CRUISE REPORT

                   R/V H.J.W. FAY - Cruise 4 - Seismic Refraction

     Dates: 20-26 October 1975
     
     Ports: Woods Hole to Woods Hole

     Ships Officers: Captain: Lawrence Buell
                     First Mate: John Ostensen
                     Third Mate: James Biller

     Scientific Party:

           Name           Organization           Responsibilities

     John Grow               USGS                Chief Scientist
     Lyle McGinnis           USGS                geophysicist
     Perry Parks             USGS                gravity technician
     Bruce Ambuter           USGS                electrical engineer/Refraction
     Bill Jaworski           USGS                geophysicist/deck systems
     Chuck Meeder            USGS                refraction technician
     Felicity Oram           USGS                watch stander
     Doug Peeler             USGS                watch stander
     Sally Wood              USGS                watch stander
     Scott Heald             USGS                resident technician
     Stan Locker             USGS                watch stander
     Gary Yerman             Bolt Inc.           2000 in3 Airgun Specialist
     Paul Passmore           Refraction Tech.    consultant
                                  Inc.

     Cruise Objectives:

          Leg 4 of the R/V FAY was set up to obtain approximately three
     reversed refraction lines on Georges Bank and to collect two to
     four long geophysical profiles over the continental slope and rise.

          The seismic refraction velocities were obtained with both
     expendable and a new recoverable sonobuoy (with an internal
     digital recorder) in order to confirm velocity models for Georges
     Bank which had been developed on the basis of earlier
     reconnaissance refraction studies in the 1950's, and on the basis
     of interval velocities obtained from multichannel seismic
     systems.

          The gravity profiles into deep water were designed to
     supplement the gravity measurements over the shallow water
     portion of Georges Bank, which were collected on FAY 3 and to
     cover several U.S.G.G. multichannel seismic lines where no or
     poor gravity data was available.

     Instrumentation:

          The underway geophysical systems employed during Leg 4 were
     gravity, magnetic gradiometry and the towed 3.5 kilohertz
     echosounder (only worked in water less than 500m).

          The expendable sonobuoys included Select Internation Inc. long
     range buoys (Model #SLF-73) with a 73 mhz FM transmitter
     and a double suspension system to isolate the hydrophone.  The
     source was a 2000 cubic inch airgun leased from Bolt
     Associates Inc. and fired at a repetition rate of 30 seconds.

          A large recording buoy was also tested on several stations.  The
     buoy is being developed with a digital magnetic tape recorder for
     long-range refraction with very high dynamic range.  The initial
     tests were plagued by hydrophone problems, but a good run was
     finally achieved on the last day.

     Cruise Log:

          The R/V FAY departed Woods Hole at 2000 GMT/20 October
     1975.  Testing of the 2000 cubic inch airgun commenced at 2130
     GMT in Vineyard Sound and were successfully completed at
     2300 GMT.  The R/V FAY then headed southwest around
     Martha's Vineyard and off towards Georges Bank.

          The FAY arrived at Station 1A (Figure 1) about 2000 GMT/21
     October.  The large recording buoy was deployed but low
     hydrophone sensitivity required its immediate recovery.
     Moderate seas (3 to 6 feet) and reluctance by the First Mate to
     use the hydraulic crane, resulted in delays and some damage to
     the buoy antenna during recovery.  Fortunately, three spare
     antennae were aboard.  An expendable buoy was launched at
     0115 GMT/22 October (buoy FAY 4-1A) on a course of 051
     degrees.  The line was reversed and buoy FAY 4-1B was deployed
     at 0408 GMT on a course of 220 degrees.  High velocity refracted
     arrivals (5-1/2 to 6 km/sec) were observed at a depth of about
     4km.
     
          At 600 GMT/22 October the seas were building and many of the
     scientific crew were seasick.  Therefore, the ship was headed out
     in deep water to pick up gravity and magnetic data along
     U.S.G.S. multichannel seismic line No. 1 (Figure 1).  This survey
     continued until 1200 GMT/23 October.

