The Nortek Signature 1000, Signature 500 and Signature 100 are equipped with active acoustics technology, commonly referred to as Echosounder. An Echosounder can generate, amplify and transmit high volume (loud), short, high-frequency, directed pulses in the water. That optional feature enables the instruments to measure the magnitude of the echo generated by the instrument pinging in high resolution. The single-beam echosounder in the Signature 1000 and 500 is a firmware sharing time slots with other measurements such as waves and ice in their central beam. In contrast, the Signature 100 has a dedicated firmware operating in typical single-beam Echosounder frequencies (70-120 kHz).

Echosounders are widely used by marine scientists to estimate plankton biomass, sediment in suspension (see this FAQ), internal waves dynamics and more. The type of target can be classified into point targets (e.g., individual fish), volume targets (e.g., zooplankton swarm, schools of fish) or boundary layers (surface or bottom). The point target is associated with the Target Strength (\(TS\)), while the volume target is linked to the volume backscatter (\(S_v\)), representing the intensity from multiple targets. Both \(TS\) and \(S_v\) are based on acoustic impedances, which is related to differences in density and, ultimately in sound speed velocities.

Ocean Contour and Signature Review (for Vessel-mounted instruments) software output processed relative (not calibrated, as opposed to absolute which is calibrated – see this FAQ) \(S_v\) and relative \(TS\) in one Matlab file. Alternately, \(S_v\) and \(TS\) can be calculated from raw amplitude data (\(R_x\)). This FAQ addresses this process.

The volume backscatter \(S_v \, [\text{dB re } 1 \, \text{m}^2/\text{m}^3]\) is the logarithmic form of the volume backscattering cross section (\(\sigma_v\)), which is calculated from the sonar equation as:

where:

• \(P_r\) is the return signal uncorrected for losses [dB]
• \(NT\) is the environmental noise [dB]
• r is the range or distance from transducer [m]
• \( \alpha \) accounts for absorption losses [dB/m]
• c is the speed of sound calculated or input by the user [m/s]
• \(\tau\) is transmit pulse length [μs]
• N is the number of frequency bins
• \(\Psi\) is the equivalent beam-angle \([\text{dB re} \text{ 1 steradian}]\) 
• PL is the power level (default value for the echosounder is 0 dB)
• \( G_{cal}\) is the gain for instrument-calibrated data for a target integrated over a volume

We will explore each one of terms in the equation above separately. The recorded amplitude can be exported in different formats depending on the software used. From raw Ocean Contour and Signature Review exported file, there are three variables available: I,Q and Magnitude. "DataI" variable should be multiplied by 0.01 to convert the signal to dB units. From Signature Deployment and Ocean Contour, the Echo variable in the Matlab structure (EchoxkHz_Data.Ampx_kHz, being x the instrument frequency) has already been converted to the decibel scale and is ready to be used. Environmental noise (NT in Eq. 1) must be removed from the recorded amplitude. To do so, follow these steps:

1. Set the Echosounder Power Level to -100 dB. This is referred to as listening mode, which can be done using MIDAS or changing the configuration in the deployment file (set PL=-100).
2. Deploy the instrument and retrieve the data.
3. Plot the Amplitude profile and averaging the top cells where the curve flattens. The resulting amplitude represents the noise threshold. This value can also be entered in Ocean Contour. When the noise level and salinity are input in Ocean Contour, the software will automatically correct for transmission loss and absorption.

The following terms in Eq. 1 account for the estimated losses for volume backscatter, which are added to the measured echosounder return signal. These include transmission losses (spreading of the sound wave and absorption by water), reflective properties of the target (target reflectivity), the position of the target in the beam (equivalent beam angle), power level, and calibration gain. 

Acoustic beam spreading occurs due to the sound wave sent by the instrument gradually encompassing a larger area as the distances from the instrument increases. The beam spreading can be understood as a “geometrical loss” as the wave signal propagates through water. It is simply a function of the distance R from transducer.

