CH Acoustic Doppler & Velocity Measurement 1 — Questions and Answers
Question 1: What physical principle does an Acoustic Doppler Current Profiler (ADCP) use to measure water velocity?
- Doppler frequency shift of acoustic pulses reflected by scatterers (Correct answer)
- Pressure differential across a Venturi constriction
- Electromagnetic induction in a conducting fluid
- Time-of-flight of laser pulses through water
Correct 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.
Question 2: Which ADCP blanking distance parameter defines the zone nearest the transducer where velocity data cannot be reliably collected?
- Bin size
- Blank before transmit (Correct answer)
- Bottom-track range
- Ensemble interval
Correct answer: Blank before transmit
The blank before transmit (blanking distance) is the near-transducer zone where ring-down interference prevents valid velocity measurements.
Question 3: A moving-vessel ADCP discharge measurement uses bottom tracking primarily to:
- Determine water temperature profiles
- Reference vessel velocity so water velocity can be isolated (Correct answer)
- Calibrate salinity sensors
- Map sediment grain size
Correct 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.
Question 4: In an ADCP measurement, what does the term 'ensemble' refer to?
- A single acoustic pulse
- The average of multiple pings collected over a short interval (Correct answer)
- The full depth profile from surface to bed
- The GPS position fix associated with one transect
Correct 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.
Question 5: Which four-beam Janus transducer configuration is most commonly used in ADCP instruments to resolve three-dimensional velocity components?
- Beams angled at 20° from vertical in opposing pairs (Correct answer)
- Beams angled at 45° and directed horizontally
- Two vertical beams and two horizontal beams
- A single rotating beam at 30° tilt
Correct 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.
Question 6: When an ADCP operates in 'water mode 12' (WM12), it is optimized for measuring velocity in:
- Deep ocean environments with strong stratification
- Shallow, fast-moving rivers with high turbulence (Correct answer)
- Tidal estuaries with bidirectional flow
- Lakes with very low backscatter
Correct 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.
CH Acoustic Doppler & Velocity Measurement 1
What physical principle does an Acoustic Doppler Current Profiler (ADCP) use to measure water velocity?