Problems with piston core sampling: Mechanical and geochemical diagnosis

A series of sampling experiments using a piston corer equipped with data logging instruments has demonstrated that representative stratigraphic intervals of soft marine sediments may never be obtained with piston corers that use a non-reference triggering system. Incidents of sediment by-passing and flow-in can occur throughout a single core, resulting in both foreshortening and stretching of sedimentary units. These problems are difficult to recognize by standard visual core logging techniques. Most of these problems are initiated by accelerations in the lowering cable that occur after tripping, with resultant accelerations in the piston, and fluctuations in the cavity pressure inside the core liner. The non-representative sampling can result in more than 1.0 m of missing stratigraphic units below the sediment interface, and as much as 1.3 m of sediment flow-in. These problems can lead to miscalculations of sediment deposition rates by as much as a factor of two, and can result in obtaining samples that are of no value for geotechnical analyses.