Correlation of Seismic Refraction Compressive Velocity and CPT Data With Particular Application to the Continuous Burial Assessment of Pipelines and Cables

Abstract

Abstract In response to the demand of the telecommunication cable market, new tools have been recently developed and introduced for burial assessment purposes which are now available to the pipeline industry. The cone penetrometer (CPT) has become the reference tool for obtaining the geotechnical parameters required for burial engineering. The bottom-towed system Gambas® provides an accurate stratigraphy of the subsoil over the target burial depth and the compressive velocity of the various layers. Correlating the seismic velocities measured on a continuous basis and the geotechnical parameters derived from CPT data at discrete locations is the key to data integration procedures and continuous burial assessment all along the route. The paper focuses on relations that can be established between the compressive velocity of subsurface marine sediments as obtained from the Gambas® system and the geotechnical parameters derived from standard CPTs. The work is based on i) an extensive analysis of several thousand kilometers of cable route surveys combining continuous seismic refraction profiles and regularly spaced 2 to 3m penetration CPTs and ii) laboratory experiments in a calibration chamber where cone penetration and seismic wave propagation tests were performed in sands under well controlled conditions. Introduction The increasing need for reliable continuous burial assessment of telecommunication cables has promoted the development of new survey methods. Data acquisition techniques and equipment are available to perform efficient burial assessment surveys in water depths of interest for the offshore and telecom industries, i.e., from shore to about 2000m. It is now recognized that standard geophysical surveys performed for route selection are insufficient and inadequate to provide reliable data for burial assessment. Specific burial assessment surveys (bas-surveys) are carried out which combine acquisition of high quality geotechnical data and continuous electronic profiling (e_bass techniques). Cone penetrometer (CPT) techniques implemented alone or in combination with high quality sampling have demonstrated their capability to provide at competitive costs the geotechnical parameters required for a proper identification of the marine sediments and the engineering parameters relevant for assessing the maximum feasible burial depth and the towing forces of plows. The bottom-towed high resolution seismic refraction system Gambas® has proved to be of prime interest to provide continuous and accurate soil layering information over the target burial depth and to characterize the various layers by their compressive velocities. Procedures for integrating data of different geophysical and geotechnical sources have been simultaneously developed. The major challenge consists of interpolating all along the route the geotechnical data acquired at discrete sampling/insitu testing locations. Establishing sound correlations between geophysical data (e.g., seismic velocities) and geotechnical data (e.g., cone resistance) is the key to spatial interpolation. This paper describes the most recent developments in burial assessment techniques and procedures and focuses on the correlations that have been established between compressive seismic velocities and CPT data in marine sediments. An original classification chart of marine sediments is proposed.