Echograms, recordings of depth or distance under water made by an echosounder, provide a means of tracking geomorphic and biological features over vast spatial and temporal domains.
Addressing the shortcomings of ADCP and echosounder technology
When echograms are coupled with current velocity data from an acoustic Doppler current profiler (ADCP), water flux rates and flux trajectories can be estimated.
However, pairing ADCP and echosounder technology has historically been problematic. Combining these two technologies has yielded echograms with low resolution, limited dynamic range and data gaps near the seabed because of competing requirements for velocity output. Nortek’s vessel-mounted ADCP, the Signature VM Coastal, was designed to address these limitations via a fifth, vertical, single-beam echosounder that occupies dedicated pings within the velocity data stream.
Monitoring all the way up to the water surface is not possible with commercially available vessel-mounted ADCPs due to limitations in the near field and blanking regions. Therefore, Nortek’s vessel-mounted system combines data with output from a second bottom-mounted Signature1000 ADCP. This enables users to see water-column dynamics from the seabed all the way to the sea surface – and this at a high spatial and temporal resolution as well as at broader scales.
Successfully combining ADCP and echosounder technology
Nortek joined forces with WaterProof BV in the Netherlands to test the performance of this novel technology. WaterProof deployed a vessel-mounted Signature1000 ADCP in conjunction with water sampling in a tidal channel in the autumn of 2019 in the Wadden Sea in the Netherlands. The sampled region, shown in Figure 5, is characterized by tidal currents, dredging activities, mud flats, ferry transport and shallow waters, making for a dynamic sampling area. The test was carried out by executing two different measurement campaigns.
The selected data subset from the first campaign was obtained over 13 hours to cover a full tidal cycle. Water samples were taken each half hour at three different water depths and analyzed for mud and sand content. A vessel-mounted Signature1000 echosounder was deployed with 0.05 m cell size and 2 Hz sampling rate in pulse compression mode. The echosounder backscatter was corrected for losses, and relative volume backscatter reflections under the altimeter-derived seabed delineation were suppressed.
During a second, multi-day campaign, a bottom-mounted Signature1000 captured data up to the sea surface. The longer timescale of this data set reflects tidal dynamics that are visible in the altimeter-derived sea surface and in oscillations in relative volume backscatter and velocity data. A subset of this data representing approximately ten days is presented below (Figures 3 and 4).
A high-resolution echogram from the vessel-mounted Signature1000 is presented here (Figure 1).