Nominal Velocity Range

Hi,

I wonder if any one can advise?

We have deployed a Vector at an estuarine field site that is new to us and the velocity range was set to 30cm/s as we were really unsure of the velocity magnitudes to expect.

Now that we've retrieved the instrument and started to look at the data it appears that actually the wave+current velocities were extremely low (<10cm/s) - a very calm day on a mudflat, with very small waves. In hindsight, it would have been much better to have chosen a much lower velocity range setting!  

Even though the velocity range was set too high, is the data that we have collected still valid? In other words, does the high range setting prevent the Vector from recording very low velocities (therefore making the observations that we have spurious/noisey).  Is there anything that we can do to correct for this range setting mistake during our post-processing?

Many thanks,

Frank

Dear Frank

Your data should be just fine " />'>

- Atle Lohrmann

Hi

I need some help. I am new with vectors and I need to understand what means the "nominal velocity range" and in what way changes in this value results in changes in horizontal and vertical velocity ranges? Why 0.3m/s gives 0.81 horizontal velocity?

At this time we have data with many spikes in maximum velocity and I don't know if these are due to the nominal velocity range I used. This was set for 3 m/s (0.81 m/s and 0.23 for horizontal and velocity range). The spikes are present when velocity is greater than 0.5 m/s. Can you tell me if the planning was wrong?

Thanks

Rafael

Hi Rafael,

The "Nominal Velocity Range" is a term coined by Nortek as a simple way of describing the maximum unambiguous velocity the instrument would be able to measure. The Nominal Velocity Range controls how far apart in time the two pulses used to determine the Doppler shift are. The further apart they are, the maximum velocity which can be measured and the noise will be lower. If they are transmitted closer together, the instrument can measure higher velocities, but suffers from slightly higher noise. This is discussed on pages 43 and 44 of the Vector manual as well. The rule of thumb is to set the velocity range as low as possible without the potential for water velocities to exceed the horizontal or vertical velocity ranges.

 

The Nominal Velocity Range is really just a convenient way of attaching one number to two different velocity ranges. I'll refer you to pages 23 and 24 of the manual for a diagram showing the Vector head and measurement volume. You'll notice the Vector actually measures velocities on a line in between the transmitter and receiver (the bistatic axis). These measured beam velocities aren't terribly useful to most researchers, so they are transformed to an orthogonal coordinate system using the transformation matrix. The two velocity ranges listed in the right most panel of the Deployment Planning dialog are estimates of the maximum horizontal and vertical velocities the Vector can measure based on the beam velocity.

 

Your data doesn't sound like it should be in too bad shape. The spikes are a characteristic of velocimeter data, and while a lower velocity range would prevent so many of them from occurring, your data should still be usable. Is your Signal to Noise Ratio (SNR) above about 15? This will tell you how much noise to expect (lower SNR, higher noise in your data). For future deployments I would try a lower velocity range if your data support this and you should see fewer spikes.

P.J.