is section compiles not only results of the data processing to determine the internal wave frequencies and whence they originated, but also some of the unexpected effects that the field deployment had on the planned missions and resulting data. ese effects are largely due to physical constraints of the AUVs and instruments and imposed effects of a dynamic ocean environment on conducting AUV missions. A brief description of the oceanographic conditions on the day of the experiment is presented first.
4.6.1 Oceanographic Conditions
Fig. 4-3 shows the morning and afternoon sound speed, temperature, salinity, and density profiles from a CTD cast from the NRV Alliance on August 13, 2010, in the GLINT ’10 operation area. e water depth at the CTD sample locations (and much of the operation area) was just over 110 m. Here a warm isothermal mixed layer can be seen near the surface of approximately 10 m depth and 24◦C resulting in a strong thermocline at about 10 m depth. e temperature then drops suddenly with depth to about 19◦C, then tapers off to about 14◦C by 60 m depth, below which the water remains isothermal. e steep thermocline near 10 m depth suggests that internal waves would be most prominently observable at that depth, if they existed. It should be noted that the high frequency variations in salinity over depth are likely due to the sensitivity of the conductivity sensor on the CTD to the rapid changes in temperature between 9 and 60 m. Sound speed was calculated using the Mackenzie sound speed equation [65]. Density was calculated from the Unesco 1983 equation of state for sea water [67].
4.6.2 Mission Execution
At the beginning of the Internal Wave Detection missions, the shallow-depth AUV was commanded to swim at the depth of the maximum gradient of the thermocline (∼ 10 m depth). is resulted in extremely poor acoustic communication observed between the shallow AUV (Unicorn, for the first mission) and the topside (via the gateway buoy) due to the fluctuating refraction direction of propagating sound waves in the steepest region of the thermocline (depth of maximum|∂T /∂z|). With Unicorn traveling at 10 m depth, 3/14 (21%) of the acoustic messages sent by Unicorn to the topside were received on the topside, while 19/38 (50%) of them were received on the topside with Unicorn traveling at 12 m depth (acoustic communication perfor- mance values based on rate 0 FH-FSK (frequency-hopped frequency shift keying) messages sent from Unicorn to the gateway buoy, data courtesy of Toby Schneider, MIT). Subsequent missions had the depth of the shal- low (constant depth) AUV changed to swim at 12 m—just below the peak gradient of the thermocline—from the start of the mission to avoid losing contact with that AUV.
e next challenge faced during deployment was a difference in speed ranges achievable by Unicorn and
Harpo. is was significant because, in order for Unicorn to trail behind Harpo without overtaking Harpo, Unicorn had to slow to its minimum speed of 1.3 m/s while Harpo had to travel at 1.3 m/s, just above Harpo’s
maximum quoted speed. When Unicorn slowed below 1.3 m/s to remain at a safe distance behind Harpo, its depth control degraded and it was observed to fluctuate involuntarily, or ‘porpoise,’ in depth by up to±0.8 m in a periodic manner, adding a detectable temperature fluctuation to its data set. Upon processing, the
1500 1520 1540 −120 −100 −80 −60 −40 −20
0Sound Speed Profile
Sound Speed (m/s) z (m) 13 Aug 2010, 05:26:33 UTC 13 Aug 2010, 13:55:15 UTC 10 15 20 25 −120 −100 −80 −60 −40 −20 0Temperature Profile Temperature (°C) 37 37.5 38 38.5 −120 −100 −80 −60 −40 −20 0 Salinity Profile Salinity 24 26 28 30 −120 −100 −80 −60 −40 −20 0 Density Profile Density Anomaly (kg/m³) ρ 0=1000.0 kg/m³
Figure 4-3: Morning and afternoon sound speed, temperature, salinity, and density profiles from a CTD cast from the NRV Alliance on August 13, 2010.
power spectral density peaks at the dominant frequencies of Unicorn’s porpoising (P SDDepth_U nicorn) were
subtracted from the temperature spectrum (P SDT emp_U nicorn) to minimize their influence on the results.
e resulting ‘pure’ temperature spectrum (P SDT emp_pure) is calculated as follows:
P SDT emp_pure= P SDT emp_U nicorn− P SDDepth_U nicorn. (4.1)
In the future, the porpoising could be avoided by adjusting the controller gains on Unicorn for smoother operation at slower speeds (there was no access to this option or time to implement and test it for these missions). Alternatively, a new loiter behavior could be written to incorporate a horizontal zigzag pattern on each loiter leg to slow down Unicorn’s forward progress, but this option was not available at the time and
the idea was to collect data along the 5 fixed headings of the pentagonal loiter to eventually back out the direction of travel of any internal waves (beyond the scope of this thesis work). During Mission 2, Unicorn’s minimum speed was not a problem because it was slowed in horizontal speed by the yo-yo depth excursions it was performing.
