Categorizing Alternative Seismic Sources Based on Their Potential to Affect Marine Mammals

Seismic airguns generate high intensity noise that can affect marine life. Emerging alternative seismic sources offer promising opportunities to reduce acoustic impacts, yet their performance and integration into regulatory processes remain poorly quantified. We evaluated four recently developed alternative seismic sources to characterize their acoustic outputs, assess their relative impacts on marine mammals, and classify them within the tiered framework of the Operational National Mitigation Protocol for Geophysical Surveys—developed collaboratively by the National Marine Fisheries Service (NMFS), Bureau of Ocean Energy Management (BOEM), and Bureau of Safety and Environmental Enforcement (BSEE). The assessment included three ultra low-frequency (ULF) pneumatic sources (Tuned Pulse SourceTM [TPSTM], Gemini SourceTM, and HarmonyTM) and a representative marine vibroseis (MV) system. Using NMFS auditory injury and behavioral disturbance thresholds alongside a new (60 s M-weighted sound exposure level) metric, we found that all alternatives produced lower 60 s energy and smaller auditory injury zones than conventional high impact airgun arrays. ULF pneumatic sources concentrate energy below 10 Hz, contributing to reduced exposure at ecologically relevant frequencies. TPSTM and the Gemini SourceTM align with moderate impact tiers, while HarmonyTM is more similar to higher impact sources. For MV systems, single and dual element configurations are placed in a lower impact tier, though an 18-element array requires further characterization before classification. Remaining uncertainties, including limited behavioral data and incomplete source characterization, highlight the need for continued empirical research. By offering a science-based framework, this work contributes to regulatory modernization, advances efforts to reduce ocean noise, and enables the responsible adoption of emerging “quieter” seismic technologies within a more consistent and streamlined approach.