10/19/2015 — admin
A Kongsberg EM710 MKII multibeam was installed aboard NOAA Ship Nancy Foster at GMD Shipyard in Brooklyn, NY in March of 2015. This system replaced the antiquated EM1002 and is the third EM710 installed on a NOAA ship. The MKII EM710 has an extended frequency range that increases the effective depth range of the system. This installation was the second MKII system delivered by Kongsberg in the United States. An unrealistic fabrication and installation period and associated shipyard delays complicated the installation of this system. Shortened sea acceptance trials were conducted during the transit to Charleston, SC. Nancy Foster was underway from NY on March 9, and arrived in Charleston on March 13. The last two days were entirely dedicated to transit in poor weather.
Acceptance testing confirmed proper operation of the EM710 between 40 meters and 2500 meters and for the primary frequency modes. Absolute and relative accuracy of the measured water depths agreed satisfactorily with reference data. Nancy Foster’s hull shape is not conducive to working in heavy
weather as waves of only a few feet produce sufficient noise to interfere with data quality. In shallow water it is recommended to limit survey speed if data quality is observed to be degraded by bottom bounce noise. Backscatter modes for the 40 kHz – 100 kHz and 70 kHz – 100 kHz frequency modes were
normalized for optimal real time backscatter.
The abbreviated time available for testing did not allow additional normalization between additional frequency modes, specifically the 40 kHz and 50 kHz settings. Had additional time been made available more thorough extinction testing could have been completed for future performance comparison. The
restricted time table did not allow for additional transit time to areas previously used for EM710 acceptance testing, specifically the areas southeast of Cape Cod where results could be compared with the EM710 recently installed aboard the R/V Sikuliaq. Using previously defined areas can improve results by limiting the uncertain associated site selection. A more quantitative assessment of the interference between the many different acoustic systems and the EM710 would also have been of benefit for future users and project planning had the testing time been made available.
Significant Findings and Recommendations
- A Kongsberg EM710 MKII was installed and tested successfully aboard Nancy Foster.
- The new multibeam echosounder meets or exceeds the specifications provided by Kongsberg.
- Insufficient sea‐time was available to fully test and categorize this system. In particular, deepwater accuracy, low frequency backscatter normalization, weather effects, potential interference between systems, and noise characterization investigations were either truncated or not performed. Allowing suitable time to properly evaluate new echosounders provides OMAO with equipment that can best mean customer’s needs, and allow for baseline information for future evaluation of mapping system health.
- Final vessel offset results were not delivered until many months after the cruise was completed. Vessel offset survey specifications need to be carefully considered for useful results, specifically including the delivery time and review of the final survey report before the ship is to get underway for testing.
- A bad element exists in the new EM710 transmitter. Kongsberg will replace the relevant module within the warranty period if NOAA coordinates this work during the next dry dock period.
- The EM710 aboard Nancy Foster is only fully operational in limited weather conditions.