04/04/2018 — admin
We installed four Kongsberg EM2040 multibeam echo sounders on each of Rainier’s four hydrographic survey launches from September 12th through September 13th in Everett, WA. We accomplished the test and qualification of these systems on Puget Sound from September 14th through September 21st. Conditions were generally agreeable for testing with some environmental effects potentially affecting a limited set of noise tests. All systems were successfully integrated. Measured performance is in compliance with vendor specifications and is suitable for NOAA hydrographic surveys.
Significant Findings and Recommendations
- Four systems were installed and integrated according to HSTB recommendations and performed as expected.
- Sounding quality and object detection of these multibeam systems is significantly higher inside 65° from nadir. We recommend limiting the operational swath to 65° in most cases.
- Sonar systems met complete coverage survey requirements when operating at 7.9 knots. Survey operations at speeds up to 10 knots will likely meet complete coverage requirements. When conducting object detection surveys, we recommend limiting survey speed to 8 kts and ensuring swath width does not exceed 65°.
- Accuracy lines were used to generate backscatter mosaics, allowing us to further examine differences between operating modes. We processed backscatter mosaics, establishing cross-correlation values between vessel, pulse length, and frequency.
- The Rainier hydrographic launches appear to have a relatively high noise floor at 400 kHz. Bottom-bounce radiated noise from the propulsion may contribute to this.
- The acceptance testing schedule occurred concurrently with an off-shore project that did not require launch work. This allowed for sufficient time to complete testing and conduct initial analysis on-site with minimal impact to sea days.
- The Shilshole reference surface, while valuable for its historical data, may have significant freshwater input from the Ballard locks and thereby experience oceanographic variability that is not ideal for calibration work.
- We recommend a 7 millisecond value for motion time delay into SIS for Rainier EM2040s.
- SV casts must be extended past the operational depth of the system prior to loading to SIS. CTD profiles that fail to cover the full depth of the survey area, extended or otherwise, may result in anomalous absorption values and interfere with reported depths.
- Because of the 7 ms timing offset between POS and the 2040s, any post-processed navigation and attitude solution should either account for this offset on export, or only the navigation (and not attitude) should be applied to the bathymetric data in post-processing. We recommend the latter.