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

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

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

Local data from NGLI Note tides plus ambient current

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

Mechanical Aanderaa Endeco Disadvantages: Savonious Rotor Impeller Fouling Dead zones

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

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

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

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)

ADCP = Acoustic Doppler Current Profiler Acoustic: typically 38-1200 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

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

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

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

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

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