NOAA Protocols for Fisheries Acoustics Surveys and Related ...

Techniques
Before conducting a calibration important issues to consider are:
1) The calibration should be conducted in the same environmental conditions (water
temperature and salinity) as are experienced during the survey.
2) Water depths must be sufficient to exceed near-field limitations and/or system limitations
for the echo sounder frequencies to be calibrated (Table 1).
3) Calibrations must be conducted before the survey begins to establish proper echo sounder
operation, and after or near the end of the survey to ensure no significant changes have
occurred. Additional calibrations during the survey are valuable for maintaining system
performance and ensuring high-quality data.
4) Calibrations must be conducted with the same pulse durations, transmit powers, and
bandwidths used during the survey.
Software
Echo sounder manufacturers provide detailed instructions and software programs for
calibrating their systems. These instructions must be followed to ensure proper calibration and
system stability. Because software upgrades occur, software version identification (both
calibration software and echo sounder software) should be documented.
Standard values
Table 1 provides a list of common standard values for calibration. The calibration sphere
target strength is dependent on the water temperature and salinity (i.e., sound speed dependent).
The copper spheres specified for each frequency have been shown to be “optimal” in that the
target strengths of the specified spheres vary minimally for a normal range of temperatures and
salinities (Foote, 1982; 1983a). However, one should confirm that the theoretical TS is valid for
the measured environmental conditions.
.
Frequency [kHz]
Calibration
Sphere
EK500 Minimum
Target Range [m]
Nominal
TS [dB]*
12 45 mm Cu 35 -40.4
18 64 mm Cu 22 -34.4
38 60 mm Cu 10 -33.6
38 38.1 mm WC 10 -42.2
120 23 mm Cu 10 -40.4
120 38.1 mm WC 10 -39.6
200 13.7 mm Cu 10 -45.0
200 38.1 mm WC 10 -39.5
Table 1. Calibration standard values.
Calibration sphere measurements are the sphere diameter. ‘Cu’ denotes a copper calibration
sphere, and ‘WC’ denotes a tungsten carbide with 6% cobalt binder calibration sphere. The
‘EK500 Minimum Target Range’ applies to the SIMRAD EK500 and was derived as a
combination of the far field of the transducer and electronic limitations of the echo sounder
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