Seafloor geodesy: Background and
Status
Dave Chadwell ([email protected])
Scripps Institution of Oceanography
Most active plate boundaries are in oceanic regions
Process Targets:
1) Gaps in rigid
plate coverage
(e.g., Pacific
north of Hawaii)
2) Mid-ocean
Ridges (how
crust adds to a
plate)
3) Transform
Faults (largely
aseismic)
4) Subducton
Zones
(continental to
oceanic coverage
now possible)
Locked and transition zones lie mostly offshore
Sub-aerial geodesy relies heavily upon electromagnetic
energy from spaced-based platforms (GPS, INSAR).
Electromagnetic
energy does not
travel an
appreciable
distant into
seawater.
( modified from Newman, 2011)
Acoustic energy does propagate efficiently through seawater
and is the basis for underwater ranging in seafloor geodesy.
Components of a Sound Wave
Speed of sound = frequency x wavelength
1500 m/s
= 12 kHz x 0.125 m
Acoustic ranging for centimeter-level positioning
Geometric range =
Travel time
(centimeter level) = (microsecond level)
Transmit
Receive
Transmit signal
x
Sound speed
(10 ppm )
Transponder
Reply signal
(Water column nominal sound speed = 1500 m/s)
~10 cm
Sound Speed vs. Depth
XBT
CTD
SOUND SPEED
Influence of sound speed on acoustic ranging
1. Main challenge is knowledge of sound speed can only be measured with
a accuracy of 30-50 ppm and repeatability of 10 ppm.
Thus, maximum acoustic range of 10 km to maintain precision of +-10 cm
2. Increasing sound velocity with depth causes upward REFRACTION
LIMITS ON ACOUSTIC RANGING…
… leads to three Geodetic Ranging Systems …
Direct path (0-2 km)
(e.g., Chadwell et al.,1999, GRL)
Indirect Direct path
(2-10 km)
(e.g., Sweeney et al., 2005,
Marine Geodesy)
GPS-surface path
(10-1000s km)
(e.g., Chadwell & Spiess., 2008, JGR)
Monitoring the south Cleft Segment of the
Juan de Fuca Ridge
Centimeter-level measurements of acoustic range
across axial valley of south Cleft Segment of the Juan
de Fuca Ridge observed no-motion over five-years.
(If full plate spreading rate was concentrated at this
fault then could expect up to 25 cm of motion.)
US Geological Survey
Direct-Path Transponders:
Transducer
Electronics and
Batteries
Onboard system included:
• Acoustic ranging
3m
• Acoustic telemetry link to ship
• Compass and tilt
Operated from 1994 to 1999
Photos courtesy of Bill Chadwick
Installation of direct-path unit
using MPL Control Vehicle (CV)
aboard R/V Roger Revelle at
2000-m depth.
Acoustic geodesy techniques for cm-level resolution
Direct path (0-2 km)
(e.g., Chadwell et al.,1999, GRL)
Indirect Direct path
(2-10 km)
(e.g., Sweeney et al., 2005,
Marine Geodesy)
GPS-surface path
(10-1000s km)
(e.g., Chadwell & Spiess., 2008, JGR)
Indirect-path acoustic
systems
AUV-based
Towed-vehicle
Bottom-moored for continuous data
Motion at the Gaviota slide?
1 km
~ 400 m depth
http://www.mbari.org/data/mapping/SBBasin/
No detected motion above 10 mm designed
sensitivity
Direct and indirect path only span small, localized regions.
Offshore GPS or GPS-Acoustics combines kinematic GPS with
precision acoustic ranging to seabed transponders to track the
horizontal motion of the seafloor.
(Newman, 2011)
Precision GPS positioning
Coded signal gives pseudorange
from satellite to receiver with mlevel precision.
Phase signal gives mm resolution of
change but has unknown number
of cycles from satellite to receiver.
Combine coded and phase signals
with satellite motion to resolve
relative positions.
How to transfer to the
sea floor?
GPS-Acoustic positioning
GPS for sub
aerial
connection.
Precision
acoustic
ranging for sub
marine
connection.
GPS-Acoustic positioning
Optical survey of phase center offsets
Precise kinematic GPS positioning
Shipboard configuration
Hydrophone inside
instrument well
(Chadwell, OE, 2003)
GPS-Acoustic positioning
Acoustic
range
ship to PXP
GPS-Acoustic positioning
Sound
speed
horizontally
stratified
Transponders deployed around circle
with radius of nominal water depth.
Internal density
waves in the
upper ocean
cause lateral
variations in
sound speed
with nominal
30 minute
periods
GPS-Acoustic positioning
Optical survey of phase center offsets
Acoustic
range
ship to PXP
Initial global
position of
seafloor
array.