A thorough calculation of sound absorption needs to take into account the absorption by water and particle in suspension. The sound absorption by water is a function of water temperature, salinity, pressure, pH and particle in suspension. It can be modeled by using different models that can be found here (Fisher and Simmons, 1977; Francois and Garrison, 1982; Ainslie and McColm, 1998). The sound absorption by particles in suspension is given by:

(Lohrman, 2021). It is necessary to integrate over the distance range as the particle concentration would most likely vary. Due to different sediment sizes, it calls for an implicit solution when using SSC, being somewhat complicated to implement. From a practical perspective, it is hard to model the absorption, and for many cases, where sediment concentration is relatively low (\(<0.1 g L^{-1}\) ) and particle size ranges between 10 and 100 \(\mu\)m (fine silt to very fine sand) this term can be ignored (Ha et al., 2011; Gartner, 2004). Alternatively, Ocean Contour processed file calculates the sound absorption as a function of instrument frequency and distance from transducer.

The target reflectivity is a function of transmit pulse length (\(\tau\)), speed of sound underwater and number of frequency bins. For the Signature systems, the velocity pulse length is not exactly matched to the cell size as the instrument sends out a chirp, as opposed to a monochromatic pulse like those sent by our legacy narrowband systems (AWAC and Aquadopp). \(\tau\) is instead the transmit length, which is selected by the user in the Echosounder tab in Signature deployment. One can also check the transmit pulse length by opening the Signature file with extension .AD2CP in a text editor like Notepad++ and looking at the deployment settings. The row starting with “GETECHO” contains the echosounder data, where “XMIT1” indicates the transmit pulse length for Echosounder 1 and “XMIT2” shows  for echosounder 2 (if present). For Signature VM measurements, the .sigVM file has to be unpacked to find the .AD2CP file. Finally, the number of frequency bins can be selected in Signature deployment for the Signature 100, while for Signature 1000 and Signature 500 it Is always 1.

The beam angle corrects for the effect that not every particle within the sampling volume contributes equally to \(P_r\), due to its displacement from the beam axis. It is a function of the instrument wavenumber (related to instrument frequency) and active transducer radius, which can be found in this FAQ. It is given by the Equation:

Where \(\kappa\) is the instrument’s frequency wavenumber \(\left(\frac{2\pi}{\lambda}\, \left[\frac{1}{m}\right]\right)\), being \(\lambda= \frac{c}{f \times 1000}\) \([m]\) and \(f\) the instrument frequency \([kHz]\), and \(A_t\) is the active transducer radius \([m]\). For the Signature 100, the frequencies can be selected as 1) 70 kHz monochromatic, 2) 120 kHz monochromatic, and 3) wide bandwidth (50%) linear chirp ranging from 68 kHz to 113 kHz centered at 90.9 kHz. The monochromatic pulses are processed internally using standard algorithms, but the chirp can be processed in one of two ways: using pulse compression or using a 5-binned frequency response.

The last two terms refer to instrument calibration, being instrument-specific. Power level relates to how much energy the instrument puts in the water, calculated from the instrument’s voltage.

In Echosounder context, Gain (G) can be used in two different contexts: (1) transducer gain, which is the ratio of the transmit acoustic intensity \(i_t[W*m^-2]\) from a real transducer and an idealized omnidirectional transducer without any losses and (2) time-varying gain which is related to the estimated losses for volume backscatter. In Eq. 1, we refer to the transducer gain, which can be estimated through calibration. Without the instrument calibration, the volume backscatter is relative (\(S'_e\)). Through calibration, \(S'_e\) is absolute, so its values can be compared to other deployments. Given the potential for transducer drift over time, recalibrating 𝐺 at regular intervals is also necessary (Demer et al., 2015). Fässler et al. (2015) provides a comprehensive guide on instrument calibration. Instrument calibration for the Signature 100 can be achieved by positioning a spherical target with known TS value at a known distance from the sensor. Giving the other terms in Eq. 1 can be easily calculated, G and PL can be estimated. It is important to notice that calibration is not always necessary.

Echosounder Target Strength

Analogous to the volume backscatter equation, estimated losses for target strength account for transmission losses (spreading of the sound wave and absorption by water), power level, and calibration gain. The target strength doesn’t take into account target reflectivity and equivalent beam angle, so that the general equation for \(TS \, [\text{dB re } 1 \, \text{m}^2]\) in the logarithmic form of the  backscattering cross-section (\(\sigma_{bs}\)) is given by:

One may notice the lack of the target reflectivity and equivalent beam angle terms in Eq. 4 compared to Eq. 1, which are due to different assumptions inherent to the measurements, such as ensonified volume.  

