Geological Hazard Surveying for Exploratory Drilling in Water Depths of 2000 Meters

Abstract

ABSTRACT Hazard surveys have been completed for exploration drilling on the U.S. Atlantic continental slope to water depths of 2000 m. Two regional surveys, near Carteret Canyon and south of Georges Bank, provided a geologic basis for 10 subsequent site-specific studies. Regional data were gathered with medium- and short range side-scan sonar, deep-tow subbottom profilers, and sparker and mini sparker lines linked to COST wells, and were supported by narrow-beam bathymetry. The regional results guided interpretations of data for individual blocks and drill sites gathered over closely spaced lines with deep-tow, high-resolution side-scan, sub bottom profiles supported by drop cores of particular geologic targets. Deep-seated geologic features were assessed using complementary mini sparker and processed sparker data. These regional and highly detailed surveys have shown that large areas of the continental slope do not contain hazards resulting from active geologic processes. Much of the present morphology of the slope is due to processes operating on geologic rather than engineering time scales. Criteria have been developed for assessment of actual drill site sea-floor conditions on the basis of subsurface structure, near-surface sediment types/thicknesses, and local morphology. INTRODUCTION In preparation for exploratory drilling, a capability has been developed to evaluate potential seafloor engineering constraints in water depths of 2000 m. Highly detailed geologic data sets, from state of the art techniques, were acquired between 1981 and 1983 in various locations along the U.S. Atlantic continental slope (Fig. 1). Begun in 1981, the East Coast Hazards Observation (ECHO) Program has now completed regional high resolution surveys for parts of the slope off New Jersey (ECHO I) and Georges Bank (ECHO II). Additionally, 10 site-specific studies have been carried out, involving a total of 3,365 km of data acquisition (Table 1). The assessment of sea-floor geologic conditions in these frontier deepwater environments required the simultaneous development of innovative survey methods, together with new procedures and concepts for data interpretation. Regional surveys were conducted to provide a considered logistical basis, and a geologic framework, for subsequent site-specific studies. Interpretation of regional data and resulting improved knowledge of sea-floor geology were used to guide site specific survey design, choice of equipment, data acquisition methods, and preliminary analyses. Site specific geologic features were considered within their regional geologic context, which, in turn, was improved as more areas were covered. A balanced, integrated hazard evaluation procedure has evolved in which field methods both contribute to and are the result of better geologic understanding. Extremely high quality data have been gathered on which evaluations of 'prospective drilling sites are based. The ECHO Program has thus provided an operational and interpretive rationale that has proved extremely useful for drill site consideration in the very complex, high relief geology off the U.S. East Coast; it is also suitable for hazards assessment in other deepwater areas where exploratory drilling is planned.