MacGillivray_NOGRF_2012 - North Slope Science Initiative

Report
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
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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)
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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:
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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

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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.
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
> 1000 m
Ocean basin
M/V Jim Kilabuk
Instrumentation
 Acoustic sensors were JASCO AMARs
 Autonomous Multichannel Acoustic Recorder
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Digital underwater sound recorders
 AMAR configuration:
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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
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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
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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:
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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:
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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:
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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?

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