Acoustic calibration on a standard spherical target

Transcription

1 Direction des navires océanographiques et de l'intervention Sous-marine Département Equipements Scientifiques et Informatique embarquée Service Développement de Logiciels Embarqués Noël Diner Christophe Durand Laurent Berger May Version DNIS/ESI/DLE/DTI/ Acoustic calibration on a standard spherical target

2 Acoustic calibration on a standard spherical target IFREMER Tous droits réservés. La loi du 11 mars 1957 interdit les copies ou reproductions destinées à une utilisation collective. Toute représentation ou reproduction intégrale ou partielle faite par quelque procédé que ce soit (machine électronique, mécanique, à photocopier, à enregistrer ou tout autre) sans le consentement de l'auteur ou de ses ayants cause, est illicite et constitue une contrefaçon sanctionnée par les articles 425 et suivants du Code Pénal. IFREMER All rights reserved. No part of this work covered by the copyrights herein may be reproduced or copied in any form or by any means electronic, graphic or mechanical, including photocopying, recording, taping or information and retrieval systems- without written permission.

3 Type de rapport : Document Technique Numéro d'identification du rapport : DITI/DSI/DTI/ Diffusion : libre restreinte interdite Validé par : C. Durand L. Berger date de publication novembre 1998 nombre de pages 22 bibliographie (Oui / Non) illustration(s) (Oui / Non) langue du rapport Titre et sous-titre du rapport : L'étalonnage sur cible étalon par le logiciel MOVIES+ Titre traduit : ACOUSTIC CALIBRATION ON A STANDARD SPHERICAL TARGET Auteur(s) principal(aux) : Noël Diner Organisme/Direction/ Service, laboratoire TMSI/TP Collaborateur(s) : nom, prénom Organisme / Direction / Service, laboratoire diplôme : établissement de soutenance : Titre du contrat : n de contrat Ifremer Organisme commanditaire : nom développé, sigle, adresse Organisme(s) réalisateur(s) : nom(s) développé(s), sigle(s), adresse(s) Responsable scientifique : Cadre de la recherche : Programme : Ressources Halieutiques Convention : Projet : Autres (préciser) : Campagne océanographique : (nom de campagne, année, nom du navire)

4 Résumé : Ce document présente les concepts de l étalonnage par cible étalon avec le logiciel Movies+. Abstract : This document introduces the concepts of echo-sounder calibration using Movies+ Mots-clés : Acoustique sous-marine, logiciel Movies+, étalonnage Keywords : Movies+ software, fishing echo-sounder, echo-integration Commentaire :

5 Table of contents 1. Introduction Calibration practice...10

6 Table of contents 1. Introduction The final goal of echo-integration is to give quantitative assessment of the fish biomass trough an evaluation on the reverberation produced by biological target. The purpose of the acoustic calibration on standard target, which acoustic characteristics are perfectly known, is to adjust the echo-sounder settings so as it delivers proper echo signal levels. MOVIES+ is processing absolute amplitude values. Concerning the SIMRAD EK500, the "V" telegrams, which are used, contain absolute values (when the Sv_Gain is correctly adjusted) and are not pre-processed by MOVIES+. The data output from OSSIAN, which are relative ones, must be corrected before any processing by MOVIES+. This needs to review the calibration procedure already designed for MOVIESB software. The calibration procedure by MOVIES+ is done only for a proper adjustment of the echo-sounder settings - "Sv-gain" or "(Sl+Vr)" so as the different of echo-integration performed by the software will deliver absolute values of reverberation. This is done by comparing the echo signal reflected by the standard target to the theoretical one, which can be calculated as the target characteristics are well known.

7 7 Theory MOVIES+ software calculatesσ (aggregate backscattering cross-section)as (cf. ag 1.2 of the manual "Shoal echo-integration" or 1 of the manual "Layer echointegration") : where : S : vessel speed 1 2 σ ag = S j T j U (1) η T : time gap between sounder pings j ε : number of samples per meter depth. i The following relation defines sa as the nautical area scattering coefficient from σ : ag s A 4π (1852) where: 2 H m = H σ m ag D H m = height of the layer in meters. D m = length of the ESDU in meters. (4) m When calibrating, the back-scattered s As by the standard target (sphere) is computed as the sum (on "i" for 1 ping) of the successive samples of the square amplitude inside a depth window defined by the operator. This window must include only the standard target echo. Relation (3) is used for this computation and gives, using the maximum value measured during a sequence, the equivalent σ ag, of the target echo. On another hand, the theoretical "Nautical area scattering coefficient" determined for the standard target is : with : s Ath 2 2 4π r0. σ bs (1852) = (5) 2 ψ. r r 0 : 1 meterσ bs : back-scattering cross section of the target as TS = 10log(σ bs ) ψ : equivalent angle (natural value) r : distance of the standard target to the transducer

