So, you’re ready to go measure your first velocity profile track using a Signature VM system. How do you know you’re really ready, and that your system is configured correctly ?
☐ Fresh water ? → consider the need for a ground-lug
☐ All instrument alignments performed, “dry-checked”, and entered into the software ?
☐ Instrument/measurement settings ? (cell size, blanking distance, salinity/speed of
☐ Recordings file location, recording duration ?
☐ Noise measurements (at least broad at -100 dB)
☐ If at sea and if possible: measure the salinity (or conductivity and temperature)
☐ Straight heading calibration line(s) (1000 m, and back again, ideally with and against the
current; you need good bottom track for these lines)
☐ “Below the bed” background noise estimation
☐ Amplitude decay with depth
INSTRUMENT ALIGNMENT "DRY-CHECK"
See Acquisition manual v 2.6, section 7.3, “Orientation of the GNSS and Signature”.
Essentially, check that GNSS Speed Over Ground (GNSS SOG) direction value and the direction for the ship’s heading (HDG) match (±1°), and that the GNSS Speed Over Ground (GNSS SOG) direction and the Signature’s Bottom Track Speed Over Ground (BT SOG) direction match (±1°).
Signature VM Review 2.6 has a procedure that can calculate the corrections from two straight tracks or sections (see reference under “Heading calibration”).
Signature VM Acquisition v2.7 will have a simplified, somewhat less accurate version of this procedure built in.
Smaller cells give better vertical resolution, but give noisier current speed values.
Larger cells improve the signal-to-noise ratio, and hence give better current speed values. They also provide significantly better range in low-scattering conditions.
Generally speaking, chose for larger cells.
Close to the transducers, there is a volume of water where you shouldn’t measure. Excluding measurements in this area also allows the transducers time to settle before the echo returns to the receiver (avoiding “ringing”).
Generally speaking, this blanking distance should be at least the size of your cells.
In case the Signature instrument is mounted behind an acoustic window, you may need to increase the blanking distance due to additional ringing.
Next to the transducer “ringing” effect, any vessel has a certain area close to the hull where the movement of the ship influences the water movement. The exact extent of this hull influence depends on factors like the mounting location and the shape of the hull. You should extend the blanking distance so this area is excluded from your measurements, too.
Salinity / speed of sound
See figure 30 (Acquisition manual v2.6) for switching between salinity and speed of sound.
RECORDINGS FILE LOCATION, RECORDING DURATION
See section 6.2 “Configuration”, Acquisition manual v2.6
See section 12, “Spectrum”, Acquisition manual v2.6
See Review manual v2.6, section 3.5 “Alignment”
"BELOW THE BED" BACKGROUND NOISE ESTIMATION
You need a recording for this.
- Open the recording in Review
- Select the “amplitude” tab, and open the vertical profile side-graph for it (a.k.a. a cross section slice)
- On the ribbon, on the tab “Processing”, (temporarily) change the “Depth” source to “Last cell”
- In the “Amplitude” plot, drag the cross-section slice along the track/time-sequence, and observe the vertical profile. We’re going to assume that the lowest value you can find, “deep below the bed”, will be the background noise level
For usable measurements, the signal amplitude needs to be at least 3 dB above this background noise level. If the signal amplitude curve is too close to the background noise level for a large part of the water column, you will not be able to get good results. (See also “Current vectors in the direction of travel”, later in this document.) Nortek instruments normally have a noise level of 20 to 30 dB. On vessels, values from 30 to 40 dB are often the best you can achieve, but it is important to get the noise level as low as possible.
AMPLITUDE DECAY WITH DEPTH
For good measurements with our instruments, you need “tracer particles” in the water, that the acoustic waves can bounce off of – not too many, or you’ll lose the acoustic energy too quickly, and not too few, or you’ll not get enough signal for your measurements. (And they should be small enough that they can be considered “passive tracers” in terms of the water movement.) Most of the time, there will be enough material in the water naturally that fulfills this role, be it organic or inorganic matter. But this may not be the case in fresh water. Crystal-clear water will indicate you’re likely to have trouble getting decent measurements.
For an ideal, homogeneous distribution of these tracer particles, you expect the signal amplitude to gradually decay with depth, following an exponential curve, with the amplitude just above the bottom still some dB above your background noise level. If your amplitude profile looks like that, things look good.
CURRENT VECTORS IN THE DIRECTION OF TRAVEL
Are you sure you’re not sailing with the current ?
Otherwise, check your noise level.
If your signal amplitudes are very close to the background noise level for a large part of your vertical water column, you will have problems. Noise correlates well with noise, so your correlation will be high. But noise averages out to 0. Your sensor travels through the water, attached to your vessel, so our software corrects the sensor measurements for this vessel speed. This means the 0 noise measurements will be corrected to the vessel speed. If this happens for a relatively large part of your water column, this effect will dominate the depth-averaged current, leading to averaged current vectors that are very similar in magnitude and direction to your vessel’s speed and direction. In a pinch, in Review you can set your amplitude minimum to a little above your background noise level. This will eliminate these 0-speed noise measurements from your depth-averaged current speed and direction. But of course, you do not have valid measurements for these noisy points.
BOTTOM TRACK (RED DOTS) AND VESSEL TRACK (BLUE/BLACK LINE) ARE AT AN ANGLE
This means the alignment isn’t correct. Redo your alignment procedure. (See the references above, under “Instrument alignment “dry-check”” and “Heading calibration”.)
Many measurement problems can be attributed to noise, and many noise problems can be attributed to the supply of power on your vessel.
If your mains power comes from a generator, or from an inverter, noise is likely.
There exist inverters that are less noisy than others are.
If you cannot get a less noisy generator and/or inverter, you could run your Signature VM Coastal system off 24 V batteries. (Of course, you should not charge or power other devices from these batteries, to avoid noise from these devices.)
Please check the power consumption details as these differ per instrument type.
For longer measurement campaigns, you may need to factor in time to recharge the batteries.