SHRIMP: A New Equipment or Precise SUbsurface Soil Investigation by High-Resolution Seismic Refraction
Authors: Pedotti G. , Dubois J-C. , Meunier J. 
Published: October 2008

A new equipment has been developed for' a precise and continuous investigation, by the high resolution seismic refraction method, of sub-bottom materials over the very shallow depths concerned by pipeline trenching projects. Qualification and full-scale tests have demonstrated the capabilities and performances of this equipment.

1. INTRODUCTION

Precise knowledge of the soil profile and geotechnical parameters, over the first few meters below sea bottom along pipeline routes is of primary importance for trenching or plowing projects. Experience shows that an improper assessment of these data is often responsible for severe problems at time of performance of the works.

For these projects, soil survey programs usually associate seismic reflexion profiling and geotechnical investigations at specific locations. In many cases these classical techniques fail to provide the necessary data:

The seismic equipments commonly used (mud penetrates, sub-bottom profilers) are not adapted to the actual problem of identifying reflectors within the first 1.3ms (about 1m) after seabed, with a precision better than 0.4ms (about 0.3m). New generation high resolution seismic equipments integrating digital acquisition, associated with sophisticated data processing, may somewhat improve the quality of structural analysis. However, the applicability of velocity inversion methods to the very shallow depths below sea bottom remains most questionable.

Geotechnical investigations provide very accurate but punctual data. They usually allow a precise determination of the geotechnical parameters of the different materials likely to be encountered but are of very little use for defining continuous profiles along a pipeline route.

A new equipment, SHRIMP, based on the seismic method, is an answer to this actual need for a tool well adapted to the precise investigation of the sub-bottom materials on a few meters penetration along continuous profiles, in water depths ranging from shore to 200m.

2. METHOD: HIGH RESOLUTION. SEISMIC REFRACTION

Investigation with the SHRIMP is performed by the high resolution seismic refraction method.

This method is widely applied onshore for site investigations related to various types of civil works, its major interest being the direct determination of velocities and depths of different sub layers identified within the investigated zone.

Further to several years of work on the offshore applications of the seismic refraction method, GEODIA, jointly with the Institute Français du Petrole (IFP) and the Institute Français de Recherché pour l'Exploitation de la Mer (IFREMER), started in 1987 a research and development program on a specific equipment adapted to the pipeline route survey. A preliminary phase of this program was aimed at concluding whether the method could provide an appropriate answer to the actual problem of very shallow sub-bottom investigation, with the following criteria:depth of investigation: 1 to a few meters,identification of layers with velocity contrasts as low as 10%.determination of contact depth with a precision better than 30cm.measurement of compression wave velocities of identified layers with a precision better than 100m/s.