Dalhousie Seismic Group Website

SCREECH OPERATIONAL OBJECTIVES

Our operational plan was to acquire detailed wide-angle OBS/H and vertical incidence MCS seismic data along three main transects across the Newfoundland margin, together with auxiliary MCS data (parallel and crossing lines) along and between transects as geological structure dictated and as time permitted. This was a two-ship program, with R/V Oceanus deploying and recovering the OBS/H, and R/V Ewing shooting to the OBS/H and recording MCS data on its 480-channel, 6-km-long streamer. Wide-angle data were recorded on 29 ocean-bottom instruments: 8 WHOI ORB, 7 WHOI OBH, 8 Geological Survey of Canada -Atlantic Region (GSCA) OBS, and 6 Dalhousie OBS. The 15 WHOI instruments recorded a single-component (hydrophone sensor) while the 14 Canadian OBS recorded 4-components (hydrophone and three orthogonal 4.5 Hz geophone sensors). Shots were fired with R/V Ewing's 20-gun, 8540 cubic inch (131 liter) airgun array.

To minimize transit time and maximize survey time, the transects were surveyed from north to south (Transect 1 to Transect 3).

Transect 1 extends from the northern margin of Flemish Cap southeast across thick continental crust of the Cap and seaward across thinned continental crust, "transitional" crust of unknown origin, and about 30 km onto known oceanic crust seaward of magnetic anomaly MO.
Transect 2 extends from continental crust at the western margin of Flemish Pass basin southeast across rifted continental crust, "transitional" crust of unknown origin, and about 60 km onto known oceanic crust seaward of magnetic anomaly MO.
Transect 3 extends from full thickness continental crust on the central Grand Banks southeast across rift basins and rifted continental crust (Jeanne d'Arc basin, Carson-Bonnition basin), "transitional" crust of unknown origin, and about 30 km onto known oceanic crust seaward of magnetic anomaly MO.

R/V Oceanus deployed the OBS/H instruments along each transect prior to R/V Ewing shooting the transect. Instruments were more closely spaced over thinner crust in deep water (typically 9-12 km) and were more widely spaced (20-50 km) over the thick continental crust of Flemish Cap and the Grand Banks. The OBS line on each transect was shot at 200 meter shot spacing in order to minimize previous-shot noise. After Ewing shot each OBS line, Oceanus retrieved the instruments and moved on to the next transect while Ewing returned along the transect shooting the MCS line at 50 meter shot spacing.

R/V Ewing conducted additional MCS surveys along portions of each transect, in part while waiting for Oceanus to redeploy OBS. These surveys concentrated first on acquiring MCS lines parallel to the main transect, and secondly on obtaining crossing lines. The grid-style survey was most extensive on Transect 2, where ODP drill sites will be proposed. At times when Oceanus was waiting for Ewing, it conducted heat-flow surveys on or near the main transects, or did magnetometer surveys if weather did not permit launching the heat-flow instrumentation.

Primary operational goals for R/V Oceanus are summarized as follows:

Deploy and recover all available OBS/H along each transect
Obtain heat flow measurements within transitional crust along each transect during shooting of transect by R/V Ewing
Obtain magnetometer profiles across M0 anomaly between transects
Maintain close communications with R/V Ewing to coordinate operations
Process OBS/H data to Seg-y format using airgun shot times and locations provided by R/V Ewing
Copy all seismic data to Exabyte tape or CD-ROM for use by participating investigators.

Primary operational goals for R/V Ewing are summarized as follows:

Fire the Ewing 20-gun array to the OBS/H at 200 m shot spacing and at 50-m shot spacing to the 6-km streamner on each transect.
Acquire auxiliary geophysical data (gravity, magnetics, multibeam bathymetry, 3.5kHz echosounding, XBT profiles) along the ship track.
Maintain close communications with RIV Oceanus to coordinate operations, and with both Oceanus and our DLC shore-based investigator (H.C. Larsen) to make strategic decisions as necessary during the cruise.
Produce near-real-time brute stacks of all MCS data.
Copy all MCS prestack data to DLT or Exabyte tapes for use by participating investigators.
Produce improved stacking velocities, stacks, and migrations of selected profiles near Transect 2 to facilitate preparation of an Ocean Drilling Progran proposal.