Introduction to

Acoustic Measuring Equipment

Klamath Falls and Chiloquin,

ORSeptember, 19 –

23, 2011

U.S. Geological SurveyTEchnical training in Support of

Native American Relations (TESNAR) - 2011

Mark Uhrich, USGS, Portland, OR (mauhrich@usgs.gov)Marc Stewart, USGS, Central Point, OR (mastewar@usgs.gov)

Glen Hess, USGS, Portland, OR (gwhess@usgs.gov)

Brief History of ADCPS

Ocean going boats used “speed logs” to measure speed of the boat.

“The first commercial ADCP, produced in the mid-1970’s,was an adaptation of a commercial speed log” (Rowe and Young, 1979).

1980s Doppler technology continue to involve Early 1990s ADCP become more widespread in the USGS

and other agencies. 2012 Acoustic based instruments become the most

common instrument type used in the USGS (Flowtrackers and ADCPs)

ACOUSTIC IN

STRUMENT

WHAT

WHY

AND WHEN

3

Much of the material in the presentation is borrowed from USGS Hydroacoustic Classes

TRDI Rio Grande ADCP

SonTek RiverSurveyor

TRDI StreamPro

Acoustic Instruments

SonTek Flow Tracker

FlowTracker (acoustic point measurements)

Ceramic transducers send and receive pulses of sound

Center transducer transmits the sound, while the transducers on the arms are receivers

Location of velocity measurement is called the sample volume

Sample volume is located about 4 inches from the transmitting transducer

Measures velocity based on the Doppler shift

Acoustic

Doppler

Current

Profiler

Sound Waves and the

Doppler Shifts are used to measure

Water Velocity

Profiles

WHAT IS AN ADCP?

LET’S LOOK AT THE NAME“A” “D” “C” “P”

Sound Waves

Crest+

-Trough

Water wave crests and troughs are points of high and low water elevations.

Sound wave “crests” and “troughs” consist of bands of high and low air or water pressure.

Trumpet ADCP Transducer

How does an ADCP instrument work?

Uses Doppler shift to measure water velocity

The Doppler effect is the change in a sound's observed pitch (frequency) caused by the relative velocities of the sound source and receiver.

fD = Doppler Shifted Frequency

fS = Source Frequency (frequency of ADCP)

V = Velocity of scatterers in water

C = Speed of Sound (dependent on water char.)

The Basic Doppler Equation

fD = fS * V/C

Scatter Velocity Assumption

V = fD / 2fS * C

V = water velocity = scatterer velocity

Important

We assume that, on average, scatterer velocity equals water velocity

Violation of this assumption will lead to errors in water velocity computation.

Note: The 2 in the equation is result of two Doppler shifts, oneas the sound goes out and another as it returns

When the scatter velocity may not be equal to the water velocity

Water-velocity measurement is biased toward the fish velocity

Water Fish:

Water

Stationary object:

Rock

Water-velocity measurement is biased toward zero

Importance of Speed of sound (C)

A temperature error of 4o C or salinity error of 12 ppt will result in a 1% velocity error

The instrument must have an accurate temperature sensor and must be configured for the correct salinity

Rule of thumb: Specific conductance generally below 5000 uS/cm should not significantly affect C

Policy: All acoustic instruments must have independent temperature check (within 2 degrees C)

V = (fD /2fS *)C

ImportantSpeed of sound (C) must be computed

accurately by the instrument.

ADCP Speed Analogy

Picture at time T2

Picture at time T1

S

V=S/(T2-T1)

To measure velocity, ADCPs listen to the returns at two separate times

Phase

Usually ADCPs use PHASE CHANGE to measure the speed, instead of measuring the change in frequency of the wave (how far the cars have travelled)

Phase is the fraction of a wave cycle elapsed relative to a point – or when thought of as the wheel on the left, how much it has rotated

Phase Change – Like measuring how much the wheels on the cars have rotated

Because we don’t know the direction the cars are traveling, we must account for both positive and negative values (we measure a

half rotation either direction) Set the time between pictures (lag) to optimize the tire rotation for

the expected speed– Longer time (lag) = more precise measurement

Too long of time between picture (long lag) may cause the distance car travels to exceeds a half rotation and result in a measurement error called ambiguity error

Short time (lag) limits precision (increased random noise), but decreases possibility of an ambiguity error

Picture 1 Picture 2

What Does This Mean? Lag = Time between pulses in a ping Long lag = accurate measurements Long lag = low ambiguity velocity Exceed ambiguity velocity = ambiguity error Ambiguity error = inaccurate measurements Lag needs to be optimized based on maximum

speed SonTek usually has short lags (no chance for

ambiguity errors but noisy – pictures close together and wheel hasn’t turned much in lower velocites)

TRDI – usually has longer lags that need to be adjusted for conditions (less noise, but chance for ambiguity errors if not adjusted correctly)

Identifying Ambiguity Errors

Correlation

S

How well the two pictures can be alignedIf there is too long of a time between pictures, cars may be in different locations relative to each other, or the to pictures could contain totally

different cars and the distance S may not be determined

Correlation Contour Plot

TransducersProduce sound waves (pulses) and then listen to returning sound

waves

ceramic element protected with a urethane coating

ADCPs use the same transducer to both transmits and receive the pulses.

