Principles of Operation:
Stage Measurement

Stage and velocity measurement are different and they require different processing. This section describes how an EasyQ's stage measurement works.

An EasyQ measures stage by measuring the travel time between the transducer and the surface. The water surface is typically smoother than the bottom, and it often looks like a mirror. A mirror-like reflector means that the echo returning to the EasyQ will look like the signal it transmits. The difficulty of making water level measurements acoustically is that the sensor can be confused by echoes from debris in the water or from out-of-range echoes. Out-of-range echoes often arise from multiple bounces between the EasyQ and the surface. An echo returning from the surface bounces back to the surface from the flat, miror-like EasyQ transducer. Because of variations in sound propagation and electronics response, it is possible for second and even third bounces to appear stronger than the first bounce.

The EasyQ uses several strategies to identify the direct echo from the surface. These include the following:

• A matched filter process enhances the echo from the surface relative to echoes from debris. A matched filter enhances echoes that match the transmit pulse, relative to echoes that have changed. The mirror-like echo from the surface looks more like the transmit pulse than does an echo from debris in the water, so the matched filter responds more strongly to the surface echo.
• The EasyQ uses a pressure measurement to place a window around the surface, rejecting echoes that fall outside the window.

Figure 1 illustrates both of the above. Note how the signal Quality (the output of the matched filter) produces a sharper response than the original signal strength. Peak A rises above the quality threshold and it lies within the pressure window, so it qualifies as a good surface measurement. Peak B is disqualified on both counts. If both peaks qualify, the EasyQ picks the peak closest to the EasyQ.

 Figure 1. Schematic of Signal Strength and Qualtity vs. Elevation above the EasyQ. Quality is the output from a matched filter.

The EasyQ makes many pings to find the surface. It averages 23 pings each second before it evaluates the echo against the quality threshold and pressure window. If the 1-s average echo passes, it is counted as a good echo. The EasyQ computes the distance to the surface using an algorithm from Visbeck and Fischer (1989). It repeats this process for every second during the measurement interval, then computes the average of all of the good echoes. The result is a measurement of stage that averages out water level fluctuations from waves and other surface disturbances, making stilling wells unnecessary.

Reference

Visbeck, M. and J. Fischer, Sea surface conditions remotely sensed by upward-looking ADCPs. J. Atmos. and Oceanic Technol., 12, 141-149, 1995.