7.1.1 Standard Targets
7.1.2 Calibration by Range (R) and Voltage (V) measurements
7.1.3 Hydrophones/Projectors
7.1.4 Intership Acoustic Calibration
7.1.5 Live Fish Calibration
The purpose of quantitative acoustic surveys of fish is to measure their abundance. Accurate repeatable calibrations are fundamental to the overall accuracy of results when estimating the size of a fish stock in this way and the required precision for different types of survey is suggested in Chapter 6.
It is necessary to derive a relationship between the output of an echo-sounder and the fish insonified by its transducer beam. The usual technique for processing echo signals from fish is echo-integration, whereby it is assumed that the acoustic energy received after correction for beam spreading and absorption losses is proportional to fish density. To obtain the constant of proportionality it is necessary to know the overall calibration of the echo-sounder ie electrical and acoustical, in addition to the target strength of the fish. The first two sections of this chapter are concerned with methods for acoustic calibration only. The factors involved are the acoustic source level, (SL) the transducer receiving sensitivity (SRT) and the equivalent beam factor of the transducer y. In later sections overall system calibration is considered.
Some common factors to successful acoustic calibration are:
(a) Always try to work in free-field conditions. Within the range of the calibration there must be no effect from boundaries, ie surface or bottom of the water, ships hull, mooring structures etc. The water should be homogeneous and also isotropic, ie having the same properties in any direction.
(b) Knowledge of the water temperature and salinity to correct for absorption losses (Figures 9, 10 a,b,c) so that a calibration for the water path of the acoustic pulses can be made.
(c) An exact measurement of distance between parts of the system for some calibration methods and a knowledge of the speed of acoustic waves (Figure 8) to measure the transit time of pulses.
(d) Transducer measurements can only be made reliably in the 'far-field' unless special acoustic field-sampling techniques for near-field measurement can be applied. Within the near-field large fluctuations of pressure maxima and minima occur (Figure 19) due to the difference in distance to the various parts of a transducer face from a particular point in its field. The minimum distance to which the near-field extends has been defined in eqn. 16, it is also the
Minimum calibration distance = 2L2/l
where
L = the length of the longest side or diameter of the transducer face
l = wavelength of the echo-sounder
both of these in similar units ie mm or m.
Thus, the narrower the beam (long transducer), the greater the distance of the near-field, see Figure 44 for practical distances at 38 and 120 kHz.
Figure 44.
(e) Calm water conditions are very important. Relative movement of any parts of the acoustic system during calibration can lead to extreme fluctuation of the signals.
(f) Noise from whatever source must be very low at the calibration site. It quickly becomes obvious when the presence of noise is preventing a satisfactory calibration.
(g) It is essential that the survey transducer is tuned to be resistive at the echo-sounder operating frequency (this should be the natural resonant frequency of the transducer). Then the transducer plus cable should be tuned to be resistive at this frequency. The principle of this operation is given in section 7.2.1. When tuning is correct, the transmitter is connected to the transducer alone and the power measured. Then, with the cable re-connected the power is measured again and from the two measurements the power loss due to the cable is calculated.
The value of the absorption loss a is sufficiently low at 38 kHz that for short ranges it is insignificant, so when calibrating acoustic survey systems a can usually be ignored at this frequency, (0.0087 dB/m when the temperature is 5°C and decreasing as the temperature increases). At a range of 12 m, the likely error through ignoring a would be 0.1 dB, and the specified target strength of standard spheres for calibration is now claimed to be within ±0.1 dB. At 120 kHz, matters are much more difficult. When t = 22.5°C, a = 40 dB/km, or 0.04 dB/m, so the absorption loss would be 0.1 dB at 2.5 m.
The principle of placing a known target, with an accurate and reliable target strength, on the axis of the beam, at a specified distance from the transducer can be used to obtain a combined figure for SL + SRT, ie the source level and receiving sensitivity, respectively. This method avoids the use of calibration projectors and hydrophones, or relying on electrical measurements, then calculating SL and SRT from formulae. It does require very accurate alignment of the target on the acoustic axis of the beam and the calibration only refers to this axis unless the beam pattern is known.
The following expression is used to obtain SL + SRT when a standard target is available.
SL + SRT = VRT + 2TL - TS (68)
where
SL = source level in dB/1m Pa/1m
SRT = sensitivity of transducer in dB/1 V/1 m Pa
VRT = voltage at the transducer due to standard target echo in dB/Volt
TL = transmission loss in dB, i.e. 20 log d + a d
d = distance between transducer face and target
TS = target strength of calibration sphere in dB.
Apart from the echo-sounder and standard target only an oscilloscope is needed to complete the equipment for this form of calibration. However, when an overall survey precision to ± 0.5 dB is necessary, great care must be taken to ensure that no error is introduced into the calibration experiment. The difficulty lies mainly in the suspension and the alignment of the standard target on the axis of the beam in the far-field. Of course, since the detailed or other necessary arrangements will vary according to the whereabouts of the transducer, it may be more difficult to perform a calibration on a hull-mounted transducer than one fitted in a towed body unless special handling equipment is available.
It is hard to make any form of attachment to a sphere without affecting its acoustic properties. A very fine mesh nylon net bag without knots is the preferred method of holding the relatively small solid copper or tungsten carbide spheres now advocated, (Foote, 1982). Fine mono-filament nylon line can then be used as the suspension material because it cannot trap air, having no interstices.
The spheres recommended for calibration purposes are:
30.4 mm diameter copper sphere with a TS of -40.7 dB at 38 kHz.
38.1 mm diameter tungsten carbide sphere having a TS of -42.4 dB at 38 kHz.
These standard targets are relatively insensitive to temperature changes, small deviations in frequency and change in pulse duration from 0.5 to 1.5 ms.
Ensuring that the target is on the acoustic axis of the beam can prove to be a difficult task. It is usually necessary to arrange for carefully controlled movement of the target in the beam to be sure that the maximum signal is obtained. This will entail a three-point suspension to prevent the target swinging excessively, which would cause the echo-signal to fluctuate. If the transducer is hull-mounted, three hand winches are required and must be operated with some precision to achieve correct positioning. The rigging for measurements can be a critical procedure which requires careful planning and execution. Foote et al. (1981), give the following description of such an exercise:
"The vessel is ideally anchored in calm and sheltered water with a depth of about 50 m. For stable measurements the stern should be tied to land or anchored." This is illustrated in Figure 45.
"Winches to guide and steer 3 lines to the sphere to centre it in the echo-sounder beam are affixed to the deck rails. This is done in accordance with detailed ship drawings. Winch No. 1, is placed in the transverse plane of the ship running through the transducer. The second and third Winches are placed on the opposite side of the boat and at equal distances from the transverse section containing the transducer and winch No. 1. Each winch is provided with a long spool of 0.60 mm - diameter, monofilament nylon marked with small lead weights at five-metre intervals, beginning 10 m from the loose end.
Prior to commencing the sphere measurements, the lines from winches 2 and 3 are drawn beneath the hull to the other winch by means of a line passed under the keel before anchoring.
The appropriate sphere with affixed loop is attached to the three suspension lines, Figure 45. It is then immersed in a solution of soap and fresh water to clean and de-aerate its surface before being lifted overboard by the attachment lines without being touched by hand. The sphere is lowered beneath the vessel to the desired depth, for example, 25 m, determined roughly by counting the lead marker-weights on each line." In fact a range of only 5 m may be satisfactory so long as the target produces a 'clean' echo which is in the far-field (see Figure 44) and the accurate T.V.G. range.
Figure 45. Measurement configuration and sphere suspension during a calibration exercise. (Illustration from FOOTE et. al. 1981).
For proper centering of the target, "The sphere is moved on to the acoustic axis of the transducer by adjustment of the several winches. This operation is coordinated by one person observing the echo waveform on an oscilloscope. The centre is reached when the slightest movement of suspension line either in or out results in a decrease in the echo signal".
The distance from transducer to the sphere is obtained by measuring the transit time of the pulse in milliseconds (ms) observed on an oscilloscope i.e. from start of transmission pulse to start of the sphere echo-signal. Using the appropriate speed of acoustic waves c, (Figure 8) the distance d, is calculated from d = c x time/2.
