Vector data quality check

Hi,

I have a quick question regarding Vector data quality checks. In my velocity time series traces (Vx) I am getting both positive and negative velocity values. During the periods in the time trace in which I am interested the Vx velocities look right, they are consistently in the range I would expect (1.2m/s), but they are all negative. Is the negative sign in relation to the phase angle produces by the acoustic beams with the scatter particles, meaning there may be data quality issues due to the change in signs or does the negative sign indicate the direction of the flow?

Hopefully I have provided you with enough information to understand my question!

Thank you,

Mairéad

Mairéad,

There are two possibilities:

1.  Make sure you know the orientation of your coordinate system and how velocities are defined relative to it. For instance in the Vector's XYZ coordinate system, positive X velocity is defined as motion away from the probe head in the direction of the marked receiver arm (page 24, Chapter 3 of the Vector manual). A negative velocity would indicate flow in the opposite direction as defined.

2.  In the Deployment Planning window of the Vector software two velocity ranges are given, one for horizontal and one for vertical. These are a function of the user selected Nominal Velocity Range. For the nominal Velocity Range you used in your deployment, check these two values and see if they are close (within a few cm/s) to your maximum velocities. If they are you likely have phase wrapping problems and will need to correct your data. I recommend checking out the just released Pulse Coherent Primer here: http://www.nortek-as.com/en/knowledge-center/technical-notes and reading on the forums how to perform this correction.

A change in sign is not a good indication of data quality. Examine your correlation and SNR values for the specific measurement to judge data qualilty.

P.J.