Application of acoustic tomography to reconstruct the horizontal flow velocity field in a shallow river

Abstract

A novel acoustic tomographic measurement system capable of resolving sound travel time in extremely shallow rivers is introduced and the results of an extensive field measurements campaign are presented and further discussed. Acoustic pulses were transmitted over a wide frequency band of 20-35 kHz between eight transducers for about a week in a meandering reach of theB asen River, Hiroshima, Japan. The purpose of the field experiment was validating the concept of acoustic tomography in rivers for visualizing current fields. The particular novelty of the experiment resides in its unusual tomographic features: subbasin scale (100 m 3 270 m) and shallowness (0.5-3.0 m) of the physical domain, frequency of the transmitted acoustic signals (central frequency of 30 kHz), and the use of small sampling intervals (105 s). Inverse techniques with no a priori statistical information were used to estimate the depth-average current velocity components from differential travel times. Zeroth-order Tikhonov regularization, in conjunction with L-curve method deployed to stabilize the solution and to determine the weighting factor appearing in the inverse analysis. Concurrent direct environmental measurements were provided in the form of ADCP readings close to the right and left bank. Very good agreement found between along-channel velocities larger than 0.2 m/s obtained from the two techniques. Inverted quantities were, however, underestimated, perhaps due to vicinity of the ADCPs to the banks and strong effect of river geometry on the readings. In general, comparing the visualized currents with direct nodal measurements illustrate the plausibility of the tomographically reconstructed flow structures.

Monitoring Inflow Dynamics in a Multipurpose Dam Based on Travel-Time Principle

“…Most of the previous monitoring applications via the FAT system were accomplished using a 30 kHz transmission frequency. However, the 30-kHz system has a certain range of transmission length between a pair of transducers, and the minimum water depth must exceed ~0.5 m for effective use (Razaz et al 2015). Consequently, in 2016, the first version of the FAT system that can be operated using 53 kHz frequency was developed for use in relatively shallower and narrower streams, with higher velocity resolution.…”

Influence of Suspended Sediment Concentration and Particle Sizes on the Sound Attenuation of the Fluvial Acoustic Tomography Technique

“…As it mentioned before, the distance between acoustic stations and the suspended solids characteristic affect FATS SNR. Equation where SL of FATS is 197 dB re 1 μPa at 1 m [28], 0.008 is α w in dB m-1 for 30-kHz acoustic waves, which is calculated by Equations 7 and 8, L 0 is assumed to be 10 dB [29], Gp is considered as 36.1 dB, which is the highest probable processing gain of M12 (Table 1), and Na presents the summation of ambient and system noises. Although the ambient noise in the oceans and seas is about 55 dB [30] and 85 dB [29] respectively, it can be adversely larger in shallow waters.…”

Mapping Small-Scale Horizontal Velocity Field in Panzhinan Waterway by Coastal Acoustic Tomography

“…Since 2010, a group of Kawanisi proposed that the fluvial acoustic tomography (FAT) system extends the applications of CAT to even shallower waters ranging from mountainous rivers to the mouth of estuaries in coastal regions (maximum 10 m deep) [ 26 , 27 ]. As a consequence, the application of CAT in shallow waters such as the marine ranch, artificial upwelling, and Autonomous Underwater Vehicle (AUV) observation has been widely investigated, and particularly the short-range velocity measurements have gradually been an important focus of research.…”

“…Due to the large environmental noise, M-sequence with an efficient autocorrelation property was generally adopted. The observation results would be directly influenced by the number of cycles used by an M-sequence bit, the order of M-sequence, the carrier frequency, and the signal repetition times [ 26 , 33 ]. Therefore, a reasonable signal design is vital in short-range CAT observation.…”