An example of a standard target calibration is given below.
Step 1 Make sure that the target is on the axis of the beam and that the signal is not fluctuating significantlyStep 2 Avoid close proximity of the target to the transducer, measure distance d
Step 3 Measure the peak-to-peak voltage of the signal from the target
Step 4 Convert the peak-to-peak voltage to an rms voltage (see section 2.3)
Step 5 Calculate SL + SRT from equation 68.
If we assume a distance, d of 3.56 m for (2) and a voltage of one volt for (4) when the target has a TS of -20 dB. In this example a is ignored.
SL + SRT = VR + 2TL - TS = 20 log 1 + 40 log 3.56 - (-20) = 0 + 22 + 20 = 42 dB
Thus the system may be calibrated without knowing the individual performance of either the transmitter or transducer. There is a need to be cautious about the signal voltage measurement because of the practical difficulty of making such a measurement directly across the transducer terminals or the cable of a towed transducer. The problem is two-fold, the usual inaccesibility of the terminals, but there is also distortion of the oscilloscope trace caused by the high voltage of the transmission pulse in relation to the small signal voltage being measured.
A practical alternative is to use the calibrated signal output of the receiver amplifier (VR) because the transmission pulse is reduced to an acceptable level in the early stages of the amplifier. In this case the gain law (TVG) and any 'static' gain must be known very accurately at the range of the target, because the rate of change is very large at short range. Static gain (SG) is defined as any gain switched into the receiving system which is not part of the actual Time Varied Gain.
If the TVG system gain is known precisely at the calibration range, TL can be omitted from eqn 68 but any static gain must be subtracted, ie SL + SRT = VR - SG - TS. A summary of the main factors to be considered in relation to standard target calibration, i.e. their advantages and disadvantages is given below.
Advantages
a) The method is relatively simple and reliable now that accurate standard targets are easily obtainable.b) A standard target is free of acoustic noise and fish behaviour problems which can be troublesome when calibrating on live fish targets.
c) Because of the repeatability of the method, it can be used for accurate monitoring of performance changes in the overall system.
d) Inter-ship calibration can be carried out irrespective of geographic position.
Disadvantages
a) The method cannot take into account the beam pattern.b) It may be difficult to align the standard target on the acoustic axis, especially with hull-mounted transducers. Needs very calm water, practically free of current. The vessel may also have to be anchored both from the bow and stern to avoid undesired movements. This problem can be significantly reduced when a dual beam method is used.
c) In practical calibrations it has been noted that fish are attracted by shiny standard targets, particularly tungsten carbide balls so care must be taken to avoid fish spoiling the calibration.
Calibration by standard target can include the receiver amplifier. In this case the combined calibration parameter becomes
SL + VR, where VR = SRT + G
G is the effective amplifier gain in dB.
However, the entire system includes the echo-integrator so an overall calibration should be related to the output, M mm of deflection, of this instrument.
10 log M = SL + VR + TS - TL + ct /2 + 10 log Gi (69)
Gi is the integrator gain.
This method by Robinson and Hood (1982) is only suitable for the calibration of transducers in towed bodies. In addition to the survey transducer mounted in its towed body with the towing cable connected it is also necessary to have a projector and a hydrophone. It is not necessary to know the characteristics of the projector or the hydrophone, neither do they need to be reciprocal devices.
The three components of the calibration: projector, hydrophone and survey transducer must be placed in a triangular formation, one at each point of the triangle, Figure 46. They must be at the same depth; the survey transducer must be mounted so that it can be rotated to transmit to the hydrophone, and the projector arranged so that it can be aligned to transmit to the hydrophone.
Figure 46.
The procedure is as follows:
Step 1 survey transducer transmitting to hydrophone: measure the voltage received at the hydrophone, convert to rms, call this VH.Step 2 switch off the survey transducer.
Step 3 transmit from projector to the hydrophone, measure the received voltage (VHP), convert to rms, call this VHP.
Step 4 rotate projector into alignment with the survey transducer and measure the voltage received, convert to rms, this is VP.
Step 5 note the exact range between the survey transducer and the hydrophone RH; the range between the projector and the hydrophone RHP; the range between the projector and the survey transducer, RP.
Step 6 obtain a figure for the absorption loss a.
It is then possible to calculate SL + SRT.
SL + SRT = 20 log(VH.VP)/VHP + 20 log(RH.RP)/RHP+2a (RH+RP-RHP) (70)
As with all similar measurements of voltage and range it is essential to ensure that the test equipment is precisely calibrated and that extreme care is taken in reading the oscilloscope.
Calibration against known hydrophones and projectors is not a preferred method in fisheries acoustics because of the lack of stability with time of these devices. The use of hydrophones and projectors is included for completeness and as a guide to measuring the separate performance of transmitting and receiving systems.
Hydrophones and projectors have known calibration factors which when these devices are used to measure energy transmitted from an echo-sounder, or to transmit to the echo-sounder, can be used in a form of the acoustic equation to obtain the equipment calibration. Although the same device may be used for both receiving and transmitting, this practice is not recommended for the calibration of fisheries acoustic equipment. Ideally, the calibration device should be rigidly fixed in relation to the face of the transducer to be calibrated, but with the need to observe a minimum distance between the two, to ensure that operation is in the far-field, Figure 44, this may pose a difficult problem.
Measurement of Source Level (SL)
This measurement requires the following items.
1. Calibrated hydrophone complete with sufficient length of cable.
2. Oscilloscope.
3. Temperature measurement.
It is assumed that the echo-sounder transmitter is set to normal pulse duration (as used for survey) and standard power output. The general arrangement for this type of calibration is shown in Figure 47 and the principle is applicable to hull mounted as well as towed transducers. As the distance (d) between the transducer face and the hydrophone is known, a time base setting for the oscilloscope can be calculated assuming an acoustic speed of 1500 m/s i.e. 1.5 metres/millisecond if d = 3 metres time = 3/1.5 = 2 ms, then the timebase speed could be set at about 0,5 ms/cm. Having once set the timebase coarse control to put the signal into a suitable position on the CRT and checked that the timebase is in calibrated mode, the precise time to the beginning of the signal is read off. The true depth, range or distance for calibration is d = c x time where c = speed of acoustic waves (c must not be assumed to equal 1500 m/s). The speed should be obtained from Figure 8 when the temperature and the local salinity have been measured.
Figure 48. Measuring SRT. (A) projector SL NOT know
Example: t = 10°C, S = 30‰ so c = 1482 m/s assuming the actual time read from the oscilloscope timebase is 2.4 ms, the true distance is calculated as d = 1482 x 2.4 x 10-3 = 3.56 metres.
If the received pulse is 3.4V peak-to-peak, we see from section 2.3 that it must be converted to rms as follows: (3.4/2) x 0.707 = 1.2 Vrms then converted to dB/Volt, i.e. 20 log 1.2 = 1.58 dB/V.
The source level of the echo-sounder can now be calculated
SL = VRT - SRT + 20 log d
it is assumed that a can be neglected, but this is not always the case
where
VRT = hydrophone output voltage in dB/V
SRT = hydrophone sensitivity from its calibration graph (in dB/Volt/1m Pa)
d = distance between hydrophone and transducer in metres
SL = source level in dB/1m Pa/metre
Assuming a figure for SRT of -208.5 dB/V/1m Pa
SL = 1.58 - (-208.5) + 20 log 3.56 = 210 + 11
SL = 221 dB/1 m Pa/m
For measurements made at angles to the acoustic axis the source level will be lower. Accurate results with the echo-integration technique require a true beam pattern to be measured at a sufficient number of points in each plane to produce a three-dimensional plot. This requires very special facilities and manufacturers figures for beam angles must normally be used to calculate a figure for y. The on-axis source level is used in the final calculation of biomass.
For the simplest method the test hydrophone used for SL measurement is transferred to a position close to and in line with the face of the survey transducer under test (not over its face). A projector is then fitted in the position previously occupied by the hydrophone, see Figure 48. This projector is supplied with a suitable drive voltage at the correct frequency and pulse duration. It is not necessary to know the projector calibration nor the amount of power applied to it for this method. The procedure is as follows:
Step 1 From the oscilloscope read the peak-to-peak voltage received at the terminals of the echo-sounder transducer under test, call this V1, assume V1 = 18 mVStep 2 Let the voltage measured across the hydrophone terminals = V2
V2 = 0.6 mVStep 3 Take the ratio V1/V2 = 18/0.6 = 30
20 log 30 = 29.54 dB.
