How can multi-frequency echosounder measurements enhance the monitoring of suspended particulate matter?

On the VM Coastal series, echosounder backscatter measurements are available as an option for studying suspended particulate matter (SPM), such as sediments, biomass, or even plastics. On the 1000 kHz VM version, enhanced functionality is available through the Multi-Frequency Echosounder License, which enables a 1000, 500 and 250 kHz echosounder ping on the vertical beam of the VM-ADCP.

 

This short article provides a brief introduction to monitoring SPM using echosounder measurements. It is intended for users who are new to the topic and looking to understand the basics and potential applications. For a more in-depth explanation of the multi-frequency method—particularly how it can reduce sensitivity to changes in particle size when using ADCP backscatter—please refer to this conference paper.

 

While ADCPs are primarily designed to measure currents, their use cases can be extended to other monitoring applications. An example is studying the presence of suspended particulate matter (SPM), like biomass, suspended sediments or even plastics. The intensity of an acoustic signal returned from SPM can be used to quantify characteristics such as SPM volume density or – using multi-frequency measurements, particle size. Traditionally, signal intensity is recorded as a data quality parameter and is recorded as a by-product for each acoustic beam in cells over depth. Combined with flow measurements, ADCP signal intensity can be a powerful tool for capturing the transport (or discharge) of SPM.

 

Historically, ADCP SPM surveys used the return signal measured by the slanted beams, which introduces two drawbacks. First, as the signal intensity is recorded as a by-product to flow measurements and the settings cannot specifically be tailored, measurements are recorded in relatively large cell sizes – required to capture velocity with sufficient precision. Second, it is not possible to measure close to boundaries as a result of the sidelobe effect. For vessel-mounted or other downlooking deployments, this means that it is not possible to sample sediment transport close to the bed, which is often the part where transport is most dynamic. 

 

To overcome these two challenges, some ADCPs – including the Nortek VM Coastal series – can be equipped with Echosounder functionality using a 5th vertical beam dedicated to backscatter measurements. When used in isolation, Echosounder equipped ADCPs are a great tool for qualitative analysis of fine SPM structures with < cm resolution, for example, to recognize and track the presence of dredge plumes.

 

When it comes to quantitative measurements of SPM, especially when monitoring suspended sediment concentration (SSC), the process is more complicated. This is because acoustic backscatter cannot be directly related to a concentration of particles: besides concentration, variables like particle size, shape and density influence the strength of the reflected signal. Also, acoustic frequency plays a role.

 

To use ADCP backscatter as an indicator – or proxy – for SPM and specifically SSC, an operational model can be set up that is site and instrument specific. In practice, this means collecting reference concentration measurements – often using water sampling – and linking these to the ADCP’s backscatter measurements. How many water samples are required will depend on the application, but generally speaking: the higher the variability in sediment

characteristics, the more samples are required. This can be a labour-intensive process, especially in coastal areas, where different types of sediments can be present during varying tidal stages.

 

To reduce the need for frequent water sampling, methods based on the use of multiple acoustic frequencies can be employed. By using empirical formulations that describe the scattering behaviour of specific types of SPM, a measured difference in backscatter intensity between the frequencies can be attributed to a theoretical particle size. While water samples are still required to tune a backscatter–SSC model to a specific deployment, particle size estimates derived from frequency-dependent signal differences can help improve correlations between ADCP backscatter signals and water sample measurements, thereby increasing confidence within the dataset.

 

While multi-frequency measurements don’t overcome the need for collecting reference concentration measurements, and introduce new challenges like echosounder calibration, it allows for an extra pair of eyes – or rather, ears – in resolving what’s present in the water column.