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Enlarge Image Figure 4: Accelerometer test for KNR191-19CDH. The upper panel shows the time series of acceleration for the release system (red) and the piston core (blue) head. The bottom panel shows the time series of movement (in meters) of the release system (red) and the core head (blue). Piston core release occurred at t = 0 seconds. The short upward acceleration of the release was 2 meters or less, which limited the movement of the piston during the coring operation. At about 4.8 seconds after release the piston reached the top of the piston core, causing nearly 3 g acceleration to the release system. Core penetration of 43 m ended about 5 seconds after release, but note that the acceleration data implies that the system continued its penetration. We believe that the jolt the system withstood when the piston reached the top of the core also affected the "zeroing" of the accelerometers, effectively ending their reliability during that operation.
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| During several coring operations we inserted accelerometers
in the release system and piston core head, which allow us to quantify the motion of the system at the time of core
release and during sediment penetration (Figure 4). At the time of piston core release, the
accelerometer on the release system recorded an upward acceleration of about 1 g, but
lasting for a short enough time to limit upward movement to less than about 2
meters. Following release, the piston core
fell, accelerating for about 2 seconds and decelerating for another 2 seconds. The core came to a "stop" five seconds after release, penetrating more than 40 meters
into the sediment.
The violent collision of the piston and the core head between t = 4
and 5 seconds suggests that the piston corer was still penetrating the
sediments when the piston reached the top of the core. It appears that
this collision may have affected the "zeroing" of the accelerometers
since the motion after t = 5 seconds implies that the core and release
both continued downward even after reaching full sediment penetration. We know
from the mud line on the exterior of the core head that penetration
ended at about 43 m below the sea floor.
The small amount of
upward movement of the release system minimized the upward movement of the piston within
the core and therefore minimized coring
disturbance of the sediments As a result, there is no evidence for sediment stretching in the upper section of the new cores.
Last updated: December 20, 2010 |
