Field observation of low-to-mid-frequency acoustic propagation characteristics of an estuarine salt wedge

Abstract

The estuarine environment often hosts a salt wedge, the stratification of which is a function of the tide's range and speed of advance, river discharge volumetric flow rate, and river mouth morphology. Competing effects of temperature and salinity on sound speed in this stratified environment control the degree of acoustic refraction occurring along an acoustic path. A field experiment was carried out in the Columbia River Estuary to test the hypothesis: the estuarine salt wedge is acoustically observable in terms of low-to-mid-frequency acoustic propagation. Linear frequencymodulated acoustic signals in the 500-2000 Hz band were transmitted during the advance and retreat of the salt wedge during May 27-29, 2013. Results demonstrate that the salt wedge front is the dominant physical mechanism controlling acoustic propagation in this environment: received signal energy is relatively stable before and after the passage of the salt wedge front when the acoustic path consists of a single medium (either entirely fresh water or entirely salt water), and suffers a 10-15 dB loss and increased variability during salt wedge front passage. Physical parameters and acoustic propagation modeling corroborate and inform the acoustic observations.

3D acoustic propagation through an estuarine salt wedge at low-to-mid-frequencies: Modeling and measurement

“…A visual comparison of ray-based model results in Figs. 5 and 6 demonstrate that the progressively refractive properties of the water column decrease the amount of acoustic energy interacting with the riverbed, consistent with Reeder (2016). Significantly, the dominant physical mechanism controlling TL shifts from bottom scatter in a non-refractive waveguide (before the entrance of the salt wedge) to 3D acoustic refraction with minimal bottom interaction in a refractive waveguide (when the salt wedge occupies the acoustic transect).…”

“…A small acoustical field experiment was carried out in May 2013 in the Mouth of the Columbia River (MCR) within the context of a larger hydrodynamic field experiment entitled Rivers and Inlets II funded by the Office of Naval Research. Reeder (2016) reported details of the experiment, a) Electronic mail: dbreeder@nps.edu acoustic data analysis, and results of two-dimensional (2D) acoustic propagation modeling, confirming the hypothesis that the estuarine salt wedge is acoustically observable at low-to-mid frequencies (500-2000 Hz). The present work presents additional insights into the physics of the acoustic propagation based upon 3D acoustic modeling, with the primary goal of establishing the contribution of horizontal refraction to total transmission loss (TL) and impulsive arrival structures.…”

“…Figure 2 (taken from Reeder, 2016) presents data collected during flood on May 27. The top panel shows sound speed as a function of depth (m) and range (m) along the acoustic path between the source at Station S2 and the receiver at Station A5.…”

“…[These modeled sound speed fields were used in Reeder (2016), but they were not used in the 3D calculations in this paper.] The middle panel shows sound speed (m/s) as a function of depth (m) from CTD profiles collected on a Castaway CTD hand-deployed over the side of the ship in approximately 15-min intervals during the 6-h acoustic transmission period from 1530Z to 2130Z on May 27.…”

“…When the fresh water is cold relative to the salt water, both temperature and salinity will contribute to create a stronger sound speed gradient compared to that observed during summer. While this nominally three-layer, very shallow water acoustic waveguide is dominated by high-angle multipath propagation at very short ranges, it has been shown that refraction occurring in the gradient layer supports ducting of low-angle energy in the upper layer, and that the statistical variability of acoustic signal increases due to the spatiotemporal dynamics of the salt wedge structure as it advances and retreats on the rising and falling tides (Reeder, 2016).…”