Acoustic shadows and array performance at offshore wind monopile foundations: quantifying trade-offs for telemetry array design

Abstract

Offshore wind energy development introduces large-scale structures into marine environments. Acoustic telemetry is increasingly employed to monitor fish responses to offshore wind infrastructure, including via regulatory monitoring programs, yet practical guidance on acoustic array design at wind farm sites is lacking. Large monopile foundations may create acoustic shadow zones where line-of-sight transmission paths are obstructed, but the effects of acoustic shadowing on detection success and time-difference-of-arrival (TDOA) positioning performance have not been systematically quantified. We deployed a dense array of acoustic receivers around an offshore wind monopile and analyzed performance using stationary reference transmitters. We evaluated 12 post-hoc receiver configurations (3–10 receivers) to assess how array size, geometry, and acoustic shadowing affect both raw detection success and TDOA positioning performance. Acoustic shadowing by the monopile reduced the contribution of an obstructed receiver by approximately 19% compared to unobstructed receivers at similar distances. Despite this suppression, the receiver positioned in the calculated shadow zone still contributed to 70–74% of positions and improved positioning precision when included in minimal arrays. Arrays with 6 or more receivers successfully calculated more than 90% of positions versus the baseline 10-receiver array, with median horizontal positioning error less than 2 m for a reference transmitter within 15 m of the foundation. Performance degraded nonlinearly below 5 receivers. One 3-receiver configuration completely failed to position a transmitter 241 m from the monopile despite successfully positioning the near-foundation transmitter, demonstrating that geometric coverage becomes critical for edge-of-array positioning. Receivers positioned in calculated shadow zones showed reduced detection rates but still contributed meaningfully to positioning, and receivers on all sides of monopiles should be included in array design to maintain positioning capability. For TDOA (e.g., VPS) studies at offshore wind sites, 6 receivers represent a practical minimum for high-quality positioning of structure-associated fish. For broad-scale movement studies, two receivers per turbine may suffice for presence-absence detection, while 3–4 receivers are recommended for studies in which individual detections are valuable (e.g., when sensor-equipped transmitters are used). These context-dependent guidelines should be scaled to site-specific detection ranges determined through range testing. This work provides evidence-based recommendations for researchers designing acoustic telemetry arrays at offshore wind installations.