Sperm whale seismic study in Gulf of Mexico-acoustics focus

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Sperm whale seismic study in Gulf of
Mexico-acoustics focus
Aaron Thode, Marine Physical Lab, SIO, UCSD

• General background
• My role 3D tracking tagged/untagged
  animalsworking a few months at a time
• Single array localization-2000
• Tag/array integration-2001
• Dual-array localization-2002-?
• Upcoming plans

Marine Fisheries Service
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Background-SWAMP cruises 2000-2001
photo Keith Mullin, SE Fisheries Service

• Sponsors
• Minerals Management Service
• International Association of Geophysical
  Contractors
• Marine Fisheries, SE, 2000-2001
• Focus effect of seismic exploration on sperm
  whales
• Endangered species
• Acoustically active
• Resident populations
• Mississippi Canyon
• De Soto

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Present SWSS study centers on two types of tags

• Bruce Mates satellite tag (STAG)
• Long duration deploymentslong-term habitat
  shift?
• Over 15 animals tagged, mostly on one day
• WHOI digital recording tag (DTAG)
• Also pressure, orientation, acceleration
• High-resolution behavioral responses, energy
  studies, 3D pseudeotracks
• Visual, biopsy, acoustic component
• Nineteen animals tagged in 2002, three
  simultaneously under controlled seismic exposure,
  many more under uncontrolled exposures.

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Gulf of Mexico an acoustically unfavorable
propagation environment
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Measured and computed detection ranges predict
3-10km, depending on source depth
Source MATLAB KRAKEN (should also Be
repeated With Bellhop)
Three different towed array systems 30-100m
depth, give 6km range
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Bottom-mounted sensors have similar predicted
ranges
Stennis Space Center (George Ioup) has placed
bottom-mounted sensors in general vicinity of
SWSS. Some coordination, little overlap so far.
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To date passive acoustic data collection program
uses towed arrays with few elements-gtclose range
work

Outgrowth of Jay Barlow work, SWFSC. Also arrays
from Ecologic, WHOI
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My focus has been tracking untagged animals,
under various conditions

• Goal statistically significant samples of
  low-resolution dive profiles under
  tagging/seismic conditions
• Complements high-resolution low volume tagging
  runs.
• Three branches
• Single array localization-2000
• Tag/array integration-2001
• Dual-array localization-2002-?

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SWSS acoustic work involves close follows at slow
speeds
NOAA ship Gordon Gunther 0.5-1.5 kts
930 m
Overnight tracking allowed biopsy, tagging in
morning July 3 typical-slow tow through middle
of traveling pod Silty/muddy flat bottom, depth
measured with fathometer Pod composition
assumed to be females and juveniles TDR was
NOT attached during this particular sequence
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tdt
tdb
tds
Bearing 1
Bearing 2
q
R
za
a
zw
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• Example from one dive
• Good depth resolution from 100 m depth
• Range uncertainty increases with animal depth
• Tracking ends when bottom returns vanish

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Ray refraction may be neglected for ranges less
than 1 km
True range (m)
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• During first stage of dive cycle inter-click
  interval is closely related to two-way travel
  time from whale to bottom.
• During second stage bottom bounces vanish and
  timing becomes irregular.
• Why so many clicks related to bottom? (New
  Zealand counterexample)

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Some work has been performed on merging tag/array
data

• Simultaneous recordings on tag/array corrects
  pseudotrack.
• Acoustics does not have control of ship during
  most tagging operations, with interesting results.

Array bearing (deg)
Time (sec)
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The inter-click interval (ICI) used to identify
tagged whale out of 7-13 other animals
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Using surface reflections only gives three
different time-of-arrivals, with two arrays
Ishmael display David Mellinger
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Two arrays can eliminate need for bottom
reflection, if array depths measured

• Restrictions
• Array depths known (difficult!)
• Depths gt 40 m (slow towing speed)
• Doesnt work broadside.
• Assumes straight-line propagation

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Sept. 5 proof-of-concept trial demonstrated
results
Range vs. time
broadside
endfire
Depth vs. time
? Array depth had to be estimated for one
array. ?Measured descent rate of 91 meters per
minute, similar to 88 meter per minute
measurements from next-day tag.
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Two arrays plus tag refines localization-leverage
s tag info
2001-Array depths still the bug-bear!
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Everything seems up in the air, as opposed to in
the water

• What ship for seismic playback?
• There was a lot of uncontrolled seismic exposures
  in 2002
• What ship for research?
• Last years ship was way too noisy
• What holes are there?
• The off axis acoustic signature of seismic
  vessels is not known.
• Modeling by IAGC free-space only, no waveguide
  effects included.
• Better arrays needed
• Bottom-mounted sensors? Stennis?