References

Ainslie, M.A., McColm, J.G., 1998. A simplified formula for viscous and chemical absorption in seawater. J. Acoust. Soc. Am. 103, 1671–1672. http://dx.doi.org/10.1121/1.421258

Demer, David A., Laurent Berger, Matteo Bernasconi, Eckhard Bethke, Kevin Boswell, Dezhang Chu, Reka Domokos et al. "Calibration of acoustic instruments." (2015).

Echoview. (2022). Echoview learner guide - Fundamentals. In.

Fässler, S., Gauthier, S., Hufnagle, L.T., Jech, J.M., Bouffant, N., Lebourges-Dhaussy, A., Lurton, X., Macaulay, G.J., Perrot, Y., Ryan, T., Parker-Stetter, S., Stienessen, S., Weber, T., Williamson, N., 2015. Calibration of acoustic instruments. CES Coop. Res. Rep. No 326. https://doi.org/10.17895/ICES.PUB.5494

Fisher F. H., Simmons V. P., "Sound absorption in seawater", Journal of the Acoustical Society of America, 62, 558-564, 1977.

Francois R. E., Garrison G. R., "Sound absorption based on ocean measurements: Part I:Pure water and magnesium sulfate contributions", Journal of the Acoustical Society of America, 72(3), 896-907, 1982.

Gartner, J. W. (2004). Estimating suspended solids concentrations from backscatter intensity measured by acoustic Doppler current profiler in San Francisco Bay, California. Marine Geology, 211(3–4), 169–187. https://doi.org/10.1016/j.margeo.2004.07.001

Ha, H. K., Hsu, W.-Y., Maa, J.-Y., Shao, Y., & Holland, C. (2009). Using ADV backscatter strength for measuring suspended cohesive sediment concentration. Continental Shelf Research, 29(10), 1310–1316. https://doi.org/10.1016/J.CSR.2009.03.001

Lohrmann, Atle. "Monitoring sediment concentration with acoustic backscattering instruments." Nortek Technical Note 3 (2001): 1-5.

Nortek, 2022. Signature Principles of Operation.

Urick, R. J. (1983). Principles of Underwater Sound (3rd ed.). McGraw-Hill. https://books.google.nl/books?id=hfxQAAAAMAAJ

Venditti,  J.  G.,  Church,  M.,  Attard,  M.  E.,  &  Haught,  D.  (2016).  Use  of  ADCPs  for  suspended  sediment  transport  monitoring:  An  empirical  approach. Water Resources Research, 52(4), 2715–2736. https://doi.org/10.1002/2015WR017348.Received

Extra material:

Cornell University, n.d. Acoustics Unpacked. URL http://www2.dnr.cornell.edu/acoustics/AcousticBackground/SONARequation.html

Deines, K. (1999). Backscatter estimation using Broadband acoustic Doppler current profilers. Paper presented at the Proceedings of the IEEE Sixth Working Conference on Current Measurement (Cat. No.99CH36331). https://doi.org/10.1109/CCM.1999.755249

Gostiaux, L., & van Haren, H. (2010). Extracting meaningful information from uncalibrated backscattered echo intensity data. Journal of Atmos-pheric and Oceanic Technology, 27(5), 943–949. https://doi.org/10.1175/2009JTECHO704.1

Lurton, X., 2010. An Introduction to Underwater Acoustics: Principles and Applications, Second Edition. Springer-Praxis Books.

Maclennan, D., 2002. A consistent approach to definitions and symbols in fisheries acoustics. ICES J. Mar. Sci. 59, 365–369. https://doi.org/10.1006/jmsc.2001.1158

Technical Guides - Speed of sound in sea water, 2023. Natl. Phys. Lab. Hampton Road Teddingt. Middx.

Thorne, P. D., & Hurther, D. (2014). An overview on the use of backscattered sound for measuring suspended particle size and concentration pro-files in non-cohesive inorganic sediment transport studies. Continental Shelf Research, 73, 97–118. https://doi.org/10.1016/j.csr.2013.10.017