BATHY 1500 Heave Compensation Data from Castle Hill Area

Evaluation of the BATHY 1500 Heave Compensation Function

By Tom Opishinski, Sr. Ocean Engineer, Ocean Data Equipment Corporation

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 below detail both corrected and uncorrected bathymetric data.
Data results are presented in four figures below. Bathymetric data is plotted for two randomly selected time periods for both 24kHz and 200KHz . In each data plot, a depth trace is shown uncorrected and corrected for heave motion. A legend is also provided that defines each colored depth trace by frequency and whether it is corrected or uncorrected. Results are extremely encouraging in that the heave compensation function removed the larger period swell and a majority of the shorter period motion caused by the wind driven waves. Previous results of heave compensation evaluations performed in our acoustic test tank can be found here.

200 kHz - Corrected Depth 200 kHz - Uncorrected Depth	Bathy-1500 Dual Channel Echo Sounder Heave Compensation Evaluation Figure 1: 200 kHz measured depth with and without heave correction applied.

24 kHz - Corrected Depth 24 kHz - Uncorrected Depth	Bathy-1500 Dual Channel Echo Sounder Heave Compensation Evaluation Figure 2: 24 kHz measured depth with and without heave correction applied.

200 kHz - Corrected Depth 200 kHz - Uncorrected Depth	Bathy-1500 Dual Channel Echo Sounder Heave Compensation Evaluation Figure 3: 200 kHz measured depth with and without heave correction applied.

24 kHz - Corrected Depth 24 kHz - Uncorrected Depth	Bathy-1500 Dual Channel Echo Sounder Heave Compensation Evaluation Figure 4: 24 kHz measured depth with and without heave correction applied.