Acoustic Doppler & Velocity Measurement Flashcards

What physical principle does an Acoustic Doppler Current Profiler (ADCP) use to measure water velocity?

Answer: Doppler frequency shift of acoustic pulses reflected by scatterers

ADCPs measure velocity by detecting the frequency shift in sound pulses backscattered by particles moving with the current.

Which ADCP blanking distance parameter defines the zone nearest the transducer where velocity data cannot be reliably collected?

Answer: Blank before transmit

The blank before transmit (blanking distance) is the near-transducer zone where ring-down interference prevents valid velocity measurements.

A moving-vessel ADCP discharge measurement uses bottom tracking primarily to:

Answer: Reference vessel velocity so water velocity can be isolated

Bottom tracking measures the vessel's speed over the riverbed, allowing the instrument to subtract boat motion from measured Doppler shifts to yield true water velocity.

In an ADCP measurement, what does the term 'ensemble' refer to?

Answer: The average of multiple pings collected over a short interval

An ensemble is the statistical average of a set of pings fired in rapid succession, reducing random noise in the velocity estimates.

Which four-beam Janus transducer configuration is most commonly used in ADCP instruments to resolve three-dimensional velocity components?

Answer: Beams angled at 20° from vertical in opposing pairs

The standard Janus configuration uses four beams tilted approximately 20° from vertical in two opposing pairs, enabling resolution of along-stream, cross-stream, and vertical velocity components.

When an ADCP operates in 'water mode 12' (WM12), it is optimized for measuring velocity in:

Answer: Shallow, fast-moving rivers with high turbulence

Water Mode 12 (broadband pulse-coherent mode) is designed for shallow, high-velocity environments common in rivers and streams.