Abstract

Deep-water surveys were completed during 1981 in the Carteret Canyon area, U.S. Atlantic East Coast, 100 miles east of Atlantic City. The surveys covered an area of approximately 16 lease blocks on the continental slope in water depths of 300-2100 meters and provided detailed high-resolution data from the shelf break to the rise. Program objectives included (a) an assessment of available deep-tow systems for detailed site surveys in deep water and (b) acquisition of geologic and geophysical data for evaluation of surface and near-surface processes relevant to platform siting. Systems included medium range side-scan sonar (27-30 kHz) with ranges of 500 and 2500 meters on line spacings of 1000 meters and 5000 meters, respectively. Short-range (100 kHz) sonars provided detail at ranges of 150 and 200 meters, with 480 meter line spacings. Deep-tow subbottom profilers (3.5 and 4.5 kHz) acquired near-surface data to 100 meters below the mudline. Deeper geologic structure was provided by 300-joule minisparker and 8-12 kjoule sparker with multifold recording and processing. Improved methods of deeply towed vehicle navigation and winch/cable handling were needed for the survey. The unparalleled high-resolution data of sea floor and near-surface features, in rugged terrain, represent a major advance in geologic site survey capability for platform installation in deep water.

Introduction

The offshore oil and gas industry has begun to exploit hydrocarbon reservoirs in deep water continental shelf margins and continental slope environments. Such deep water sites pose challenging problems; the recent geologic history and the nature of contemporary sea floor processes may impose engineering design constraints. Acquiring high-resolution geophysical data on near surface sediments and contemporary sea floor processes in water depths exceeding approximately 600 meters poses problems that differ significantly from those .commonly associated with shallow water surveys. The East Coast Hazards Observation (ECHO) Program was conceived to examine the capability for acquisition of sufficiently high-resolution data to promote progress in the understanding of the geologic context for future engineering in a deep water (continental slope) setting off the U.S. East Coast (Fig. 1), Figure 2 shows the detail limits of the survey grid.

A well defined, closely-grided, high-resolution geophysical survey can determine the presence or absence of diagnostic bottom features that indicate processes that may impose stresses on bottom supported structures or pipelines. The data can also be used to interpret the three-dimensional geometry of surface and near-surface features, slopes, and sedimentary units. The ECHO program focused on two main objectives:

  1. Develop procedures for deep water surveys. Previous engineering site surveys have been completed in relatively shallow water and thus there is a need to develop optimum cost-efficient techniques for deep water sites. While state-of-the-art prototype deep-tow survey systems have been developed by most of the major equipment companies, there existed a need to examine the relative capabilities of these systems and their use for marine surveys on closely spaced grids. A major goal was to demonstrate that industry can conduct high precision site evaluations, comparable to those routinely applied in shallow water regions, in water depths consistent with future leasing plans.

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