Calibration of acoustic instruments

Abstract

Acoustic sampling has long been a standard tool for estimating the abundance and distribution of fish, zooplankton, and their seabed habitats (Kimura, 1929; Sund, 1935; Holliday, 1972a; Nielson et al., 1980). Typically, acoustic surveys are conducted using multifrequency echosounders that transmit sound pulses downward beneath the ship and receive echoes from animals and the seabed along the path of the sound waves (Simmonds and MacLennan, 2005).

For surveys of animals, the backscatter signal is normalized to the range-dependent observational volume, yielding the volume backscattering coefficient, which provides indications of target type and behavior. Objects scatter sound if the product of their mass density and sound speed differs from that of the surrounding medium. A fish with a swim bladder has a large acoustic-impedance contrast (Foote, 1980), resulting in a large backscattering cross-section. Plankton, such as krill and salps, generally have lower acoustic-impedance contrasts but can produce large volume backscattering coefficients when aggregated at high densities (Hewitt and Demer, 1991, 2000).

Under certain conditions, the summed and averaged volume backscattering coefficients are linearly related to the density of the fish or plankton aggregations that produced the echoes (Foote, 1983a). The number density can be estimated by dividing the integrated volume backscattering coefficient from an aggregation of target species by the average backscattering cross-section from a representative animal (Ehrenberg and Lytle, 1972). An estimate of animal abundance is then obtained by multiplying the average estimated fish density by the survey area.

Increasingly, multifrequency echosounder surveys are being augmented with samples from other acoustic instruments such as multibeam echosounders (Gerlotto et al., 1999; Simmonds et al., 1999; Berger et al., 2009; Colbo et al., 2014), multibeam imaging sonars (Korneliussen et al., 2009; Patel and Ona, 2009), and long-range scanning sonars (Bernasconi et al., 2009; Nishimori et al., 2009; Stockwell et al., 2013). Use of these instruments provides additional information on many aspects of the biotic and abiotic environment, such as bathymetry, seabed classification (Humborstad et al., 2004; Cutter and Demer, 2014), oceanographic fronts (Wade and Heywood, 2001), mixed-layer depths, anoxic regions, internal waves (Lavery et al., 2010a), turbulence (Stanton et al., 1994), currents, and methane seeps, all contributing to a broader ecosystem perspective (Demer et al., 2009a).

In each case, quantitative use of the data requires that the acoustic instrument be calibrated. Instrument calibration involves characterizing measurement accuracy (bias or systematic error) and precision (variability or random error). Sampling with the calibrated instrument introduces additional systematic and random error (Demer, 2004). Calibration accuracy is estimated and optimized by comparing measured and assumed values for a standard and correcting for the difference. The selection and characterization of a calibration standard is therefore essential for accurate instrument calibration (Foote and MacLennan, 1984). Calibration precision is estimated by comparing multiple measurements of a standard.

The performance of an instrument, and therefore its calibration accuracy and precision, may change over time or with environmental conditions (Demer and Hewitt, 1993; Brierley et al., 1998a; Nam et al., 2007). For this reason, instruments should be calibrated frequently within the range of environments in which they are used to make calibrated measurements (Demer and Renfree, 2008). If frequent calibration is not possible, any changes in the instrument or environment that significantly affect calibration accuracy and precision should be accounted for.

This report includes general instructions and current best practices for calibrating a selection of acoustic instruments commonly used in fishery science and surveys. It also describes less-developed protocols for other acoustic instruments. Not all fishery acoustic instruments are included for practical reasons.

The remainder of Chapter 1 summarizes theoretical principles of acoustic instruments used in fishery research and surveys, describes commonly used instruments and their deployment platforms, and briefly introduces common methods for calibrating acoustic instruments. Readers seeking only calibration protocols for echosounders may skip directly to the relevant sections using the table of contents.

Chapter 2 details the sphere calibration method. Chapter 3 explores uncertainty in sphere calibrations. Chapter 4 describes protocols for calibrating commonly used echosounders. Chapter 5 describes emerging protocols for other acoustic instruments. Chapter 6 acknowledges valuable contributions to this report by individuals not listed as authors.