Comparison of CTD vs XBT data LCDR John Whelan Cruise dates: 23-30 January 2007

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

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.

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.

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-7 Max Depth: 760 m

Good Error Too far (Not used)

Good Error Too far?

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

Analysis Representative plot:

Analysis EOF for CTD for 21 pairs

Analysis EOF for XBT for 21 pairs

Analysis EOF for Temp Difference for 21 pairs

Analysis EOF for Sound Velocity Difference for 21 pairs

Analysis Mean and Standard Deviation for 21 pairs - Temperature

Analysis Mean and Standard Deviation for 21 pairs – Sound Velocity

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

Analysis EOF for CTD for 16 pairs EOF for CTD for 21 pairs

Analysis EOF for XBT for 16 pairs EOF for XBT for 21 pairs

Analysis EOF for Temp Difference for 16 pairs

Analysis EOF for Sound Velocity Difference for 16 pairs

Analysis Mean and Standard Deviation for 21 pairs - Temperature

Analysis Mean and Standard Deviation for 16 pairs – Sound Velocity

Conclusions XBT probes measure 0.034 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.

References http://www.seabird.com/products/spec_sheets/911data.htm http://www.sippican.com/stuff/contentmgr/files/0dad831400ede7b5f71cf7885fdeb110/sheet/xbtxsv92005.pdf Professor Collins!

Questions?