Hi, my name is Nick Stasulis and I am a hydrologic technician with the Maine office of the New
England Water Science Center.
In this screencast I will discuss how you can identify site interference with the StreamPro
in SxS Pro.
A recent evaluation of StreamPro data collected throughout the USGS indicated that the StreamPro
is susceptible to electromagnetic interference at some sites, more than other ADCPs.
The StreamPro ADCP is more susceptible to EMI because the transducer is separated from
the electronics by a cable.
Think of this cable as an antenna, and the longer the cable is, the more potentially
susceptible the unit is to interference.
While we believe that this problem is mostly confined to StreamPro ADCPs, it is conceivable
that other ADCPs may experience the problem.
First, let’s discuss the interference itself.
This interference is due to sources specific to a site or measuring section.
So, just because a unit experiences interference at one site or one measurement cross section,
doesn’t mean it would absolutely experience interference at another site or in a different
cross section at the same site.
Also, sites with a low backscatter condition are more susceptible to interference as the
EMI can more easily overwhelm the returns of the true signal transmitted by the ADCP.
The cause of this interference is site-specific electromagnetic interference, or EMI.
EMI sources could include television, AM, FM and satellite transmissions, solar magnetic
storms, lightning, power transmission lines, airport radar, railroad and mass transit infrastructure,
and several others not listed here.
Now, let’s talk about how to identify interference in the SxS Pro software.
First, interference usually causes erroneous or very odd looking data, particularly unusual
patterns that change with depth, and using the simple review steps outlined in an earlier
screencast should help identify situations where interference exists.
This is a pretty classic example of a measurement with interference.
First, notice how the there is a very clear increase in velocities vertically in the contour
plot, velocities at the bottom are much slower than the velocities at the surface.
Next, notice the velocity profile for this ice measurement appears to be non-standard.
Intensity values look reasonable, but switching to correlation shows an increase with depth,
which is not what we’d expect in a normal measurement.
Another sign of interference could be high error velocities, and in this case, they are
fairly consistent in the contour plot, with only a few outliers.
The most telling sign that interference is possible is the PT3 test, collected as part
of the ADCP test.
In this case, viewing the ADCP test shows correlation values that stay nearly the same
over some number of lags.
In normal conditions, the correlation values in all four beams would decrease to 15 or
so by about lag 3 and remain low.
If interference is expected, you can’t rely on one of these checks alone, you have to
evaluate each and determine if interference is likely.
In this case, we have an oddly shape velocity profile vertically, unexpected correlation
values, and a non-standard PT3 test result, though our error velocities look reasonable.
If you identify interference, you may be able to use water up and water error thresholds
to remove the erroneous data, but this may screen out too much data to allow the measurement
to be used.
Your best option is likely to try moving a substantial distance to a different measurement
section, or switch instruments.
As of Spring 2015, TRDI is working on a hardware solution to help minimize this issue.
If you have questionable data, consider consulting with the USGS Hydroacoustics Workgroup using
the email address shown.