# A Wide-Area Deep Ocean Floor Mapping System: Design and Sea Tests
## 1 Introduction  
As the ability to map large sections of the seafloor has increased over time, so has this technology's impact on a broad range of scientific disciplines. For example, recent advances in coverage rate and resolution of undersea sensing capabilities have inspired new methods for identifying and tracking hazardous geological processes and vulnerable ecosystems and habitats [1; 2; 3]. Large-scale high-resolution mapping can enable safe navigation for submysibles, improved climate modeling, infrastructure monitoring, and disaster response efforts such as locating missing objects. However, the difficulty of recovery tasks in deep and wide areas of the ocean has been highlighted by recent tragedies such as the disappearance of Malaysia Airlines flight 370 in the South Indian Ocean [2; 4]. A significant challenge in continuing to advance seafloor mapping is that no technology exists in water deeper than 1 km to obtain meter-scale resolution maps of the seafloor from the ocean surface [4; 5; 6]. Current technology capable of finding human-made objects on the seabed or identifying bathymetric features at meter-scale has a maximum range of less than 1 km through the water [7; 8]. Because the ocean has an average depth of 3.7 km, high-resolution mapping for the vast majority of the sea requires sending a vehicle underwater, typically within a few hundred meters of the seafloor. Deploying undreresa vehicles, even uncrewed ones, to the deep ocean is a time-consuming and expensive endeavor, severely limiting the area such vehicles can map [9; 10; 11]. Therefore, wide-area, deep ocean surveysare currently forced to accept lower-resolution goals that surface-vessel-based multibeam sonars can achieve. For instance, the Seabed 2030 project [12] used 400 m \(\times\) 400 m grids as the baseline seabed cell resolution for water depths from 3 to 5.75 km. Surface-vessel-based multibeam sonars can map the seabed with a fast coverage rate compared to sonars mounted on undersea vehicles but at a significantly lower resolution [7; 13]. To employ sonar ranging through a deep water column, high-amplitude transducers operating at relatively low center frequency must be selected to reduce transmission loss. The selection of frequency determines the maximum range of a sonar array and an upper bound of the resolution set by diffraction limit [14]. Therefore, low-frequency sonar operation that can reach the deep ocean typically yields low-resolution imagery. Each pixel on the map generated by a surface-vessel-based multibeam sonar such as Kongsberg EM302 represents approximately the size of a football field on the deep seabed. By contrast, higher frequency sonars on vehicles that operate closer to the sea bottom, such as Kongsberg EM2040, can achieve a 1 m resolution, resulting in 10,000\(\times\) more detailed imagery. This work aims to develop and demonstrate a cost-effective technology for simultaneous high-resolution and rapid seafloor mapping in deep water. To generate comparable resolutions to the high-frequency systems that operate near the bottom of the ocean, a surface-based sonar must have an aperture approximately 100\(\times\) the size of those currently in use. However, the sonar arrays installed on existing large ocean mapping ships are already at the maximum size supported by their hulls [12; 15]. To achieve a sonar array that is both cost-effective and 100\(\times\) larger than a ship hull, our design concept uses a small swarm of widely spaced ASVs, each hosting a small local sonar array, as shown in Figure 1.  
In this concept, ASVs would work collaboratively to create a large effective array aperture, spanning hundreds of meters. However, for cost reasons, only a limited number of ASVs would be used to synthesize the array, on the order of tens of ASVs separated tens of meters from each other in a random pattern. With this sparse spatial sampling, the array achieves a lower signal-to-noise ratio (SNR) and is less able to reject noise coming from undesired directions because of high sidelobes [14].