b'TECHNICHEADLINEAL TIPSHomogenous ConDitions Turbulence / EDDiesOne of the basic assumptions for measuring with anyWhen there is significant turbulence or eddies present in multi-beam acoustic Doppler current measurementthe channel, the individual beams may be measuring in system is that the individual beams are measuring invery different conditions (thus violating the homogenous similar conditions, so that the velocity averaged from theconditions assumption), resulting in their averaged individual beams will provide an accurate mean velocity. velocity being significantly different than the actualmean velocity.Spatial AveragingFor example, there could be situations where a large With a multi-beam acoustic Doppler current measurementeddy results in beams measuring velocities in opposite system such as the RiverSurveyor S5/M9, SonTek-SL,directions, resulting in the averaged velocity being zero.and SonTek-IQ, the reported velocity is an average of the velocity measured by the individual acoustic beams,There is usually some level of turbulence or eddies in a which are very narrow. channel, especially a natural channel, but averaging the velocity data over an appropriately long period can help The reported velocity approximates a spatial averageto improve the results. Parameters such as Velocity Error computed from the velocities measured by 2, 3, or 4and Correlation will give an indication of the homogeneity beams, and the area averaged increases with distanceof the measurement.from the system.Magnetic InfluenceThe off-axis beam angle for the SonTek systems is 25 degrees*, so at any specific distance from the system Another site selection consideration is localized magnetic (i.e., range), the beams are separated by (0.93 x range).fields, which can affect systems that include a compass, For example, with the 2-beam SonTek-SL system, at a 10msuch as the RiverSurveyor S5/M9. Sources of magnetic range the beams are separated by 9.3m.interference can include steel bridges, the steel rebar used in concrete bridges or structures, as well aspower lines. The example below shows the boat tracks with velocity vectors for a river transect, where a nearby bridge pillar N caused magnetic interference with the compass:Multi-cell Blanking Distance1 Cell Begin Bottom track ship track (does Cell2 not use compass information)GNSS-derived ship track 3 (uses compass information)N Multi-cells Size45 System ENUIntegratedGPS-GGA NUMulti-cells Bottom-Track ENU57556346 0.1 m/sVelocity Cell The effects Grid: 10.0m 7 of magnetic 8 Cell End interference on data 9575563210 Center bridge V pillar affecting X compassV5755630 y Diagram for illustration only.System ENUFlow GPS-GGA NUBottom-Track ENU0.5 m/sDepending on the available measurement sites,Grid: 2.0mthe above recommendations and considerations Example of Spatial Averaging for a 2-beam system. may not always be feasible. No site is ideal, but it is important to keep the basic assumptions in mind * Note:IQ skew beams are 60 degrees off axis when selecting a site.For Customer Support Services: SonTek.com/Total-CareWhos Minding the Planet? 59'