Dalhousie Seismic Group Website

SCREECH PRELIMINARY CRUISE ASSESSMENT

Although we experienced a variety of difficulties during the cruise, and some data sets may be considered less than perfect, we are optimistic that the data will meet or exceed our goals in understanding the crustal structure across the Newfoundland margin, and we judge that our field work has been a notable success. Factors responsible for achieving our goals included: 1) adequate planning to accommodate the vagaries of two-ship operations, and especially inclusion of contingency time to cover unforeseen circumstances, 2) flexibility in survey plans that allowed each of the two ships to accommodate changes or in some cases even to assist one another, 3) generally good weather conditions throughout the cruise, and 4) the highly professional ship's crew and LDEO technical staff on Ewing, who did everything possible to help us achieve our goals and ensure a safe cruise.

Notable successes for R/V Ewing included the following:

Complete OBS and MCS lines along all three main transacts, plus grid MCS survey and tie lines to address special geological problems of interest and to define ocean drilling objectives.
High quality of the MCS data. The airgun array produced a very sharp and clean pulse, and the 480-channel streamer produces closely spaced CMPs and highfold data, contributing to a high-resolution reflection record.
Near real-time brute stacks of the MCS data. We stacked the entire data set (480 channels, 16 sec records) and copied all prestack data to DLT tapes as data came off the system, and we were easily able to keep up with the data flow rate.
Efficient recovery of one GSC OBS in heavy fog and with the streamer deployed, and return of the instrument to Oceanus several days later via release from the Ewing fantail and pickup by Oceanus.

Notable successes for R/V Oceanus included the following:

Complete OBS and MCS coverage along all three main transects, plus grid MCS survey and tie lines to address special geological problems of interest and to define ocean drilling objectives.
High quality of the OBS data. The OBS records were generally of high signal-to-noise and the airgun array produced a strong and clean pulse.
Processing of most OBS data and combination with shot data to Segy-y format during the cruise.
Successful use of the new Dalhousie heat flow probe on transects 1 and 3.
Successful magnetometer profiles across anomaly M0 south of transect 1.

Principal operational difficulties for R/V Ewing included the following:

Fishing long-lines. Commercial fishermen deploy these along the warm-to-coldwater edge of currents in the Newfoundland Basin. They often extend 10-20 miles or more and cannot be crossed because of the possibility of fouling the Ewing's airgun array or streamer. Long-lines were directly in our path as we began to shoot Line 1-OBS, and we were forced to divert around them, sacrificing 200-meter-spaced shots to OBIUS instruments 27 to 29. Fortunately, we were able to shoot to these instruments at 50-meter shot spacing on the return line (I-MCS) by arranging with Oceanus to delay instrument recovery, although the 50-meter spacing is less than optimal because of previous-shot noise. Farther up Line 1-OBS, we snagged abandoned net-and-float flotsam on the streamer's tailbuoy. This forced us to pull in the full streamer, and we lost the tailbuoy as we attempted to bring it onboard. We decided to shoot the rest of the line without the streamer deployed, so the latter part of Line I-OBS has no associated multichannel data (this is not a significant problem, since MCS data at 50 m shot spacing were later acquired on that portion of the line). Finally, we encountered long-lines that were about to be deployed when we were shooting Line 104, which ties Transect I to Transect 2. We were able to communicate by radio with the commercial fishermen involved and arrange plans that minimally impacted each of us. We had no further problems with fishing gear on Transects 2 or 3.
OBH/S instrumentation. The OBH/S portion of the project can only be considered a qualified success, due to the relatively high rate of instrument losses and failures. We note the primary difficulties here, although they are covered in more detail in the Oceanus cruise report. The Dalhousie and GSC instruments presented new and unusual problems in ship-to-instrument communication during recovery, whereas in previous use these instruments had always been extremely reliable. Three of the instruments were lost, several were recovered on backup-timer release, one failed to record, and one had poor data quality. The WHOI ORB instruments failed to record in four instances. One WHOI ORB was lost because it could not be tracked on the surface during inclement weather (ORBs have no radio transmitter). One WHOI OBH also was lost; Oceanus was unable to communicate with the instrument when it attempted recovery. In the final analysis, Oceanus obtained 24 OBS/H records in 29 deployments on LinelOBS (plus one additional, poor-quality record), 24 OBS/H records in 27deployments on Line 2-OBS, and 21 OBS/H records in 24 deployments on Line 3-OBS.
Weather. We were fortunate to have acceptable to good weather conditions throughout most of the cruise. We normally towed the hydrophone streamer at 7.5 meters depth with little noise from surface waves, but on some occasions deepened it to 10 meters when wave noise increased. On 31 July to 1 August we lost about 24 hours of shooting time. Winds and seas were tangling the airgun array and threatening to damage it, so we pulled in the array until the weather subsided. On 11 August we pulled the airguns and streamer about 12 hours before the planned end of our survey in order to depart the area early and avoid the predicted arrival of hurricane Alberto. We had already completed all of our major objectives, so the early termination did not adversely affect our scientific program.

