Independent of and prior to any activities related to implementing either standalone study or an EIA process, some form of high level ‘screening’, or review activity, is undertaken early in a project planning process, prior to initiating more detailed assessment studies, to identify key environmental aspects of a project and sensitivities of the planned project location. This identifies the level of detail and complexity of analysis likely to be needed for the given project.
A screening process may identify the spatial and temporal extent of underwater sound propagation from a given activity relative to known marine species’ sensitivities to help determine whether underwater sound generated by an activity poses a potential risk to marine species.
Screening processes are likely to vary between organizations, but all aim to identify activities with the greatest and/or longest duration sound emissions that may coincide with marine sensitivities and provide an early view on the potential level of risk of potential impacts to marine life associated with underwater sound. This information helps to inform decisions regarding the appropriate level and extent of risk or impact assessment and/or additional studies that may support the project activities.
The proximity of project activities to known sensitive marine species (e.g., species type, hearing frequency range, conservation status) and/or protected/sensitive areas (designated for the purposes of sensitive marine species habitat).
Anticipated sound emission characteristics (e.g., sound type, sound level, frequency range, received sound level vs distance).
Underwater Sound Assessment Process
The figure below shows generic steps used for assessing potential impacts of underwater sound and the types of information or activities that may influence each step.
Assessments begin with the identification of project activities that are likely to generate underwater sound, such as geophysical surveys, drilling and infrastructure construction, which may include activities such as piling, dredging and pipelaying.
A number of review studies are available describe types of sound sources associated with oil and gas E&P activities and example source and sound levels derived from existing measurement studies. There are also web-based resources available, which provide an overview of the fundamentals of underwater sound and propagation (source) (source).
Location, Physical Footprint, Timeline/Schedule & Duration
In addition to the type of sound sources and likely sound intensity associated with the various sources, other information such as activity location, physical footprint, timelines, and duration of activities are important to help inform the assessment process.
Sound from E&P activities can be characterized as being either impulsive or continuous. Impulsive sounds are pressure pulses with rapid rise times, followed by decay that may include oscillating maximum to minimum pressure. Sounds made by seismic air gun source arrays are impulsive, with durations typically much less than a second, and with a repetition rate or ‘duty cycle’ of up to ten seconds for most marine survey applications. Other impulsive sounds include pile strikes and some imaging sonars.
Continuous sounds do not have the rapid rise time of a distinct pulse, but can persist for long periods of time (hours or days). Examples of continuous sounds include drilling, excavation, ships, and mechanical equipment.
The sound level from a given sound source (typically called source sound level and denoted in terms of dB … @1m) is determined by the physical characteristics of the sound source. For example, for a conventional marine seismic source array, this is a function of the number and volume sizes of source elements.
The amplitude of sound level generally decreases with increasing distance from a given sound source due to transmission loss as a sound wave travels away from the source and absorption/scattering effects occur. The variation of sound level with distance is a function of various physical environmental factors, such as water depth, temperature and salinity and seabed characteristics.
Different metrics, such as Sound Pressure Level (SPL) and Sound Exposure Level (SEL) both use the decibel (dB) unit. This is further complicated as sound pressure level can be defined as a peak, peak-to-peak, zero-to-peak or RMS (route mean square) value depending on whether a sound is considered continuous or impulsive. The dB unit is used whether sound is traveling through air or water, but comparing sound in air and water must be done carefully. Sound waves with the same intensity will have a higher dB value in water than in air. This is primarily due to the difference in density between water and air.
Likewise, care must be taken when comparing between sound level values expressed in different metrics, such as SPL and SEL, SPLpk and SPLrms, SELss and SELcum
To assess potential impacts of underwater sound on marine species and/or habitat areas, it is necessary to estimate how sound level varies with distance from a sound source. This can be achieved by either using simplified propagation functions (for example cylindrical or spherical spreading) or more complex modelling methods and site-specific data (depending on the complexity of the area of interest and the data available).
The manner in which sound propagates through the ocean depends on a number of factors that must be accounted for by the model. These parameters include the bathymetry of the ocean floor, the physical properties of the sea bottom (sand, silt, or hard rock) as well as the temperature and salinity profiles of the water column. Models may be required to be constructed for several representative sites that cover typical water depths and sea bottom conditions across the area of interest to test the extent of possible impacts.
