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 sensitivity Measure 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 technique Projector Hydrophone Reciprocal transducer Input known voltage V PH V PT Input same voltage (V T ) V TH Response H ≈ (V TH V PH /V PT V T )

Two transducer reciprocity Two identical transducers (often determined by calibrated hydrophone comparison method) Response H ≈ (V TH / V T ) H V

Self reciprocity Only need transducer to be calibrated Perfect reflector –Flat surface –Metal-backed corprene Must use pulsed signals Response H ≈ (V TH / V T ) 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 f 0 is echosounder frequency c speed of sound in seawater R opt = 2d 2 f 0 /c 38 kHz, 12º m 70 kHz, 7º m 120 kHz, 7º m 200 kHz, 7º m Outside near field, but easy to control sphere position

Calibrating for echo energy integration S A or S V correction, also called C R t =c(t h -t del )/2 (target range) C =E t R t 2 /  t E t = measured from the target sphere  t = acoustic cross section of the target sphere

Calibrating for single target measures TS correction, also called C C =E t /  t E t = measured from the target sphere  t = 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