Principal operational difficulties for R/V Oceanus included the following:

In summary, OBH/S suffered from a higher than normal rate of instrument loss and some additional problems with recovery. These difficulties were exacerbated by the need to maintain coordinated schedules with R/V Ewing. In a total of 82 deployments, we obtained 24 OBS/H records in 29 deployments on Line 1 (plus one additional, poor-quality record); 24 OBS/H records in 27 deployments on Line 2; and 23 OBS/H records in 26 deployments on Line 3 for an overall data recovery rate of 87%.
Five instruments were lost during recovery (1-WHOI ORB, 1-WHOI OBH, 1-GSCA OBS and 2-DAL OBS) for an overall instrument recovery rate of 94%. Of these, 2 losses of the DAL-GSCA OBS were due to delayed release from the bottom; one WHOI ORB was lost in bad weather and strong currents when it could not be located on the surface; and one DAL OBS and one WHOI OBH were lost when there was no response.
The Dalhousie and GSC OBS experienced some unpredictable delays in releasing from the bottom. We learned following the cruise that similar difficulties had been encountered during previous deployments of similar OBS within Orphan Basin (north of Flemish Cap). Thus, we believe that this was caused by peculiar characteristics of the bottom sediment within this region. Three OBS with release delays of up to several hours were successfully recovered by returning to the sites following recovery of adjacent instruments. Two OBS (one Dalhousie and one GSCA) with much longer delays (up to ~12 hours) were lost when they had drifted too far away from their deployed position to be located by the time Oceanus could return.
Four DAL-GSCA OBS had to be retrieved using their backup timed releases. All but one of these occurred for instruments deployed on the continental slope and most likely were due to acoustic interference from near-bottom side echoes. On transect 2, there was an unexpected need to simultaneously recover two OBS on timed release that were far apart. A probable loss of one of these OBS was avoided when R/V Ewing was able to recover one of the OBS during a bad weather break in its operations.
Four WHOI ORB failed to record. The ORB were often difficult to locate on the surface particularly when they had drifted off site during ascent in strong surface currents or when surface visibility was poor in fog or at night. The need to recover in good daylight conditions sometimes complicated the scheduling of instrument recovery.
The Dalhousie heat flow pressure case flooded during initial pressure testing. Once the cause of the leak was identified, the instrument worked without fault. However, deployment on deck through the aft A-frame using the modified track system was difficult at best and was restricted to good weather conditions.
The small size and maneuverability of the R/V Oceanus were particularly well suited to deployment and recovery of the OBS/H. However, some reduction in operations also resulted during marginal weather conditions, and particularly while the R/V Ewing was shooting transect 2. In addition, the need to return for delayed pickup of some OBS due to various difficulties with recovery or to accommodate operations of the R/V Ewing (i.e. need to re-record W-end of transect 1 during MCS shoot), added significantly to the time spent in transit and limited the time available for heat flow and magnetometer measurements.

Preliminary scientific results include:

OBS record sections with generally high data quality and high signal-to-noise ratio. A number of these sections show evidence for very thin crust within the transition regions.
Heat flow measurements on oceanic crust at W-end of transect 1 (12 measurements) and in two areas of the transition region on transect 3 (12 plus 8 measurements). These later measurements were the first use of the system with an extended number of thermistors.
Two crossings of magnetic anomaly M-0 south of transect 1 which show no evidence for a continuous feature.
Basement character changes markedly across the transacts, with four basement types visible from west to east: (1) a relatively smooth, high-amplitude reflector beneath continental shelves, (2) an apparently block-faulted geometry in rifted continental crust, (3) smooth, low-reflection-ainplitude basement of uncertain affinity beneath the "U" reflection, and (4) rougher, possibly block-faulted basement seaward of the "U" pinchout, which may be serpentinized mantle or oceanic crust. This general morphological pattern exists on all three transacts, but each transect differs in detail.
At the current stage of processing, there is no evidence in our data that U is an unconformity; rather, it seems to conformably overlie a remarkably flat and layered sequence of possibly high-velocity material. The extent of the "U reflection" is very well defined by seaward pinchouts well landward of Srivastava's MO picks and by landward limits against large, apparently continental fault blocks.
The basement surface below "U" shows little impedance contrast with the overlying sub-U material, and it thus is defined more by an absence of reflections than by a distinct, regionally correlatable reflection. The basement surface below "U" tends to be deeper and to show less roughness than basement either landward or seaward of the "U" pinchouts. The cross-isochron width of the "U" zone increases steadily southward from Transect I to Transect 3.
Hummocky basement that has relatively low-amplitude roughness and is characteristic of ocean crust occurs at the seaward ends of Transects I and 3. At the seaward end of Transect 2, however, the presumed ocean crust seaward of MO has very high amplitude roughness, suggesting that it was formed under conditions where tectonic extension dominated over magmatic accretion.
Coherent intra-basement reflections sporadically appear outside the zone of the "U reflection". In the seaward basement province, these reflections appear to dip landward more frequently than seaward. These may eventually prove to be interpretable as fault surfaces and/or Moho reflections. These reflections are much clearer in reprocessed sections than in the brute stacks. We therefore expect that other intra-basement reflectors may be discovered as more lines are processed beyond the brute stack stage.
A distinct and widespread set of sedimentary sequences is developed above "U". From the base upward, these sequences include: 1) Flat-lying, weakly laminated sediments capped by a flat-lying and highly reflective sequence, all apparently deposited from turbidity flows. 2) Sculpted, pinch and swell reflections that appear to reflect deposition and local erosion controlled by abyssal currents; the base of this sequence may be equivalent to Horizon Au farther south in the western North Atlantic. 3) A sequence of contorted reflections including apparent channel development and channel fill; this sequence is interpreted as fan deposition. 4) A sequence of flat-lying and highly stratified sediments that lap onto the fan deposits; these are abyssal plain turbidites.