Multifrequency Acoustics
Multifrequency acoustics is a method for estimating suspended sediment.
The multifrequency acoustic method can be used if sand content is of interest and the river depth and width is adequate to profile in multiple “bins”, and there is a large range in sand grain size, or a large ratio in the ratio of fines (silt and clay) to sand.
Since single frequency methods may not be adequate in systems with highly variable grain size within the ensonified measurement volume, a method using multiple acoustic frequencies should be used. When multiple frequencies are available to invert acoustic signals, and the horizontal measurement volumes ideally overlap in space and close in time, the uniqueness of each acoustic response for each frequency can be used to invert N-1 unknown parameters, where N is the number of frequencies used. This method solves for suspended-sediment concentration (SSC) for each frequency separately, and the two frequencies are used to solve for median sand size and D50 corrected sand concentration.
The Grand Canyon Monitoring and Research Center website delivers interactive continuous sediment transport data for sediment management purposes as well as some multifrequency site data.
Software: The use of multifrequency acoustic profilers for calculating suspended-sediment loads allows for the median grain size of the suspended sand to be calculated in addition to the sand concentration and silt/clay concentration. While the multifrequency sidelooking method offers the advantage of calculated median grain size and the removal of sand grain-size dependent bias in the calculated sand concentration, implementation of the method has been slowed by the complexities of the method over the single frequency approach. To support use of the multifrequency method developed by Topping and Wright (2016), several MATLAB® scripts have been developed for site-specific calibration of the acoustic instruments to physically collected suspended-sediment samples. Additional scripts have been developed to process calibrated multifrequency acoustic profiler data for suspended silt/clay concentrations, suspended sand concentrations, and suspended sand median grain size. Please reachout to Ron Griffiths (rgriffiths@usgs.gov) for questions and updates.