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Data Processing
The CHIRP, Geopulse (1996 data only), and water-gun
subbottom data were logged digitally and processed using the PROMAX
(Advanced Geophysical Corporation) software package and mapped using
the Landmark software package. An Ormsby minimum-phase band-pass
filter (90-100-580-600 Hz) was applied to the water-gun data in
order to reduce ambient noise. A minimum-phase spiking deconvolution
also was applied to the water-gun data collected in 1996 to remove
reverberation. The CHIRP subbottom data were processed by decimating
the traces by 4 (keeping every forth trace), combining and averaging
2 traces, and application of a time-varied gain at a 1.5 ms time
power constant from 0-100 ms to further enhance internal reflectors.
A static correction for tow-fish depth was added to the CHIRP subbottom
data collected in 1996; this information was not available for the
data collected in 1995. The Geopulse boomer data were processed
by combining and averaging 4 traces and using a minimum phase spiking
deconvolution (operator length = 10 ms) to remove reverberation,
an Ormsby minimum-phase band-pass filter (325-3000 Hz), and application
of time-varying gain from 0-25 ms (>25 ms, constant gain). Subbottom
data were gridded and contoured using Earthvision (Dynamics
Graphics, Inc www.dgi.com) and the Landmark (www.lgc.com)
software packages.
Bathymetric data were
collected digitally at 0.17 samples per minute using a 200 kHz fathometer
and were processed to remove water depth variations due to vessel
heave, transducer draft, and tidal fluctuations. The data were reduced
by a factor of 2 in the along-track direction in order to minimize
trackline bias within the data and gridded and contoured using the
EarthVision software package (Dynamic Graphics, Inc www.dgi.com), using a 300-m grid cell size. Although the vertical
resolution of the bathymetric data along-track is approximately
0.5 m, these data were contoured in 2-m intervals in order to account
for interpolation between tracklines (across-track).
The sidescan-sonar data
were logged digitally at a sample rate yielding a 0.18-m pixel size
in the across-track direction and approximately 0.14-m in the along-track
direction following the methodology outlined by Danforth and others
(1991). A median filtering routine (Malinverno and others, 1990)
was applied to the sidescan-sonar data to remove speckle noise,
resulting in a 0.73-m pixel size. The data were further processed
and digitally mosaicked using procedures described in Danforth and
others (1991), Danforth (1997), and Paskevich (1992) resulting in
an enhanced, geographically correct, sidescan-sonar mosaic with
4 m/pixel resolution.
The multibeam data were
acquired digitally and processed using software developed by the
Ocean Mapping Group, University of New Brunswick; see World Wide Wed URL www.omg.unb.ca/omg
. Tidal elevations measured at the NOAA Sandy Hook station were
used to adjust the data for the tidal fluctuations and reference
the data to mean lower low water.
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