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Acoustic Doppler Current Profiler (ADCP): Data Acquisition and Basic Processing

Christian Mohn & Martin White

SMARTSkills Workshop for Vessel Users and Researchers,Marine Institute, Galway 29th April 2016

• Data acquisition and on‐board visualization• Processing of shipboard ADCP data: Error

sources and main steps• Software tools for processing moving vessel

ADCP data

Overview

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Part 1: Vessel‐mounted ADCP data acquisition and on‐board visualization

The primary line of blue‐water shipboard ADCPs

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Vessel‐mounted ADCP: Which information is needed?

Earth CoordinatesShip SpeedGPS time, position

Ship HeadingShip SpeedPitch and Roll

ADCP (Beam) VelocitiesError VelocityEcho Intensity

Data Acquisition System (DAS)Output: Raw and Corrected ADCP

Velocities in Earth CoordinatesVADCP = Vship + Vwater

Vessel‐mounted ADCP: Data collection

Data Acquisition System (DAS):

• Set the ADCP’s operating parameters (cell size, number of cells, file directories etc.)

• Record data from the ADCP and ancillary sensors

UHDAS (University of Hawaii)VMDAS (RD Instruments)

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VMDAS: Setting ADCP operating parameters

VMDAS: Setting ADCP operating parameters

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VMDAS: Setting ADCP operating parameters

VMDAS: Setting ADCP operating parameters

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VMDAS: Setting ADCP operating parameters

VMDAS: Setting ADCP operating parameters

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General recommendations for ADCP operation (Firing & Hummon 2010)

• Choose a single, reasonable set of instrument setup parameters, and stick with it.

• Use bottom tracking only when needed for checking the transducer orientation or other aspects of instrument performance.

• Monitor the sensors.

Software Scope Origin

WinADCP Graphical plotting and data

output of ADCP binary data files.

Not freely accessible, but free of

charge (requires RDI account).

RD Instruments, USA.

On‐board visualization

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Part 2: Post‐processing of shipboard ADCP data

Data Output

Data file Processing Level

.ENR Single ping raw ADCP data in ADCP beam

coordinates

.ENS Single ping ADCP data in ADCP beam

coordinates including statistical error

screening and navigation

.ENX Single ping ADCP data transformed to

Earth coordinates and screened for error

velocity, vertical velocity, false targets.

.LTA Long‐Term‐Average: Time averaged

(Ensemble) ADCP data in beam

coordinates including navigation

.STA Short‐Term‐Average: Time averaged

(Ensemble) ADCP data in beam

coordinates including navigation

Data Acquisition System (DAS)

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Shipboard ADCP data: Why processing –Error sources, problems and pitfalls

• Random noise in single ping ADCP data• Sound absorption• Misalignment of the transducer relative to the ship’s

keel• Interference by physical objects and ship‐induced

turbulence• Ship motion

Ignoring these error sources may lead to incorrect estimates of current speeds and directions.

Step 1: Ensemble Averaging

• Ensemble averaging reduces the impact of measurement errors and random noise.

• Choose averaging period according to scientific needs of spatial and temporal resolution.

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Ensemble Averaging

10 kts

Δx = 1500 m

ADCP current,5 min average

Ensemble Averaging: Reconsider setup over rough topography

10 kts

Δx = 1500 mADCP current,5 min average

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Step 2: From Beam to Earth Coordinates –Check/Correct Transducer Offset

• Calibrating the ADCP: Setting / calculating transducer orientation relative to the ship heading

• Important to obtain accurate estimates of flow magnitude and direction

• Most critical step in ADCP data processing

Step 2: From Beam to Earth Coordinates –Identify hull‐mounted Transducer Offset

3

4

21

+

-

0

Ship Velocity

Water Velocity

Beam Velocity(= Ship Velocity + Water Velocity)

3

2

4

1

North

Transducer aligned with ship heading

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Step 2: From Beam to Earth Coordinates –Identify hull‐mounted Transducer Offset