During the second mission in which Unicorn was adapting its yo-yo depth range to focus around the thermocline, hysteresis was observed in the temperature data (see Fig. 4-4). As Unicorn ascended through the 12-meter depth mark, the temperature was consistently observed to be lower than the AUV’s subsequent descent through the 12-meter depth mark. In Unicorn, the CT sensor is mounted on top, mid-way between the nose and tail of the AUV, and the pressure sensor (giving depth readings) is mounted in the bottom of the aft section of the AUV. us, if there were any appreciable lag between sensor readings of temperature and pressure at 12 m, the temperature reading at 12 m would be expected to be higher on the ascent (CT sensor at the mid-section is higher in the water column than the aft pressure sensor) and lower on the descent, which is the opposite of what had been observed. e Sea-Bird Electronics, Inc., model SBE 37-SI CT sensor on
Unicorn has an acquisition time of 1.0–2.6 seconds/sample [82], which is comparable to the∼1.5 s it takes
the pressure sensor to catch up in depth to where the previous temperature measurement was taken, which may account for some of the discrepancy, and thus, the hysteresis. e resolution of the temperature sensor on
Unicorn is specified as 0.0001◦C [82], while the Paroscientific, Inc., Digiquartz depth sensor resolution is at 0.1 mm or better with and accuracy of 0.02% or less and hysteresis≤ ±10 cm [83]. us, this temperature fluctuation is not due to the resolution of the temperature or depth sensor. is leaves the only probable explanation of the temperature fluctuation as hysteresis between the CT and pressure sensors due to the slow acquisition time of the temperature sensor and the hysteresis in the pressure sensor. One way to adjust for this in post-processing is to find the average temperature difference between each instance of shoaling and diving through the 12-meter depth mark, and add (subtract) half the difference to (from) the temperature measurement on the ascent (descent). e best way to prevent the majority of this hysteresis is to use a pumped CTD (or CT plus depth) sensor instead of a flow-through CT sensor plus depth sensor. In this case, the CT and depth sensors were the instruments available on the AUVs, and the mounting locations are fixed. e thermistor chain was deployed throughout both successful AUV missions, however it was only sam- pling at a 30-second interval compared to the approximately 10 Hz and 4 Hz sampling frequencies of Unicorn and Harpo, respectively. is means that the thermistor data spectra are resolved for a much lower frequency range than the spectra from the AUVs’ data (see Figs. 4-7, 4-9, and 4-11), allowing us to detect any possible lower-frequency internal waves.
1 1.05 1.1 1.15 1.2 1.25 1.3 x 104 14 16 18 20 22 24
Time (s) since 07:50:05 UTC
C T D _ T EMPER AT U R E ( C ) o
Hysteresis when Diving/Shoaling
CTD_TEMPERATURE Temperature at 12m +/−0.1m 1 1.05 1.1 1.15 1.2 1.25 1.3 x 104 80 60 40 20 0
Time (s) since 07:50:05 UTC
N AV_ D EPT H (m) NAV_DEPTH Depth at 12m +/−0.1m
Figure 4-4: Hysteresis is seen in Unicorn’s temperature data (CTD_TEMPERATURE) while preforming yo- yos through the water column. NAV_Z values are the negative of Unicorns measured depth values. e stars signify temperature and depth measurements taken when Unicorn is at 12±0.1 m depth. It has been verified that the±0.1 m depth range allowed is not the cause of the hysteresis.
Finally, atmospheric weather conditions can also affect underwater measurements through surface inter- actions of wind and waves. From approximately 0900–0930 UTC, or 1100–1130 local time (∼30–60 min into Mission 1), a storm system passed over the ship and AUV operation area. Storms frequently sustain higher winds than clear-weather conditions, and introduce an influx of fresh water to the otherwise salty sea surface. Depending on the severity of the storm, its effects on the underwater environment may lag the storm and persist from hours to weeks after the storm has passed. In this case, the storm only covered a local area of about 200 km2with squalls of very heavy rain, and it did not appear to cause an appreciable change in the
temperature at the thermocline immediately following the storm’s passing. Over the course of the the entire day (end of Mission 1 and through Mission 2, about 4.5 hours), however, there was an overall decrease in temperature of∼0.5◦C by the end of Mission 2. It is unlikely that this temperature decrease is due to the storm, since a deluge of 10 cm of water at 14◦C advected into the surface mixed layer (10 m deep, 24◦C) over the storm’s area would only decrease the mixed-layer temperature by about 0.1◦C or less. us, it is more likely that this drop in mixed-layer temperature is due to surface cooling as the post-storm sunshine waned going into the mid-afternoon (local time).