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Data from the cruise!

Cruise Participants Return From Spain

Yosio Nakamura, James A. Austin and Ben Yates return to UTIG following participation on a cruise on the R/V Maurice Ewing as part of a project headed by Rice University's Dale Sawyer. The project titled: The S reflector and rifting processes: Iberia Abyssal Plain/Galicia Bank is part of the Margins Initiative.

Cruise Reports

Cruise reports from Yosio follow these cruise reports from Dale.

From Chief Scientist, Dale Sawyer

Weekly Cruise Report I
Ewing 97-05
Sunday 13 July 1997 10:30 AM
We departed Ponta Delgada, The Azores, at 1200 on 10 July 1997 for transit to first OBS drop site on Galicia Bank. Along the way we conducted a test of the GEOMAR acoustic releases, involving two release rack lowerings to 3000 m. The first of 25 OBS deployments was completed at 0137 on 13 July 1997 and 13 total OBS deployments have been completed as of the time of this report. We are using 14 GEOMAR instruments and 11 UTIG instruments. We expect to have the OBS array deployed by 2000 today and begin MCS streamer deployment and balancing.

We have not encountered any problems and the weather has been good. Several members of the science party have commented to me about the extraordinary helpfulness, skill, and professionalism of the Ewing crew.

Weekly Cruise Report II
Ewing 97-05
Sunday 20 July 1997 13:40
We have been engaged during the first weeks of the cruise in studying the S reflector, located on Galicia Bank, offshore Spain in the Atlantic Ocean. The S reflector is a very bright, roughly horizontal, intracrustal reflector, normally interpreted to be a major crustal detachment fault formed during the last stages of rifting between Iberia and the Grand Banks of Newfoundland. We have acquired a combination of multichannel seismic (MCS) reflection profiles and long-offset seismic profiles using Ocean Bottom Seismographs (OBS's) and Hydrophones (OBH's) to try to determine the seismic velocities of the rocks directly above and below the S-reflector where it is well developed and how they vary laterally. We are also seeking to image the structures associated with the edges of the S-reflector, as we predict that these will tell us the most about the processes of rifting in general and the formation of S in particular.

We completed deployment of an array of 11 University of Texas Institute for Geophysics (UTIG) OBS's and 14 Kiel University Marine laboratory (GEOMAR) OBH's on 13 July. The instruments are located mostly along east-west Line 1. A few instruments are located on crossing Lines 5 and 6 to the north and south of Line 1.

We deployed and balanced the MCS streamer on 13 -14 July. The streamer required the addition and replacement of some hydrophone sections, the addition of depth control "bird" collars, filling some sections with buoyant fluid, and the addition of lead weights. At the end of this process, the wind increased and the sea state did not permit us to proceed with airgun array deployment and seismic profiling. We waited for about 12 hours and changed the order of our planned acquisition before deploying the airgun array. Over the next 4 days we shot MCS profiles along Lines 1, 4, 5, and 6. Line 1 was shot twice using different shot intervals, one interval thought best for the MCS profiling and the other thought best for the OBS's. On 19 July we recovered the airguns and the streamer and began picking up the OBS's. As of the writing of this report we have recovered 7 of 14 GEOMAR and 10 of 11 UTIG OBS's. Among these, there have been only two data failures; the GEOMAR OBH at location 119 returned with its tape jammed and apparently recorded no data, and the UTIG OBS at location 126 experienced disk write errors and may have returned no useful data. The remaining instruments should be recovered by tomorrow morning, 21 July.

A portion of the shots fired by the R/V Ewing this week were also recorded by an array of about 20 land recording stations deployed onshore in Galicia, Spain.

I enclose tables of OBS deployment locations and MCS seismic reflection profiles acquired during the past week.

Weekly Cruise Report III
Ewing 97-05
Sunday 27 July 1997 12:00
During the past week we completed the first full deployment and recovery of our Ocean Bottom Seismograph (OBS) array and the corresponding Multichannel Seismic (MCS) acquisition. We have had a quick look at most of the OBS data acquired and are quite pleased with the results. Of the 25 instruments deployed, 23 appear to have recorded the data completely. One instrument failed to record anything and one instrument experienced numerous "disk write errors." We hope to be able to recover some data from the latter instrument.