8 The proper settings - Sv_gain for EK500 or (Sl+Vr) for OSSIAN will be found when the ratio : S As R c = 10 log[ ] (6) S Ath expressed in db, will be equal to 0 db. 8 The " σ ag " value in is the maximum value obtained, using one or more consecutive pings, from the standard target when it is situated on the acoustic axis of the transducer. The soft MOVIES+ keeps automatically this maximum value and determines "R c " corresponding value. As it appears (cf. figure 1) that there are some fluctuations from ping to ping in the acoustic performances of an echo-sounder, it seems better to calculated the " σ ag " maximum value by using a maximum mean value of reflected echoes of the target when it remains on the acoustic axis. It appears that (figure 1), a mean for 5 pings is OK. In presence of bad weather conditions, it may occur that the standard target does not remain 5 successive pings centred; in that case, the user must reduce this number to 3 or even 1. In this last case, it appears that the echo-sounder performances could be over estimated. Ecart (db) rdb1 rdb3 rdb4 rdb Ecart db Ping n Figure 1 : Variation in db of the received aggregate backscattering cross-section, σ ag, from the standard target during a calibration phase (38 khz), when the target remained perfectly on the acoustic axis of the transducer (controlled by operating

9 the MOVIES TS analysis function). There are some fluctuations (> ± 0.1 db) from ping to ping. By computing the mean σ ag on 5 or even 3 successive pings, these fluctuations are lowered to less than ± 0.03 db. 9

10 10 2. Calibration practice The calibration function included in MOVIES+ is provided, in the frame of a sphere acoustic calibration, for determining the proper echo-sounder settings. In a first phase, it is necessary to put the acoustic target exactly on the axis of the acoustic beam. Attention : the target depth will be high enough so that its distance to the transducer must exceed about twice the near field distance and TVG minimum range. When this operation is done, the different settings of the echo-sounder are adjusted : transmitted power, pulse duration, gain,... It is then possible for the soft to determine a preliminary factor "R c ", corresponding to the current settings of the echo-sounder. So as to obtain a "0" value for "R c ", it is necessary to modify "(Sl+Vr)" for OSSIAN or "Sv_gain" for EK500. The correction value in db is of the same level than previous "R c ". A new calibration step with MOVIES+ is then operated so as to verify that a "0" value is reached for "R c ". At the end of this calibration phase, the operator can archive, in an "Excel" compatible file, all the pertinent information of the equipment. In a new stage, it is then possible to change some echo-sounder settings (pulse duration for instance) and determine their corresponding "(Sl+Vr)" or "Sv_gain". The calibration function can be operated from the "Tools" menu. When this function is validated, a first window appears (Fig.2) for defining the location and the name of the Excel file where the calibration data will be stored.

11 11 Figure 2 : Dialog box for the definition of the Excel file where will be stored the calibration data. When the operator click on "SAVE" in the lower part of the calibration window, the following data, spaced by ";", are stored in the Excel file : Survey name, Date and time, sounder name, frequency, value of (Sl+Vr) or "Sv_gain", 10logψ, pulse duration, transmitted power, TVG function, Depth and width of the target window, TS and maximum energy of the target, "R c " factor On the left part of the screen, a special window is opened where are displayed from top to bottom (Fig.3) : settings of the used sounder, extracted from the received telegrams : identification, level "(Sl+Vr)" or "Sv_gain", 10logψ, pulse duration, secondary windows where it is possible to settle information concerning the standard target : target strength, upper limit and width of the target window. Using the mouse, it is possible to modify these 2 last parameters by dragging the green and red bars displayed in the oscilloscope window when "Apply" has been validated. Attention : the depth "r" of the standard target, used in formula 5 to calculate s Ath is corresponding to the depth of this window, i.e.

12 the depth of the green bar. In this window, you can also set the number of pings you want to be used to determine the maximum σ ag (i.e. calculated by using a mean on N consecutive pings), N can be set between 1 and 20, 1 corresponding to rough sea conditions and 20 to fair sea conditions (5 seems practically sufficient). This allows to take in account slight variations in the echo-sounder performances (cf. figure 1). data concerning the current calibration, particularly the received echoes inside the target window : aggregate backscattering cross-section, σ ag, of the last ping and maximum aggregate backscattering cross-section of the current sequence. By a click on the "Reset" button, all these values are put to "0" and a new sequence can start again the result, i.e. the R c coefficient. The "Save" button allows the storage in an Excel file of all the parameters and measurements of the sequence. 12 Figure 3: Copy of the screen in calibration mode. The special calibration window can be seen on the left. In the oscilloscope window, the green and red bars show the practical limits of the window enclosing the target echo.

13 13 The following organization chart summarize the different steps of the calibration with MOVIES+ : 1 - Standard target 1.1 Putting the target on the acoustic axis 2 - Sounder 2.1 Settings adjustments : power, pulse duration, gain, Preliminary values for (Sl+Vr) or Sv_gain 3 MOVIES+ 3.1 Calibration adjustment : TS and target window 3.2 "Apply" for taking in account the target parameters 3.3 "Reset" for starting the sequence with correct echo values of the target 3.4 Determination of a R cp 3.5 Adjustment of proper (Sl+Vr) or Sv_gain "Reset" on MOVIES+ 3.4 Control on MOVIES+ that the R c value is "0" 3.5 "Save" for the storage of different information in Excel file 3.6 Back to point 2.1 for another stage with new sounder settings