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Winch length vs. CTD depth, e.g. on Gaia Blu

The winch indicated a cable length that was not matching the CTD depth. The value was less important than depth (e.g. 48 m of cable, 50 m for CTD depth), and the difference increased when the profile was deeper. The CTD depth is robust and reliable, inferred from the pressure sensor (REF: decibar incertitudes, depth equivalence, TEOS functions), while the length of the cable, digitally calculated from the winch parameters, has proven to be unreliable from a safety point of view. The risk is to bring the probe to the bottom if used without the CTD information. Repeated observations have shown that the difference increases with depth (e.g. approximately 5 m difference at 100 m depth, 50 m at 1000 m, and up to 300 m at 3000 m, see Fig. 1.).

Example: the bottom is at 3000m, the winch payout is programmed at 2900m "to stop before", but in reality the bottom will reached at 2700m of cable.

The main recommendation that emerged from the practice on board is to check systematically the difference between the unrolled cable length of the wind and the CTD depth given from the pressure sensor. Once a stable correspondence between the cable length and the CTD depth has been established empirically, a simple procedure could be, for each profile:

  1. check the local depth using the acoustic multibeam datae.g. 677m
  2. round down to the nearest decadee.g. if 677m consider 670m
  3. subtract a safety distancee.g. 40m, corresponding to the altimeter range: 670m - 40m = 630m
  4. find its correspondence with the cable length (empiric)e.g. according to our observations, 630m of depth corresponds to 600m of cable
  5. report the value of 600 in the payout field, and not 630, in order to ensure that the probe will safely stop at a maximum depth reasonably distant from the bottom
  6. start the profileslowly first, at 20 m/min, then faster, at 40 m/min, then up to 60 m/min = 1 m/s.
  7. continue to monitor the CTD value and the difference with the winch length
  8. once close to the bottom areae.g. the winch unrolled 550m and the downcast continuesslow the descent to 40 m/min, then 20 m/min, and increment the payout value with precautione.g. from 600 to 620 and be ready to stop at anytime
  9. check if the altimeter catches the bottom: the bottom should be detected at a distance of 50 to 30 meters
  10. if the altimeter does not catch the bottom: stop the profile, proceed to the final operations, and prepare to upcast
  11. if the altimeter responds well, use its value to finish the profile by stopping at a security distance of 10-15 meters
  12. proceed to the final operations (measurements, closure of bottles, ...), and prepare to upcast

Fig. 1. Example of difference between winch length and CTD depth. The difference is not constant, but increases with depth.