The instruments deployed over the S reflector show the S reflector clearly at pre- and post-critical offsets. In some examples, S is apparently a single reflector and in others it appears to be several reflectors. The pattern in its variation laterally is not what we expected. Most of the instruments show first arrivals out to 80 km or more. We expect that these will yield a good velocity model into the upper mantle.

We have now moved into the Second Deployment of our OBS array. The main objective of this deployment is to determine the crust and upper mantle structure under the Galicia Interior Basin (GIB). The GIB lies between the highly extended continental crust of the Galicia Bank Basin and the unextended continental crust of Iberia. It may be analogous in tectonic history with the Grand Banks of Newfoundland, which were affected by a series of rifting events over 50 million years prior to the rifting event which culminated with the formation of oceanic crust between Iberia and North America. Although there have been a number of prior efforts to determine the crust and upper mantle structure under the Galicia Bank Basin, there have been no long-offset seismic studies of the GIB. We have deployed OBS's along three profiles across the GIB, two offshore of Spain and one offshore of Portugal. Our Spanish colleagues from Barcelona and Madrid have deployed approximately 30 land stations in Spain and Portugal along these profiles. They will record shots from our airgun array and attempt to carry the image of the crust and upper mantle, obtained offshore by the OBS array, across the beach and under the continent.

We completed deployment of an array of 11 University of Texas Institute for Geophysics (UTIG) OBS's and 14 Kiel University Marine laboratory (GEOMAR) OBH's on 23 July. The instruments are located along east-west Lines 17, 1, and 12.

While deploying the OBS array, we noted that there was significant along-shore traffic east of 10W. We deemed use of the 4 km streamer in this area risky and elected to shorten the streamer to 1 km (40 channels) for the portions of lines 12, 14, 1, and 17 in the affected area. We have completed that acquisition and are currently redeploying the streamer to its 4 km (160 channel) configuration.

I enclose tables of OBS deployment locations and MCS seismic reflection profiles acquired during the past week.

Weekly Cruise Report IV
Ewing 97-05
Sunday 3 August 1997 10:00
During the past week we acquired multichannel seismic reflection (MCS) data and recovered our array of ocean bottom seismographs (OBS's). This completes the second, of three, deployments of the instruments. We are now transiting to Lisbon Portugal to offload the GEOMAR instruments, their storage van, and personnel (Reston, Ye, Domaschk, Walther, Gusine, and Hojka). Gustavson-Unger of Rice University and Teixeira of University of Aveiro, Portugal, will also be getting off, and we will be taking on James Austin, from the University of Texas, and Mercedes Romero Pascual, from the University of Madrid.

At the time of the last weekly report, we were changing the streamer configuration from 1 km length, for maneuvering close to shore, to the full 4 km length. That operation was completed without incident. We acquired MCS data in nearly ideal weather until the evening of 30 July. The profiles acquired this week are tabulated below.

The winds and seas picked up a bit during OBS recoveries. We recovered 25 instruments of 25 deployed. One UTIG instrument (Site 232) did not surface on time and we had to leave it to make other recoveries. We returned to Site 232 about 13 hours later and found the instrument using the radio direction finder. We appreciate the skill of the Ewing crew in tracking down the wayward UTIG OBS

Of those recovered, 19 seem to have recorded data fully. Apparent instrument failures include:

3 GEOMAR OBH's stopped recording data due to tape drive failures. We do not expect to get any useful data from these.

3 UTIG OBS's stopped due to battery failures. We believe all the failures occurred late in the deployment and most or all useful data will be recovered.

The approximately 21,000 shots fired during Deployment 2 were recorded onshore by groups from the Universities of Barcelona and Madrid. They report that initial analysis of data from some instruments show clear arrivals from our shots. Many of the land stations will also be recording the shots from Deployment 3.

