Model-based Estimation of Noise Impact Zones for Deep Offshore Seismic Surveys Alexander MacGillivray, Marie-Noël R. Matthews JASCO Applied Sciences, Victoria BC NORTHERN OIL AND GAS RESEARCH FORUM 2012 Overview The Project: Acoustic modelling and measurement of underwater noise from a deep-water marine seismic survey (Chevron Sirluaq 2012) The Objective: To verify pre-season model estimates of marine mammal exclusion zones for airgun arrays JASCO used computer-based modelling to forecast exclusion zones for marine mammals Our results showed good agreement between modelled and measured sound levels The deep water environment (500 m – 1500 m) was challenging for performing acoustic measurements The Outcome: Showed computer-based modelling is an effective tool for forecasting underwater noise levels from deep-water seismic surveys Background: Regulatory Context Noise from marine seismic surveys can potentially have negative effects on marine mammals: 1. 2. Behavioural disturbance (harassment) Auditory injury (PTS) Seismic operators implement exclusion zones and other mitigation practices (e.g., soft start) to limit potential impacts In US and Canada, permit applications and environmental assessments require advance estimates of noise impact zones Marine Mammal Impact Zones Regulatory agencies (e.g., NMFS, DFO) use standard sound pressure level (SPL) thresholds to define noise impact zones Although there are minor differences between Canada and the US, the most commonly applied thresholds are as follows: Auditory Injury (level A take): 180 dB SPL (rms) re 1 μPa for Whales 190 dB SPL (rms) re 1 μPa for Seals, Walrus, and Bears Behavioural Disturbance (level B take): 160 dB SPL (rms) re 1 μPa for Whales 120 dB SPL (rms) re 1 μPa for Bowhead cow-calf pairs The size of these zones is not static… different for each survey Sound levels strongly depend on two factors: 1. 2. The sound output of the seismic source (e.g., airgun array design) The environment where the source is operating (e.g., water depth) Methods for Estimating Impact Zones FIELD MEASUREMENTS During survey operations, sound source verification (SSV) measurements are used to determine distances to impact zones Marine SSVs have been done for nearly all Arctic seismic programs over the last 6 years SSV measurements are carried out at the start of a survey (1-2 weeks to complete, typically) MODELLING Computer-based prediction tools Underwater sound propagation is very complex Physics-based acoustic models must be used to accurately predict noise footprints Requires detailed description of source and environment Imperfect knowledge limits model accuracy Sirluaq 3-D Survey 2012 Chevron conducted Sirluaq 3-D survey in Canadian Beaufort Sea during summer 2012 Survey operator was WesternGeco (M/V Western Neptune) JASCO performed environmental acoustics studies: 1. 2. Pre-season acoustic modelling Sound source verification measurements Sirluaq prospect area (EL460) located in very deep water Continental slope and ocean basin (> 800 m) Deep ocean = unique measurement and modelling challenges… Pre-Season Modelling (MONM) JASCO modelled acoustic footprint of airgun arrays (2011) at 5 different locations in survey area using our standard acoustic models: 1. Marine Operations Noise Model (MONM) – Propagation Model 2. Airgun Array Source Model (AASM) – Source Model Model inputs include the following: High resolution digital bathymetry Sound speed profiles in water Geoacoustics of seabed Airgun array design Maps below show contours of SPL around airguns Sound emissions from airguns are anisotropic Airgun arrays are directional Environment is heterogeous Sound Source Verification JASCO performed SSV measurements at start of Sirluaq survey We measured sound levels during 8-15 Aug 2012 using five autonomous recorders We measured sound levels at distances of 50 m to 50 km Two sets of measurements were carried out in distinct water depth regimes Intermediate depth: 1. 500-1000 m Continental slope Deep water: 2. > 1000 m Ocean basin M/V Jim Kilabuk Instrumentation Acoustic sensors were JASCO AMARs Autonomous Multichannel Acoustic Recorder Digital underwater sound recorders AMAR configuration: Calibrated M8E/M8K reference hydrophones Recording bandwidth: 0.01-32 kHz 24-bit 64 kHz audio recording ~30 days of continuous recording (1 TB) AMAR suspended in water column Target recording depth 50-100 m We used two different methods to deploy the AMARs: 1. Moored to bottom at intermediate depth (< 800 m) 2. Towed from vessel in deep water (> 1 km) Bottom Moored AMARs (< 800 m depth) INTERMEDIATE DEPTHS AMAR was suspended in water column using floatation and anchor line Tandem acoustic releases were used to retrieve AMAR 5 recorders were deployed simultaneously to measure sound levels at multiple distances and directions from survey line One mooring was lost during intermediate depth measurements Possible failure of acoustic release system Four remaining recorders was sufficient to characterize footprint of airgun array Towed from Vessel (> 1 km depth) DEEP WATER AMAR was suspended from surface float, connected to vessel via tow line Vessel drifting while recording CTD loggers used to record depth of hydrophone To reduce noise interference from vessel: 1. 2. Vessel drifting with engines off Hydrophone isolated from surface waves with suspension system Sampled at ~15 locations to measure different distances and directions SSV Measurement Locations Data Processing Data were downloaded from AMARs after completion of measurements at each site Data were processed using JASCO’s custom data analysis suite: 1. 2. Airgun pulses automatically identified using feature extraction algorithm SPLs for each pulse computed according to standard methods Pistonphone calibrations performed before and after AMAR deployment to ensure accurate sound level reporting Examples of Airgun Sounds 1 km 5 km 10 km 50 km Model vs. Data Comparison Plots show comparison of model (black) and data (green) Plots show data from multiple recording locations Lower thin line represents SPL at 50 m depth Distance scale is logarithmic Overall model data agreement was good down to 160 dB SPL Model accurately predicted propagation loss trend < 20 km Model predicted shadow zone at ~1-2 km Convergence zone at ~3.5 km range not predicted by MONM – related to imperfect environmental model Critical reflection from seabed? Refraction in water column? Challenges of Deep Water Acoustic Measurements Towed measurements cannot be performed within ~1 km of 3D survey: vessel collision with streamers is major hazard Moorings have many advantages over towed hydrophones: Multiple instruments can be deployed at once (faster data collection) Hydrophones can sample very close to airguns (as close as 50 m) Higher quality acoustic data (less noise) Design of moored hydrophone systems are very complex: Floatation and instruments must be rated for extreme depths Long mooring cables must use low-weight, low-drag materials Deployment of > 1 km mooring from vessel is complex Accurate positioning of hydrophone is difficult Greater risk of equipment loss JASCO is developing deep-water mooring designs for future deep-sea SSV measurements Conclusions Modelling and measurements provide complementary methods for estimating marine mammal impact zones for seismic surveys: Models allow forecasting of impact zones and noise “footprints” SSV measurements allow ground-truthing of model estimates Regulatory compliance often requires that both methods be used Results from Sirluaq 2012 survey show that modelling is an effective method for predicting impact zones in deep water However, acoustic measurements are particularly challenging in deep water environments: More logistically challenging Engineering of moorings is more complex Risk of equipment loss is greater Acknowledgements: • Thanks to Party Chief and Crew of M/V Western Neptune (WesternGeco) • Thanks to Captain and Crew of the M/V Jim Kilabuk (NTCL) Questions?