Acoustic backscatter from krill and silverfish in McMurdo Sound from 2014-2015

<p>Krill and fish were sampled acoustically and visually beneath the fast ice using the tethered SCINI ROV, which was deployed and operated through a 25 cm diameter hole drilled through the sea ice. SCINI contained cameras and thrusters, and towed a sensor package consisting of a WET Labs fluorometer (ECO-AFL/FL) and a single-beam Biosonics 120 kHz DT-X echosounder. Visual targets were identified to the lowest taxon possible; these observations were used primarily to verify classification of acoustic signals. The echosounder operated at a nominal ping rate of 1 ping s-1; however, this rate was occasionally adjusted if false bottom signals were observed. The general profile of a dive included a surface transect of ~300 m horizontal distance, where the acoustic transducer faced downward, and also a dive to ~120 m if conditions allowed. Echogram data were saved to a depth of 500 m, and background noise was removed. Given the effective range of the transducer of approximately 100 m (resolving -80 dB targets), surveys characterized the upper 200 m of the water column.</p>本研究通过钻穿海冰的25厘米直径冰孔部署并操控系留式SCINI远程操作水下机器人（ROV），借助该设备在固定冰下方采用声学与视觉联合采样的方式对磷虾（Krill）和鱼类开展数据采集。该SCINI ROV搭载摄像机与推进器，并拖曳一套传感器组件，组件包含WET Labs公司生产的荧光计（fluorometer，型号ECO-AFL/FL）以及单波束Biosonics 120 kHz DT-X型回声测深仪（echosounder）。研究人员尽可能将视觉观测到的目标鉴定至最低分类单元（taxon），此类观测结果主要用于验证声学信号的分类结果。该回声测深仪的标称脉冲重复频率为1次/秒，但当观测到假底回波信号时，该频率会被临时调整。单次下潜的常规流程包含一段约300米水平距离的水面测线阶段，此时声学换能器朝下；若环境条件允许，还会下潜至约120米深度。回声图数据的保存深度范围为500米，并已完成背景噪声剔除处理。考虑到该换能器的有效作用范围约为100米（可分辨-80分贝的目标），本次调查覆盖并表征了水柱上层200米的区域。