Acoustic turbidity measurement
What is turbidity (optical vs acoustic) ?
The turbidity (the cloudiness of water) is due to the presence of suspended matter or sediments composed by organic and inorganic particles, flocs or vesicles.
This physical effect was initially linked with the optical turbidity which depends on the colour, the size and the shape of the suspended sediments. Optical turbidity can, after adequate calibration, be linearly linked to the suspended sediments concentration. But OBS (Optical Backscattering Sensors) do not see large particles because their sensitivity decreases inversely as the particles size increases. However, optical turbidity is a point measurement which might not be representative of the entire depth of the water column. Another major drawback is its sensitivity to biofouling which leads to a weak signal.
The acoustic turbidity measurement is based on the capacity of the particles to diffuse the acoustic wave. It is determined by the ratio of energy received by the transducer (backscattered wave), over the emitted energy, corrected with the dispersion of the acoustic wave. Based on the backscattered amplitude measurement, it corrects all the effects of the transducer and of the electronics to bring a value that only depends on the medium and the suspended particles. So the acoustic turbidity is the ratio of the energy diffused by the particles per distance unit (distance to the transducer) over the emitted energy. The acoustic turbidity delivered by our devices is thus given in m-1. The acoustic turbidity allows to monitor the suspended sediment concentration (SSC) and thus the quality of water. As for OBS there is a link between the particle size and the sensitivity of the ABS (Acoustic Backscattering Sensors). The sensitivity of Ubertone’s profilers goes from 10 to 1000 microns (depending on the emitting frequency).
The acoustic turbidity is measured along the acoustic beam and can potentially be inverted in order to give a concentration profile.
In a homogeneous suspension where concentration and particle size can be considered uniform, the theoretical relation between turbidity and sediment concentration at a given emitting frequency is given by:
T(r) = β CV e-4 ( α CV + αw ) r
where β and α are the backscattering and attenuation coefficients of the particles, Cv the volumetric concentration and αw the attenuation coefficient of the fluid. Thus in a homogeneous suspension, the logarithm of the turbidity is a straight line.