Figure 49. Measuring SRT. (B) projector SL known.
We know that the hydrophone SRTh = -208.5 dB/V/1m Pa and from the measurements above it is clear that the survey transducer is 29.54 dB more sensitive than the test hydrophone.
SRT = SRTh + 29.54 = -208.5 + 29.54 = -178.96 dB/V/1m Pa
Measurement of Receiving Sensitivity. Method (B)
This method does not require the test hydrophone but the source level of the projector SLp must be known, or obtained. Calibration is usually presented in the form where a given driving voltage, or current, applied to the projector, will result in a particular source level
ie XdB/1 m Pa/metre/per Volt or Amp
(current is more difficult to measure accurately than voltage).
The physical arrangement for the calibration is shown in Figure 49. If the test projector does not have its own transmitter, take the following steps:
Step 1 disconnect the echo-sounder transmitter from the survey transducer
Step 2 connect this transmitter to the test projector
Step 3 before switching the transmitter ON, reduce power to at least 1/10 of normal.
SRT of the survey transducer = VRT + 20 log d - SLp (71)
A typical calibration figure for the test projector might be 120 dB/1m Pa/m/V.
If the rms voltage from the transmitter is 30V
SLp = 120 + 20 log 30 = 149.5 dB/1m Pa/m
The oscilloscope is then used to measure the voltage received across the terminals of the survey transducer, eg 26.7 mV, peak-to-peak.
Vrms = (26.7/2) x 0.707 = 9.4 mV
VRT = 20 log 9.4 x 10-3 = -40.5 dB/V
SRT = -40.5 + 20 log d - 149.5
if d = 3.56 m, 20 log d = 11 dB
SRT = -40.5 + 11 - 149.5 = -179 dB/V/1m Pa.
It should be noted that section 7.1.3 is included so that all of the calibration methods can be understood. Although they suffer from the inherent poor stability of wideband devices, the test projectors and hydrophones do allow a separate assessment of receiving and transmitting characteristics of the survey transducer to be made.
In the previous sections of this chapter we have looked at methods for the calibration of parts of the survey system, or the complete system. Despite great care in making these calibrations it is often uncertain how the results from one vessel compare with those of another. Now that electronic units are very stable and standard targets of high accuracy are available for on-axis calibration it should be possible in theory to obtain very close agreement between acoustic survey systems on different ships. One problem is lack of knowledge of the actual beam pattern, a vitally important factor for the echo-integration technique. The concept of checking calibration of the complete acoustic equipment i.e. echo-sounder and echo-integrator on one vessel against that of another is simple and attractive in principle but it contains practical difficulties.
It would be ideal if the vessels could travel side-by-side over an area of static targets whose numbers (density) increased or decreased gradually along the survey track. This would allow direct comparison of results over the full dynamic range of signals entering the echo-sounder. Such a situation is not possible and for intership calibration we must consider the following factors.
1. Is there a suitable fish stock aggregated at a moderate density over a significant area (several nautical miles)?2. Are the fish likely to be scared?, e.g. are they too close to the surface? (the effect of scaring may be small but variable from ship to ship).
3. If the fish are in discrete schools how large and how mobile are these? These questions are particularly important if schools are scarce, one vessel may sample a significantly different part of the school to the other vessel, simply because of the time lapse or distance between them.
Intership calibration procedure is commonly carried out for the purpose of comparison, or the direct transfer of results from one accurately calibrated research vessel to another vessel (or vessels). This is desirable because experience shows that considerable time, effort and repeated testing is necessary before the engineer/scientist is satisfied with the level of accuracy and stability he wants to attain with calibrations prior to actual fish density measurements at sea. In the case of multi-ship surveys the question of accuracy may not predominate, and inter-ship calibration may merely compare different equipments to establish a consistent and reliable data collection for subsequent analysis and the plotting of charts showing relative abundance of fish. An intercalibration between two or more vessels is based on the fundamental assumption that the respective acoustic systems are
(a) echo-sounding on the same fish aggregation or
(b) echo-sounding on an equivalent sea bed.
i) Inter-ship calibration on fish layers
This has been practised for some years especially by the Institute of Marine Research, Bergen, Norway (Røttingen, 1978). Also, Icelandic and Norwegian research vessels have intercalibrated on dispersed fish concentrations as part of a joint survey of the Icelandic capelin stock (Vilhjalmsson et al., 1982). Such calibrations have occasionally been carried out in FAO projects since 1973 although not reported. Using the same notation as before, the mathematical basis for the intercalibration is
(72)
(73)
On the assumption that: 
,
i.e. both vessels will on average measure equal fish density, the scaling factor
for the non-calibrated vessel can be calculated directly from
(74)
In other words the vessel subjected to calibration will have a scaling factor
(C') which is equal to the scaling factor (Cc) of the pre-calibrated
vessel after multiplication by the ratio between mean integrator values
.
Thus, two factors become apparent
(a) the validity of this kind of inter-ship calibration depends entirely on
the fish stock, ie if its geographic extent and dispersion permits
(b) if the M-values obtained are wide ranging, the simple arithmetical mean
may not be suitable and C'
must be established through regression analysis of the data. Røttingen
(1981) shows inter-ship calibration results between R.V. G. O. SARS and R.V.
JOHAN HJORT, first using a standard copper sphere and subsequently repeated
on a scattering layer of capelin in the Barents Sea. Differences in results
varying from approximately 17-23% depending on the depth of the different fish
layers are discussed but not satisfactorily explained. An overall appraisal
of the results gave the two calibration methods equal weight and 'C' was taken
as the mean value of both experiments.
An example of good calibration results is presented in Vilhjálmsson et al. (1981) and illustrated in Figure 50a and b. The wide spread in M-values is clearly demonstrated so the data are well adapted for regression analysis. A confidence interval for the estimated ratio (MGOSMBS) is not given but a high correlation (r = 0.98) points towards a relatively high-precision inter-ship calibration.
The field procedure for the inter-ship calibration can be summarised by three steps
Step 1 Locate the target stock and determine the time of the day when its distribution pattern is suitableStep 2 Put all equipment in to normal operational mode as for actual survey work
Step 3 Run the two vessels at a fixed inter-ship distance and over a sufficient area (or length of time) obtain representative sets of M-data.
In regard to (3) optimum vessel speed should be determined and the geometry of the inter-ship positions related to the experimental aim. If close simulation of survey conditions is important the vessel's speed should be the same as during a normal survey. This would dictate a larger inter-ship distance (to avoid undesired effects of noise and vessel wakes and to maintain navigational safety). But, the possibility of the two vessels insonifying the same, or equivalent, fish layer(s) would be reduced. There are conflicting criteria and optimum arrangements can only be established on the basis of repeated field experiments. Success of such experiments depends on several factors, the vessel size and noise level, depth distribution of the fish layer, geographic extent and mobility or the fish stock; its biological characteristics and sensitivity (fright reaction) to a moving vessel, as well as weather and sea conditions.
Fig. 50. (a) Integrator values vs. distance. 4 August 1979
Fig. 50. (b) Linear regression of integrator values. October 1981
As an approach to the experimental design, three different inter-ship sailing patterns are illustrated in Figure 51 and relate to different vessel speeds; (a) appears to give best results in Norway (Røttingen, 1982); (b) may give good results for smaller vessels and when the target stock is relatively stationary; (c) refers to a situation when the available fish layer has a very limited geographic distribution and may have to be criss-crossed several times to obtain the necessary data. The purpose of reversing vessel positions is to check if vessel noise and possible wake effects will influence the measurements. Compared to the standard target calibrations this method has the advantage that it utilizes natural targets in real-life survey conditions and it includes all equipment parameters. It can be used to intercalibrate two integrator systems of different frequency. However, the two vessels do not integrate exactly the same targets so the assumption of equal densities may, or may not, be justified.
ii) Inter-ship calibration on the sea-bed echo
The nature of the sea-bed as an acoustic scatterer is described in Urick (1975). As a 'standard target' for inter-ship calibration (or transducer comparison) it is attractive for the following reasons:
a. it is an effective reflector and acoustic scattererb. little or no frequency dependance appears to exist for 'rock', 'sand and rock', and 'silt and shell' bottoms
c. different depth ranges or even variable depths can be selected
d. normally, it can be considered a stable echo reflector for months or years
e. it has practically no geographic limits.
