The second stage of the expedition on-board ASV started on July 1 from the port of La Spezia, Italy. The goal was to compare horizontal resolution of different tomographic techiques and decide on their suitability for mapping the ocean mesoscale structure. The prerequisite for such study was an appropriate mesoscale inhomogeneity, which should be strong enough to give measureable effect on acoustic images. The CTD survey performed during MOST experiment (Sect. 184.108.40.206) revealed relatively weak and smooth horizontal dependence of the sound speed field. The only remarkable feature was the North-Balearic front observed on the S-H leg. It was not found on the H-W3 section, hence, is located west of this line. The seasonal heating has been shading the front and it was not clear if it still existed in July. We were uncertain in choosing the experimental site and requested our partners from of IfM, Kiel, to analyse the data of satellite imaging to find a suitable ocean structure. By putting out from La Spezia, Dr. Uwe Send, the Project Co-ordinator of THETIS-2, notified us that there are some evidences of an inhomogeneity North-East of the Balearic Islands. However, it was not well defined that prevented Dr. Send from giving detailed recommendations.
Two different sources indicated that a suitable ocean structure may be found in the square E by N. According to the Levitus climatology for the Western Mediterranean, and also to theoretical speculations and data analysis by [25, 26], there is a steady cyclonic water circulation in the same region. Together, these pointers motivated us to select this area for the experiment.
Prior to commencing a CTD survey, the R/V had to carry out two extemporaneous tasks. First, it was already mentioned that the transmission level of the W3 source was 12-15 dB below the nominal power. The low transmission level prevents from using the reciprocal data to remove clock drift effects. So, all the data recorded by W3 would rely on somewhat questionable assumption of constant drift rate. There was a vital need to check the W3 operation and to correct the internal clock, if necessary. To fulfill this work, G. Krahmann and M. Schartau of IfM, Kiel, arrived to ASV during the stay in La Spezia. After leaving the port, the R/V steered to the W3 location. On July 4 the German specialists with assistance from our team fulfilled the necessary clock calibration. The same evening they left ASV for Sardinia on a boat of Italian coast-guard.
Second, according to the data of a THETIS-2 monitoring station at Le Brusc [20, Sub-task 1.5,] the most important high-power transceiver H ceased transmissions on July 3. It put under question the continuation of the whole THETIS-2 experiment. IfM, Kiel, would have to organize an urgent expensive expedition ad hoc to retrieve, repair, and repeatedly deploy the transceiver. Having been notified about this failure, our group on-board ASV fulfilled special observations of the signals in the frequency domain of interest. It turned out that the source works according to its usual schedule. The emitted M-sequences were successfully correlated against the nominal replica giving a reasonable arriving pattern. We informed Dr. U. Send that the device operates regularly, a recovery cruise is unnecessary and seemingly the Le Brusc monitoring station is in trouble.