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SO173-1, 2nd weekly report, July 20, 2003

During the 2nd week at sea we first surveyed a depth seismics refraction profile seawards of the trench, where there are deep faults in the oceanic plate. 12 ocean bottom stations that had registered the air gun shots along a profile of 45 nm length could be recovered and contained good data. First estimates of seismic velocities have not shown marked anomalies, but a thorough analysis has still to be made. Then, after having anchored the current meter (to be recovered in the course of the 3rd leg) on Mound Culebra, we could eventually put the Posidonia System on board in Caldera, where we were lying at anchor around noon on Wednesday. We made the first calibration of the system successfully during the transit to our working area "Decollement".

After that, we redeployed 12 ocean bottom stations, however, only one cable apart this time. In addition, we surveyed several profiles by using different air guns, the side-scan sonar system, and deep-towed seismic streamers (DTS). The Posidonia navigation system operated only for a short time when first used; we have not found the reason for failure yet, but after the installation of spare parts the system is in working condition again. The images the side-scan sonar system produced of this region are little spectacular.

After we had recovered the OBH/S - extremely fast (5 minutes per instrument!) as we had been using the inflatable boat, too - we made the transit to the working area around mound 11 and 12 where we deployed 8 OBH/S. We had planned the survey to be made with deep-towed seismic streamers arranged with small space in between. Unfortunately, the DTS did not respond so that we had to interrupt the program after 3 side-scan profiles that showed - apart from mound 11 and 12 - further mounds, unknown so far but clearly smaller. We hope to be able to make the survey as scheduled as soon as the streamer has been repaired.

For the present, we have started side-scan mapping in the adjacent "BGR slide area". The sea is very calm, temperatures are similar to those in Central Europe, and rain has spared us for a few days. All aboard are fine and send their regards to those at home.

At sea, 09:10 N, 84:40 W 20.07.03

E. Flüh

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