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The Santa Barbara Channel Experiment (SBCX) |
During SBCX, many CTD casts were taken in the area of the Santa Barbara Channel. All were from the Danny C by Mark Stevenson. CTD data were furnished to SAIC, then to MIT by Newell Booth of SPAWAR. Fifty nine CTD's were taken over six days of the experiment. Casts were limited to daylight hours, in the areas where shipping was not present.
The spring of 1998 saw unusually wet and cool weather in southern California. During the experiment, rain fell frequently in the Point Mugu area; the hills surrounding the coast were green; they had not dried to yellow. Consequently, one cannot reliably use historical databases for sound speeds in the water column.
CTD's are typically taken by deploying an XBT (with a rope, making it non-expendable) from the side of a ship, letting it fall to the bottom. The sensor is hoisted (by winch, unless the water shallow) back onto the deck. At regular time intervals, the sensor records pressure (depth), temperature, and salinity of the water. The speed of sound through water is calculated for each depth; this quantity is also recorded.
Generally, both the upward and downward casts of SVP data are recorded. The downward cast has less winch noise present, so it is generally thought to be of better quality than data recorded during the sensor ascent.
During SBCX, the downward and upward casts sometimes varied by several meters/sec near the surface. Interpolation of the downward and upward casts was thought not to be oceanographically accurate, so downward and upward casts were treated as separate sound velocity profiles.
The third and fifth columns of the CTD log file contain the depth (meters) and the sound speed (meters/second). It is unclear what information the other columns contain; presumably pressure, salinity, temperature, or other information. Plotted on a Cartesian scale, it becomes apparent the sound speed results were sampled at regular temporal intervals; these do not translate to regular spatial intervals. The data was first separated into upward/downward casts, then linearly interpolated and re-sampled at 1 meter depth intervals.
Most of the files had incomplete downward or upward casts. The locations of each cast differed; the bottom depth varied with the bathymetry. From these casts, a mean sound velocity profile was calculated. This was determined using only the data available for each sampled depth point.
| Last updated: 000218
Comments/Questions: pmd@mit.edu |
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