Published results on the use of the sea-bed as a target for intercalibration purposes are few. However, this type of calibration has been practiced over the years in FAO projects, for example in the FAO/NORWAY Regional Acoustic Research Centre for Latin America, based in Lima, Peru from April 1975 to August 1980. Numerous comparative calibrations of transducers were made with the sea-bed as a reference target and also to determine the difference in performance of two integrator systems alternatively switched to the same transducer.
Experience from these calibrations suggests that because of the relatively low (say, 8-16%) coefficient of variation associated with bottom echo integration (and hence independence of geographic variability within a properly selected calibration area) the sea bed is a useful 'standard target'. It is suitable for the intercalibration of two transducers, two integrator systems using the same transducer, or two, or more ships. The data collected by the FAO/NORWAY Centre did not permit detailed quantitative analysis of the relative merits of the different targets (spheres, natural fish distributions, caged live fish and the seabed), but it did greatly encourage the use of the seabed as one approach to inter-ship calibration. Some examples are given below:
Example 1: Inter-transducer Calibration in Indonesia
Place: Bitung Harbour, North Sulawesi
Date: April 29, 1982
Vessel: R.V. TENGGIRI
Target: Seabed echo from 23-27 m
|
EKS-120 echo-sounder |
QM-MK-II Integrator | ||
|
Gain: |
-20 dB/20 log R |
Gain: |
20 dB |
|
Pulse: |
1.0 ms |
Expander: |
x 10 |
|
Power: |
1/1 |
Range gate: |
23-27 m |
|
Range: |
0-50 m |
Others: |
as proper and convenient |
Results
|
External Transducer |
Ships's Transducer | ||||||
|
(120 kHz, 10 cm F) |
(120 kHz, 10 cm F) | ||||||
|
Channel A |
Channel B | ||||||
|
Obs. |
M |
Obs. |
M |
Obs. |
M |
Obs. |
M |
|
(No.) |
(mm) |
(No.) |
(mm) |
(No.) |
(mm) |
(No.) |
(mm) |
|
1 |
516 |
9 |
508 |
1 |
306 |
9 |
350 |
|
2 |
515 |
10 |
562 |
2 |
375 |
10 |
348 |
|
3 |
437 |
11 |
472 |
3 |
327 |
11 |
348 |
|
4 |
467 |
12 |
480 |
4 |
332 |
12 |
334 |
|
5 |
443 |
13 |
512 |
5 |
337 |
13 |
315 |
|
6 |
479 |
14 |
473 |
6 |
354 |
14 |
310 |
|
7 |
523 |
15 |
503 |
7 |
408 |
15 |
369 |
|
8 |
494 |
|
|
8 |
364 |
|
|
|
Mean value: |
Mean value: | ||||||
|
Coeff. of variation: C.V. = 6.6% |
Coeff. of variation: C.V. = 7.8% | ||||||
The external transducer suspended on a nylon rope was lowered into the water close to the front of the hull-mounted transducer so as to give exactly the same depth sounding. These transducers were alternatively switched to the echo-integrator system to obtain comparative integrator values corresponding to 500 soundings, controlled by an electronic counter.
It was noted that during the measurements the vessel was slowly moving over the bottom about its anchor chain so, for practical purposes, the assumption can be made that the two transducers were sounding on the same target. The ratio between the transducers was estimated as
a confidence interval for the ratio was calculated from C.I. = R ± t Ö Var (R)
where t = 2.15 corresponding to 95% confidence probability and 14 degrees of freedom, and Var (R) is calculated from
(75)
Thus yielding the confidence limits
Upper limit: Ru = 1.497
Lower limit: Ri = 1.355
corresponding to ± 5% from R = 1.462, suggesting that inter-transducer calibration (or inter-ship) on the seabed can be carried out with fairly high precision. Similar calibrations have been made, e.g. in FAO projects in Peru, Uruguay and Brazil with coefficient of variation (C.V.) ranging from 5-15%.
Example 2: Inter-system calibration on anchovy layer
Place: outside Matarani Harbour, Southern Peru
Date: June 9, 1975
Vessel: R.V. SNP-1
Target: fairly uniform anchovy layer, 10-40 m depth
The 120 kHz integrator systems were intercalibrated, one using the hull-mounted transducer while the other was served by a similar transducer in a towed body of the 'Shark' type. The comparative (M) measurements are tabulated below in millimetres.
|
|
|
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
|
System |
I |
114 |
88 |
121 |
128 |
55 |
50 |
103 |
134 |
54 |
72 |
50 |
64 |
|
System |
II |
56 |
39 |
55 |
102 |
21 |
15 |
78 |
80 |
33 |
43 |
29 |
46 |
From the above data we obtain
= 86.1 with C.V. = 37.8% and
= 49.8 with C.V. = 52.2%
In the above case the vessel was steaming at approximately 8 knots and M-readings (mm) were taken every 6 minutes. It is also important to note that pulse durations were the same at 0.3 ms.
When comparing the results of this experiment with those in example 1 it becomes clear that even a fairly uniform fish layer yields much higher coefficient of variation than obtained from seabed calibrations.
Example 3: Inter-system calibration on caged live fish
Place: Islo Lobos de Afuera, Peru
Date: March 21, 1977
Vessel: R.V. TAREQ II
Target: live sardine of size range 6-9 cm and density corresponding to 23,670 tonnes/mile2
A pulse duration of 0.3 ms was used as for the calibration on the anchovy layer (example 2) but each integrator reading was the accumulation of 1000 sounding integrals instead of 576 as before.
|
|
1 |
1 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
|
M38 |
345 |
365 |
354 |
378 |
375 |
403 |
353 |
284 |
302 |
334 |
352 |
331 |
|
M120 |
126 |
138 |
133 |
148 |
143 |
153 |
160 |
153 |
142 |
157 |
161 |
131 |
Giving the mean and C.V. values:
|
38 kHz system: |
|
|
120 kHz system: |
|
The above values of the coefficients of variation are similar to those obtained from seabed intercalibrations but again much lower than associated with intercalibrations on a natural fish layer. In some areas (e.g. West Africa, data from the FAO/UNDP Regional Fisheries Survey of W. Africa) plankton layers of highly uniform density and thickness are available for intercalibration. While such layers offer low variability, they can only be used to intercalibrate systems of the same working frequencies because their volume backscattering is likely to differ significantly to that of the fish layers of interest.
There are many problems in the determination of target strength for particular species and size ranges of fish. Complex experiments to make the necessary TS measurements are beyond the terms of reference and often the scope of FAO projects, so a method of calibration has often been adopted which uses a known number, or quantity of fish. The purpose of this calibration is to obtain the constant 'C' to convert the echo-integrator reading into fish density using the relationship r = CM from Midttun and Nakken (1971). Results can then be expressed in tonnes/square mile of sea surface area.
This type of experimental calibration requires careful organisation and preparation; in particular, a net of suitable material (acoustically transparent) and dimensions must be constructed. Through experience a design has evolved which is known as the standard cage, it has a volume of 6 m3.
Fish must be caught in sufficient numbers and kept in good condition ready for transfer to the 'standard cage' either to start the calibration, or when the density of fish needs to be increased. Success for this type of calibration is critically dependent on fish behaviour, a factor which varies with species and environmental conditions. Fish may need to adapt to a shallow depth, they may be sensitive to strong light, exhibit unnatural swimming patterns, or show distress if they are too roughly handled, or become too densely packed. Regular observation is important to ensure that the measurements do not become unreasonably biassed through behavioural factors. This is often possible by means of underwater closed-circuit television or by direct observation using divers. If the cage is open at the top and it projects out of the water, handling of fish from the keep net into the cage is simplified. A calibration system of this type, using two different frequency echo-sounders simultaneously is shown in Figure 52.
