Circular Synthetic Aperture Sonar (CSAS) provides a 360° azimuth view of the seabed, surpassing the limited aperture and mono-view image of conventional side-scan SAS. This makes CSAS a valuable tool for target recognition in mine warfare where the diversity of point of view is essential for reducing false alarms. CSAS processing typically produces a very high-resolution two-dimensional image. However, the parallax introduced by the circular displacement of the illuminator fill-in the shadow regions, and the shadow cast by an object on the seafloor is lost in favor of azimuth coverage and resolution. Yet the shadows provide complementary information on target shape useful for target recognition. In this paper, we explore a way to retrieve shadow information from CSAS data to improve target analysis and carry 3D reconstruction. Sub-aperture filtering is used to get a collection of images at various points of view along the circular trajectory and fixed focus shadow enhancement (FFSE) is applied to obtain sharp shadows. An interactive interface is also proposed to allow human operators to visualize these shadows along the circular trajectory. A space-carving reconstruction method is applied to infer the 3D shape of the object from the segmented shadows. The results demonstrate the potential of shadows in circular SAS for improving target analysis and 3D reconstruction.

翻译：环形合成孔径声呐（CSAS）能够提供海底的360°方位视角，超越了传统侧扫SAS的有限孔径和单一视角成像。这使得CSAS成为水雷战目标识别的宝贵工具，其中视角多样性对于降低虚警率至关重要。CSAS处理通常生成极高分辨率的二维图像。然而，由照射源圆周位移引入的视差会填充阴影区域，海底物体投射的阴影因追求方位覆盖和分辨率而丢失。但阴影为目标识别提供了关于目标形状的补充信息。本文探索了一种从CSAS数据中提取阴影信息以改进目标分析并实现三维重建的方法。通过子孔径滤波获取沿圆周轨迹不同视角的图像集合，并应用固定焦点阴影增强（FFSE）技术获得清晰阴影。同时提出交互式界面，便于操作人员沿圆周轨迹可视化这些阴影。采用空间雕刻重建方法从分割的阴影中推断物体的三维形状。实验结果证明了环形SAS中阴影信息在改进目标分析和三维重建方面的潜力。