Onboard data display, analysis, and preliminary interpretation, show that we recorded interesting data during Deployment 1. The S reflector is well imaged in both near vertical and long offset data. We have processed, through stacking and time migration, a sample of one of the MCS profiles across the S reflector. Although this portion of the profile was acquired in the highest sea state encountered during the cruise, the image looks to be very good. We have developed a preliminary velocit structure model for the main east/west line of OBS's in Deployment 1.

I enclose a table of MCS seismic reflection profiles acquired during the past week. All were acquired using the 4 km, 160 channel, digital streamer. The twenty-five OBS's listed in this spot last week were still on the bottom and recording (mostly) throughout.

Weekly Cruise Report V
Ewing 97-05
Sunday 10 August 1997 16:00
We docked in Lisbon at 1045 on Monday 4 August. Customs formalities were completed by 1200. The crane brought to unload the GEOMAR container was too small. A larger crane arrived at 1600 and unloaded the container. The 6 GEOMAR scientists (Reston, Ye, Domaschk, Walther, Gusine, and Hojka), 1 Rice scientist (Gustavson-Unger), and 1 Portuguese scientist (Teixeira) left the ship. One UTIG scientist (Austin) and 1 Spanish scientist (Romero Pasqual) boarded the ship. The science party enjoyed a sunny and bright afternoon in Lisbon (the naughtier ones of us even drank beer!). We departed Lisbon at 1800 and transited to the south end of Line 9. We deployed the 11 UTIG OBS's to the north along the line, deployed the MCS streamer and guns, and began MCS profiling. The weather has been excellent since leaving Lisbon. We will enjoy our first and last cookout on the Bridge deck this afternoon.

The third deployment of the cruise is designed to study the along strike variation of the Iberia rifted margin and to acquire a few short lines in the area of Ocean Drilling Program (ODP) Legs 149 & 173. The deployment consists of a north-south OBS/MCS line extending from the Galicia Bank to the Iberia Abyssal Plain. We expect it to cross 30 km thick crust in the north to 5-8 km thick crust in the south. From these data we are interested in learning whether the change in crustal thickness along strike is gradual or abrupt. This should tell us whether rifting progressed steadily or eposidically to the north. If it was episodic, we would like to locate the boundary between rift episodes. The deployment also includes a series of MCS profiles across the Ocean Continent Transition at varying latitude. These will be used to study rifting style along the strike of the margin. The line locations were chosen to complement existing MCS data. The final parts of Deployment 3 are several MCS profiles near the ODP Iberia Abyssal Plain drilling transect. These lines are designed to extend our knowledge of the basement structure around the drilling sites, and assist in interpreting the lithologic observations made at the sites.

Bosun John Santini left the ship in Lisbon to get medical care for a strained back, and then to return to the US. The science party will miss him and hope he recovers, and returns to the R/V Ewing soon.

I enclose tables of OBS deployment locations and MCS seismic reflection profiles acquired during the past week. All the reflection profiles were acquired using the 4100 m, 164 channel streamer.

From Yosio Nakamura at Sea

Report #1
Date: Sun, 13 Jul 1997 17:44:48 +0000
It's been a busy week since we arrived at Ponta Delgada, Azores, but as we have just completed the first deployment of our (UTIG) OBSs, I finally have time to sit down and send you this report. Below is a brief summary of what's happening so far:

Ben, Stephen (Clark) and I arrived at Ponta Delgada safely on Sunday (July 6) night after overshooting our destination by two time zones and coming back one at a time. [Atlanta ---> Madrid; Ponta Delgada <- Lisbon <- Madrid] Maybe this is a good way to reduce the effect of jet-lag.

We spent most of Monday and Tuesday in unpacking, setting up and checking out our equipment on board R/V Ewing in port. Some damages during the transit were found but not bad enough that Ben couldn't fix.

On Wednesday, we finalized our experiment plan for the first deployment with the chief scientist Dale Sawyer of Rice, who arrived the previous night, and Tim Reston of GEOMAR. With equipment preparation essentially completed by noon, Ben and Stephen had a chance to see the island in the afternoon.

