11/19/2014 — admin
The Kongsberg Maritime (KM) EM120 multibeam echosounder (MBES) aboard the RVIB Nathaniel B. Palmer (NBP), installed in 2002, recently received two upgrades through the installation of a KM Seapath 330 position and attitude sensor (November, 2013) and replacement of the original EM120 transceiver with an EM122 transceiver (June, 2014). In conjunction with the EM122 Harbor Acceptance Trials (HAT) and Sea Acceptance Trials (SAT) performed by KM field engineers, the Multibeam Advisory Committee (MAC) participated in cruise NBP1405 to review the MBES system configuration, perform a calibration for angular offsets between the Seapath 330 and MBES, and assess swath coverage and bathymetric accuracy. The configuration review and calibration proceeded normally, yielding sensor offsets which should be maintained until sensors are moved or new surveys are performed. Conversely, results of the MBES performance assessment require immediate attention for the efficiency and effectiveness of upcoming scientific mapping programs aboard the NBP.
Of primary concern, MBES swath coverage and accuracy are degraded compared to manufacturer specifications, original SAT performance in 2002, and an EM122 recently installed aboard another UNOLS vessel. Symptoms include a 33% reduction in effective swath width in waters deeper than ~1500 m and a 50% increase in standard deviation of depths reported across the swath compared to data collected with fully functional systems. Unlike typical MAC evaluations, the limitations in swath width and artifacts in depths observed during NBP1405 required modification of survey lines for data collection and filtering of outliers for data analysis.
The symptoms documented in this report are consistent with weak transmission signal strength related primarily to failure of transmit (TX) array elements. According to KM analysis during NBP1405, about 11% of TX array elements exhibited impedance anomalies; this finding exceeds the manufacturer specifications for a fully functional TX array. Other factors compounding the weak transmission signal strength include damage to the TX array ice window and acoustic attenuation from bubbles swept beneath the arrays.
Of particular note, the EM122 transceiver is capable of frequency-modulated (FM) transmission modes which are expected to increase the effective swath width compared to the original EM120. FM transmission is used widely amongst other UNOLS vessels. It is possible to operate the NBP’s
EM120 TX array in FM mode, however, this would likely accelerate degradation of array elements and was not attempted during NBP1405 at the recommendation of KM field engineers. Based on the observed high rate of EM120 TX array element failure and likely prohibitive cost of individual module replacement, it is recommended that the EM120 TX array be replaced with an EM122 TX array at the next opportunity. Replacement would also address damage to the existing ice window and facilitate improvements in swath width using FM transmission. Additional consideration should be given to placement of both the TX and RX arrays to minimize exposure to bubble sweep. To ensure a comprehensive record of sensor offsets, a survey of MBES arrays, motion sensors, and antennae should be performed in dry dock and reported in a reference frame using KM conventions. Finally, a schedule should be implemented for routine Built-In Self-Tests (BISTs) to document element conditions over the effective lifecycle of the MBES arrays.