Combining geophysical-survey technologies in the deepwater GOM - Gulf of Mexico - Statistical Data Included

World Oil, Sept, 1999 by Art Kleiner, Brian Anderson, Lindsay Gee

When bringing together the newest advances in multibeam bathymetry, acoustic reflectivity, gravity, magnetics and interactive 3-D viewing, the result is more than the sum of its parts

E&P in the deepwater Gulf of Mexico (GOM) is hampered by an absence of existing geophysical-survey data. In the late 1980s and early 1990s, NOAA (National Oceanographic and Atmospheric Administration) ventured to map the deepwater GOM with multibeam sonar. Unfortunately, this visionary project lacked both funding for completion and the technology to provide the survey accuracy and resolution presently desired.

An aggressive campaign has begun to acquire and evaluate collocated multibeam bathymetry, acoustic reflectivity, gravity, magnetic and sub-bottom data across 17,000 [mi.sup.2] of uncharted GOM deep water. 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 and production analysis, and allow for the establishment of geologic correlation.

Interrelationships associated with collecting, processing and interpreting these data sets and the benefit of multibeam bathymetry on the 3-D Bouguer correction are discussed in this article.

NOAA SURVEYS

NOAA was the forerunner in applying multibeam technology in the GOM. From 1989 to 1992, it conducted a successful campaign of mapping the Gulf beyond the continental shelf. A lack of governmental funding brought the program to a halt before 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.

[Figure 1 ILLUSTRATION OMITTED]

The NOAA program incorporated two different multibeam bathymetry systems that were state-of-the-art at the time. For waters shallower than 1,000 m, a 36-kHz system was used. It provided 17 beams across a swath 2.5 times the water depth. For waters deeper than 1,000 m, a 12-kHz system was employed. It provided an array of 16 beams across a swath 0.7 times the water depth (see table).

Comparison of acquisition systems

Property          NOAA, < 1,000 m   NOAA, > 1000 m   Simrad EM-300

Frequency, kHz        36                12              30
Positioning, m     [  or -] 15       [  or -] 15      [  or -] 2
Pitch, deg        [  or -] 0.50     [  or -] 0.25    [  or -] 0.05
Roll, deg         [  or -] 0.15     [  or -] 0.25    [  or -] 0.05
Yaw, deg          [  or -] 0.75     [  or -] 0.75    [  or -] 0.05

The sounding density that resulted varies as a function of water depth, vessel speed and the system used, but a 250-m bin is the most common filter applied to this data. Across-track, raw, sounding densities ranged from about 75 to 130 m. The 250-m filter provides multiple soundings per bin, allowing statistical leverage for an accurate sounding selection.

Similar calibration methods were employed for the 36-kHz and 12-kHz systems. These comprised iterations that included: 1) collecting sounding data on predefined survey lines; 2) comparing charted, overlaid contours of sounding data; 3) scaling offsets from charts; and 4) entering offset values into spreadsheets for evaluation.[1,2]

Considering the technical limitations of positioning systems, motion sensors, processing systems and multibeam systems employed at the time, this calibration method served its purpose quite well. Some organizations still employ this method in cases of shallow waters and narrow swaths.

Although these surveys were collected employing tolerances considered low by today's standards, from a regional perspective they are astounding. They comprise the foremost GOM bathymetric database in existence today and are used by the oil and gas industry throughout many E&P stages.

They create an extraordinary panorama of the deepwater Gulf, which aids in regional construction and development of offshore oil leases. Unfortunately, their limited accuracy reduces their effectiveness as a tool for detailed exploration and production analysis.

NEW INSTRUMENTATION

C & C Technologies, Inc., in conjunction with LCT, Inc., is "filling the gap" in GOM, NOAA multibeam data using new technology that includes multibeam bathymetry, multibeam imagery (Fig. 2), gravity, magnetics and chirp sub-bottom profiling. The R/V Ocean Alert, a retired, 235-ft Canadian Coast Guard vessel, is fitted with the newest scientific hardware, including a Simrad EM-300 multibeam system, ZLS La Coste & Romberg marine gravity meter, GEM Systems Overhauser magnetometer, Datasonics Chirp II pinger and POS/MV, GPS/Inertial navigation system.

[Figure 2 ILLUSTRATION OMITTED]

The Simrad EM-300 (see table) furnishes collocated bathymetry and imagery at an operating frequency of 30 kHz, providing an angular coverage sector of up to 150 [degrees] with 135 simultaneous 1 [degrees] X 2 [degrees] beams. The angular-coverage sector and beam-pointing angles are variable to maximize the number of usable beams. Equidistant and equiangle modes are selectable. All beams are steered electronically for roll, pitch and yaw. Operating depths ranged 10 to 4,500 m below the transducer, accurate to the greater of 0.3% of water depth or 15 cm throughout the swath.