Piston Coring Capabilities at SACLANTCEN: Minimizing and Assessing Core Disturbance

摘要

Piston and gravity coring are techniques used to collect samples of seabed material. There are many variables that can be adjusted when operating a piston corer, such as free-fall height, core liner material, weight, piston friction, rigging, and winch speed. In order to develop a capability at SACLANTCEN to collect longer cores with minimal disturbance, the aforementioned variables were adjusted in a systematic manner in order to determine their relative effects. During the Coring Engineering Trial in March 1999, multiple cores were collected at an experimental site in the Capraia basin, north of Elba Island, where a geo-acoustic model has been developed based on a time domain inversion of wide-angle reflection data and frequency domain modelling of bottom loss data. The disturbance of the seabed material during the coring process may have an adverse effect on its physical properties, such as sound speed, magnetic susceptibility, and bulk density. Accordingly the amount of compression in each core has been estimated by an analysis of magnetic susceptibility data, correlating and aligning nulls in the response. Laboratory measurements of sound speed and bulk density on three of the cores have been compared with each other and with the gen-acoustic model. From a seabed dominated by silt and clay material with some thin layers containing shells, the properly configured piston corer was able to recover cores of 5 to 6 m in length with a compression of approximately 10%. When it is not properly configured or does not function properly, the compression may be considerably higher, 25 to 35%. Critical factors in the operation of the piston corer include the piston design, the piston friction against the liner, the strength of the shear pins, the cable lengths, and the winch speed. Factors that are less significant include the liner material and free-fall height.