CE17-01 MVP Data

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MVP Sound Speed Data
RV Celtic Explorer Cruise CE17-01
Chief Scientist: Kevin Sheehan, MI
July 22nd to August 6th 2017

John E. Hughes Clarke
Anand Hiroji
Jose Cordero Ros
Center for Coastal and Ocean Mapping
University of New Hampshire

direct comparison - EK60 multi-frequency scattering section v. continuous MVP profiling

Contents:

Overview
Data Output
Standard Operational Procedures
Recommendations.

Overview:
This cruise marked the first time that the MVP-200 had been used on the Celtic Explorer in ~ 10 years. Originally purchased by the Marine Institute in ~ 2003, the fish had only been deployed for a few cruises before it was lost. Subsequently (~2004-2007?), all CE mapping surveys used a stationary conventional winch deployed SVP. The outer shelf INFOMAR surveys have only just restarted (CE16-01 last September was the first survey). The hope is that the MVP can be reintroduced to increase the efficiency of survey.

As part of CCOM experiments into imaging the thermocline variability, the MVP was requested for this survey. The idea was that the MVP could act as a groundtruth, defining where the thermocline really way as a comparison against the acoustic scattering layers (see animation above).

Darrell Groom of AML (formerly of Rolls Royce, ODIM and originally Brooke Ocean) undertook an inspection and maintenance of the hardware (which had been kept in a warehouse for a decade). A single sensor fish was still available and two probes were re-calibrated. They were a :

AML microCTD
AML Smart SVP

The original idea was to use the CTD to distinguish temperature from salinity controls on sound speed. However, while the sensor gave good values when stationary, as soon as the fish was underway, the salinity values dropped (from the real 34-25 ppt to ~ 1 ppt). This was traced to a clear micro-fracture in the glass of the conductivity probe. As a result ,the SVP probe was used instead. Unfortunately, this is one of the old probes that only measures SV without a temperature sensor. Thus it was not possible to invert for salinity. To compensate for the lack of oceanographic data, occasional Valeport or Seabird CTD casts were made.

The fish was deployed almost continuously at survey speed of 7.5-8 knots. The recovery messenger was deliberately located so that the fish would sit at ~ 4.5-5.5m at normal tow speed. This matches the depth of the EM302 array. Standardly the MVP was launched to ~ 10m off the bottom every ~ half an hour. As the survey consisted of ~ 2.1 hour lines this typically provided 4-5 dips in every line.

In total, over 500 profiles were obtained in 14 days of survey. The fish was only brought in in the rare event that the vessel was moving through a new area within which the density of fishing gear had not yet been established.

Data Output:

The following plots show all the SV profiles taken over the ~14 days of operations:

This first plot emphasizes the location and thickness of the thermocline over the 2 week period.

Time Series of SV structure
The thermocline is represented by the colour range (1494.75 to 1511 m/s)
greyscales above and below represent very different local ranges
(lower: 1491.8 to 1494.75 m/s --- upper: 1511 to 1513.7 m/s)

As can be seen, over the two week period, the men thermocline deepened (with lots of local oscillations, the focus of this study). And the surface mixed layer, started off warming and stratifying during the first 3 days of calm weather. The first gale, mixed up again and cooled the whole layer. The inter-storm period allowed a partial restratification of the surface mixed layer.
Throughout the whole period, the bottom mixed layer gradually warmed. Superimposed on all these temporal trends, there is a clear east-west gradient in both the surface and bottom mixed layers, presumably reflecting gradients toward and away from the distant UK coast.

This second set of plots illustrates the time variability in the thermal gradients in the layer above the thermocline:

	Time Series of upper layer thermal structure The colour range goes from : 1512 to 1514 m/s contours at 1 m/s intervals
This plot emphasized the variability in the average temperature, and amount of weak stratification in the surface mixed layer. As mentioned above, the initial calm period promoted the development of a shallow secondary thermocline. This was destroyed by the first gale. To place the MVP variability in context, the plots below illustrate the day-night cyclicity seen in the EK60 tri-frequency imaging over the identical time period.
	EK60 - RGB image of the same time period

Standard Operational Procedures

As the MVP hadn't been used for a decade, the aim was to come up with an optimal safe deprecating procedure that would allow maximum benefit while minimizing the risk of towbod y loss.

The following linked documentation has been provided.

Recommendations

The following changes are suggested:

Video Coverage of the Winch - at this time it is not possible to assess the state of the winch drum or messenger status without physically go out and up to the instrument,. Having at leasty two real-time video feeds - one looknig inside the winch drum, and one looking at the outer block - would greatly increase the inside operator's awareness of the instrument state.
Dual Sensor Towbody - it would be nice to have both SV and CT sensors on the same fish. This would allow redundancy, cross-calibration checks (of SV v. derived SV and pressure v. Pressure) and an ability to investigate oceanographic controls.

page composed offshore -- JEHC