The Ewing set sail at noon on Thursday, July 10, heading to the experiment site on the Iberia abyssal plain and Galicia Bank just off Portugal. Science party includes five people from Rice, three from UT, six from GEOMAR, one from Spain and one from Portugal. We started and programmed each OBS as soon as we left the port. Since all of the UTIG OBSs were found functioning well, we decided to deploy all eleven OBSs, including a spare.

On Friday afternoon, we stopped at the mid point to our destination and run GEOMAR OBH release tests. This was a test to make sure that all acoustic releases were working properly, but I understand that they seldom fail and thus this test is s sort of a ritual to perform before every GEOMAR OBH experiment.

With the help of a good weather, we gained some time during the transit; and thus we decided to advance the start of our experiment by five hours from our original plan. Accordingly, we downloaded a new 'shot table' (recording schedule) to each of our OBSs and advanced the resetting of backup release timers by five hours. [We can do all these without opening the spheres now.]

We started deploying GEOMAR OBHs at 1:30 this morning, completing 7 deployments by 7 a.m. Deployments of our OBSs followed, and all 11 of them were completed by 2 p.m. Now the Germans are deploying the remaining 7 units, and should complete them by late tonight. This will be followed by deploying and balancing of the streamer and deployment of the air-gun array. The shooting is expected to start late tomorrow night. Each of our OBSs is programmed to start recording at noon tomorrow for a five-and-a-half day continuous recording on all 4 channels (3-component geophone and a hydrophone) at 5 ms sample interval, except for two units, which will record 4-channels for two and a half days and then switch to 2-channel (V & P) recording because of a limitation in disk capacity for these units.

Both Ben and Steve are working very hard. Ben now needs some rest after working continuously to prepare all OBSs for deployment. Steve is on regular watch in addition to helping us in OBS preparation, and thus cannot rest yet.

That's all for today. The next report will probably be after the first recovery, hopefully with good news.

Report #2
Date: Wed, 23 Jul 1997 20:55:48 +0000
Now, the first part of our experiment has been successfully completed and the shooting has just begun for the second part, I am sending this report to you as I promised earlier.

All of the 11 UTIG OBSs and 14 GEOMAR OBHs have been recovered from our first deployment over the Iberia Abyssal Plain, and all but one of our instruments recorded full data. (The only exception was a unit with a new IBM disk drive, which started to malfunction in the middle of the recording and numerous intermittent disk-write failures caused a loss of about 30% of the data.) The acquired data are being processed on board, and preliminary record sections from some of the instruments show clear reflections and refractions from the S reflector.

There are a couple of good news with our OBS:

(1) This was the first experiment after we installed temperature-controlled crystal oscillators (TCXOs) on all of our OBSs to improve their timing accuracy. [We tried it on one OBS during the Chicxulub experiment with full success.] The total clock drift after seven days of deployment ranged from 3 milliseconds to 142 milliseconds, again with one exception. One clock drifted 21 seconds in a week, but the drift rate before deployment and after recovery was normal. Obviously we replaced this clock with a spare clock board for the second deployment, and we are examining this clock to find out what happened.

(2) We tried an acoustic release for the first time on one of our OBSs, and it worked!! This is a new modification to our OBS Ben has been working hard to complete in time for this cruise, and he should be commended for the success in the first sea trial. Our version of acoustic release intercepts the signal received by our regular hydrophone, processes it with an electronic circuit board from Benthos, and sends the release command to our normal release circuitry. Since the only additional part is the circuit board, it costs only about one twentieth of the price of a commercially available complete acoustic release unit, which usually costs more than the cost of each of our OBS.

Our second and current experiment is mostly over the Galicia Bank, closer to the shore. Again we deployed 11 UTIG OBSs and 14 GEOMAR OBHs. The shooting will last for eight and a half days, and after the recovery of instruments, we will head for a short port call in Lisbon to drop off the German scientists and equipment, which is planned for August 4.

The sea has not been very calm, with waves ranging from 4 to 6 feet (this is my guess), which, I understand, is unusual for this time of the year in this area. Waves wash the Ewing's deck (and our feet and legs, and sometimes more) while we deploy and retrieve OBSs. Hopefully, more usual sea will return in the coming weeks.

