A General Guide for Deriving Abundance Estimates from Hydroacoustic Data




















Passive Data Monitoring

When a transducer is switched to “passive” mode, it listens but does not transmit (Fig. 26).  This function allows the user to measure ambient background noise or to determine the source of detected noise.  Passive mode should be used prior to the commencement of a survey to determine whether vessel noise levels are acceptable.  In general, stationary noise levels of -140 dB (TSu domain) or less are attainable with proper grounding.



Procedures for the collection of vessel noise data are available from the manufacturers.  Recommendations from Mitson (1995) and Simrad suggest that passive data should be collected under the following conditions:

  • vessel stationary and running
  • incrementally increasing the vessel to survey speed
  • vessel running at survey speed
  • incrementally decreasing the vessel to stationary
  • towing trawls (midwater and/or bottom) to be used during the survey
  • If increasing noise is observed with increasing speed, and vice versa with decreasing speed, the propeller is causing the noise.  It is then also advisable to collect passive data under different pitch and propeller speeds.  A minimum of 2-5 minutes of passive noise collection under each condition/speed is recommended. 

    By using the echogram color scheme, the user may evaluate whether noise levels will mask the targets of interest.
    The following example is taken from Simrad EK60 Manual:

    If the desired lower Sv limit is -70 dB, and a Signal-to-Noise Ratio (SNR) of 10 dB is required, set the minimum color scale for the 20logR TVG to -80 dB.  The point at which shading in the echogram (e.g., gray) begins is the depth at which the Sv limit is obtained with the 10 dB SNR.

    The same can be done using the 40logR TVG echogram for TSm.   

    Passive data collected with 120 kHz echosounder, showing increases in noise with depth.  Lower threshold is -90 dB. 

     Passive Data Monitoring
    Noise levels at 1 m (Sv at 1 m) observed during survey conditions should be recorded and reported in papers that utilize the survey data.  Note that noise level measured in the TSu domain is not the same as noise levels in the Sv domain (Equation [24]).