The transducer/s must be fixed with each beam exactly vertical and its axis perpendicular to the centre of the cage. The active face of the transducer must be at a sufficient distance to place the fish targets in the far-field, see Figure 44. Positioning should be such as to avoid any interaction between the main beam (or sidelobes) and the framework of the cage. This means that the cross-sectional area of the beam, perpendicular to its axis, should be smaller than any internal cross-sectional area of the cage but it should insonify the maximum possible volume within the cage. It is desirable to know the beam pattern so that with slight adjustment of the transducer, up or down, or even tilting, the effect of sidelobes interacting with the framework may be minimised.
The minimum operating range of the TVG is critical; for marine systems it is about 3 m. At short ranges, the rate of change of gain with time is at its greatest so the TVG law must be known precisely, even over the distance of a 1 m integration interval for cage experiments, there is a significant change. Because the greatest rate of change occurs at short range the maximum deviation to the gain/range law is likely and this may lead to serious errors unless taken into account.
Before starting calibration measurements, the instruments must be given time to 'warm up' so that their functions can stabilise, eg timebase, TVG, integrator drift, etc. Then a note must be made of all the control settings, remembering that there may be a need to adjust 'static' gain during the initial test period before starting to run the experiment in order to avoid amplifier saturation.
Figure 52. FAO Calibration Method
tonnes/nautical mile2
|
Echo-sounder |
|
|
Gain |
dB |
|
TVG |
20 log R |
|
Power |
1/1 |
|
Pulse |
ms |
|
Bandwidth |
|
|
Range |
m |
|
Echo-integrator |
|
|
Gain |
dB |
|
Threshold |
|
|
Depth |
m |
|
Interval |
m |
|
Speed comp. |
10 k |
|
Expansion |
|
If a single set of observations are obtained, the calibration constant Cc is
tonnes/mile2/mm
where
N = number of fish within the calibration cage
v = effective volume of the cage in m3
D Rc = integrator range gate in mmean weight of fish in the cage
3.43 = conversion factor to raise km2 to nautical miles2
M = the integrator reading in mm
In a more general form
Cs = Cc(Sc/Ss)(Pc/Ps) 10-0.1(D Gi+D Gee)
in log form
10 logCs = 10 logCc + 10 log(Sc/Ss) + 10 log(Pc/Ps)-D Gi-D Ge
where
Cs = the constant related to actual control settings during survey.
Sc/Ss = ratio of the number of 'pings' calibration/survey
Pc/Ps = numerical relation between pulse duration, calibration and survey
D Gi = Gs - Gc = difference (dB) in integrator gain, calibration and survey
D Ge = Gs - Gc = difference (dB) in echo-sounder gain, calibration and survey.
Rc and Cc as previously given.
7.2.1 Transmitter/transducer
7.2.2 Receiver/Time Varied Gain
7.2.3 Time Varied Gain Amplifier Calibration
7.2.4 Integrator
Calibration of the electronic circuits of the echo-sounder and echo-integrator does not remain constant with time. It is just as important to check these as it is to calibrate the acoustic parts of the system. Electronic calibration checks are in principle easier to perform than those of acoustics because the difficulties associated with water as a propagation medium do not apply.
The test instruments needed for any calibration are discussed in section 3.4 and they will be referred to without further explanation in the following sections. Examples of test and calibration procedures refer to the Simrad EK scientific echo-sounders and the QM echo-integrator but are similar in principle for any equipment of this type.
The following must be checked to ensure correct functioning of the transmitter.
(a) Transmitter Output pulse amplitude (see (i) below)
(b) pulse duration (see (ii) below)
(c) carrier frequency (see (iii) below)
(d) receiver self noise (measured in position 'test').
The usual transmitter performance tests are (a, b, c) but (d) is a part of the receiver test procedure, it is mentioned here because a 'noisy' transmitter, especially if it is due to the output stage, may be the cause of a high noise level in the receiver. Even though this fault exists, the transmitter may operate satisfactorily to the specifications measured under points a, b and c. Measurement of receiver noise is given in section 7.2.2.
i) Transmitter output pulse amplitude
Take the following steps;
Step 1 Set the Test/Operate selector to 'TEST' and switch on the echo-sounder. Select longest duration transmitter pulse (a 60 ohm dummy load is connected to the transmitter output in the TEST position). NOTE: The position of the TEST/OPERATE switch should not be changed whilst the echo-sounder is working or the output stage may be damaged.Step 2 Connect the oscilloscope across the dummy load and measure the voltage Vp-p as shown in Figure 53 below.
Figure 53.
Typical measured values are
|
Transmitter output power control |
Echo-sounder frequency |
|
|
38 kHz |
120 kHz |
|
|
1/1 |
35 VP-P |
28 V |
|
1/10 |
10.5 VP-P |
9.5 V |
The signal is attenuated by about 25 dB, or about 17 times in amplitude before appearing at the TEST SIGNAL socket.
The power may be calculated from
P = V2/R where V = the 35V measured x 17 to raise to its actual peak-to-peak value. But in order to get the rms value (see section 2.3) the p-p voltage is divided by 2.828.
V = (35 x 17)/2.828 = 210.4 V
P = (210.4)2/60 = 737.8 W
Note: this is the power into the dummy load - not into the transducer.
It is instructive to calculate the change in power for a small change of voltage, eg if Vp-p increases by 2.5 V, P = 847 W.
ii) Measurement and adjustment of the pulse duration
Measure the duration of the pulse in each of the four positions of the 'bandwidth and pulse duration' selector. These should be within ± 5% of the nominal values which should occur within the limits t p to t R as shown in Figure 53.
|
Pulse duration selector |
Frequency of echo-sounder (kHz) |
|
|
38 or 50 |
120 |
|
|
Position |
pulse duration (ms) |
|
|
1 |
0.3 |
0.1 |
|
2 |
0.6 |
0.3 |
|
3 |
1 |
0.6 |
|
4 |
3 |
1 |
The pulse duration selector on the Simrad EK model, is marked with the numbers 1-4 as shown in the table. On the EK-R and EK-S models, the control is marked with the nominal pulse duration. The EK-R and EK-S models are equipped with a trimming potentiometer (R01, PCB 25H43) for adjustment of the pulse duration. This potentiometer affects all four pulse durations in equal proportion. The 0.6 ms pulse is adjusted to its nominal value at the factory and other pulses then normally come close to their nominal values. If separate calibration of each pulse is necessary, the capacitors C104-C107 on the pulse duration selector must be changed.
If it is found necessary to make a correction in the EK model, components must be replaced (R 601 on the transmitter board (PCB 81...6) or C508-C511 on the pulse selector). Changing the resistor value alters all four pulse durations in the same proportion, while replacement of capacitors permits individual correction for each position.
iii) Measurement and adjustment of carrier frequency
The oscillator operates only for sufficient time to produce the transmitter pulse and this has such a short duration that it is not possible to use an electronic counter to measure oscillator frequency directly. Under NO circumstances should the oscillator be forced to run continuously or the output transistors will be damaged. It is necessary to measure the oscillator frequency under normal operating conditions of the echo-sounder, but during any essential adjustment of frequency, no test probes and/or instruments should be connected directly to the oscillator circuit, this is likely to change the operating frequency.
Checking and adjusting the oscillator (carrier) frequency:
Step 1 Set the oscilloscope controls for 'external horizontal' operation. Connect the SG to the external 'X' deflection terminals.Step 2 Adjust the output level from the signal generator and the horizontal sensitivity of the oscilloscope until a horizontal line of approximately 4 divisions is presented on the oscilloscope screen.
Step 3 Position the line in the centre of the screen by adjusting horizontal and vertical position controls.
Step 4 Start the echo-sounder. For each transmission a vertical deflection is observed. Increase the intensity or brilliance, until the flashes due to this deflection are clearly visible on the CRT. If the oscilloscope is equipped with a storage mode this may be used by adjusting the persistence control so that each transmitter pulse is visible for the maximum possible time, but is erased before the next pulse. Be careful to keep the intensity as low as possible to avoid damage to the storage screen.