Various consultant/contractor resources offer sound source and propagation modelling services using either proprietary or commercially available software applications, which in turn utilize a variety of algorithms or techniques (such as ray tracing, finite difference/wave equations, and/or parabolic equations). All of these have various capabilities and limitations that should be considered relative to both the sound source being modelled and the hearing sensitivities of the marine species being assessed.
Complex sound modelling methods will require additional site-specific information such as bathymetry/water depth, sound speed profiles and geoacoustic characteristics of seabed sediments.
Previous sound measurement datasets may be available for the specific project or neighbouring areas, which may provide information related to background sound level and general soundscape characteristics.
A comparison between modelled ‘received’ sound levels and background sound levels may help assess at what distances sound associated with a specific activity may decrease to below background levels and therefore no longer likely to be detected by marine species.
Location of protected areas and/or distribution, abundance and seasonal behaviour information for marine mammal and other marine species or habitats relative to the planned activity may be available via national or international database tools or archives. Hearing sensitivity are available in the literature for various marine mammal species groups.
For areas where such information is limited, additional baseline data collection may be considered to help inform knowledge and understanding of marine species presence/absence and seasonal variability within and around the project area.
Understanding potential effects of underwater sound on marine life is an area of active and ongoing research. Therefore, a review of up to date and relevant scientific literature should be conducted to provide additional qualitative information to help inform the overall assessment process.
Potential effects from sound on marine mammals, turtles and fish are generally categorized in terms of:
physical auditory injury/impairment or
behavioural disturbance, such as changes in movement or interruption of activity
masking, where sounds of interest may be difficult to detect or interpret due to the presence of competing sound
Acoustic exposure estimation
Species – Sound level vs threshold comparison
The primary factors determining whether an animal can sense a sound signal, and potentially react to it, are the frequency of the signal and the strength of the signal in relation to the natural background sound level.
A comparison between the hearing sensitivities of marine species expected to be present in the area with the sound levels and signal characteristics, such as frequency range, will identify whether sound from a given activity may be detected.
Models are commonly used to estimate what sound levels animals in a specific area may be exposed to during operations and how the intensity of the sound attenuates with distance away from its source. Model results are interpreted for the species groups that may be present in an operational area. This analysis is conducted taking into account auditory weighting functions (estimates of how loud a sound must be at a particular frequency to be heard by an animal) to assess the impact on marine fauna in different hearing groups. Different animal groups have different regions of best hearing. Outside this frequency range, they cannot hear the sound, much like humans cannot hear a very high frequency dog whistle. As scientists are continuously learning more about the hearing of animals the curves are refined and the ability to predict potential impacts is improved.
Marine mammals are typically divided into four hearing groups based on their range of best hearing:
Population – Area/Species Density vs Population Consequence
If marine species density data is available in terms of numbers of individuals per km2 for an area where activities are planned, total numbers of individual animals that may be exposed to sound may be calculated for a given spatial footprint of sound level above a given threshold value. Alternatively, and more realistically, estimated numbers of individuals may be determined using sound exposure modelling accounting for animal movement characteristics.
In both cases, the numbers of animals can be compared to overall population numbers to indicate a percentage of population that may be impacted by sound from a given activity. The number of animals or percentage of population that may be potentially impacted by sound can then be compared to levels that may be defined for some species at a national level to safeguard a species’ conservation status. In the absence of such national criteria, potential impacts at a population level can be assessed qualitatively.
Assessing potential population or ecosystem level impacts is an area of ongoing research. The science used to investigate this significance is called the “Population Consequence of Disturbance” (PCoD). Such methods look at whether impacts to individuals can affect health or the ability to reproduce and ultimately whether that is likely to affect population dynamics for the species.
The potential consequence of a disturbance to a population depends on wider, ecosystem-related factors, such as animal health and life stage, activity (feeding, breeding, migrating), and the health of the animal’s local environment (prey location and availability, toxins, chronic stressors). Very detailed field studies, coupled with sophisticated models, seek to understand the key sensitivities that, if avoided, prevent a disturbance from have any impact at the population level. The impacts considered can be both acute (resulting in mortality) and chronic (repeated small impacts or chronic stressors).
