Cours sur les méthodes d'évaluation acoustique ... - Fao - Copemed

CalibrationThe purpose of the swivels is threefold:- to unravel rotation of the nylon line- to mark distances on the line- to add weight so that the line sinks in waterAttaching the spherePrior to commencing the sphere measurements, a rope should be drawnbeneath the hull from the first winch to the second and third winch beforeanchoring. Use this rope to pull the line from the first winch beneath the hull tothe side with the second and third winches.Attach the appropriate sphere, with affixed loop, to the three suspensionlines, refer to figure 1. For the smaller spheres it may be necessary to add aweight to keep the sphere stable. This is done via a second line attached tothe three suspension lines. The length of the line must be at least two pulselengths, so that the echo from the additional weight does not interfere withthe sphere echo. Immerse the sphere in a solution of dishwashing detergentand freshwater and lift it overboard by the fastened lines without touchingit. The soap helps to eliminate air bubbles attached to the sphere.Lowering the sphereLower the sphere beneath the vessel to the desired distance, for example25 m, which is determined roughly by counting the swivels on each line.In general, one should use sphere distances of 15 m or more for 38 kHz orhigher frequencies. This in order to reduce the effect of pulse rise time andresolution in distance measurements on the calibration results. Softwareversion 5.30 has corrected for these effects on the TS and s A calibration.Two further considerations in choosing the range are the transducerbeamwidth and vessel geometry. The physical width of the beam, whichincreases linearly with range, should be sufficiently great so that the sphereecho is unaffected by the small, perhaps pendular movements to which it isinevitably subjected. The minimal range must also be convenient withrespect to the vessel geometry. In particular, if the suspension lines do nothang freely, then control of the sphere may be hindered by friction or possibleobstructions on the hull. Despite the number and variety of these considerations,it is seldom difficult in practice to find a suitable range which satisfiesall of the above criteria.6 P2260/E