Thursday, August 27, 2009

MED-09 Oceanographic Measurements

Today was the first of two days transit to our new operating area in the Tyrrhenian Sea. We continued both visual and acoustic observations while traveling and were quite interested in what we might see or hear because the areas we are passing through have not been well-studied. Some dolphin groups were detected, but not much else. We had reasonable seas and the wind was just 10-12 knots, but it was straight from the east and on our bow so effectively closer to 20 knots which made things tough for the visual team. As we transit, we continue to process all of the biological and oceanographic data that have been collected thus far.

An important and interesting kind of oceanographic sampling that is being done is to measure different properties of sea water at various depths in areas where we have been operating. We use a device to measure the conductivity (a proxy for salinity), temperature, and depth (measured with pressure) of the water (called a "CTD"). Measurements are made by lowering the CTD slowly in the water column to provide real-time data about biologically-important ocean parameters. The CTD has two salinity probes, two temperature sensors, and one pressure sensor; it passes through and samples the water for these parameters at approx one meter/sec. It also operates a pumped system from the different sampling tubes seen in the picture here to guarantee that the same sample of water is measured at specified times. Additionally the Alliance CTD includes two sensors, which directly measures the oxygen content in the water column. In this way we can fully characterize the oceanographic parameters of the water column and can also calculate how sound travels at different depths in real-time using the salinity, temperature, and depth information.

In the images here you can see four different plots from a recent deployment of the CTD to 1000m, each of them showing a different measurement with increasing depth (downward on the plots). The temperature plot shows the much warmer surface water and colder deep water; note that below about 200m there is almost no change in the ocean temperature here. The salinity trace shows another common pattern in this area which is fresher water near the surface and saltier water with depth. The oxygen plot shows higher levels near the surface (where waves mix the water) and lower levels deeper. Finally, the sound velocity profile resulting from the other measurements shows more rapid sound speed at the surface and deeper, with a minimum sound speed around 150m. Like light, sound bends to areas where it travels more slowly and the minimum seen here is called a "sound channel" that would tend to carry sound greater distances than where the sound velocity is higher.

By making these measurements in different areas of interest, and at depths down to 1000m (over 3,000 feet), and comparing them to where we see and hear different marine mammals, we can get a better sense of which kinds of environmental conditions are most important for different species. Additionally, the sound velocity profiles are useful in predicting broadly the underwater sound fields during controlled exposure experiments using real-time oceanographic measurements; these are also directly measured at different places with other sensors (e.g, acoustic tags, sonobuoys).