UTIG RESEARCH PROJECTS ARCHIVE

Dates: June 7 - June 10, 1999

PI's: John Goff, University of Texas Institute for Geophysics, and Neal Driscoll, Woods Hole Oceanographic Institution

Additional Participants: Laurie Duncan (UTIG Student), Wayne Spencer, chirp technician (among many hats).

R/V Onrust Crew: Steve Cluett (Captain), Adam Jenkins

Funding Agency: Joint Oceanographic Institutions, Site Survey Augmentation Award

The chirp seismic reflection program (Figure 1) on the New Jersey shelf was the second phase of an integrated JOI Site Survey Augmentation program.  Phase I, which occurred in September, 1998, involved sediment grab sampling and grain size analysis for the purpose of ground truthing the sidescan backscatter data (collected in 1996 as part of the ONR STRATAFORM program).  The chirp data were intended to probe the internal structure of inner- and mid-shelf sand ridges, the Mid-Shelf shore, and to extend our picture of Quaternary stratigraphy shoreward from a previous Huntec high resolution seismic survey that extended from the Mid Shelf shore to the outer shelf (the Huntec data are being interpreted by Jamie Austin and Laurie Duncan).   Several of our deeper lines were exact duplicates of Huntec lines, so that a direct comparison of structure and resolution can be obtained.  We also towed a dual frequency sidescan sonar system (100 and 500 kHz), which will be useful for comparing against the 1996 survey and for investigating frequency dependence in backscatter response to sediments.

The cruise went along swimmingly for a day and a half.  Anticipating some extra time and with a good weather report, we started to head out to the outer shelf to obtain a few lines along some planned deep coring sites on the Hudson Apron (jointly funded by JOI and ONR).  However, the weather gods were against us (again) as an unforecast "back door" cold front hit us, bringing winds from calm to 25 kts within an hour.  The Onrust is a sturdy but small research vessel, and for safety's sake we had to return to Barnegat Light.  The seas were sufficiently bad that we could not finish our planned crossing lines (we could zig, but not zag).  We otherwise satisfied our primary objectives, ending up with  four closely space profiles through the swath mapping corridor and through the three proposed jack-up drilling sites.  The chirp system performed admirably in the harsh conditions, though by the time we reached the end of the corridor the wear was beginning to show in the quality of the data.  Overall the data are quite spectacular (Figure 2).  Good results to follow.

Figure 1.  Planned track lines (yellow) overlain on the sidescan backscatter data (lighter shades - higher backscatter) and regional bathymetry (meters).

Figure 2.  Example of some of the phenomenal seismic imagery that we were able to obtain with the chirp system - here a buried river channel with finely laminated internal stratigraphy.  Bottom reflector is the paleo-Hudson valley.  Scale at right is meters below instrument (assuming 1500 m/s).

Photo Gallery - click on thumb nail for full size picture and caption