          Beginning about 1445 GMT/23 October, three expendable buoys
     were launched on a course of 230 degrees (FAY 4-2A, B, C).  Buoy
     FAY 4-2C was launched at 1717 GMT and was the best of these
     buoys.  A compressor failure caused some interruptions in the
     line, but it was completed at 2000 GMT.  The large buoy (FAY
     4-2D) was deployed at 2100 GMT but again had hydrophone
     problems and had to be recovered.  An expendable buoy (FAY
     4-2E) was launched at 0129 GMT/24 October on a course of 050
     degrees and a good run was completed at 0400 GMT.  A local
     magnetic and gravity survey was accomplished during the night
     and the R/V FAY arrived at station FAY4-3 about 1200 GMT/24
     October.  The large buoy was launched at 1421 GMT (Sonobuoy
     FAY 4-3A) and the ship commenced a course of 052 degrees.  An
     expendable buoy was launched at 1526Z (FAY 4-3B) and was
     somewhat quiter than the big buoy.  The course was reversed at
     1749 GMT.  An airgun misfiring problem developed at 1850 GMT
     which ruined the ends of 4-3C, D.  The big buoy was recovered at
     2030 GMT.  At 2330 GMT on 24 October.  The FAY headed on a
     course of 070 degreees to get in position to reshoot 4-3C, D.  The
     new buoy (4-3E) was launched at 0001 GMT/25 October on a
     course of 230 degrees and was sucessfully completed at 0420
     GMT.

          The ship then headed north and at morning launched the big
     buoy again at 1234 GMT at position (4-4A in Figure 1).  This
     time the buoy worked well and the run was commenced on a
     course of 050 degrees.  At 1437 GMT the ship slowed and
     manuevered to attempt some "stacked" signals (i.e. repeated
     airgun blasts at the same location to improve signal-to-noise
     enhancements).  At 1715 GMT on 25 October the ship reversed
     course to 221 degrees and launched buoys 4-4B and 4-4C.  Buoy
     4-4B was the best and is labelled on Figure 1.  The large buoy
     was recovered after some searching at 2100 GMT.

          At 2111 GMT/25 October all refraction buoy work had been
     completed and time remained to make another pair of gravity
     and magnetic profiles into deep water.  Therefore, the ship
     headed south.  By about 0230 GMT/26 October the seas had built
     significantly.  The ship was pitching severely into 8 to 12 foot seas
     and had to slow to 6 knots.  At that slow speed, the ship could not
     complete the deep water lines and report back into Woods Hole
     by the required time of 1200 GMT/27 October.  Since everybody
     was seasick, including the Chief Scientist, the ship turned west
     and headed for Woods Hole, order to assure an early arrival.  The
     seas to the west and over the shelf calmed considerably in the
     next 6 hours and the FAY completed Leg 4, arriving in Woods
     Hole at 2100 GMT/26 October 1975.

     Results:

          Although on three reversed refraction lines were planned for this
     leg and it was shorter than originally scheduled, four lines were
     run (Fig. 1).  These refraction lines all were near U.S.G.S.
     multichannel seismic reflection Line #1 (Figs. 1 and 2).  The
     interval velocity data from U.S.G.S. Line #1 showed very high
     velocity sediments 5.6-5.9 km/sec between 4 to 8 km depth
     beneath Georges Bank and a 5.2 km/sec layer at a depth of 2 km
     at the outermost edge of the shelf.

          Earlier refraction data is shown at the bottom of Figure 2, which
     was generally consistent with the interval velocities but left a
     large gap in the middle shelf.  The new clay from FAY 4-1 to 4-4
     has only had prelimnary analysis but is penciled in on the
     refraction settings of the figure.  It is clear that the correlation
     with interval velocities is remarkably good.  The high velocity
     sedimentary rock layer (5.6-5.9 km/sec) was confirmed and
     probably represents early Cretaceous or Jurassic limestones and
     dolomites.  The 5.0 to 5.4 km/sec material beneath the outher
     edge of the shelf and upper continental slope probably also
     represents a carbonate bank or reef which existed at the shelf
     edge in the early or Middle Cretaceous period.

     Cruise Statistics

      Gravity Data                   1640km
      Bathymetry Data (3.5 kHz)       500km
      Magnetics                       600km
      Refraction Station             4 reversed profiles