06/04/2012 — admin
A sea acceptance trial (SAT) was conducted to evaluate the performance of the EM302 multibeam echosounder (MBES) onboard the R/V Thomas G. Thompson in October 2010 off the coast of Washington state. Excellent sea conditions permitted continuous acquisition with all planned survey sites being successfully mapped. It is the intent of this report to document the results of the SAT and to provide recommendations on further improvements that can be made to maximize the performance and accuracy of the system for both bathymetric and acoustic backscatter measurements.
Verification of the configuration of the MBES and all ancillary sensors was done during transit to the deep water patch test calibration site (the same site used for the Thompson’s EM300 SAT in 2002) with minor discrepancies being found. These were brought to the attention of the Kongsberg Maritime (KM) personnel who then corrected the configuration in the KM acquisition software (SIS). Patch test operations allowed for the estimation of residual timing and angular misalignments between the MBES and motion reference unit (MRU). Data from the patch test were independently evaluated by the authors and the KM personnel with similar results.
A series of bathymetric surveys were conducted after the patch test over a range of water depths. A deep water survey (1400m-2500m) was conducted using a 5 by 5 survey line grid with survey line directions for survey differing by ninety degrees. Results from the surveys run in different directions confirm the internal consistency of the system and indicate that the MBES and all ancillary systems are being correctly integrated to provide georeferenced soundings. The deep water site was chosen to match the same area mapped by the Thompson during the 2002 EM300 SAT; the 2002 and 2010 grids were statistically consistent within the expected noise level of the system and are a good indication of the absolute accuracy of the system. Backscatter data were acquired over the deep water site as well with results being consistent between the two 2010 surveys, differing in survey direction, and between the 2002 and 2010 surveys. Minor beam pattern artifacts were observed, these were readily removed via standard normalization techniques.
Two shallower water surveys were conducted in ~150 meters of water on the continental shelf and in Puget Sound. Cross lines were acquired during the shelf survey and allow for a quantitative assessment of the system’s performance in shallower water. Results were within expected uncertainty levels across the majority of the swath with some minor bottom mistracking issues in the nadir region. Significant beam pattern type anomalies were observed in the transmit sectors immediately adjacent to the nadir region with some discrepancies between the swaths of the dual swath geometry; these are the likely cause of the nadir mistracking issues mentioned earlier. The beam pattern type artifacts are consistent with an incorrect transmitter source level and/or beam steering configuration both of which can be rectified through corrections applied in a configuration file on the sounder.
The shallow survey conducted in Puget Sound attempted to mitigate the nadir mistracking issues by intentional forward steering of the transmitter fan by 6º to 8º. This technique, though successful in minimizing the nadir mistracking, introduced a “hump” type artifact, likely due to bottom tracking algorithms locking on to the transmitter sidelobe response from nadir. Optimization of this technique may yield a configuration that minimizes both of these effects, however, further testing is required. It should be noted that the same backscatter anomalies observed during the shelf survey were also observed during the Puget Sound survey and also throughout the transit data acquired in and out of Puget Sound.
Acoustic noise level measurements were conducted immediately after the deep water survey with the vessel at rest in the water and at a variety of vessel speeds. Results indicate that flow and machinery noise are consistent at speeds below 8kts and only increase slightly (~5dB) at 10 and 12 kts. The low noise level at high speeds and the dual swath capability of the EM302 allow for potentially higher survey speeds as compared to the older EM300 configuration. It should be noted that noise tests in foul weather were not conducted.
The results from all tests and analyses indicate that the EM302 system is performing as expected with minor issues associated with transmit sector configurations affecting bottom tracking performance and quality of acoustic backscatter measurements in the shallow mode of operation. Further testing can be done to address these deficiencies and improve the overall performance of the system.