# A Wide-Area Deep Ocean Floor Mapping System: Design and Sea Tests
## 4 Sparse-Aperture Sonar: Ocean Test  
The tank testing shows that the sparse-aperture concept itself is viable. However, tank experiments cannot fully simulate the ocean's complex and dynamic acoustic propagation conditions and the relative position variations in distributed arrays resulting from ocean waves and currents. To address the former, we built an oceangoing large sparse-aperture sonar testbed, shown in Figure 11. The metal frame ensures that the hydrophones and transmitters are locked in position and that the array geometry varies by less than a tenth of the wavelength (4 mm), even in rough seas.  
Figure 11: (**a**) The initial deployment of the oceangoing sparse-aperture sonar. (**b**) The sonar is towed out to the ocean through the Boston Harbor on 17 September 2020.  
Figure 10: A comparison of the results of 0.05 cm (**a,b**), 0.1 cm (**c,d**), 0.5 cm (**e,f**), and 1 cm (**g,h**) resolution imaging. The images lose details at resolutions larger than 0.1 cm, but the finer 0.05 cm resolution image provides no further benefit.  
The frame holds together a sparse array of 6 transmitters and 19 cross-shaped receiving subarrays (with a total of 247 hydrophones). The frame also includes two inertial measurement units and a waterproof server rack that houses the data acquisition system with signal conditioners, signal converters, pulse generators for transmitters, and control PCs. All the sensors and electronics are remotely controllable via Ethernet. When assembled, the array is 8 m \(\times\) 8 m (24' \(\times\) 24'), but it can be separated into three units to fit onto a single 53' flatbed trailer. The system operates at 33 kHz and is designed for an initial operating depth of 150 m. Figure 12 shows the sparse array layout, range profiles of all transmitter-receiver pairs, and a bathymetry point cloud from pier-side sensor data acquisition testing and validation.  
We identified sunken objects near Boston Harbor to image with our sonar array for an initial at-sea demonstration of the sparse-aperture concept. We installed a reference fathometer and ran Humminbird HELIX 7 side-scan sonar to generate reference imagery of the seabed. This side-scan sonar operates at 462 kHz and has a maximum range of 60 m. With 14\(\times\) the operating frequency of our sparse array, the side-scan sonar is better equipped to resolve small features. Figure 13 shows the reference side-scan imagery and the imagery generated from our sparse sonar array of a sunken barge outside Boston Harbor. The sparse sonar array operates as a multibeam sonar. It can produce a full 3D reconstruction of the seabed, whereas the side scan generates an intensity versus range plot with narrow strips. Because of this difference in image formation techniques, a direct comparison of the two outputs is difficult. Qualitatively, however, the image resolution we are obtaining from our array is similar to that of the side-scan system. A more capable reference multibeam system will be used for further validation and quantitative comparison of the resolution.  
Figure 12: Initial pier-side test results of the oceangoing sparse-aperture sonar. The array layout and range profiles from all transmitter–receiver pairs are shown in **(a,b)**, respectively. The reconstruction point cloud of the pier bottom is shown in **(c)**.