Five of the instruments on this mooring carried pressure sensors
that worked, although one failed after about 10 days. Looking at
rest depth, two things stand out:
1. Depth decreased by about 3 meters during the first 3 or 4 days,
and by another meter during the next 10 days. This does not look
like line stretch, since it is fairly uniform with depth. On the
other hand, current speeds were not high during this time so it
seems unlikely that the mooring was dragged - especially unlikely
that it was dragged uphill.
2. Sometime between 6 January 2005 and 16 January the depth of the
mooring increased by about 7 meters.
Again we will use the initial rest depth - in this case the rest
depth during the first day - to determine the initial seafloor depth
and an initial depth for each instrument. Comparing the nominal and
measured depths of the five instruments (and rounding to the nearest
meter) we have
nominal depth measured depth difference
455 m 444 m 11 m
456 m 452 m 4 m
655 m 646 m 9 m
1056 m 1044 m 12 m
1555 m 1057 m -2 m
For the top two instruments, which we know were only a meter apart,
the problem obviously is inaccurate calibrations - which may or may
not be true for the other instruments. In any case, giving equal
weight to the differences, we get an average difference of about
7 meters - on average the instruments were 7 meters shallower than
the nominal depths.
The installation and recovery logs put the seafloor at a depth of
1756 meters. The best we can do here is place it at 1756 - 7 = 1749
meters and take (nominal depth - 7) meters for the various instrument
depths.
Move up one level.