Theoretical comparison of seafloor surface renders from multibeam sonar and 3D seismic exploration data

Abstract

Abstract Detailed sea floor surface renders (i.e. bathymetric/ morphologic maps) and amplitude backscatter maps are extremely valuable and indeed necessary for offshore oil exploration and production, for both geohazard and environmental site assessment and monitoring. The continental slope off Nova Scotia has been a region of active geophysical exploration over the past three decades, but in particular over the last three years. In the summer of 2000, in excess of 22,000 km2 of multibeam hydrographic (EM-300 and EM-1002) data were acquired. These data are part of a program to understand sedimentary processes, geohazards and environmental hazards in this slope environment. The program includes significant amounts of high-resolution subbottom reflection data and shallow piston cores. At the same time, large tracts of this same ground have been covered with 3D seismic exploration data. In deep water environments (>500 m) surface renders generated by (1) multibeam sonar and (2) first-return picks of 3D seismic reflection data over the same ground are remarkably similar on visual inspection; yet the underlying physical principals of the two technologies are quite different. Although both are based on acoustic reflection principles, the systems operate at different frequencies and different angles of incidence. This paper addresses the theoretical comparison of the two technologies and their usefulness as tools for geohazard and geo-environmental assessment. It forms part of a larger study of the physical, statistical, and empirical attributes of surface renders and backscatter amplitude data generated by the two technologies in deep water environments on the Scotian Slope. Introduction Early hydrocarbon exploration on the Scotian Slope was carried out in the late 1970's and early 1980's with the completion of five deep-water wells. Exploration then stalled, but has been renewed in the last few years. Most of the slope has been leased, geophysical exploration has been intensive, and the first wells since the mid-1980's are presently being drilled. In preparation for this drilling, an industry-government partnership acquired multibeam bathymetry from the slope1 that was followed up by acquisition of high-resolution seismic reflection data and piston cores for near surface geology and geohazard studies2. Concurrently, industry has collected 3D seismic exploration data over large tracts of the central Scotian Slope, overlapping the territory mapped with the multibeam sonar. Seafloor morphologic renders can be compiled from the seafloor pick of 3D seismic data sets, and from multibeam sonar data. These data are frequently used in geohazard and environmental assessments to characterize the seafloor. In general, the finer the scale of resolution achievable, the better the assessment. In deep water, the coverage of depth soundings by 3D seismics and multibeam sonar are at a similar scale, yet the physical principals of operation of the two systems are distinctly different. It is the purpose of this study to present theoretical aspects of the distinctions of the two data types, especially with respect to vertical and horizontal resolution and precision as it pertains to the deep water regions of the Scotian Slope.