Acoustic Doppler Current Profiler (ADCP): Principles of Operation and Setup

Transcription

1 SMARTSkills Workshop for Vessel Users and Researchers, Marine Institute, Galway 29th April 2016 Acoustic Doppler Current Profiler (ADCP): Principles of Operation and Setup Christian Mohn & Martin White Overview Principles of operation ADCP deployents, setup and systes Fro an acoustic ping to a velocity profile Biological easureents Trade-offs 1

2 Part 1: Principles of Operation Physical Processes in the Ocean: A Myriad of Tie and Space Scales White et al

3 Why ADCP? Measuring currents is fundaental to understand ecosyste dynaics, nutrient and organic atter cycling. High-resolution and ability to saple deep within the ocean interior. Measures currents at ore than one location at the sae tie. Coon ADCP specs and ocean processes Frequency Range Resolution λ (c) Exaples of processes 2 MHz Turbulence 1.2 MHz BBL and Sedient Dynaics 600 khz Tides, Internal Waves, Sub-Mesoscale Near Surface/Botto Currents 300 khz Meso-scale Near-Surface and Botto currents, Planktonic Scatterers 75 khz khz Large-Scale Upper Ocean Currents, MLD, Shelf-Slope Dynaics, Planktonic Scatterers 8 4 Large-scale Upper and Interior Ocean currents, Planktonic Scatterers 3

4 Coon ADCP specs and ocean processes Frequency Range Resolution λ (c) Exaples of processes 2 MHz Turbulence 1.2 MHz BBL and Sedient Dynaics 600 khz Tides, Internal Waves, Sub-Mesoscale Near Surface/Botto Currents 300 khz Meso-scale Near-Surface and Botto currents, Planktonic Scatterers 75 khz khz Large-Scale Upper Ocean Currents, MLD, Shelf-Slope Dynaics, Planktonic Scatterers 8 4 Large-scale Upper and Interior Ocean currents, Planktonic Scatterers Coon ADCP specs and ocean processes Frequency Range Resolution λ (c) Exaples of processes 2 MHz Turbulence 1.2 MHz BBL and Sedient Dynaics 600 khz Tides, Internal Waves, Sub-Mesoscale Near Surface/Botto Currents 300 khz Meso-scale Near-Surface and Botto currents, Planktonic Scatterers 75 khz khz Large-Scale Upper Ocean Currents, MLD, Shelf-Slope Dynaics, Planktonic Scatterers 8 4 Large-scale Upper and Interior Ocean currents, Planktonic Scatterers 4

5 Coon ADCP specs and ocean processes Frequency Range Resolution λ (c) Exaples of processes 2 MHz Turbulence 1.2 MHz BBL and Sedient Dynaics 600 khz Tides, Internal Waves, Sub-Mesoscale Near Surface/Botto Currents 300 khz Meso-scale Near-Surface and Botto currents, Planktonic Scatterers 75 khz khz Large-Scale Upper Ocean Currents, MLD, Shelf-Slope Dynaics, Planktonic Scatterers 8 4 Large-scale Upper and Interior Ocean currents, Planktonic Scatterers Coon ADCP specs and ocean processes Frequency Range Resolution λ (c) Exaples of processes 2 MHz Turbulence 1.2 MHz BBL and Sedient Dynaics 600 khz Tides, Internal Waves, Sub-Mesoscale Near Surface/Botto Currents 300 khz Meso-scale Near-Surface and Botto currents, Planktonic Scatterers 75 khz khz Large-Scale Upper Ocean Currents, MLD, Shelf-Slope Dynaics, Planktonic Scatterers 8 4 Large-scale Upper and Interior Ocean currents, Planktonic Scatterers 5

6 Profiling ADCP: Transducers Monostatic: Transit and recieve sound waves Vibrating ceraic eleent protected by urethane A Brief ADCP History 1970s The first ADCP was produced as an adaptation of a coercial Doppler speed log (Rowe and Young, 1979). 1980s A range of coercial ADCPs becoes available (self-contained, ship-based, different frequencies). 1990s ADCPs becoe popular in the scientific counity and environental agencies. > 2000s Acoustic based instruents becoe the ost coon instruent type for flow easureents. 6

7 The Doppler Effect Train approaches Pitch higher than transitted Train recedes Pitch lower than transitted The Basic Doppler Equation f D = f S * V/C f D = Doppler Shifted Frequency (easured) f S = ADCP (Source) Frequency V = Water velocity C = Speed of Sound (dependent on water T/S) 7

8 ADCP: Water velocity fro passive sound scatterers Pteropod Euphasiid Copepod Assuption: On average scatterers ove at the sae horizontal velocity as the water. ADCP: Water velocity fro sound scatterers Scatterer is oving Received signal f D toward f D > f S away f D < f S Transitted pulse f S across/stationary f D = f S 8

9 ADCP and Sound: Narrowband and Broadband Narrowband : One siple tone burst Doppler Frequency Shift of the return signal Broadband: One phase coded pulse pair Phase Shift of the return signals ADCP and Sound: Broadband Technology Higher precision but lower range than Narrowband 9

10 Iportance of Speed of sound (C) V = f D / f S * C Speed of sound (C) ust be coputed accurately by the ADCP. A teperature error of 2 C or a salinity error of 5 ppt would result in a 1 % error in easured velocity. The ADCP ust have an accurate teperature sensor and ust be configured for a representative salinity. Fish: When the scatter velocity ay not be equal to the water velocity Water Water velocity easureent is biased toward the fish velocity Stationary objects: Rock Water Water-velocity easureent is biased toward zero 10