I hope you all have well settled in our new place by now. I am definitely enjoying the hard work on board the Ewing much better than the final 30-day period of "Move" until I was excused from it by coming here. I now have time to look at some data and catch up on some reading.

Report #3
I hope various glitches associated with the move have gone and everything is well at UTIG. We have just started shooting for the third phase of this experiment, and our instruments will be recording the shots for the next seven days.

There were both low and high times during the last 5 days, when we recovered OBSs, refurbished and redeployed them. The low time came when a few OBSs came back with the main clock silent. It turned out that a few alkaline batteries died prematurely, thus providing insufficient power to keep the CPU running. This was followed by an OBS not returning to surface at all at the expected time and during the one hour wait at the site before our giving up to proceed to the next station.

The high time came during the following day, when we decided to go back to the lost OBS site on our way to a port call in Lisbon and heard the radio beacon signal calling us to come pick her up. We were all so happy to hear this signal again, and elated when the instrument was sighted. The instrument was functioning perfectly, and we are puzzled as to what caused the delay in surfacing. During the deployment, however, we did notice that one of the anchor frames had a slightly oval upper ring, which caused a relatively tight fit of the sphere and the protective yellow hat to the frame. Thus, this may have been the cause of the delayed release, but we are not certain of it.

The dead CPU batteries turned out to be not much of a problem either. The CPUs stopped 3 to 15 hour short of the full 9-day recording period, and by then either the shooting had already stopped or final shots were being fired along a line off and far away from the OBSs. The loss of the clock calibration at recovery would be a problem with our old clock, but with the new TCXO-based clock this is less of a problem.

The acoustic release worked well again, this time at a shallow (146 m) depth. The water depth was over 5000 m last time. We are now testing it at an intermediate depth of about 2000 m for the current deployment.

During the first two phases of the experiment, we deployed and recovered a total of 50 OBS/Hs, 22 of ours and 28 of GEOMAR's. All were recovered and all but five had practically full data - four of GEOMAR's had tape errors which prevented them to write any useful data and one of ours had numerous disk write errors. We are studying the preliminary record section plots made on board.

Jamie came on board in Lisbon with a fresh supply of batteries and tapes (we were short of them because we have been collecting much more data on our instruments than we originally planned), but without clean clothes. His baggage, unfortunately, did not make to Lisbon, and he may has to depend on supply of T-shirts etc. from the ship's slop shop for the next ten days.

The wind from north continues to be rather strong, but not enough to disrupt our operation. Ben and Stony (Steven Clark) are working hard (Ben sealed up 11 spheres in 8 hours, from 9 p.m. to 5 a.m., for the current deployment), and Jamie helped us to tape up the spheres and to deploy OBSs.

Report #4
I regret to start this report with a sad note. Two of our OBSs from the final deployment failed to return. The captain was kind enough to postpone our arrival at Lisbon by six hours to return to the sites after the last OBS was recovered to look for them, but the search was unsuccessful. All indications are that they failed to surface, because the sea condition was very good for sighting of the instruments. This is highly unusual for our current instrument, and I have no explanation. It tells you that even when you think all the bugs have been eliminated from an instrument, there still remain some unexpected ones.

Each of the remaining nine instruments from the final deployment contained full data - not even a single disk-write error, and no premature battery failure. Preliminary record sections look excellent. Arrivals are observed well beyond 100 km, with clear PmP and Pn phases on this line across the ocean-continent boundary. We are talking about letting Stoney (Stephen Clark) analyze this data set for his MS thesis.

Overall, we made 33 deployments of UTIG OBSs and recovered 31 with a total of over 23 giga bytes of raw OBS data, which is more than the planned 30 drops with 500 Mbyte per OBS drop.

We are now on our way back to Lisbon, with expected arrival at 3 o'clock this afternoon. Since today is a national holiday in Portugal, Ben and I need to wait till Monday before our shipping agent picks up our equipment. We plan to head back to Austin the following day.