During the Fall months of 1998, engineers from Ocean Data Equipment Corporation were
conducting operational tests on the new Bathy-1500 survey echo
sounder. As part of the functional test plan, a survey of the Castle Hill area was
conducted. The Bathy-1500 was equipped with a 200kHz transducer.
A heave sensor was incorporated to allow heave corrections to be performed in real time.
Data was recorded in ODEC HEAVE format to the internal Flash memory of the Bathy-1500
and downloaded to a laptop computer after the survey was completed.
Following planned survey lines (see Figure 1)
the Bathy-1500 recorded simultaneous measurement of time, depth,
heave and location. The ODEC HEAVE formatted data was directly imported and processed
using "Surfer® for Windows"
by Golden Software. Surfer creates a representative grid of regularly spaced data from a
data set containing irregularly spaced (x, y, z) data. In order to increase the accuracy
of the gridded data, the coastline of Castle Hill (zero depth or mean sea level) that fell
within the grid boundaries, was introduced into the depth grid. The coastline data points
were extracted from a digital coastline file of Narragansett Bay.
Castle Hill escarpment
is located in the entrance to the east passage of Narragansett Bay (See map Figure 2). The
entrance to the passage is very narrow, being bounded on either side by Newport and
Jamestown. The narrow dimensions of the passage cause tidal currents to move very swiftly
as they pass in and out of the bay. Over the years the swift currents have carved out the
sea floor thereby deepening the waters near Castle Hill. These unique features make it a
prime location for conducting a hydrographic survey to examine the features of the
escarpment. In addition, the dynamic sea floor features of the Castle Hill escarpment
offers an excellent opportunity to test the reliability and functionality of an echo
sounder.
Once a file of regularly spaced data is
generated by Surfer, the results may be viewed in a variety of formats in Surfer®.
Example formats include two-dimensional contour mapping, post and classified-post mapping,
shaded relief mapping, and three-dimensional surface contour maps. In addition, data may
be viewed as a 'slice' along any user defined transect or two or more display formats may
be combined. As an example, a surface contour map may be combined with a two-dimensional
contour map to generate a three-dimensional color surface contour map. Figure 3 combines
both two- and three-dimensional contour maps and also a post map. The small "+"
marks shown in the figure are locations where a depth measurement was taken and are
generated from a post map. The "+" are shown only for every 75th data point to
avoid cluttering the figure.
The two-dimensional contour map of
Castle Hill escarpment, shown in Figure 4 illustrates some interesting features of Castle
Hill. There is a steep gradient surrounding the shores of Castle Hill. From the water line
(MSL) the water depth increases rapidly to 40 or 50 meters deep. The closely packed
contour lines surrounding Castle Hill detail this feature. The "cliff' bounds one
side of a deep trench which runs through the escarpment parallel to the shoreline of
Castle Hill. On the northwest side of the trench the sea floor slopes upward at a gentler
grade. In the northwest comer of the grid there is a large flat area. This feature is not
representative of the actual bathymetry, but is caused by the absence of any measured data
in this area. The main feature observed is the deep hole located immediately to the west
of the northern tip of Castle Hill. The depth of the hole is approximately 60 meters (195
feet).
Data collected from the Castle Hill survey has shown
that the Bathy-1500 is capable of providing valid hydrographic
data in areas of widely varying bathymetry. During the survey the Bathy-1500
continually tracked the sea floor without loss of data. The resulting data set was easily
imported into a post-processing package (Surfer® for Windows) for further
analysis and presentation. Presently the Bathy-1500 Survey Echo
Sounder provides data formats that are compatible with Hypack software by Coastal
Oceanographics, HYDROpro by Trimble Navigation, and most data spreadsheet/plotting
software packages (e.g., Surfer® for Windows®, Microsoft
Excel®, Corel Quattro Pro®, etc.).