Step 5 Adjust the vertical sensitivity until the deflection is approximately equal to the horizontal deflection (4 divisions). The trace on the oscilloscope should now be an approximate square appearing once for each transmitter pulse. If, by chance, the frequency of the signal generator happens to be exactly the same as the oscillator frequency, a Lissajou figure is observed Figure 54.
Step 6 If this is not seen, slowly adjust the frequency of the signal generator, which should be close to the nominal frequency of the echo-sounder, watching the oscilloscope screen. Adjust until the Lissajou figure is as crisp and clear as possible (see Figure 54b and c), when the transmitter oscillator will be equal in frequency to the signal generator. The latter can then be switched into CW mode and measured by the electronic counter. The accuracy of this indirect method of measuring the carrier frequency of the echo-sounder transmitter increases with the duration of the transmitter pulse, therefore the longest pulse duration should be used. Working with extreme care an accuracy of ± 0.05% or better may be obtained.
Figure 54.(a)
Figure 54. (b)
Figure 54. (c)
If adjustment of the oscillator frequency is necessary, take the following steps:
Step 1 Switch off the echo-sounder and wait for 20 seconds.Step 2 Unplug the transmitter board (PCB 25H43)(EK-71-6).
Step 3 Plug in the extension board in the place of the transmitter and then fit the transmitter board into the socket at the top of the extension board. The core of T01 (EK-T601) is now accessible for adjustment of frequency.
Step 4 Switch on the echo-sounder.
Step 5 Set the frequency of the signal generator to the nominal frequency of the echosounder and then adjust the core of T.01 (EK-601) until the Lissajou figure, as illustrated in Figure 54(c), is obtained.
Step 6 Switch off the echo-sounder and wait for 20 seconds.
Step 7 Remove the extension board and the transmitter.
Step 8 Replace the transmitter board in its original socket.
Step 9 Repeat the frequency check.
Note: It might be observed, particularly with the 120 kHz sounder, that the oscillator frequency alters slightly when the extension board is removed. This may be compensated for by setting the frequency of the signal generator a little to the side of the nominal frequency while tuning the oscillator core. The checking and adjustment procedure is then continued until the frequency is correct with the transmitter in its normal socket.
iv) Transducer
It has already been shown in this section that very little change in voltage output from the transmitter causes a significant change in power output. However, it is also important that the transducer should present a constant loading (which is resistive) to the transmitter. Transducers are electroacoustic devices and the acoustic conditions at the transducer face determine the electrical characteristics which are measured across the terminals. In some instances paint on the face will significantly affect the transducer impedance, changing the amount of power accepted from the transmitter, hence the acoustic source level because the efficiency of the transducer is reduced. A fairing over the transducer face on a towed body can affect the transducer impedance. Magnetostrictive types of transducer need long soaking periods before any form of measurement is made but all transducer faces must be carefully degreased for calibration purposes.
Transducers must be electrically matched to the cable/transmitter combination. This also involves tuning but both processes are specialised and are usually arranged or taken into account by the echo-sounder manufacturers. To check that the matching and tuning are correct it is necessary to know
1. that the current and voltage are in phase
2. there is no significant distortion of either waveform.
Photographs of actual waveforms appear in Figure 55 below.
Figure 55. Superimposed voltage and current wave-forms
Figure 55. Part of (a) expanded
These matters can be seen if the relevant waveforms are compared on an oscilloscope. If the power into the transducer is to be measured the radiation resistance RR, or impedance must be known accurately.
To check 1 and 2 above, obtain current and voltage probes and check that these are matched to the dual input oscilloscope.
Connect the current probe over one of the insulated conductor wires of the transducer cable. Connect the voltage probe across the two conductor wires of the transducer cable. Adjust the timebase/s of the oscilloscope so that both current and voltage waveforms start at precisely the same instant, then examine each individual waveform for distortion. It is difficult to estimate percentage distortion but an attempt should be made to do so and an allowance then made when calculating power. Severe distortion indicates that a fault or faults exist. Next compare the voltage and current waveforms by adjusting their vertical position until one is superimposed on the other. Inspect the waveforms to see that peaks and troughs are co-incident, ie in-phase. If they are not, measure the phase difference and note it down.
The intention in this section is to briefly mention the important factors which must be correctly set if the signals are to remain proportional to the echoes.
Items of test equipment needed
1. Oscilloscope and calibrated probes
2. Signal generator
3. Multi-meter
4. High impedance voltmeter.
These items of equipment must themselves be in a fully calibrated condition as discussed in section 3.4.7.
NOTE A: Before making adjustment to any part of the equipment it is essential to be certain that the adjustment is absolutely necessary. Always re-check the control settings of the unit being tested, and those of the test equipment itself before carrying out an adjustment.NOTE B: It is advisable to switch off the echo-sounding system, and to wait about 20 seconds before removing, or installing a printed circuit board (PCB). The equipment should never be switched ON whilst any board is out of its socket.
i) Receiver Bandwidth
In acoustic systems used for quantitative measurements it is important that the transmitter generates a well-shaped pulse, as square as possible, and that the receiver system has the necessary bandwidth to preserve this shape. Theoretically, if a perfect square-wave is transmitted and its shape is to be preserved in the receiver, the bandwidth of the receiver must be infinite, because a square-wave is composed of a fundamental frequency and an infinite number of harmonics. In practise, a relatively limited bandwidth will reproduce a good approximation to a square-wave. This is fortunate because the wider the bandwidth, the more noise enters the system.
To preserve a reasonable pulse shape and amplitude the minimum bandwidth BW is
BW = 2t -1
For a 1 ms duration pulse, the minimum bandwidth is
BW = 2/1 x 10-3 = 2000 Hz, or 2 kHz.
In practice the bandwidth is not always matched to the pulse duration and Table 6 shows the compromise made with the 38 kHz and 120 kHz EK systems.
Table 6.
|
Frequency |
Pulse duration |
Actual BW |
kHz BW for |
|
|
kHz |
t (ms) |
kHz |
1/t |
2/t |
|
38 |
0.3 |
3.0 |
3.33 |
6.66 |
|
0.6 |
" |
1.66 |
3.33 |
|
|
1.0 |
1.0 |
1.00 |
2.00 |
|
|
3.0 |
" |
0.33 |
0.66 |
|
|
120 |
0.1 |
9.4 |
10.00 |
20.00 |
|
0.3 |
|
3.33 |
6.66 |
|
|
0.6 |
3.8 |
1.66 |
3.33 |
|
|
1.0 |
|
1.00 |
2.00 |
|
The figures given in the 'Actual Bandwidth' column are those to which the system was designed and no attempt should be made to change to the figures in the columns 'Matched Bandwidth'. Despite being labelled 'Actual Bandwidth', the figures given in this column are nominal, and, whilst every effort should be made to keep to them, in practise some variation will occur. The average of 'Actual Bandwidth' figures measured from a number of echo-sounders at 38 and 120 kHz are shown below.
Table 7.
|
Frequency |
Nominal Bandwidth (kHz) |
Measured Bandwidth (kHz) | ||
|
kHz |
NARROW |
WIDE |
NARROW |
WIDE |
|
38 |
1.00 |
3.00 |
0.985 |
2.93 |
|
120 |
3.00 |
10.00 |
3.82 |
9.32 |
Bandwidth should never be altered unless the centre frequency has shifted, or it is evident that the shape of the bandwidth response is badly distorted. A description of a typical checking and alignment procedure follows.
With a signal generator (SG) connected to the input or TEST SIGNAL socket and a voltmeter, or oscilloscope to CALIBRATED OUTPUT, the bandwidth can be checked. The signal input level should be sufficiently great to prevent 'ragged' (noisy) edges on the CW or pulse output, but not so great as to overload the receiver at the selected GAIN position.
