Introduction to Acoustic Measuring Equipment Klamath Falls and Chiloquin, OR September, 19 – 23,...
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Transcript of Introduction to Acoustic Measuring Equipment Klamath Falls and Chiloquin, OR September, 19 – 23,...
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 ([email protected])Marc Stewart, USGS, Central Point, OR ([email protected])
Glen Hess, USGS, Portland, OR ([email protected])
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?