# A Vector Sensor-Based Acoustic Characterization System for Marine Renewable Energy
## 1 Introduction  
The market acceleration and adoption of marine renewable energy (MRE) technologies requires that hurdles related to their perceived environmental effects are better understood, characterized, and mitigated [1, 2]. A key environmental stressor of concern is the acoustic impacts of MRE, given that sound is an important mechanism via which marine animals find prey, communicate, and engage in social behavior. With ongoing progress in MRE device development, there is a growing concern over noise levels emanating from these devices. Sound emissions related to MRE devices are regulated by the National Oceanic and Atmospheric Administration National Marine Fisheries Service [3], which sets limits on appropriately weighted sound pressure levels above which sound emissions can be considered detrimental to marine mammals. As a consequence, and because determining actual noise levels in the marine environment is difficult and often cost-prohibitive, a number of notable efforts to measure and monitor noise from MRE devices are under development. [PERSON] et al. [4] recently tested a real-time acoustic observing system (RAOS) that consists of a surface telemetry buoy coupled to a bottom-mounted, self-powered acoustic hydrophone and modem package. The bottom-mounted hydrophone unit archives and relays processed acoustic data digests via an acoustic communication link to a surface buoy, from where it is transmitted, via a satellite link to a land-based station. With the bottom device and surface buoy, RAOS requires a doubling of deployment and recovery effort, with the further constraint that the measurement consists of only a single spatial measurement of acoustic pressure. To obtain location estimates of MRE devices, the developers proposed the simultaneous deployment of four RAOS devices around a waveenergy converter installation. This approach, similar to [PERSON] et al. [5], offers considerable acoustic travel-time differences for location estimates of low-frequency sound. Location estimates, however, are not available in real time, and the presence of independent processing units can lead to difficulties with clock synchronization. [PERSON] et al. [6] developed a tethered underwater remotely operated vehicle (ROV) with a suite of oceanographic sensors including a hydrophone, a sonar system, cameras, velocimeter, and current profiler. This futuristic integrated monitoring system includes a bottom-mounted cabled docking station. While the ROV is designed to make mobile acoustic measurements in the immediate vicinity of an MRE device, the presence of a tether can cause unwanted complications when operating in the vicinity of MRE devices consisting of several berths, connected with tendon-like cables. Additionally, given the low noise emitted by MRE devices, the presence of multiple thrusters and ballast pumps can significantly reduce the signal-to-noise ratio, a problem common in glider-based acoustic measurement platforms [7]. Furthermore, location estimation using a single pressure-only hydrophone remains a challenge, and the engineering costs involved in designing, maintaining, and deploying an ROV can be significant. [PERSON] et al. [8] developed a low-cost drifting acoustic instrumentation system that can be deployed as small swarms around MRE devices to measure and localize device sound. By virtue of being drifting units, contamination by flow noise is significantly reduced in high-current regions, while a heave plate provides vertical stabilization in wave-dominated environments. However, the drifting nature makes long-term operational measurements cumbersome, if not impossible, while the use of hydrophones requires the use of several drifting units for accurate source localization. Two vector-sensor based acoustic monitoring systems that were employed to provide location estimates of vocalizing baleen whales are those designed by [PERSON] et al. [5] and [PERSON] et al. [9].