In recent years, significant progress has been made in the application of the PCoD analysis method via two pathways. The first is a data-driven or ‘bottom-up’ approach that uses bioenergetic and population dynamic models to identify population-level responses for given disturbance scenarios. This model has been applied to several marine mammal species for which long term empirical data is available. The second is an ‘interim’ or ‘top-down’ approach that uses an expert elicitation process in the absence of species specific data, whereby opinions of multiple experts are used to quantify how much temporal disturbance may cause changes in vital rates that in turn may result in population changes. The resultant relationship functions are then combined with statistical analysis and population modelling methods. The latter is commonly referred to as ‘Interim PCoD’.
Such analysis typically involves significant levels of baseline population and behavioural response data being available for the species of interest, which may not be available during the planning phases of a project. The applicability and use of such analysis methods should be considered on a case by case basis depending on the scale of activity, sensitivity of receptors and availability of appropriate information for the identified receptors.
Stage 4a: Impact scoring/evaluation
Scoring or rating criteria for both ‘Impact’ significance in terms of magnitude and receptor sensitivity and/or ‘Risk’ in terms of consequence and probability are likely to vary between organizations. However, the intent of the process is to identify and prioritize potential impacts or risks and then in turn prioritize appropriate mitigation measures to reduce them.
Evaluation of either impact significance or risk is often presented on a matrix (Figures XXX).
Stage 4b: Cumulative impacts
Cumulative impacts are often assessed in terms of spatial and temporal combined or additive effects the planned activity with other existing or planned activities at the same time or location.
Continued advancements in acoustic modelling, and the applicability of PCoD type analysis methods, are expected to contribute to assessing cumulative impacts in the future.
Understanding the spatial and temporal characteristics of the sound generated by a sound-producing operation facilitates the design of mitigation measures to prevent both injury and population-level disturbance of marine fauna during operations, such as designating exclusion zones that extend well beyond the distance where injury or significant disturbance can occur.
Distance calculations to ‘impact’ threshold levels may be used to determine distances from or zones around an activity for which some level of monitoring (typically visual and/or acoustic) and/or mitigation actions may be implemented. For example, if an animal is observed within the calculated distances at which hearing impairment may occur, operations may be delayed or shutdown. Such actions reduce the likelihood of an animal experiencing high sound levels and therefore avoid likelihood of hearing impairment occurring.
In some geographic areas, there may be national regulatory requirements or permitting conditions that include generic mitigation and monitoring activities to be implemented during specific offshore activities. In the absence of national requirements, industry recommended measures may be adopted with any additional measures that are identified during the Sound Disturbance Assessment process.
Mitigation measures may include both pre-activity planning informed by additional baseline data collection as well as operational measures.
Pre-activity planning may include:
Where significant knowledge gaps are identified through project screening or assessment studies, baseline data studies may conducted. For example, marine mammal monitoring to identify/confirm seasonal presence/absence of marine species. Or soundscape measurements to verify sound source modelling assumptions.
Temporal and spatial planning for operations to ensure avoidance of environmentally sensitive areas during key times of year if sound may influence breeding or disrupt migrations.
Operational mitigation measures are carefully designed and implemented to address site-specific environmental conditions of each sound-producing operation to avoid exposing marine fauna to sound or, in case avoidance is not possible, to ensure that sound exposures do not harm marine fauna. Mitigation measures need to be appropriate to the identified level of impactor risk of impact, cost-effective, and operationally practical to implement.
Common operational mitigation measures designed to reduce the risk of harm include:
Soft starts: gradual increase in sound levels, designed to gently clear the immediate area around the sound source of most fish.
Survey equipment: designed to avoid injuring marine fauna or polluting the habitat. Turtle guards on towed equipment prevent accidental entanglement and drowning. Gel-filled streamers avoid spills into the ocean in the event of accidental cable damage.
Visual observers: identify marine fauna, record sightings and behaviour and ensure exclusion zones are clear before soft starts.
Passive Acoustic Monitoring (PAM) can be used to complement Visual Observers. Sound sensors are placed in the water and specially trained operators listen for the vocalizations of marine mammals, which allows detection and estimated location of certain species of cetaceans at night and when visibility is poor.
Adaptive risk management improves the effectiveness and efficiency of mitigation measures during operations. One key advantage of adaptive risk management is the ability to observe the effectiveness and efficiency of mitigation measures during operations in a specific site and then optimize the measures for the next phase of operations either at that site or similar sites. As additional data becomes available, it may show that actual risk is different than originally perceived and inform how mitigation measures can be refined.