With the bandwidth control set to NARROW take the following steps:
Step 1 Tune the SG through the nominal centre frequency and back again, noting the frequency of maximum response (f0)Step 2 Reduce the frequency of the SG until the echo-sounder output level has dropped by 3 dB, ie 0.707 of the maximum, note this frequency (f1)
Step 3 Increase the SG frequency through the position of maximum response, until the output is again -3 dB relative to maximum then call the frequency at which this occurs (f2)
Step 4 The bandwidth is f2 - f1
Step 5 The centre frequency is (f1 + f2)/2, which should be the same as f0
Step 6 With the bandwidth control set to WIDE
Step 7 Repeat the steps 1 to 5, but note that there are two frequencies with maximum amplitude of response now, neither of these correspond to f0. The procedure in this case is to find the peak on the lowest side of the nominal frequency, then reduce frequency until the response is -3 dB relative to maximum, call this fwl
Step 8 Increase frequency through the first peak, on to the second and continue until the response is -3 dB relative to the second peak (highest frequency). Call this fw2
Step 9 The WIDE bandwidth and centre frequency can be calculated in a similar manner to 4 and 5 above.
ii) Pre-amplifier GAIN control check
Take the following steps:
Step 1 Connect the test instruments as shown in the manufacturers handbook
Step 2 Set the echo-sounder controls as below
|
TVG and GAIN |
0 dB (any of the two 0 dB positions) |
|
TEST/OPERATE |
TEST |
|
BANDWIDTH/PULSE DURATON |
NARROW: note that the bandwidth selector of the EK is marked NARROW and WIDE, whereas the EK-R and EK-S are marked with the nominal bandwidth. Any of the 3 positions of NARROW bandwidth can be used. |
|
EK-S models |
|
|
(a) Basic range selector |
0 |
|
(b) Disconnect tx trigger cable at recorder main terminal. | |
Step 3 Switch on the echo-sounder
Step 4 Adjust the SG to the nominal working frequency of the echo-sounder
Step 5 Set the function selector of the test set to Rec. The voltmeter will now measure the calibrated output of the echo-sounder
Step 6 Adjust the attenuator and/or level control of the SG until the voltmeter reads 0 dB (1 Vrms)
Step 7 Set function selector of the test set to Gen. The voltmeter will now show the output from the generator (input to 'test signal')
Step 8 Read the voltmeter and then calculate the gain
G = A - Vin + VR
where
G = voltage gain in dB
A = attenuation in the echo-sounder (dB)
Vin = level of input signal to test signal point
VR = level of signal at calibrated output dB/V
The nominal gain of the pre-amplifier is 85 ± 0.5 dB
NOTE: the trimmer control R 35 or R734 (depending on the echo-sounder), is for adjustment of pre-amplifier gain.
Step 9 Set the function selector of the test set to Rec., the voltmeter should read 0 dB. If the reading differs more than 0.2 dB from this repeat steps 4-8
Step 10 Change the 'TVG and GAIN' control to one of the positions -20 dB, the voltage reading should drop by approximately -20 dB. If the reading falls from 0 dB to 19.7 dB the gain of the pre-amplifier is reduced by 19.7 dB when the TVG and GAIN is set to -20 dB. Note the measured value of the attenuator
Step 11 Set TVG and GAIN back to one of the 0 dB positions and the voltmeter deflection should be 0 dB. If this is not so repeat steps 4, 5, 6, 10
Step 12 Change the bandwidth selector to one of the four positions giving WIDE bandwidth. The difference in reading should be very small when changing between NARROW and WIDE. Note the difference if any, if more than 0.5 dB check the complete bandwidth characteristic of the receiver
Step 13 Repeat the steps 4, 5, 6, with selector in WIDE position. Then repeat step 10 and 11. Note reduction in gain for -20 dB position of TVG and GAIN control. It should be about equal to the result for NARROW bandwidth.
NOTE: Allow time for the equipment to warm-up before making any measurements (about 10 minutes).
The echo-sounder and echo-integrator can be calibrated utilizing standard targets or measurements on live fish. These calibrations are valid at the depth of the experiment if the TVG function of the echo-sounder is known to be correct at that depth. But it is also necessary to ensure that the calibration is valid for the whole depth range covered by the TVG. The manufacturer specifies the accuracy of the TVG function to be within ±1 dB of the appropriate law but, even so, this can result in an intensity difference of 2 dB between the shallow water, where the standard target or the live fish calibration experiment was carried out and the depth where fish concentrations are found during survey. As 2 dB corresponds to a factor of 1.58 this could lead to a large error in the estimate of a fish stock. It is necessary to keep errors to the minimum possible, but although it is desirable to have a TVG function with a maximum deviation of less than 0.5 dB (± 0.25 dB) this is not practical with equipment prior to 1980.
The EK equipment is designed to have an accuracy of ± 1 dB, so it is necessary to measure the TVG function and introduce a correction in the acoustic estimates according to the difference between the theoretical TVG function and that actually measured. Several methods of measurement may be utilised, depending upon the equipment available. All use the gain of the TVG amplifier (pre-amplifier) at a certain depth for reference and compare this with the gain at other depths.
i) TVG Calibration Method ONE
This TVG measurement utilises a Continuous Wave (CW) signal generator, an electronic counter, AC voltmeter and oscilloscope. Take the following steps:
Step 1 Connect test the instruments as shown in Figure 56.
Step 2 Prepare a table similar to table 5 by calculating the theoretical values for 20 logR + 2a R or 40 logR + 2a R at suitable range increments. Remember that the value of a is dependent on temperature and salinity.Step 3 Adjust the frequency of the signal generator to the centre operating frequency of the echo-sounder, and its amplitude to the reference signal level (say -40 dB/V).
If the noise level permits, a lower reference level ie -60 dB/V may be used. The lower the input, the less chance there is of over-heating the receiver input stage during the gain measurement, this helps to preserve the accuracy at short ranges. Theoretical values have been calculated for both -40 dB/V (A) and -60 dB/V (B) reference levels.
Step 4 Select the TVG function to be measured (20 logR or 40 logR).
Step 5 Adjust the polarity and triggering level of the oscilloscope, so that stable triggering is obtained from the positive leading edge of the transmitter trigger pulse. Make sure that the timebase of the oscilloscope will cover a sufficient range.
Step 6 Adjust the oscilloscope vertical sensitivity until the picture at the reference depth eg 75 m (100 ms) from the start, exactly fills the screen between the two main horizontal division lines (one at the top, one at the bottom of the screen) Figure 57. The vertical sensitivity of the oscilloscope should be left untouched during the rest of the measurements.
Figure 57.
Step 7 Now alter the level of the signal generator output until the reference amplitude is obtained at one of the other ranges given in the table. Read off the signal level and note down the result in the table. Note: To make it easier to read the voltmeter, stop the recorder of the echo-sounder while the voltmeter readings are taken.Step 8 Repeat step 7 until all points in the table have been measured. Try to spend the minimum time possible at short ranges. The CW signal output of the generator is high, and the transistors may heat up slightly, thus causing a change in their characteristics and upsetting the measurements.
Example 1:
We are measuring the 20 logR + 2a R function of a 120 kHz echo-sounder. A reference level of -40 dB/V is selected, and the oscilloscope adjusted to give reference amplitude at 100 ms.
The output level from the signal generator is then reduced until the reference amplitude is presented at the oscilloscope screen at 133 ms. Stop the echo-sounder recorder and read the output level of the signal generator on the voltmeter. Write down the result eg (-44.9 dB) in the table. Start the recorder and increase the signal level until reference amplitude is obtained at 60 ms. The signal generator output level is again measured and the result put into the table. Then repeating the same procedure for all points in the table we have the following result. Note: for simplicity, a has been ignored in this example.
Table 8.
|
Range |
Theoretical dB |
Measured value dB |
Difference dB | |||
|
metres |
ms |
A |
B |
A |
B |
A |
|
3 |
4.0 |
-5.4 |
-25.4 |
-4.5 |
|
+0.9 |
|
4.5 |
6.0 |
-9.1 |
-29.1 |
-2.3 |
|
+0.8 |
|
7.5 |
10.0 |
-13.8 |
-33.8 |
-13.2 |
|
+0.6 |
|
15.0 |
20.0 |
-20.5 |
-40.5 |
-20.3 |
|
+0.2 |
|
37.5 |
50.0 |
-30.5 |
-50.5 |
-31.0 |
|
-0.5 |
|
45.0 |
60.0 |
-32.8 |
-57.8 |
-32.9 |
|
-0.1 |
|
75.0 |
100.0 |
-40.0 |
-60 |
-40.0 |
|
0 |
|
100 |
133 |
-44.8 |
|
-44.9 |
|
-0.1 |
The difference is calculated as:
Difference = measured value - theoretical value.
