Geotechnical Sampling of the Sea Floor
Authors: Ross D.I. 
Published: March 2010

Introduction

A part of exploration for petroleum resources is the development of a regional geological model of a prospect area. Such a model requires ground truthing of geophysical data through sampling of bedrock sequences. This ground truthing is provided by borehole drilling and, in areas where the bedrock closely approaches the seafloor, can be achieved cheaply and effectively with remotely operated seafloor drilling systems.

Development of offshore petroleum plays requires a knowledge of the engineering properties of the seafloor sediments at the location of the potential development. Offshore Canada, the need for detailed geotechnical knowledge of the seafloor sediments is increased by the requirement to provide protection for pipelines and production systems from impact of ice and icebergs. Information is required for foundation studies, excavation/trenching criteria and the more general consideration of effective loading on buried systems resulting from iceberg impact on the overlying seabed.

Generally, geotechnical information of this kind has been obtained from borehole drilling and associated in-situ geotechnical measurements. In the offshore, this is expensive if carried out from a surface drilling vessel, particularly in water depths greater than 150 m, where the use of a dynamically positioned drill vessel becomes almost essential. As a result, increased emphasis has recently been placed on the refinement of geophysical tools and the development of remotely operated portable seafloor- mounted sampling and measurement systems for cost-effective preliminary assessment of extended areas.

Acoustic "Sampling"

Considerable work has been carried out on the relationship between acoustic velocity and impedance and other physical properties of marine sediments (e.g., Laughton, 1954; Hamilton et al., 1956; Hamilton, 1970; Sutton et al., 1957; Morris et al., 1978). This work was originally aimed at accurately predicting sound velocity in seafloor sediments from a knowledge of average grain size and sediment type. In recent years, the emphasis has' been on the remote measurement of acoustic properties for the estimation of the physical properties of the sediments (e.g., Li and Taylor Smith, 1969; Lee et al., 1977; Maclssac and Dunsiger, 1979).

(figure in full paper)

acoustic impedance mismatches resulting from changes in the physical properties of the sediment within a sedimentary sequence to map stratigraphic boundaries. Measurement of the reflected energy enables the acoustic impedance of the sedimentary interface to be calculated and thus changes in the physical properties of the sediments to be predicted.

Advances in the design and manufacture of high-resolution marine seismic systems have led to the development of systems with well-defined and repeatable broadband output pulses. Sound sources such as the Huntec Deep Two Seismic System have enabled quantitative measurements of acoustic reflectivity and online graphic display of reflectivity indices to be included as part of the standard survey data acquisition (Parrott el al., 1979, 1980). The reflectivity data, coupled with the acoustic character of the seismic record has proved effective in defining sediments type and mapping the boundaries of geological units on the seafloor.