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Bransfield Strait Cruise Progress Reports
Progress Report #1: NBP00-02/GO-306-O
Bransfield Strait: Ocean Bottom Seismograph Profiling
Chief Scientist: James A. Austin, Jr., The University of Texas at Austin, Institute for Geophysics
April 10
We are presently in the lee of King George Island, deploying the Palmer's new 6-gun, 3000 cu. in. array, in order to test it prior to beginning to acquire our first OBS oblique reflection/refraction profile.
We were due to leave Punta Arenas at noon on 5 April, but we were delayed until 1920 hours on 6 April because of the need to wait for hand-delivery of gun synchronization parts from the United States. We completed the transit from South America to Bransfield Strait, in generally favorable weather, in ~3 days. However, we have been forced to wait on weather since yesterday evening, ~12-hours, because of winds in excess of 30 knots and seas of ~5-6 m in the deployment area.
During the transit, we completed readying our OBSs for deployment along our first profile, which will cross the eastern part of the Strait. This involves general checking of electronics, installing batteries, setting recording windows, synchronizing internal clocks, and placing electronics packages within glass spheres for deployment. The science party also gave a series of talks for ship's personnel and members of RPSC on the geologic relationship of Bransfield Strait to Andean evolution, our previous geophysical (multichannel seismic) investigation of the area (conducted aboard the R/V Maurice Ewing in 1991), and our planned operations strategy for deploying and recovering OBSs from the ship.
After a ~6-hour test of the array under actual towing conditions, including testing at both 90-s and 120-s shot intervals, we will recover the array, and transit to the beginning (SE end) of our first profile to begin to deploy OBSs. We should be deploying instruments by early tomorrow morning (April 11).
Progress Report #2: NBP00-02/GO-306-O
Bransfield Strait: Ocean Bottom Seismograph Profiling
Chief Scientist: James A. Austin, Jr., The University of Texas at Austin, Institute for Geophysics
April 17
We are presently recovering OBSs along Line 2, the second of eight oblique reflection/refraction profiles proposed for collection in the Bransfield Strait region. Line 2 runs down the axis of the Strait. We expect the recovery operation to be completed by sometime tomorrow morning (April 18). Ice conditions are excellent; only scattered bergs have been sighted. Weather in the Strait has also been good - light winds and calm seas, with moderate visibility (some fog and scattered snow showers). Our next operation will be to reconnoiter water depth and ice conditions in Nelson Strait, through which we have to acquire Line 3.
After collecting two profiles, our results are mixed. Our biggest operational complexity thus far has been the air guns, a 3000 cu. in. in-line array consisting of 6 guns (#1-1000, #2-500, #3-450, #4-400, #5-350, #6-300). First, deploying and recovering the array is labor-intensive; guns and harnesses are dragged across the deck, attached to an extremely heavy umbilical (necessary armoring against ice). We have routinely had between 7 and 9 people on deck during these operations. (RPSC has four techs assigned to this task, supplemented by our party.) To save time, the guns are being stored on deck when not in use. The down side is that gun repairs, when necessary, are also being made on deck; this will be difficult to impossible in bad weather (some of which we experienced in the Drake Passage at the seaward end of Line 1). Second, although we tested the array prior to deployment for Line 1, gun reliability has been generally poor to date. The most conspicuous of a variety of both air and electrical problems has been that gun #1 (the largest gun, nearest the ship) air lines have continued to fail. Although we built in a 12-hour period of OBS recording "flexibility" on Line 1, a combination of bad weather in the Drake Passage and gun #1 problems forced us to collect approximately 65% of that profile without gun #1. As gun #1 represents 1/3 of the array's total capacity, and the source of most of the array's low frequencies (critical for the science that we are trying to do), we may be forced to collect Line 1 again (see below). The gun array also failed twice during the acquisition of Line 2; ~17 hrs of an expanded (24-hr) "flexibility" window in OBS recording was necessary to fix the guns, get them back in the water, and close recording gaps. One encouraging note is that the last recorded segment of Line 2 is also the longest - more than 500 shots, ~17 hrs) without a failure.
The performance of our OBSs has also been uneven. Four of eleven instruments deployed on Line 1 returned with noisy but usable data, and one instrument did not record any data at all (a computer problem which can be fixed). These results, along with diminished array performance on Line #1 (see above), all argue for reacquiring Line 1. Thus far, one instrument has not responded to acoustic recall (the primary system) on Line 2. However, we are confident that back-up timed-releases on that instrument should allow us to recover it early tomorrow.
Progress Report #3: NBP00-02/GO-306-0
Bransfield Strait: Ocean Bottom Seismograph Profiling
Chief Scientist: James A. Austin, Jr., The University of Texas at Austin, Institute for Geophysics
April 24
We are shooting to our OBSs along Line 4, a profile parallel to the northern margin of the Antarctic Peninsula. If the guns cooperate (they have now been operating flawlessly for more than 14 hours), we should complete the shooting phase of this profile by tomorrow afternoon (April 25). Then, we will begin OBS recovery operations. Weather and ice conditions in Bransfield Strait continue to be optimal.
We have now collected three of eight profiles proposed for collection in and around Bransfield Strait. Unfortunately, Lines 1-3 have all been affected to varying degrees by unreliable gun/array performance. We will almost certainly have to collect Line 1 again (see April 17 report), if cruise time aboard the Palmer becomes available next fall. Recording of lines 2 and 3 has been completed successfully only by programming longer than normal recording times into our OBSs; 36 hours longer than the projected shooting schedule (@4 kts, 120-s repetition rate) in each case. This has had the effect of slowing down our overall operation. We have lost ~19.5 hrs to gun-related problems during Line 2, and ~16.25 hrs during Line 3.
Of the 33 OBS deployments thus far, we have had only two mishaps. OBS 22 (Line 2), deployed at the northeast end of Bransfield Strait, failed to answer an acoustic recall; fortunately, that OBS surfaced later, and was recovered off King George Island. OBS 32 (Line 3), deployed on the open shelf seaward of the South Shetlands, did not return at all. We think that the frame may have turned over in an area of rough seafloor. Our examination of recordcd data thus far suggests that instruments deployed in shallow water (e.g., in the vicinity of Nelson Strait) are noisy; ground coupling may also be a problem where the seafloor is hard/winnowed. Other instruments (e.g., many along Line 3) have recorded data of high-quality, including suspected returns from the Moho.
We hope to finish Line 4 and one more profile (Line 5, another crossing of the Bransfield Strait/South Shetland Island pedestal) before we must depart for Punta Arenas on or around May 2.