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Jamie's Cruise Reports

Progress Report #1:  NBP00-07A/GO-306-0 Bransfield Strait:  Ocean Bottom Seismograph Profiling (Part 2) Chief Scientist:  James A. Austin, Jr., The University of Texas at Austin, Institute for Geophysics November 2      We are recovering OBSs along Line 6, an oblique reflection/refraction profile along the inner (Bransfield Strait) margin of the South Shetland Islands.  As this report is being written, we have recovered five of eleven instruments deployed along this line.  According to the present schedule, recovery will be completed by early this evening.  Following a brief hiatus to rest personnel, during which a test of Palmer's single-channel profiling capability will be conducted, deployment of OBSs along Line 7, a re-shoot of a dip profile (Line 1) crossing Bransfield Strait at its northeastern end, will begin tomorrow morning (November 3).  Our intention is to complete eight total profiles in this region within approximately the next two weeks, a field effort begun last April-May.   The redesigned 6-airgun array is a significant improvement over the initial design.  Gun deployment and recovery are now both more efficient and safer for deck personnel.  The guns performed flawlessly for almost 1500 shots (at a 90 s repetition rate), before an auto-fire problem with one of the guns (#5, 350 cu. in. capacity) forced a break in data acquisition.  The problem was finally traced to a damaged O-ring in the solenoid, and was easily fixed.  This was the only gun-related malfunction during almost 48 hours of shooting along Line 6.  We anticipate continued reliable performance for the remainder of the field effort.  Both Edison-Chouest and Raytheon personnel are to be commended for design changes which have resulted in improved airgun array performance aboard the Palmer. Our initial examination of OBS data recovered thus far along Line 6 (three instruments) reveals that although quality is variable, returns from the Moho have been received on each instrument.  Returns are visible to horizontal ranges of up to 140 km.

Progress Report #2:  NBP00-07A/GO-306-0 Bransfield Strait:  Ocean Bottom Seismograph Profiling (Part 2) Chief Scientist:  James A. Austin, Jr., The University of Texas at Austin, Institute for Geophysics November 9  We are deploying OBSs along Line 10, an oblique reflection- refraction profile along the Peninsular margin of Bransfield Strait.  As this report is being written, we have deployed three of eleven instruments along this line.  According to the present schedule, deployments will be completed by the end of the day.  Following a brief hiatus to rest personnel, we will shoot to this strike profile, the fourth that we have acquired along the trend of the Strait.  Line 10 is also the eighth OBS profile in this region that we have collected during two field efforts, this one and one last April-May.  We expect to finish Line 10 on November 12 or 13, then transit back to Punta Arenas. The redesigned 6-airgun array has performed almost flawlessly.  We had a total of less than 3 hours of gun-related delays along the previous strike profile, Line 9.  The culprit was a broken o-ring on the solenoid of gun #5 (350 cu. in.), which caused the gun to auto-fire.  This o-ring has failed twice.  We have not diagnosed the reason for this repeated failure, but fixing the problem has been easy.  The Syntron gun-controller has also presented undiagnosed  "faults."  The most consistent is its tendency to announce gun auto-fires, when no such auto-fires are occurring.  There are also indications for cross-talk on the fire-circuit.  However, the guns themselves appear unaffected by these intermittent electrical anomalies.  Cooling the Syntron unit appears to reduce its misbehavior.  Gun deployment and recovery are now extremely efficient, albeit personnel intensive; such operations routinely take less than 30 minutes with 5-6 people. Our initial examination of OBS data recovered thus far along lines 6 (a strike profile along the inner edge of the South Shetlands pedestal), 7 (a re-shoot of dip Line 1, which crosses northeastern Bransfield Strait) and 9 reveals uniformly high quality.  Along Line 9, reflection returns from the Moho have been received on each instrument.  Returns are routinely discernible to horizontal ranges of 100-120 km.  We attribute these excellent records to improved gun performance and uniformly superb weather for this kind of operation - light winds and little or no sea.

Progress Report #3:  NBP00-07A/GO-306-0 Bransfield Strait:  Ocean Bottom Seismograph Profiling (Part 2) Chief Scientist:  James A. Austin, Jr., The University of Texas at Austin, Institute for Geophysics November 15          We arrived at the dock in Punta Arenas at 0730 hrs local time this morning, ending NBP00-07A.  We finished recovering the last instrument along Line 10 southeast of Bridgeman Island a little before 0600 hrs local time, November 12.  Our transit across the Drake Passage was routine. The cruise was an outstanding success.  We deployed and recovered 44 OBSs along four profiles, one along the inner margin of the South Shetland Islands pedestal, one crossing the northeastern Bransfield Strait, and two along the Peninsular margin, to complement four similar profiles collected last April-May.  All OBSs were recovered, and all recorded good-excellent data.  On Line 10, the final profile acquired, the 6-airgun array performed flawlessly for the first time, for a continuous period of more than 30 hours (at a 90-s repetition rate).  Weather was optimal throughout the ~14 days of data acquisition, which helped all aspects of the operation.  Few icebergs were encountered. Preliminary examination of our results suggests that crustal thicknesses in Bransfield Strait increase generally from northeast to southwest.  Thicknesses in the northeast range from 12 km (in the vicinity of the neovolcanic zone along the axis of the Strait) to 16-18 km beneath the Peninsular margin.  To the southwest, thicknesses range from 16 to 22 km.  The inner part of the South Shetland Islands pedestal is characterized by crustal thicknesses of ~23 km..  A wide area characterized by upper mantle velocities of ~7.5 km/s underlies the boundary between the deep part of Bransfield Strait and the Peninsular margin.  The presence of such velocities suggests either elevated mantle temperatures, some degree of partial melt, or a combination of the two.