3

4

2

+

-

0

Ship Velocity

Water Velocity

Beam Velocity(= Ship Velocity + Water Velocity)

3

24

1

North

Transducer MISaligned (45°) with ship heading

1

3

2

4

1

North

Step 2: Correcting remaining transducer offset from ship heading errors

Gyro Heading

True Heading 1 – 3°

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Step 2: Bottom / Water Track calibration

• Obtain accurate estimates of flow magnitude and direction (For a ship travelling at 5m/s (10 kts), a 1°degree error in heading causes a cross‐track velocity error of 10cm/s).

• Bottom tracking often not available or too short ‐Water track calibration is the most commonly used method.

• Calibration requires many twists, turns, starts and stops of the ship – the more the better.

• Calibration not needed for transducer offsets <= 0.5°.

An example from the NE Atlantic continental margin west of Ireland

30 cm/s

°Longitude

°Longitude

°Latitud

e°Latitu

de

Transducer misalignment: 53°(+ small offset from shipheading error)

Transducer misalignmentcorrected

30 cm/s

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Step 3: Quality Control / Editing

• Scan and eliminate bad ADCP profiles/bins.

• Typically, only bins and profiles with a ‘total percent good data’ value > 25% are retained.

• Available software tools: Gautoedit (CODAS, University Hawaii), ADCPtool (University of Graz, Austria), Velocity Mapping Toolbox (University of Hull, UK; USGS)

Step 3: Quality Control / Editing: Gautoedit

http://currents.soest.hawaii.edu/docs/doc/codas_doc/edit_doc/index.html

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http://currents.soest.hawaii.edu/docs/doc/codas_doc/edit_doc/index.html

http://currents.soest.hawaii.edu/docs/doc/codas_doc/edit_doc/index.html

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Step 4: Getting the absolute water currents

• Calculating absolute current velocities by removing the best estimate of ship velocities per ensemble from the corresponding relative ADCP velocities

Uwater = UADCP – USHIP

Vwater = VADCP – VSHIP

• Smooth absolute velocities to reduce the effects of remaining noise in the ship’s position data

DONE

Part 3: Software tools for processing of moving vessel ADCP data

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Software Scope Origin

CODAS Most comprehensive shipboard ADCP

processing software suite on the

market (python); focus on data editing

/ processing, but visualizing tools also

available .

Physical Oceanography Division,

SOEST, University of Hawaii, USA.

http://currents.soest.hawaii.edu/docs/

doc/index.html

VMT

(Velocity

Mapping

Toolbox)

Processing and visualizing ADCP data

collected along transects in rivers or

other bodies of water. Matlab based.

Department of Geography, University

of Hull, UK; Hydoacoustics, USGS, USA.

http://hydroacoustics.usgs.gov/movin

gboat/VMT/VMT.shtml

ADCPtool Open Source software to analyze,

process and export ADCP

measurement data (python).

Reference Manual in English.

Hydraulic Engineering and Water

Resources Management, Technical

University Graz, Austria.

http://portal.tugraz.at/portal/page/po

rtal/TU_Graz/Einrichtungen/Institute/

Homepages/i2130/software/ADCPtool

Data Processing / Analysis

Spatial analysis and interpolation

Senghor Seamount, 2/2013, 75 kHz ADCP, 2 min ensembles

The Task: Regriddingspatially scattered data

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Software Scope Origin

DIVA (Data‐

Interpolating

Variational

Analysis)

Software for gridding

spatially scattered in situ

data. Spatial interpolation

of data in an optimal way.

Binaries for different OS,

online version without the

need to install.

GHER, University of

Liège, Belgium;

SeaDataNet, EU.

http://www.seadatanet.o

rg/Standards‐

Software/Software/DIVA

Spatial analysis and interpolation: The DIVA package

Spatial analysis and interpolation

DIVA interpolated ADCP data

Velocities (cm/s) Relative Error

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Thanks a lot

Questions ?