1. Comparison of CTD vs XBT data
LCDR John Whelan
Cruise dates: January 2007

2. Outline I. Motive II. Equipment Used III. Data Quality Control
IV. Analysis
V. Conclusions

3. Motive
Conductivity Temperature Depth (CTD) profilers provide an accurate method of determining temperature and salinity with depth. CTD data is then used to compute sound velocity profiles which are of significant importance to naval forces in predicting acoustic characteristics/performance.
Unfortunately, CTD systems are very expensive, time consuming to operate, and require a stationary platform to collect data.
Expendable Bathythermographs (XBT) provide a quick and low cost method of determining temperature versus depth. Using an assumed salinity, sound velocity profiles (SVP) can easily be generated.
XBT can be dropped while underway and are not affected by sea state.

4. Motive
Since XBTs are less sophisticated than CTDs there is a concern that XBTs are not as accurate in determining temperature or may exhibit data bias which would ultimately result in SVP errors.
The intent of this project is to determine if there are significant differences between XBT and CTD temperature measurements.

5. LM-3A Hand Held Launcher
Equipment Used
Sea Bird 911+CTD/Rosette (12 position) with standard sensor suite.
Max cast this study: 2127 m
LM-3A Hand Held Launcher
Sippican Mark 12 XBT system
XBT: T Max Depth: 760 m

6. Good
Error
Too far
(Not used)

7. Good
Error
Too far?

8. Data Quality Control Initially 25 possible pairs 1 CTD failure
1 XBT failure
1 XBT processed as shorter depth T-6 vice T-7
1 pair 4 km apart
21 pairs of CTD/XBT remain for analysis
Only downcast data for CTD used
Data was plotted and inspected for obvious errors
Several XBT datasets had temperature spike at end of record removed

9. Analysis
Representative plot:

10. Analysis
EOF for CTD for 21 pairs

11. Analysis
EOF for XBT for 21 pairs

12. Analysis
EOF for Temp Difference for 21 pairs

13. Analysis
EOF for Sound Velocity Difference for 21 pairs

14. Analysis
Mean and Standard Deviation for 21 pairs - Temperature

15. Analysis
Mean and Standard Deviation for 21 pairs – Sound Velocity

16. Data QC
During course of analysis noticed some pairs were separated by over 1 km
Re-analyzed data excluding pairs more than 1 km apart

17. Analysis
EOF for CTD for 16 pairs
EOF for CTD for 21 pairs

18. Analysis
EOF for XBT for 16 pairs
EOF for XBT for 21 pairs

19. Analysis EOF for Temp Difference for 16 pairs

20. Analysis EOF for Sound Velocity Difference for 16 pairs

21. Analysis Mean and Standard Deviation for 21 pairs - Temperature

22. Analysis Mean and Standard Deviation for 16 pairs – Sound Velocity

23. Conclusions
XBT probes measure C warmer than CTD with standard deviation of 0.1 C (exactly same as listed in Sippican’s product brochure)
XBTs exhibit a warm bias above 200 m and below 700 m
Resulting higher temperature measurements by XBT result in an increase in SVP of approximately 5 m/s
Difference is not operationally significant – XBT remain a viable method for sampling the ocean environment for acoustic propagation predictions.

24. References http://www.seabird.com/products/spec_sheets/911data.htm
Professor Collins!

25. Questions?