Studying Volcanoes With InSAR: Where Have We Been and Where Are We Going?

Howard Zebker, Cody WorthamStanford University

10 Years Ago: Where Were We?• Anticipated several new space radar systems for monitoring

and forecasting of volcanic events• Developing inverse methods to reveal the details of faulting

or pressure changes at depth, and better describe precise magma chamber geometries

• Beginning reliable interferometric imaging, m-scale resolution of mm-scale deformation over wide areas

• Proposed high-res stereo radar for 3-D topographic maps fast events, e.g. rapid dome growth

• Foresaw data collection over all of Earth’s 600 potentially active volcanoes weekly or even daily

• Designing satellite constellations in along-track interferometric formation to map flow velocities

Reality: What actually happened• Anticipated several new space radar systems for monitoring

and forecasting of volcanic events• Developing inverse methods to reveal the details of faulting

or pressure changes at depth, and better describe precise magma chamber geometries

• Beginning reliable interferometric imaging, m-scale resolution of mm-scale deformation over wide areas

• Proposed high-res stereo radar for 3-D topographic maps fast events, e.g. rapid dome growth

• Foresaw data collection over all of Earth’s 600 potentially active volcanoes weekly or even daily

• Designing satellite constellations in along-track interferometric formation to map flow velocities

Reliable imaging• Technology has advanced to where we know

how to do this• Want long wavelength, high resolution, rapid

repeat times• Systems beginning to reflect community

knowledge• Still waiting for the “perfect” system

Longer wavelengths yield higher correlation

ALOS Kilauea interferogram: 460 day separation, 1490 m baseline

Wortham et al., 2010

Time series volcano observationsEyjafjallajokull

PS deformation

SBAS deformation

Time series 1993-2000

From Hooper, 2008

PS image of San Andreas Fault - ERS satellite

PS spacing is ~1km

PS performance

RMS error ~1 mm/yr

Precision of PS method

Higher resolution promotes time series analyses

A coarse resolution cell

Brightest scatterer Brightest

scatterer

A fine resolution cell

• Same brightest scatterer, much less background

• Pixel does not scintillate, is “persistent”

•Bright scatterer perhaps less than rest of background

•Pixel scintillates over time from background signal

New spaceborne systems

• Radar systems launched this decade have expanded the data modalities for InSAR

• New frequency bands and orbit repeat geometries

• ALOS PALSAR (Japan) launched 2006• TerraSAR-X (Germany) launched 2007,

Tandem-X satellite launched 2010• Some commercial systems as well

ALOS/PALSAR

• ALOS satellite, PALSAR radar instrument• L-band, wavelength 24 cm• Repeat period 46 days• Multipolarization• 10-20 m resolution• Very high correlation• Just ended successful

mission

TerraSAR-X

• X-band, wavelength 3 cm• Repeat period 11 days• Multipolarization• 1-20 m resolution• Ideal for time-series observations

Current research …

• Most exciting area is time series analysis• Modeling continues to advance• Tandem satellites supersede previous desire

for radar stereo• New measurements and methods will yield

new descriptors

Time series mimics GPS imaging

From Lundgren et al.

Modeling of Yellowstone caldera

Interferograms: C-band

Interpretation: Inflating sill

From Wicks et al., 2006

Modeling Uzon caldera, Kamchatka

Radarsat measurements2000-2005

(a) Distributed opening model, (b) distributed crack model,

(c) depth slice of model bLundgren and Lu, 2006

Probabilistic modeling of Etna activity• Predict eruptive activity from

observed deformation and thermal flux

• Highest activity from coincident increases

• PDF derived from spaceborne data only

From Patrick et al., 2006

Tandem observations for DEMs

• Higher resolution and accuracy than traditional stereo• Could produce radar stereo, but this method is superior

Mt. Merapi Digital Elevation Model from Tandem TerraSAR-X observations

From DLR

New local measurements

From C. Werner, Gamma Res.

… but some things still lacking

• Enabling technology is coverage, temporal and geographic

• Optimizing designs for InSAR– Orbits: poor north retrieval– System parameters– High resolution, long wavelength helps

• Future mission prospect good/bad/?

Vector deformationSBAS vector solution for average deformation rate at Kilauea, HI

•ALOS yields fairly high correlation over 2 year time span•Near polar orbit results in poor northing component retrieval

Wortham et al., 2010

Poor retrieval – northing componentKilauea: GPS – black line, InSAR – Red symbols

Up

N

orth

E

ast

Wortham et al., 2010

Multiple Aperture InSAR (MAI) Method

N. