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