MacGillivray_NOGRF_2012 - North Slope Science Initiative
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Transcript MacGillivray_NOGRF_2012 - North Slope Science Initiative
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?