Model-based Estimation of Noise Impact Zones for Deep Offshore Seismic Surveys Alexander...
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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
Model-based Estimation of Noise Impact Zones for Deep Offshore Seismic Surveys Alexander MacGillivray, Marie-Noël R. Matthews JASCO Applied Sciences,
Model-based Estimation of Noise Impact Zones for Deep Offshore
Seismic Surveys Alexander MacGillivray, Marie-Nol R. Matthews JASCO
Applied Sciences, Victoria BC N ORTHERN O IL AND G AS R ESEARCH F
ORUM 2012
Slide 3
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
Slide 4
Background: Regulatory Context Noise from marine seismic
surveys can potentially have negative effects on marine mammals: 1.
Behavioural disturbance (harassment) 2. 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
Slide 5
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 Pa
for Whales 190 dB SPL (rms) re 1 Pa for Seals, Walrus, and Bears
Behavioural Disturbance (level B take): 160 dB SPL (rms) re 1 Pa
for Whales 120 dB SPL (rms) re 1 Pa 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. The sound output of the
seismic source (e.g., airgun array design) 2. The environment where
the source is operating (e.g., water depth)
Slide 6
Methods for Estimating Impact Zones F IELD M EASUREMENTS 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) M ODELLING 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
Slide 7
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. Pre-season acoustic modelling 2. 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
Slide 8
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 1. Marine Operations Noise Model (MONM) Propagation Model 2.
Airgun Array Source 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
Slide 9
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 1. Intermediate depth: 500-1000
m Continental slope 2. Deep water: > 1000 m Ocean basin M/V Jim
Kilabuk
Slide 10
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)
Slide 11
Bottom Moored AMARs (< 800 m depth) I NTERMEDIATE D EPTHS
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
Slide 12
Towed from Vessel (> 1 km depth) D EEP W ATER 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. Vessel
drifting with engines off 2. Hydrophone isolated from surface waves
with suspension system Sampled at ~15 locations to measure
different distances and directions
Slide 13
SSV Measurement Locations
Slide 14
Data Processing Data were downloaded from AMARs after
completion of measurements at each site Data were processed using
JASCOs custom data analysis suite: 1. Airgun pulses automatically
identified using feature extraction algorithm 2. SPLs for each
pulse computed according to standard methods Pistonphone
calibrations performed before and after AMAR deployment to ensure
accurate sound level reporting
Slide 15
Examples of Airgun Sounds 1 km5 km10 km50 km
Slide 16
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?
Slide 17
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
Slide 18
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
Slide 19
Questions? Acknowledgements: Thanks to Party Chief and Crew of
M/V Western Neptune (WesternGeco)Thanks to Party Chief and Crew of
M/V Western Neptune (WesternGeco) Thanks to Captain and Crew of the
M/V Jim Kilabuk (NTCL)Thanks to Captain and Crew of the M/V Jim
Kilabuk (NTCL)