11 Part 2: ADCP deployents and systes ADCP deployents 11

12 Vessel ounted systes S-ADCP: Long-range profiling over ranges > 1000 L-ADCP: Long-range profiling over entire depth SV-ADCP: Short-range profiling over entire depth Self-contained, fixed position systes Anchored surface/sub-surface ooring Botto ounted lander/frae Horizontal ADCP 12

13 ADCP deployents: Advantages/disadvantages Vessel-ounted Fixed position + 3D currents Tie series + Transport, discharge, flux easureents Near-botto, near-surface currents - Ship/vessel otion Battery life Part 3: Fro an acoustic ping to a velocity profile 13

14 Profiling ADCP: What is easured? Doppler frequency shift between ADCP and scatterer Strength of the acoustic backscatter (echo aplitude) Water teperature at the ADCP Orientation of the ADCP Ancillary data (position, orientation and speed of the vessel) Profiling ADCP: What is derived? Water velocity (east, north, up) in ADCP coordinates (attention: the coordinate syste of the ADCP ight be different fro earth coordinates) Quality statistics (bea correlation, error velocity) Relative oveent (speed) of the ADCP over ground (botto track) 14

15 Profiling ADCP: Multiple Beas ADCP only sees velocity of scatterers parallel to the bea. But: Bea is tilted - Water velocity in the horizontal fro trigonoetric relationships. One bea is required for each velocity coponent (east, north, up) Why four beas? Error velocity Assuption: Water layer seen by the ADCP is hoogenous Error velocity: Difference of vertical velocity between 2 beas 15

16 Error Velocity Differences in vertical velocities caused by alfunctioning equipent, sall-scale turbulence, oving objects (fish, litter, etc.) Should be randoly distributed Behind bridge pier Getting a velocity profile: Depth cells and range gating Distance fro ADCP Blanking Transitting Gate 1 start end A B C echo echo cell 1 cell 2 cell 3 cell 4 Gate 4 Gate 3 Gate 2 echo echo Tie Blank Bin 1 Bin 2 Bin 3 Bin 4 16

17 Uneasured parts of the water colun Blanking distance (recovery tie after ping) Transducer depth Side lobe (lower sound intensity) Side lobe (lower sound intensity) Main bea (higher sound intensity) good velocity profile Area of side lobe interference ADCP velocity profile Blank Distance + Transducer Depth Depth Bin V U W Loss of Data Side Lobe Interference Distance: (1-cos(bea angle))*depth Benefit: Velocity averaged over entire depth cell Trade-off: 6 12 % of the profile cannot be used 17

18 Part IV: Biological easureents Biological easureents The strength of the backscattered signal can provide very useful estiates of biological bioass, distribution and behaviour or suspended particulate atter (SPM). Wavelength λ of the acoustic signal (frequency, sound absorption) deterines the iniu size of the sound scatterers seen by the ADCP Miniu size () = 0.25 λ 0.5 λ Exaple: 75 khz ADCP resolve scatterers >

19 Diel vertical igration about a tall isolated seaount Senghor Seaount, Cape Verde, North Atlantic Z () Z () Decial day Repeated transect across seaount suit, vessel ounted 75 khz ADCP, scatterers > 1 c (what?) Diel vertical igration at a cold water coral reef Tisler Reef, Skagerrak, Baltic Sea Tie series, stationary upward looking 300 khz ADCP, scatterers > c (icrozooplankton) 19

20 Part V: Trade-offs Trade-offs 20

21 ADCP setup and Trade-offs Goal ADCP setup Trade-offs Better Depth Resolution Reduce Rando Noise (Depth) Reduce Rando Noise (Distance) Longer Profiling Range Saller Depth Cells Larger Depth Cells Tie-average Profiles (Enseble Averaging) Operate in Narrowband ode Reduction in Profiling Range Lower depth resolution Lower Horizontal Resolution (if ship is underway) More Rando Noise ADCP setup and Trade-offs Goal ADCP setup Trade-offs Better Depth Resolution Reduce Rando Noise (Depth) Reduce Rando Noise (Distance) Longer Profiling Range Saller Depth Cells Larger Depth Cells Tie-average Profiles (Enseble Averaging) Operate in Narrowband ode Reduction in Profiling Range Lower depth resolution Lower Horizontal Resolution (if ship is underway) More Rando Noise 21

22 ADCP setup and Trade-offs Goal ADCP setup Trade-offs Better Depth Resolution Reduce Rando Noise (Depth) Reduce Rando Noise (Distance) Longer Profiling Range Saller Depth Cells Larger Depth Cells Tie-average Profiles (Enseble Averaging) Operate in Narrowband ode Reduction in Profiling Range Lower depth resolution Lower Horizontal Resolution (if ship is underway) More Rando Noise ADCP setup and Trade-offs Goal ADCP setup Trade-offs Better Depth Resolution Reduce Rando Noise (Depth) Reduce Rando Noise (Distance) Longer Profiling Range Saller Depth Cells Larger Depth Cells Tie-average Profiles (Enseble Averaging) Operate in Narrowband ode Reduction in Profiling Range Lower depth resolution Lower Horizontal Resolution (if ship is underway) More Rando Noise 22

23 Other Considerations Ship Speed: Slow speed reduces the ean error in flow calculation. Diension of the cells: Cells with a sall size reduce the profiling range but give velocity easureents closer to the surface, the botto and the shore. Environental Factors: Profiling range is enhanced by colder and fresher water and by ore suspended aterial and scatterers. A brief suary Water velocity is easured with respect to the ADCP (bea coordinates). Velocity is easured taking advantage of the suspended/passive particles in the water colun. The velocity of the ADCP is also easured (botto track). Measureent gaps at the surface and botto. 23

24 Interesting online resources GO-SHIP (The Global Ocean Ship-Based Hydrographic Investigations Progra): Thanks a lot 24