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SO173-1, 3rd weekly report, July 27, 2003

The last week was finally a successful one for us - we can report that all systems operated as intended without any problems. Thanks to the efficiency of many people aboard, we could persuade our DTS system to operate perfectly again.

On Monday and Tuesday we shot a series of side-scan profiles in the "BGR slide area" and around mound 11 and 12. We used both the 75 and the 400 kHz side-scan sonar system as well as the surface streamer and the Gi-Gun. Further, but smaller mounds were located and the extent of the edge broken away at the BGR slide could be charted. The edge is approx. two kilometres wide and its morphology is complex.

After having recovered the nine OBH/S from mounds 11/12, we used a transit of about 200 nm across the oceanic crust (using magnetics) seawards of the trench off Nicoya to subject the deep-towed streamer to further tests.

Off Nicoya, at the same location where a seismic reflection profile had been recorded during the EWING cruise in 1999, we recorded a wide-angle profile 80 nm in length in order to determine the velocity field of the crust and of the outer mantle. For this purpose, we deployed 18 OBH/S approx. 3 nm apart and shot across the profile by using three big bolt guns. Some additional Gi-gun profiles will enable us to determine the orientation of the seismometers. In the evening of 25th July all instruments were back on board; their data are of good quality. Unfortunately, as was the case with the first profile, no sub-sedimentary shear waves can be seen. These would considerably contribute to solving the question of possible serpentinization of the outer mantle.

Then we deployed four OBH/S to observe microseismicity in the area, where the oceanic crust bends at its maximum before the deep-sea trench. These OBH/S are to be recovered at the end of the 2nd leg.

On Saturday morning, we started a DTS survey of Mound Culebra. This survey will be continued until Monday. We detected at least two further - but much smaller - mounds beside Mound Culebra. Seismics revealed many structures, some reaching deeply down, and bathymetry showed faults at the side of Mound Culebra. These were recorded by the side-scan sonar system and by a seismic survey we made across them in order to chart their origin and their deeper strike.

The sea is still very calm, temperatures are pleasantly high, and only now and then does it rain. All aboard are fine and send their regards to those at home.

At sea, 10:15 N, 86:15 W, den 27.07.03
E. Flüh

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