Argonaut-SW "Goes all the way"

The Argonaut-SW (Shallow Water) uses three acoustic beams typically pointing up. Two of the beams are set at a 45-degree angle and are used to measure water velocity (see drawing at right). Ordinarily, using beams set to this angle would run the risk of sidelobe interference for the near surface measurements.

To evaluate potential sidelobe interference, Argonaut-SW data has been collected in a variety of environments. When possible, reference velocity data from an independent sensor has been collected as well. To improve evaluation of velocity data, a special de-bugging mode has been developed in the Argonaut-SW. This allows us to record the raw velocity profile with 3.4-cm resolution. This high-resolution profile can be compared with reference velocity data to look for any sidelobe interference.

A sample velocity profile is shown in the figure below. The data were collected in a re-circulating flume with a water depth of about 0.7 m. The Argonaut-SW was mounted on the bottom of the flume, slightly off center. A FlowTracker Handheld ADV was manually raised and lowered to measure the current profile at the same location along the length of the flume; the ADV measurement location was 0.2 m to the side of the Argonaut-SW. The data shown below represents the average profile over a period of more than one hour.

The offset between Argonaut-SW and ADV velocity data (≈2 cm/s) is attributed to variations in the velocity field across the width of the flume, and is consistent with other flume data. The important comparison is the consistent shape of the velocity profile, particularly in the top half of the water column. The Argonaut-SW is able to accurately measure water velocity all the way to the water surface, with no evidence of sidelobe interference. Numerous tests in other flow conditions have shown similar results, without any evidence of sidelobe interference.

SonTek has been manufacturing acoustic transducers with very low sidelobe energy levels for many years. The proprietary technology that lowers the side lobe energy in our transducers is also a key reason for the success of our side-looking Doppler Instruments (Argonaut-SL and SL-ADP). This same advantage enables the Argonaut-SW to make velocity measurements all the way to the surface.

• Argonaut-SW