1. Measuring the flow of water
LaGrangian
Follow a parcel of water over time
Use satellite tagged drifters
Eulerian
Meausure the flow of water at a single point over time
Usually from a mooring
Combined?
Measure flow from a moving ship
Calculate the flow based on glider drift

2. Drifters Usually at the surface
High-drag so that they attach to the flow
Usually include both GPS and Argos positioning but only Argos is required

3. How it works
Polar Orbiting Satellites
Receiving stations
Positions calculated by meauring Doppler shift from consecutive transmissions

4. Local data from NGLI
Note tides plus ambient current

5. Eulerian: Moored Mechanical (rotors or propellers) Electromagnetic
Acoustic
Travel time
Doppler
Radar (“Codar”)

6. Mechanical Aanderaa Endeco Disadvantages: Savonious Rotor Impeller
Fouling
Dead zones

7. Electromagnetic Interocean S-4 Marsh McBirney
The S4 Electromagnetic Current Meter measures the voltage resulting from the motion of a conductor (water flow velocity) through a magnetic field according to Faraday's law of electromagnetic induction. Simply stated, Faraday's law defines the voltage produced in a conductor as the product of the speed of the conductor (water flow velocity) times the magnitude of the magnetic field times the length of the conductor. In the case of the S4, the conductor length is the effective path between the sensing electrodes. The magnetic field intensity is generated by a circular coil, internal to the S4, driven by a precisely regulated alternating current. The use of an alternating magnetic field and synchronous detection techniques to measure the voltage at the sensing electrodes provides an extremely stable, low noise current measurement. Two orthogonal pairs of electrodes and an internal flux gate compass provide the current vector.
- Interoceansystems.com
Electromagnetic
Interocean S-4
Marsh McBirney

8. Acoustic: travel time BASS: Benthic Acoustic Stress Sensor
MAVS: Modular Acoustic Velocity Sensor

9. MAVS Specifications Toughness Fouling sensitivity
Carried by submarine landslide 550 m down Monterey Canyon. Still recording pressure although sensor rings sheared off and tube bent 90°.
Fouling sensitivity
Stops measuring only when flow is totally blocked as in illustration
Insensitive to particles, bubbles, or perfectly clear water. Only hard obstruction or swim bladder stops acoustic signal

10. Acoustic Doppler Measure Doppler shift in 4 directions Single depth
Low vulnerability to fouling
Somewhat dependent on particles
Can be done with lasers too (LDV)

11. ADCP = Acoustic Doppler Current Profiler
Acoustic: typically kHz
Doppler: uses Doppler shift to measure speed
Current: measures the flow of water
Profiler: measures current vector simultaneously in as many as 100 distance bins
Distance can be depth below ship or height above bottom
More bins isn’t always better; statistics

12. Brands RD instruments (RDI) Sontek LinkQuest AAnderAA? Simrad? Others
Oldest, first, biggest
Sontek
Smaller instruments
LinkQuest
New on the market; quality?
AAnderAA? Simrad? Others

15. Echoes from particles or just the water
Doppler shift
Timing for distance

16. Tradeoff between resolution and range
Frequencies
Tradeoff between resolution and range
Depth and currents
Size:
Low F = Big ducers

17. Applications Current profiling from mooring
Current profiling from ship
Lateral flow
Tracking
AUV/ROV

18. Data!
0-100m
Flow vs time
Lots of engineering data too