CE17-01 EK60 Imaging

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EK60 Thermocline Imaging
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

Contents:

Overview
Data Presentation
Sonar Settings
Synchronization
Diurnal Signature
Daily Sections
Line by Line Sections

Overview:
One of the aims of this cruise was to see if we could remotely and robustly detect undulations in the thermocline. From the point of view of bathymetric surveying, it is these periodic variations (on timescales ranging from minutes to hours) that have the greatest impact on the refracted ray path. Failure to adequately monitor its location though sufficient physical sampling usually results in poor quality bathymetry, particularly in the outer parts of the swath.
As the standard MI operational approach is to run with only 10-15% overlap using angular sectors of 65-70 degrees, this has a major impact on the vertical accuracy of the data. Ultimately, insufficient sound speed profiles will result in the data not being to acceptable standards (typically IHO-Order 1, allowing ~ +/-1.3% uncertainty at the 95% level).

Data Presentation:
For every line, there are three available views of the water column acoustic scattering - at 18, 38 and 120 kHz. Each one has information unique to the distribution of scatterers that respond preferentially at the corresponding acoustic wavelengths (~8,3, 4.0 and 1.3 cm). It would be excessively space-consuming in this report to provide all three profiles separately. Thus, using the standard false- colour imaging approach using in multi-spectral remote sensing, a colour composite was generated using the following colour allocation: red - 18 kHz, green 38 kHz, blue 120 kHz. The dynamic range of scattering for each frequency was chosen arbitrarily to represent the main range of volume scattering strength observed. Example profiles are illustrated below.

Note that, as the MVP dips are time and location tagged, they may be standardly overprinted on the EK imagery to ascertain how well the scattering layer represents either the top or bottom of the thermocline.

Ideal Case: Unambiguous definition of the base of the thermocline at all 3 freq.
Transitional: not clear what happens to the right. rises (38 kHz) or descends (018 kHz)?
Ambiguous: Level or ascending?

The data can be provided as time or distance sections. To look at time variations (such as the diurnal signature), time plots were used. To look at relationships to the seabed topography, standard distance sections were generated where the profile data are geographically registered so that, irrespective of whether the lines are run to the east or west, they appear in the same geometry.

Synchronization Issues
Based on the data collected in the CE16-01 survey in September 2016 when none of the shipboard sonars were synchronized, the decision was made to use the K-Sync. In the CE16-01 data, the 38 kHz was heavily overprinted by interference from the EM302. Similarly the 200 kHz was overprinted by interference from the EM2040. As the aim here was to use the data for thermocline tracking, the interference needed to be minimized.
Therecommended approach from Kongsberg was to have the two EM's fire in one group, followed by th EKs in a second group. This unfortunately results in a very low ping rate for all sonars. The configuration adopted for this cruse was to have all of the EM302, 2040 and EK fire in a single group. That provided a better ping rate (~ 1.3 Hz in 120 m of water). About one third of the way through the cruise however, it was noted that the ping rate was primarily limited by the EM302. This being despite the fact that the EM302 was utilizing the same angular sector as the EM20400 (+/-65-70 degrees). As a result ,the EM302 was taken of synchronization and allowed to run independently. This resulted in an acceptable ping rate of 2.3 Hz. Note that, as the EKs only need to listen to ~ 1.1 x the water depth (defined in the Runtime settings of K-Sync as a multiplication factor of 1.1), their ping rate would be higher than the EM2040 and thus not slow down the EM2040.
Notably, for standard Fisheries operations, the EK's are set to fire at a standard time interval (1or 2 seconds?) and log to a fixed range (375m). In this case, the EK ping rate was variable (slave to the EM2040), but was generally about 2Hz. Only the first 150m was logged as that was more than the maximum depth in the survey area.

Sonar Settings
As the EM2040 was operated in its 200 kHz mode, it was found that the 200 kHz EK-60 channel, if synchronized, would cause the EM2040 to track slightly deep at nadir. Thus the 200 kHz channel was set to passive to not interfere with the prime bathymetric mandate of the Marine Institute. Thus only 3 frequencies are available for thermocline tracking.

All channels utilized a 1ms (1024 microsec) pulse at max power setting (18 and 38 kHz : 2000W, 120 kHz 500 W). A fixed range of 150m was logged.

Diurnal Signature:
The most obvious variability is imposed by the diurnal light variations. As expected, the zooplankton rise at dusk and descend at dawn. As we were logged 24 hours a day for 2 weeks ,we can look at how this oscillation is modulated as a function of seastate and region. The plot below shows the full 2 week period.

Daily Sections:

For each 24 hour day (presented below from 1800 the previous day to 0600 the following day), a standard section is illustrated. It shows the diurnal signature with the majority of the scatterers above the thermocline at night ,and the low frequency scatterers below during the day. There is a clear period of about an hour and dusk and dawn where the scatters are rising and falling through the thermocline. This will create the greatest challenge to automatically picking the thermocline layer.

for data prior to start of main survey, see the discussion of the Celtic Sea Front imaging.
	1800 ------------------- midnight ------------------- 0600 ----------------------------1200 -------------------------- 1800 ------------------- midnight --------------- 0600
JD204
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