Test methodology for evaluation of linearity of multibeam echosounder backscatter performance

Multibeam echosounders (MBES) are widely used for hydrographic surveys and are currently installed on numerous operational platforms. Many of these systems are capable of recording intensity information from the returned acoustic signal in addition to the detected depth solution. There is increasing interest in using this intensity data to remotely characterize the seafloor. Depending on the processing methodology used for this analysis, it may be necessary to compensate the system output for the characteristics of the sonar as well as operation parameters such as power, gain, and pulse width. Calibration techniques for MBES are a focus of current research. At one end of these efforts is a full calibration of the installed system such that the effects of beam patterns, hydrophone sensitivities, system gains, and all other operational parameters are known and can be compensated. Calibrations have been done with systems mounted in specially instrumented test facilities, but removal of an operational system for calibration at a specialized facility may not be practical. In addition, the installation environment of the system may have system may have significant effects on its operational characteristics. Field calibrations of installed systems have only recently been performed on a limited number of installed systems, but these techniques may not be applicable to all systems. This paper proposes a more limited test that does not achieve a full calibration, but does help characterize the linearity and relative consistency of an installed MBES installed on an operational platform over the operational range. This test consists of varying the operating parameters over the operation range and analyzing the system output. Though this procedure does not serve to calibrate the sonar in an absolute sense, it does provide a relative calibration. It allows the data from one set of operational parameters to be compared with data from another set of operational parameters. The eff- ct of adjustments to operating parameters on the returned signal can be directly observed. In addition, this test can indicate constraints on the linear operation of the sonar and has been shown to diagnostic of certain faults. This test can be performed while the vessel is at the pier and uses only the seafloor for as an acoustic target. This test was carried out on five Reson 7125 MBES installed on different vessels. Significant departures from linearity are observed in some operating conditions. Results are compared with observations made in a test tank environment. This method has the advantage of evaluating the MBES performance as installed as well as being simple and quick. It does not require the ship to be underway and does not require dedicated test areas. Some of the pier side environments were found to be more suitable than others however. Certain mud seafloors appeared to have variable target strengths due to the variable presence of bubbles. Material in the water column (presumed to be fish) also had a negative effect in some locations. Until practical techniques are developed to enable full calibrations of MBES installed in the field, accessible methods of characterizing at least some aspects of MBES intensity data will be advantageous for interpretation of this data set. The method described in this paper has been demonstrated to be effective for one MBES model installed on various platforms.