07/13/2012 — admin
The EM122 and EM710 systems both appear to be configured correctly and, for the most part, adequately calibrated. Small MRU roll alignment residuals do exist and it is recommended that these be confirmed as soon as possible through acquisition of a pair of reciprocal survey lines over a flat and featureless seafloor. Verification lines should have been run for calibration tests for both systems, however, time constraints on the cruise schedule precluded this. It is standard practice to run verification lines.
The EM122 performance issues appear to have been resolved: 3 of 8 RX modules were wired with reverse polarity, Kongsberg Norway has issued a software patch to reverse the polarity in software prior to beamforming. It is the author’s understanding that a set of jumper cables are in the process of being made to fix the problem, at which point the software patch would presumably no longer be required. It is recommended that Kilo%Moana personnel obtain a copy of the software patch and that procedures for the software patch installation should be documented such that the EM122 can be restored to its current working state in the event of a TRU software reWinstallation. It is also recommended that MAC personnel
be kept informed of the hardware/software configuration changes when the jumper cables are put in place.
The accuracy (repeatability, strictly speaking) analysis indicates that both systems are performing within expected levels. Further testing should be done in the various modes not investigated during the sea acceptance trials prior to committing either of these systems to cruises where an untested mode of operation will be used. Having a pair of multibeam echosounders allows the luxury of choosing the most optimal system for a given water depth. The question then becomes at which depth should mapping operations switch between the two systems. Previous studies of this issue, conducted by Hughes Clarke for NavO vessels having the same pair of systems, indicates that the crossover depth from EM710 to EM122 should be when the EM710 switches to “Deep” mode, this occurs at 300 m depth according to the EM710 specifications. Even though the EM710 can achieve coverage over its entire angular sector beyond this depth, the use of FM pulse waveforms in the outer sectors degrades the data quality in this mode of operation. In these same depths, the EM122 continues to use CW pulse waveforms and does not suffer the same degradation in sounding accuracy. That being said, the EM710 central sector is still CW in “Deep” mode and will provide better data than the EM122. A further complicating factor is that the EM122 has a longer wavelength, thus seafloors appear smoother relative to the EM710, resulting in typically steeper seafloor angular response curve at nadir. Under conditions of poor sidelobe suppression, due for example to aging of receiver elements, soWcalled “Erik’s Horns” artifacts can result. Clearly there is no straightforward answer and this should be perhaps dealt with by testing both systems prior to surveying and determining the optimal configuration in the field.
For optimal data quality, EM710 operations in shallow, continental shelf waters will likely require: (1) GPS differential corrections provided to the POS/MV, at least DGPS, and perhaps (2) application of Applanix “True Heave” in post-processing. Logging/archival procedures should be established for Applanix “True Heave” either through SIS or through the Applanix POS/MV monitoring software. Processing procedures for “True Heave” should be investigated as well to allow for real-time QA/QC by technicians and/or cruise participants. The datagram distribution service can potentially overload the EM710 TRU as it is currently configured. It is recommended to remove duplicate datagram subscriptions for the two HMRG Seafloor Mosaic Display systems; the redundancy provided by the two HMRG systems could be kept in place with the backup system
changing its IP address in event of a failure of the primary HMRG machine. The removal of duplicate datagrams subscriptions should be repeated for the EM122. On a technical note, it was observed that the EM710 Hydrographic Work Station (HWS) is reporting SMART errors for the primary hard drive (C:). This drive should be backed up and replaced ASAP. Knowledge of multibeam data transfer/archival scripts implemented by the MAC (P. Johnson) during Portland/Honolulu transit in March 2012 has not been adequately transmitted to OTG technicians in crew change handovers. These should be internally documented and explained to all staff.
Coverage performance of the EM122 is as expected and is consistent with Lurton’s model predictions. In its current configuration, gains in coverage can only be made by reducing acoustic noise levels. There is much focus on the achievable coverage for these systems as this is the metric with which many science personnel judge the capabilities of a system. It should be pointed out that the sounding uncertainty at the outermost edges of the swath, perhaps at 70°, can be nearly an order of magnitude larger than what is achievable at nadir (see Fig. 5). Though it is tempting to wish for a system that can image over a larger sector, the additional data that are gained are typically of very poor quality and would likely be omitted from any scientific analysis on the data. Clearly, other methods and metrics are required to report the useable swath width. For example, Ifremer measures the coverage of their multibeam systems by the swath width that exhibits less than 0.3% w.d. sounding uncertainty at the 95% confidence level. For the EM122 investigation done in this work, this would be equivalent to a sector of +/W50° in “Deep” mode in 4,700 m of water (this will vary, of course, with depth). This gives a much more realistic and practical measure of the useable swath width.