To measure the distance to the bed from the central transducer, an acoustic pulse is sent out separate from the normal velocity sampling pulses. The return from this pulse is listened to over some time to obtain an amplitude profile. The amplitude profile is checked for peaks, and an interpolation scheme is used to locate the peak location based on the largest amplitudes recorded.
The Distance Check function is available in the Vectrino Plus firmware, the Vector and the Vectrino Profiler, but NOT on the Vectrino Standard software. The standard Vectrino firmware does not allow for the boundary distance mode, and this is of course reflected in the software. Immediately preceding the measurements the Vectrino does however measure the distance to the boundary, and it adjusts the selected pulse separation (= velocity range) slightly if the distance conflicts with the pulse separation. This is done automatically, and the measured distance is not reported anywhere, except for in the data table in the software.
The Vector reports the distance to the boundary at the start and end of each burst, or at the start of data collection for continuous operation. Thus you could do very short burst and get almost simultaneous measurements (requires Firmware V1.20 or higher). Only the Vector is capable of burst measurements among the Nortek acoustic velocimeters.
The Vectrino+ firmware comes with a separate boundary distance mode (10 Hz), where the distance to the boundary is measured, but with no velocity readings. This is used in experiments where bottom changes as a function of time are needed, or when the Vectrino+ is mounted on traverse systems for bottom topography measurements. The Vectrino Profiler has an interleaved boundary distance functionality. In other words, you get BOTH velocity profiles and distance to the bottom. The reasons for this are both that we could do it technically, and also that the problem of weak spots is much more challenging with the Vectrino Profiler than with a single point instrument. Hence, in order to operate the Vectrino Profiler properly we depended on interleaving velocity and distance measurements, such that the time lag between the two pulses in every pulse pair can be adjusted adaptively.
The accuracy of the distance check is +/- 0.5 mm, half the measurement cell size. In practice, it is actually a bit better, around +/- 0.3-0.4 mm over smooth boundaries owing to the curve fit done on the data.
The maximum and minimum distances that can be measured are approximately as listed below, but note that it will depend a lot on the hardness of the bottom.
Vector: 4 cm - 45 cm
Vectrino+: 1 cm - 35 cm
Vectrino Profiler: 20 mm - 2 m. The Vectrino Profiler can be configured where to start collecting bottom check data. Refer to SW user guide (available within the software) for more details.
We provide all of the data used to determine the peak location is users need to create their own bottom detection algorithm. This is available as the probe check data in the Vector and Vectrino, and if enabled for the Vectrino Profiler the bottom detection fields in the Data structure. This applies to our other instruments as well; you can use the amplitude return to formulate your own boundary detection algorithm. You will just need to make sure the profile extends into the viscous mud layer. Acoustic pulse with lower carrier frequency (eg. 1 or 2 MHz) has stronger penetration that can go through the fluid mud layer. There are also sampling considerations when using a mono-static system that make it more difficult to enable a separate measurement, independent of the velocity measurements, than with a single point, bi-static sensor like the Vector or Vectrino.
Quality (relevant for Vectrino+)
Quality is a number indicating how valid the distance measurement is. Above 100 should be a good distance measurement. It's always good to have an independent estimate of position though, so we usually recommend using the probe check feature of the Vectrino software to get a ballpark number.
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