Underwater Sound Field Working Group

In 2011, the U.S. National Oceanic and Atmospheric Administration (NOAA) convened a working group to develop tools to map the contribution of human sound sources to underwater ocean noise in U.S. waters. The specific objective of the NOAA Underwater Sound Field Mapping Working Group (SoundMap) was to apply mapping methods to depict temporal, spatial, and spectral characteristics of resulting underwater noise. These tools use environmental descriptors and the distribution, density, and acoustic characteristics of human activities within U.S. waters to develop first-order estimates of their contribution to ambient noise levels at multiple frequencies, depths and spatial/temporal scales.

The SoundMap products can be viewed and downloaded, with accompanying metadata, here. The following overview is provided to facilitate understanding and appropriate use of these products.

Spectral resolution:

The emphasis of SoundMap modeling on broad-scale and long term (seasonal to annual) noise exposure resulted in a focus on low frequencies, ranging from 50 to 1000 Hz (with several specific exceptions), since higher frequencies are subject to strong absorption effects and are more local in effect. Broader band levels (1/3rd–octave) were estimated based on modeled frequencies to assist interpretation relative to mammalian hearing systems.

Spatial resolution:

SoundMap modeling focused on coastal waters at least 5 m in depth out to the 200 nm U.S. EEZ boundary at a 0.1° x 0.1° (~100 km2 at the equator) grid size. Additionally, due to the emphasis on low frequencies and the lack of a hard boundary for noise at 200 nm, some sources of chronic noise at greater ranges were modeled for larger portions of ocean basins at 1° x 1° (~10,000 km2 at the equator). To capture differences in sound propagation and how this can influence interactions with marine wildlife that spend time at different depths, modeling was conducted at discrete depths between 5 m and (up to) 1000 m.

Temporal resolution:

The central SoundMap products are predicted noise levels maps for U.S. EEZ waters of the continental U.S., Hawai’i and Alaska. These maps depict predictions of wide-ranging contributions from “chronic” anthropogenic sources of underwater noise, including vessels (merchant shipping, ocean-going passenger vessels and mid-sized service, fishing and passenger vessels in regions where data was available) and sustained areas of offshore energy exploration (seismic surveys). Predicted received levels are expressed as equivalent, unweighted sound pressure levels (Leq), which are averages of aggregated sound levels. Averaging time varies according to the appropriate timescales for the activities of interest, with a focus on annual averages from year-round activities (e.g., merchant shipping in most regions), and shorter scales for activities or events which are seasonal (e.g., in sometimes ice-covered areas).

Additionally, mapping efforts were conducted for four localized and transient events that are more episodic or seasonal; these were selected to reflect major acute sources of human-induced noise in areas of biological importance to marine mammals, including: 1) a military active sonar training exercise in Hawai’i; 2) a period of seismic exploration in the Beaufort Sea; 3) the installation of an alternative energy platform off New England; and 4) the decommissioning of an oil platform in the Gulf of Mexico.

Key discussions of the working group focused on each of the transient event scenarios, in particular methods for summing energy from chronic and intermittent sources during the events, and presenting cumulative energy averages over days to months when some sources were intermittent during those time periods. The group wanted to avoid averaging over “dead periods” between noisy events (especially very long events) and not retaining duration information, given the ultimate goal of integrating this meaningfully with biologics.

Thus, events were divided into appropriate number of acoustic “states” characterized by combinations of sources that are coincident over discrete time periods (i.e. staging prior to driving a pile, then driving a pile, then a break, then driving a pile etc.). Duration information associated with these “states” can be retained and exemplary output maps can be created for each.

Methodology and Metadata:

SoundMap focused on developing feasible methods that could be implemented within a one year analytical effort. A variety of informed approximations were necessarily applied to enhance computational feasibility and to bridge data gaps. All extrapolations and assumptions made in producing these products have been explicitly documented in methodology summaries. These summaries are intended to assist users in understanding the current status of the data used here, methodologies applied and requirements for producing different or higher resolution products in the future.

Next Steps:

We welcome your feedback in prioritizing next steps for development of these tools. Feedback can be submitted using this form.

Acknowledgements:

We are grateful for financial support for this effort provided by U.S. NOAA, U.S. Navy and U.S. Bureau of Ocean Energy Management to Heat, Light and Sound Research, Inc., and for additional funding provided by Dokumentes des Meeres to C. Kappel. In addition, we are grateful to the Cumulative Effects Working Group (convened by U.C. Santa Barbara) and JASCO Applied Sciences for providing access to modeling results from the Beaufort Sea. Finally, the working group chairs (L. Hatch and B. Southall) thank all participants for the time and enthusiasm they have provided for this effort.



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