Abstract

Echosounders may be calibrated by suspending a solid metal sphere in the acoustic beam and adjusting the system gain so that the measured target strength (TS) equals the theoretical value for the sphere. The suspension apparatus (i.e. net bag or loop of line glued into a hole, and control lines) may appreciably scatter sound, potentially affecting the sphere-echo intensity and phase, and hence the calibration uncertainty. Here, we characterize the effects of conventional suspension materials and configurations on measurements of sphere TS and recommend approaches to mitigate the observed effects on echosounder calibrations. We show that the effects of suspension lines on both calibration accuracy and precision change with line type and increase with line diameter and acoustic frequencies above ∼100 kHz. Compared to commonly used polyamide (nylon) line, ultra-high-molecular-weight polyethylene (PE) line has an acoustic impedance closer to that of water and therefore has a lessened effect on measurements of sphere TS. The effects may be mitigated in a practical manner using a 0.38-mm diameter, multifilament, PE line for the suspension.

中文翻译：

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球体悬挂对回声测深的影响

摘要

可以通过将固体金属球悬挂在声束中并调整系统增益来校准回声器，以使测得的目标强度（TS）等于该球的理论值。悬挂装置（即粘在洞中的网袋或线环和控制线）可能会散射声音，从而潜在地影响球面回波强度和相位，从而影响校准不确定性。在这里，我们表征了常规悬浮材料和结构对球体TS的测量的影响，并推荐了减轻在回声测深校准中观察到的影响的方法。我们表明，悬挂线对校准精度和精度的影响随线型而变化，并随线径和约100 kHz以上的声频而增大。与常用的聚酰胺（尼龙）线相比，超高分子量聚乙烯（PE）线的声阻抗更接近于水，因此对球体TS的测量影响较小。使用直径为0.38 mm的复丝PE线进行悬浮，可以以实际的方式减轻这种影响。