The accuracy of length measurements made using imaging SONAR is inversely proportional to the beam width

Data collection To investigate measurement accuracy, three different Blueprint Oculus imaging SONAR systems were used in this study, models: M750d, M1200d, and M3000d. Between them, they operated at four different (centre) frequencies: 750 kHz, 1.2 MHz, 2.1 MHz, and 3 MHz. These imaging SONAR systems were used to measure the length of three rectangular targets of nominal lengths: 10 cm, 20 cm, and 1 m. The targets were designed to present consistent, uniform acoustic scattering. The measurements were carried out in a (freshwater) swimming pool on 16th September 2021. The targets were created from layers of hollow polycarbonate sheets, with the edges sealed to maintain the airspaces and backed with a 1 mm thick aluminium sheet. The targets were completely wrapped in black PVC duct tape (Figure 3). The actual final length of each nominal target size was 1 m = 1005 mm, 20 cm = 195 mm, 10 cm = 105 mm, but are referred to by their nominal lengths. The height of each target was 1 m = 255 mm, 20 cm = 115 mm, 10 cm = 55 mm. For the 1 m and the 20 cm targets, the thickness was 33 mm, and 25 mm for the 10 cm target. The targets were attached to fishing line and orientated so that the length was perpendicular to the face of the imaging SONAR (Figure 3). Surfactant was applied to the fishing line immediately prior to measurements, to mitigate any contribution bubbles on the line could have to “elongating the target” as per standard underwater acoustic protocols. The Blueprint Oculus imaging SONAR systems were connected through a RJE Oceanbotics™ SRV-8 ROV and recorded using Oculus Viewpoint software. The sound velocity of the freshwater pool was calculated through a temperature probe integrated into the Oculus systems, and was also independently verified. Imaging SONAR data of the targets were collected at ranges between 1 m and 15.5 m, by moving the targets either closer to or further away from the ROV, which remained at a fixed location. At each range step (nominally 1 m steps), the ROV would pan horizontally to collect measurements at different bearings over the full swath width. The ROV was paused when necessary to allow targets to be perpendicular to the SONAR. This was repeated for the four different system frequencies. The swimming pool depth was approximately 1.8 m, and there was minimal reverberation noise. Length measurements The imaging SONAR data were analyzed by someone who was not present during the data collection and did not know the length of the targets, so as not to introduce bias. The measurements were made using the measurement tool in Oculus Viewpoint software, and the co-registered video from the ROV was used to confirm the targets were present. For each length measurement, the frequency, range setting, and measured range of the target were recorded. For the 750 kHz and 1.2 MHz systems, the bearing of the target was recorded if it was measured at greater than +/-32.5o, as beyond this, the beam width was more than 20% of that of the centre beam for those systems and the effects of larger beam widths on the imagery could be seen at those bearings. These bearings were displayed on the software, making it easy for the user to distinguish this area. The measurements were repeated for consistency, and the mean length of the repeated measurements was used in the analysis.数据集采集 为探究测量精度，本研究采用三台不同型号的Blueprint Oculus成像声呐（Blueprint Oculus imaging SONAR）系统，型号分别为M750d、M1200d及M3000d。上述系统的工作中心频率涵盖四档：750 kHz、1.2 MHz、2.1 MHz与3 MHz。本研究使用该系列成像声呐对三个标称长度分别为10 cm、20 cm及1 m的矩形靶标开展长度测量，靶标设计为具备一致均匀的声学散射特性。测量于2021年9月16日在淡水泳池内完成。靶标由多层中空聚碳酸酯板材制成，边缘密封以维持内部气腔，背面贴合1 mm厚铝板，并全程以黑色PVC电工胶带包裹（Figure 3）。各标称尺寸靶标的实际最终长度分别为：1 m对应1005 mm、20 cm对应195 mm、10 cm对应105 mm，后续仍以标称长度指代。各靶标的高度分别为：1 m对应255 mm、20 cm对应115 mm、10 cm对应55 mm；1 m与20 cm靶标的厚度为33 mm，10 cm靶标的厚度为25 mm。靶标通过钓鱼线悬挂固定，调整方位使其长度方向垂直于成像声呐的探测面（Figure 3）。根据标准水下声学规程，测量前需在钓鱼线上涂抹表面活性剂，以消除钓线表面气泡对“靶标伸长”效应的干扰。Blueprint Oculus成像声呐系统通过RJE Oceanbotics™ SRV-8水下机器人（ROV）连接，并借助Oculus Viewpoint软件完成数据录制。泳池淡水的声速通过集成于Oculus系统的温度探头计算得到，并经独立方式验证。通过调整靶标与固定位置的ROV之间的距离，在1 m至15.5 m的探测范围内完成靶标的成像声呐数据采集。每间隔标称1 m的距离档位，ROV将水平旋转以采集全测绘带宽度内不同方位角的测量数据；当需要靶标保持与声呐垂直时，暂停ROV移动。针对四档不同的系统频率，重复上述采集流程。泳池水深约1.8 m，背景混响噪声极低。长度测量 数据由未参与现场采集且不知晓靶标实际长度的人员进行分析，以避免引入观测偏差。测量采用Oculus Viewpoint软件自带的测量工具完成，并结合ROV的共配准视频确认靶标存在。每次长度测量均需记录对应频率、量程设置及靶标的实测距离。对于750 kHz与1.2 MHz系统，当靶标方位角大于±32.5°时需记录方位，因为在此范围外，上述系统的波束宽度超过中心波束宽度的20%，该方位下波束宽度对成像的影响可被观测到；该软件会直接显示此类方位区域，便于用户区分。为保证一致性，测量重复多次，最终取重复测量的平均长度用于后续分析。