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Calibration methods
• Calibration – experiment conducted to determine the correct value of the scale reading of an instrument
• Need to know– Sensitivity– Beam pattern– Signal characteristics
How often?
• Before and after each cruise standard for fisheries applications (legal protection)
• Necessary – Once/year or Twice/year in seasonal differences
How
• Tanks– Reflections (walls, surface, bottom)– Large tanks– Anechoic tanks– Baffles
Calibrated hydrophone
• Need transmitter/receiver with known response over the frequency range of interest
Calibrated transmitter(known signal strength and frequency)
Known range and orientation
Measure on axis sensitivityMeasure receiving beam angle
Calibrated hydrophone
• Need transmitter/receiver with known response over the frequency range of interest
Calibrated receiver(known signal strength and frequency)
Known range and orientation
Measure transmission sensitivity, frequency response, beam angle
Calibrated hydrophone comparison
Range need not be known
Source of unknown
characteristics
Compare levels received
Calibrated hydrophone
• Pros– Simple
• Cons– Requires calibrated transmitter/receiver– Difficult to do at sea– Requires large tank
Reciprocity technique
• Based on electroacoustic reciprocity principle
• To be reciprocal, transducer must be– Linear– Passive– Reversible
– Satisfied by piezoelectric elements
Reciprocity techniqueProjector
HydrophoneReciprocal transducer
Input known voltage
VPH VPT
Input same voltage (VT)
VTH
ResponseH ≈ (VTH VPH/VPT VT)
Two transducer reciprocity
• Two identical transducers (often determined by calibrated hydrophone comparison method)
ResponseH ≈ (VTH / VT)
H V
Self reciprocity• Only need transducer to be calibrated• Perfect reflector
– Flat surface– Metal-backed corprene
• Must use pulsed signals
ResponseH ≈ (VTH / VT)
H V
Perfect reflector
Reciprocity technique
• Pros– Does not require calibrated hydrophone– Self reciprocity good for measuring frequency
response for broadband measurements– Very accurate measurements
• Cons– Lengthy– Requires reciprocal transducer– Difficult to measure beam pattern
Standard target method• Spheres
– Orientation unimportant– Must minimize hardware for attachment
• Pros– Accurate– Simple to apply in the field– Calibration same as field survey set up– Measure
• Combined transmit-receive sensitivity (including gain and noise and frequency response)
• Beam angle
• Cons– Need to control target position relative to beam
Spheres
• Copper or tungsten carbide
• Note difference in units
• TS well understood and easily predicted based on radius and material
Tungsten carbide
Copper
Field set up
Simple field set up
• Split-beam only so can measure position in beam
• Calm currents
Set up for towed body
How far away?
• d largest width of transducer face• f0 is echosounder frequency• c speed of sound in seawater
Ropt = 2d2f0/c
38 kHz, 12º - 5.1 m
70 kHz, 7º - 7.3 m
120 kHz, 7º - 5.2 m
200 kHz, 7º - 3.8 m
Outside near field, but easy
to control sphere position
Calibrating for echo energy integration
• SA or SV correction, also called C
Rt =c(th-tdel)/2 (target range)
C =EtRt2/t
Et = measured from the target spheret = acoustic cross section of the target sphere
Calibrating for single target measures
• TS correction, also called C
C =Et/t
Et = measured from the target spheret = acoustic cross section of the target sphere
Equivalent beam angle • Crucial for echo energy integration• Predictions from theoretical >20% off real measurements• Constant for a given transducer unless damaged• Difficult experiment for single beam transducers• Need to be ±2%• Measurements usually provided by manufacturer
Measured beam patterns from 2 transducers with the same
Multibeam calibration
Calibrating ADCPs
• Tow tank– No current– Seed tank with backscattering particles– Tow ADCP at known speeds in different directions for
relatively long distances– Mostly factory cal’d, not user
• Calibration of gyro-compass• Backscatter measurement not intended
– Techniques to calibrate backscatter counts not established