Acoustic wave propagation in rivers: an experimental study

Abstract

Abstract. This research has been conducted to develop the use of passive acoustic monitoring (PAM) in rivers, a surrogate method for bedload monitoring. PAM consists in measuring the underwater noise naturally generated by bedload particles when impacting the river bed. Monitored bedload acoustic signals depend on bedload characteristics (e.g., grain size distribution, fluxes) but are also affected by the environment in which the acoustic waves are propagated. This study focuses on the determination of propagation effects in rivers. An experimental approach has been conducted in several streams to estimate acoustic propagation laws in field conditions. It is found that acoustic waves are differently propagated according to their frequency. As reported in other studies, acoustic waves are affected by the existence of a cutoff frequency in the kilohertz region. This cutoff frequency is inversely proportional to the water depth: larger water depth enables a better propagation of the acoustic waves at low frequency. Above the cutoff frequency, attenuation coefficients are found to increase linearly with frequency. The power of bedload sounds is more attenuated at higher frequencies than at low frequencies, which means that, above the cutoff frequency, sounds of big particles are better propagated than sounds of small particles. Finally, it is observed that attenuation coefficients are variable within 2 orders of magnitude from one river to another. Attenuation coefficients are compared to several characteristics of the river (e.g., bed slope, surface grain size). It is found that acoustic waves are better propagated in rivers characterized by smaller bed slopes. Bed roughness and the presence of air bubbles in the water column are suspected to constrain the attenuation of acoustic wave in rivers.

Passive Acoustic Measurement of Bedload Transport: Toward a Global Calibration Curve?

“…Journal of Geophysical Research: Earth Surface setup (Geay et al, 2019;, and we believe that hydrophone signals could be corrected for attenuation effects, leading to a better estimate of bedload fluxes with hydrophones.…”

“…However, bed roughness (Wren et al, 2015) or water depth have been observed to control the attenuation of acoustic waves: A larger roughness induces larger attenuation and smaller water depths increase attenuation at low frequencies. Propagation laws are variable from one river to another (Geay et al, 2019), complicating the relationship between the acoustic power measured by a hydrophone and bedload fluxes.…”

“…According to a recent study focused on the characterization of acoustic wave propagation in rivers (Geay et al, 2019), it was observed that (1) sound attenuation increases with frequency and that (2) the increase in attenuation is larger in steeper rivers. The entrainment of air bubbles (Norton & Novarini, 2001) and the increase of bed roughness (Wren et al, 2015) are suspected to affect the propagation of sounds (especially above 10 kHz).…”

“…As discussed in section 4.2, the highest frequency range is modified by propagation effects related to attenuation. Steeper rivers induce higher attenuation (Geay et al, 2019), leading to weaker acoustic power measured by the hydrophone. These hypotheses are schematically presented in Figure 8b.…”

“…However, it was shown by studying the frequency content of bedload sounds that these acoustic measurements are affected by processes related to the propagation of the acoustic waves. As propagation properties (i.e., attenuation coefficients) vary from one river to another (Geay et al, 2019), it can be concluded that a part of the scatter observed in the calibration curve (Figure 4) should be attributed to propagation effects. Attenuation coefficients can be determined for a specific experimental Figure 4.…”