Evaluation of the BATHY 1500 Heave
Compensation Function
By Tom Opishinski, Sr. Ocean
Engineer
While Hurricane Dennis was off
the coast of the US Carolinas during August 1999, we took the opportunity to evaluate the
BATHY 1500 heave compensation function. The conditions were ideal as the force of
the distant hurricane generated rough seas in our local New England waters. We
departed from Newport, RI aboard a 24 foot recreational fishing vessel and headed south to
a location offshore Castle Hill beach, facing the open waters of the Atlantic. The
rocky shoreline surrounding the beach created a perfect test site as the small vessel was
subjected to long period wave swell combined with shorter period wind driven waves.
Our BATHY 1500 was outfitted with 24 and 200 kHz
transducers in order to test the heave compensation function in both single and dual
channel operational modes. The transducers were located on the starboard side of the
vessel using a pole mounted configuration.
Dual channel operation also allows for a
direct comparison of results from each individual channel. Peripheral components
connected to the BATHY 1500 included a differential GPS that provided position data and a
Seatex Motion Reference Unit (model MRU-5). The MRU-5 provides the raw heave data
used by the BATHY 1500 to compensate the measured water depth for the vertical heave
motion of the vessel created by the ocean waves. The heave sensor was located on the
starboard side of the vessel as close as possible to the transducers. Location of
the motion sensor near the transducers, rather than at the center of mass of the vessel,
provides the most accurate results. This is because the measured heave motion
closely matches that of the transducers. It also minimizes potential vertical heave
components resulting from pitch and roll moments of the vessel when the motion sensor is
not located at the transducers.
- Data from this survey was stored internally by the BATHY 1500 in
non-volatile flash memory and downloaded to a laptop computer following the survey using a
standard terminal emulator program (e.g., HyperTerminal, Procomm Plus).
- The internal data storage format, implemented by the BATHY 1500,
is ASCII based and includes:
date/time, position, depth measured by each
transducer (corrected for heave motion),
heave correction value (half the sum of raw heave value at transmit and receive) applied
to each measured depth, vessel speed and course, operational measurement units and
fix/event mark indicator.
The format,
which is the identical format the system uses to store data on an optional external mass
storage device, is designed to allow the user a great deal of flexibility during post
processing. For example, because the format is ASCII based and comma delimited, it
can easily be imported in spreadsheet software (as was performed for this analysis) and
/or into many two- and three-dimensional plotting packages. In addition, the
availability of the heave correction terms allows one to remove the heave correction if it
is determined, after a survey, that the motion sensor provided faulty data due to
malfunction or calibration error. Availability of the heave correction values also
allows the user to look at the corrected and uncorrected bathymetric data to see how well
the correction is working. For example, the figures available at our website detail
both corrected and uncorrected bathymetric data
.