Precise kinematic GPS positioning
Shipboard
hydrophone is
common
spatial point
observed both
by GPS from
shore and
acoustic
triangulation
from seafloor
transponders
H(GPS) = A + B
H(PXP) = C+(D + Δ)
Global position of seafloor array is estimated by determining the corrections Δ to D
from H(GPS) = H(PXP) or A + B = C + (D+Δ)
Estimate of East and North corrections (Δ) for each
acoustic interrogation (ping)
Amplitudes from ~ 40 cm with ~30 minute periods
Multiple-hour average reduces effect of internal waves
0.1352 m
0.0573 m
Resolution ≈10 cm / √ 2 x hours
= ~ 7. 5 mm for 90 hours
Contraction at the Nazca – South America plate
Inter-seismic deformation consistent with
convergence rate and stick-slip contact
along the thrust fault.
(Gagnon, Chadwell, Norabuena, Nature, March 2005)
Co-seismic slip – Lesson from Tohoku-Oki Earthquake.
Estimated by terrestrial GPS data
and seafloor GPS/Acoustic data
+
Maximum 56m
(Newman, 2011)
(from GSI web site)
More GPS-A sites in Japan …
An additional ~20 sites (for a
total of ~ 30) to be deployed
in the offshore region of the
Tohoku EQ.
(slide from Ishikawa et al. 2012)
provisional plan
New site
Existing site
(JCG & Tohoku Univ.)
More GPS-A sites in Japan …
Existing station
New station
(Keiichi Tadokora on Tuesday)
TOKAI
Nagoya Univ.
Tohoku Univ.
TONANKAI
NANKAI
Jan. 2012
The JCG installed eight new stations along Nankai Trough
(slide from Ishikawa et al. 2012)
Operational barriers to more GPS-Acoustics
Need less
expensive
alternatives to large
ships for the
surface platform.
Need
continuous
presence.
Need longer
lasting, less
expensive markers
on the seafloor.
Projects addressing these issues ….
Concept …. Moored-buoy for continuous centimeter-level measurements of:
Sea
water
pressure
gauge
•
Horizontal seafloor deformation
•
Vertical seafloor deformation
•
Height of sea surface
•
Water column density
Implementation ….
~2 m
Acoustic
hydrophone
GPS antenna
mounts
Solar panels
Electronics
housings
Battery bank
(SIO: Chadwell, Send, Tryon, )
Deep water (~1200 m) test offshore San Diego
(NSF-OCE-0551363 from OTIC)
Results
…. Seafloor array position updates every 60 sec.)
5 – 10 cm RMS
ping to ping
Wave Glider for GPS-Acoustics
Copyright 2010 Liquid Robotics Inc.
7m
2m
•
•
•
•
Differential wave motion with depth provides propulsion.
Vertical motion of an
Solar panels provide electrical power.
ocean surface wave
Iridium satellites used for command & control from shore.
Rudder on glider steers course.
Harvesting of renewable energy
sources allow the Wave Glider to
operate for months at sea.
Permanent seafloor Benchmarks
GOAL: To provide horizontal and vertical reference marks on the seafloor for the long-term.
In-expensive,
permanent seafloor
benchmarks allow
seafloor transponders
to be placed and
removed with mm
registration.
(Chadwell, Webb, Nooner)
(NSF-OCE-1155305 OTIC)
Scripps Institution of Oceanography:
Seafloor Geodesy
While all (almost) geodetic data onshore the majority of slip-rate deficit occurs offshore.
(McCaffrey et al., GJI, 2007)
Cascadia Subduction Zone
2014-2016 ..
Establish two new site and
restart an existing site on
the Juan de Fuca Plate
Funded by grants from NSF-GeoPRISMS
Vertical geodesy techniques for cm-level resolution
(e.g., Phillips et al.,2008, JGR)
Vertical deformation from sea water pressure
Vertical (pressure) measurements
How is the flank collapsing at
an volcanic island?
(Phillips, Chadwell, Hildebrand, JGR, 2008)
Vertical : GPS + water column density +
water pressure at seafloor.
Surface
~ 8 cm
T/S (11m)
T/S (21m)
T/S (36m)
T/S (51m)
T/S (76m)
T/S (101m)
T/S (151m)
T/S (201m)
T/S (301m)
T/S (402m)
T/S (502m)
T/S (702m)
T/S (902m)
• Density change equivalent to 8 cm vertical
motion.
• GPS recovers sea surface with +/- 6 cm
epoch-by-epoch, and +/- 5 mm over
months compared to nearby NOAA tide
Bottom: 1190m
Will it be possible to
control long-term drift of
sensor that is presently
~ 10 cm/yr?
Can absolute reference
surface be realized over
100s km?
(Eiichiro Araki – DONET Tuesday Morning.)