Measures Velocity Parallel to Beam

Need Multiple Transducer

4 beams can be resolved into: x, y, z and

error velocities

Since each Transducer only measures the velocity component parallelto the beam, multiple transducers are needed

RiverRay forms 4 beams from the single phased array

transducer

M9 only uses 4 beams at a time

to compute a velocity

Velocity Errors

Homogeneous(Low error velocity)

Non-homogeneous(High error or D velocity)

?

The difference betweenbeam pair vertical velocitiesis reported in software as

Error or D Velocity

Error Velocity Contour Error velocities should be

randomly distributed areas of high area error

velocities may occur when water is not flowing at similar magnitudes and direction in all beams (example: turbulence)

Error velocities may also be the result of an instrument measuring one beam velocity wrong

Behind Bridge Pier

Types of Pulses

How the pulses are transmitted into the water and sampled can vary and be optimized for the conditions

This configuration is commonly called “water mode”

Some of the newer ADCPs automatically adjust the configurations for the environment on the fly

Until recently the majority of ADCPs currently in use must be set up prior to data collection

Depth Cells (Bins)

cell 1

cell 2

cell 3

echo echo echo echo

Transmitting

start end

Gate 1

Gate 2

Gate 3

Gate 4 Time

Blank

Bin 1

Bin 2

Bin 3

Bin 4

Distan

ce F

rom A

DC

P

cell 4

Blanking

A B C

Depth Cell

ADCP’s Profile (Ensemble)

ADCP Measured Water Velocity

The faster the boat travels,the faster the velocity of the water relative to the ADCP.

Boat Speed (Bottom Tracking)

ADCP’s can also measure the speed of the instrument or boat by measuring the Doppler shift of a pulse off of the bottom

This is called bottom tracking and assumes that the streambed is stationary

Sediment transport on or near the streambed can affect the Doppler shift of the bottom-tracking pulses, which can result in the measured boat velocity being biased in the opposite direction of the sediment movement. This is referred to as a Moving Bed condition

Depths

Bottom Track pulses are also used to measure depth

Typically 4 beam depths are averaged SonTek also can use a vertical beam

dedicated to depth

Acoustic Profiler Discharge Measurement

A single pass across the river is called a transect, a discharge measurement is usually comprised of multiple transects averaged together

32

Computation of Discharge

Measured Q = ∑(V x A) V = Velocity perpendicular to

boat path for the ensemble A = Depth Cell Size x

Width Width = boat speed x

time since last valid ensemble

Assumption made: the measured boat and water velocities are representative of the boat and water velocities since the last valid data. The longer it has been since the last valid data, the greater the error may be in this assumption

The above is equal to the cross product of the boat and water speed x depth cell size and the time since last ensemble, which is how software computes Q in a depth cell

Unmeasured Top and Bottom

Measured and Unmeasured Areas

DepthValid Ensembles(Profiles)

Top (Estimated) Layer

Middle (Measured) Layer

Bottom (Estimated) Layer

Edges (Estimated)

How To Estimate Top and Bottom Q?

Velocity cross product (X-value), ft2/s2

f1

f3

f4

f5

fn

f2

MeasuredEstimatedPower fit

Free surface

Dis

tan

ce fr

om

the

be

d (Z

), fe

et

Q in the top and bottom unmeasured areas is estimated for each ensemble, based on the measured data

The typical method is to use a power fit of the measured data, but other options are available when this is not valid

0 0(-) (+)(-) (+)

Dis

tanc

e fr

om t

he b

ed (

Z),

fee

t

Unidirectional Flow Bi-directional Flow

Velocity cross product (f-value), ft2/s2

Dis

tanc

e fr

om t

he b

ed (

Z),

fee

t

Power Curve Limitations

Examples of profiles affected by Wind

Ran

ge

fro

m B

ott

om

Ran

ge

fro

m B

ott

om

Water Velocity Water Velocity

Depending on direction, wind can either cause the profile to bend either way at the water surface

The magnitude of this may cause the standard power fit to be a poor choice for top extrapolation, in this case the software has options to only use data near thesurface for estimating the top Q

Estimating Shore Discharges

dm=last measured depth

dm

Measured by ADCP

Measured by User

L = distance from last ensemble to edge of water

L

The averaged measured velocity is multiplied by the averaged measured depth, the measured length, and finally by a coefficient to account for the shape of the edge (.35 for triangle and .91 for square)

Average multiple ensembles to get an accurate depth and velocity

MEASURE the edge distance.

Vm = last measured velocity

Vm

Ideal Reach

From: Water Resources Investigations Report 00-4036. By

K. M. Nolan and ShieldsOnline Training Class SW1271

Computation of Total Discharge

Right Q

Left Q

Bottom Q

Middle Q

Total Q = Left Q + Right Q + Top Q + Bottom Q + Middle Q

Top Q

Where ….Ideal Reach

From: Water Resources Investigations Report 00-4036. By

K. M. Nolan and ShieldsOnline Training Class SW1271

Reach - Straight and uniform for a distance that provides for uniform flow

Streambed - stable free of large rocks, weeks or other obstructions

A poor cross section = poor measurement regardless of the accuracy of your point velocities

Site Selection Still Critical

Reach - Straight and uniform for a distance that provides for uniform flow

Streambed - stable free of large rocks, weeks or other obstructions

A poor cross section = poor measurement regardless of the accuracy of your point velocities

Questions?