Step 9 The difference between the measured value and theoretical value is now plotted in the graph. Figure 58 and this graph can then be used to correct the calibration constant to ensure that it is valid for the depth where the major fish concentrations are registered.
Example 2:
Using the 120 kHz echo-sounder from the previous example, we have made a live fish calibration. Distance from the transducer face to the centre of the integration interval was 4 metres during the calibration. When using the same equipment at sea for an abundance survey, heavy fish concentrations at between 50 and 60 metres depth are found in one area. We want to find the correct calibration constant to convert the echo-integrator deflection 'M' corresponding to these fish concentrations to absolute values.
Solution: From the graph we find that the 'error' of the TVG at the depth of the calibration (4 m) hence our reference is +0.83 dB. On survey the fish are found at a depth of between 50 and 60 m. The average depth of the fish, (55 m) corresponds on the graph to a TVG 'error' of -0.03 dB so the total difference is therefore (-0.03) + (-0.83) dB = -0.86 dB. Consequently, the constant 'C' must be reduced by 0.86 dB, or be divided by 1.22. The same result is obtained if the calibration constant kept at its original value but the integrator deflections are divided by 1.22.
ii) TVG Calibration Method TWO
Again measuring the TVG function using a CW signal generator, an electronic counter, an A.C. voltmeter, an oscilloscope and an echo-integrator. This method is similar to that just described, the difference lies in the use of the interval pulse from the echo-integrator, to trigger the oscilloscope, so only a small part of the total TVG function is presented on the screen. The procedure is:
Step 1 Connect the instruments as shown in Figure 59.Step 2 Adjust frequency of signal generator to the centre operating frequency of the echo-sounder and its amplitude level to the reference given in the manufacturers table.
Step 3 Start the echo-sounder and display the interval pulse from the echo-integrator on an oscilloscope. Adjust triggering level of the oscilloscope so that it occurs at the leading edge of the interval pulse.
Step 4 Select a 2 m interval and adjust the timebase of the oscilloscope so that the interval corresponds to the total width of the screen (10 divisions). The leading edge of the interval pulse corresponds to the depth selected and the trailing edge to this depth plus two metres, the centre of the screen corresponds to the selected depth plus one metre.
Step 5 Now display the signal from the calibrated output. Set the depth selector of the echo-integrator to one metre less than the reference depth, and the latter is then displayed at the centre of the screen.
Step 6 Adjust the vertical sensitivity of the oscilloscope until the centre of the displayed picture covers the whole screen, minus one division above and below the curve (Figure 60). Do not alter the oscilloscope controls after it has been calibrated as described above.
Figure 60.
Figure 61.
Step 7 The rest of the procedure is similar to that described in method ONE. From the table, another point on the curve is selected and the depth control of the echo-integrator is set to 1 metre less than the depth found in the table.Output from the signal generator is adjusted until the reference amplitude is obtained at the centre of the oscilloscope screen, the recorder of the echo-sounder is then stopped and the signal level read from the AC voltmeter. These results are written down in the table. Some of the measurements in the table refer to half metres (for example 37.5 metres). In this case set the depth selector of the echo-integrator to a depth of 1.5 metres less than the depth found in the table (for example 36 m). This depth is now found midway between the centre and the right hand side of the screen. (For a screen with 10 divisions, at division 7.5). In this case adjust the level of the signal generator until reference amplitude is found at this position of the screen, Figure 61.
Step 8 Proceed with the measurements in similar manner as described for method ONE.
Calculate the difference between the measured and the theoretical value and plot the error curve.
iii) TVG Calibration Method THREE
Measurement of the TVG function using a pulsed signal generator, electronic counter, A.C. voltmeter and an oscilloscope.
This method is similar to the previous two. The difference is that the continuous wave (CW) signal generator, is replaced by one which may be triggered or amplitude modulated in such a way that a signal pulse or burst is produced. This avoids heating up of the amplifying transistors due to continuous high signal input. The TVG amplifier is therefore checked under normal operating conditions and a more exact result expected.
Step 1 Connect the instruments as shown in Figure 62.
Step 2 Start echo-sounder and switch on the echo-integrator.Step 3 Select CW mode of operation for the signal generator and adjust its frequency to coincide with the echo-sounder operating frequency and its output level to 0 dB.
Step 4 Adjust oscilloscope triggering level (external) and polarity (+) until a stable triggering occurs. Use dual beam mode, and select sweep speed and vertical sensitivity so that the signal generator output and the echo-integrator interval pulse are both clearly visible, as shown in Figure 63.
Step 5 Select triggered mode and external gate control at signal generator, then adjust triggering level and polarity (-) until the generator produces a stable signal pulse, Figure 64.
Step 6 Now connect the oscilloscope channel A vertical input, to the echo-sounder calibrated output. The pulse signal should appear on the screen, although its amplitude will be much higher and the signal generator output level should be reduced until the whole pulse is within the screen. The instrument connections should now be as shown in Figure 65.
Step 7 Select the Channel A only mode on the oscilloscope and adjust vertical position of the pulse until it is centred on the screen.Step 8 Stop recorder (without switching off the echo-sounder).
Step 9 Set signal generator mode selector to CW. Check that the generator frequency is still equal to echo-sounder operating frequency. Correct if necessary. Adjust output level of generator to reference level.
Step 10 Start recorder of echo-sounder. Select triggered mode of signal generator. Set depth selector of the echo-integrator to one metre less than the reference depth and the interval selector of the same channel to 2 m.
Step 11 Trigger the oscilloscope from the Channel A interval pulse of the echo-integrator.
Step 12 Adjust vertical sensitivity and sweep speed of the oscilloscope until a picture similar to the one shown in Figure 66 is obtained. Do not alter the oscilloscope settings after they have been adjusted in this way.
Figure 66.
Step 13 Change the depth selector on the echo-integrator to a position one metre * less than the depth indicated for one of the other measuring points in the table. Adjust the generator output level until reference amplitude is obtained at the centre of the oscilloscope screen, stop the recorder, change generator mode to CW and read off the level on voltmeter. Go back to triggered mode, start echo-sounder recorder and repeat this step until enough points on the TVG curve have been measured. Note down the results in the table. Continue as described in method ONE from step 9 and beyond.(*If the depth to a measurement point is now a half metre, eg 37.5 m, select one and a half metre less than in the table and adjust the output level of the signal generator until reference amplitude is obtained on the oscilloscope screen, midway between the centre of the pulse and its end).NOTE: Other methods of TVG calibration have been developed since the material given in 7.2.3 was prepared by B. H. Larsen but these require specialised instruments. One such was reported by Lee (1982) and Foote (pers. comm.) stated that H. P. Knudsen of the Institute of Marine Research in Bergen has designed and built the necessary circuits for automated measurements. Automated calibration is also mentioned by Hood and Huggins (1983).
Correct functioning of echo-integrators can be checked by various methods involving the application of pulse or continuous wave signals at the input terminals and monitoring the output by digital voltmeter or chart recorder (Simrad QM). Actual setting up of the analog circuits is a difficult and lengthy procedure but a quick overall system check for the QM echo-integrator is as follows
Set the controls to
|
Depth |
012 metres |
|
Interval |
030 metres |
|
Gain |
10 dB |
|
Threshold |
Zero |
|
Bottom Stop |
OFF |
|
Operate/test |
test |
|
Speed (Manual) |
10 knots |
Monitor the following
|
Signal |
3.16 positive pulse |
|
Interval |
+15V to 0V, duration 40 ms, delay 9 ms |
|
Sounding |
3.64V positive pulse, approx 100 ms |
|
Nautical mile |
0.32V steps rising to 10V and resetting |
The nautical mile output takes 31 trigger pulses to reach 10V. Look at the record on the QM recorder and count the steps.
The Simrad QD system has a built in test panel which allows a number of functions to be monitored but of course they are of a different type to the analog systems. In this case for example the signal from the echo-sounder is in the form of a 12-bit word. The pre-integrated squared signal can be checked to least significant and most significant bits and the status of all the controls bits can be observed by means of LED lights.
= 492.3
= 345.1
= 374.9 and
C.V. = 9.4%
= 145.4 and
C.V. = 8.1%
