustic Reflectivity, Gravity, and Magnetics in the Deepwater Gulf of Mexico

Abstract

Abstract Exploration and production in the deepwater Gulf of Mexicois hampered by an absence of existing survey data. In the late 1980s and early 1990s, NOAA ventured to map the deep Gulf of Mexico with multibeam sonar. Unfortunately, this visionary project lacked both the funding for completion and the technology to provide the survey accuracy and resolution presently desired. An aggressive campaign has been initiated to acquire and evaluate collocated multibeam bathymetry, acoustic reflectivity, gravity, magnetic, and sub-bottom data across 17,000 square miles of the uncharted deepwater Gulf of Mexico. Evaluation of these data is being accomplished with new technology developed by the University of New Brunswick, Canada, which interactively integrates them into one common graphical environment. "Fly-throughs" of geo-referenced data provide construction planning, production analysis, and allow for the establishment of geologic correlation. Interrelationships associated with the collection, processing, and interpretation of these various data sets and the benefit of multibeam bathymetry on the 3D Bouguer correction are discussed. NOAA Surveys The National Oceanographic and Atmospheric Administration (NOAA) was forerunner in the application of multibeam technology in the Gulf of Mexico. From 1989 to 1992, NOAA conducted a successful campaign of mapping the Gulf beyond the continental shelf. A lack of governmental funding brought the program to a halt prior to its completion. Consequently, a large "data gap" exists in the areas of East Breaks, Alaminos Canyon, and the western half of Garden Banks (Fig. 1). The NOAA program involved the incorporation of two different multibeam bathymetry systems that were state-of-the-art at the time. For waters shallower than 1000 meters, the Hydrochart II was employed. This system operated at 36 kHz and provided 17 beams across a swath of 2.5 times the water depth. A 12 kHz Sea Beam system was employed for waters deeper than 1000 meters. It provided an array of 16 beams across a swath of 0.7 times the water depth. The sounding density that resulted varies as a function of the water depth, vessel speed, and the system used, but a 250-meter bin is the most common filter applied to these data (across-track raw sounding densities range from approximately 75 meters to 130 meters). The 250-meter filter provides multiple soundings per bin, allowing statistical leverage for an accurate sounding selection. Similar calibration methods were employed for the Hydrochart II and the 12 kHz Sea Beam systems. These comprised of iterations that included:Collecting sounding data on predefined survey lines.Comparing charted, overlaid contours of sounding data.Scaling offsets from charts.Entering offset values into LOTUS-123 spreadsheets for evaluation.1,2 Considering the technical limitations of the positioning systems, motion sensors, processing systems, and multibeam systems employed at the time, this method of calibration served its purpose quite well (some organizations still employ this method in cases of shallow waters and narrow swaths).