Various observations pointed out that cores performed with gravity piston corer show significant distortions mainly located at the top of the core. A series of 15 cores were performed at the same location on a submarine sand wave (Var canyon, France). Six different settings of the corer "three freefall heights and three slacks of the piston cable" were tested, including duplicates. Two accelerometers recorded simultaneously the movements of the core tube and the movements of the triggering arm. Then the z displacements were obtained by a double integration versus time of the measured acceleration. The analyses of results allowed the authors to estimate the amplitude and the duration of the elastic recoil of the aramid cable, and to distinguish four steps during the 4 seconds of penetration, including a distortion phase followed by a normal sampling phase linked to the status of the piston.
The analyses of the quality and benchmark layers from recovered cores highlight the major role of the piston driven by the lengths of the counterweight and piston cables. The recovered thickness of a given layer can vary from 0.8 to 1.3 depending to the settings.
A cone penetrometer test (CPT) trial at the same location gives a good estimation of the absolute geometry of the layers. The settings for cores with geotechnical purpose (better quality) will be different from settings for cores with sedimentological or palaeoclimatological purposes (better geometry). A compromise is proposed.
Gravity coring with stationary piston is an efficient way to recover long sedimentary cores1. However, various observations pointed out consequent distortions2 based on different techniques: magnetic orientation3, and comparison of different corers or with sub-bottom profiler4. Authors described the ?over-sampling? and the ?under-sampling5, and others proposed recommendations for improvements of corers6.
The Stacor corer gets round these disturbances with a truly stationary piston7, 8. The device provides high quality Sampling 9 especially for soil investigation10, But the duration of the deployment (8–10hr) and the size of vessels and cranes are constraining factors for the use of such a device by scientific community.
The trial of a new aramid cable onboard the R/V Le Suroit was an opportunity to examine the effect of the elastic recoil of such a cable on the recovered sediment. The use of accelerometers allowed for the recording the behaviour of a corer and understanding the effect of settings on the recovered sediment.
The stationary piston corer of the R/V Le Suroit consists of a 994kg weight and a 10m steel tube with a plastic liner (maximum recoverable length of sediment of 9.55m), as well as a platform for the release or triggering arm linked to the main cable and the counterweight linked to the plat-form with a cable (Figure 1). The piston is linked to the platform via the piston cable and slides freely inside the liner. The main cable is a 17mm aramid cable with a weight in water of 0.056kg/m.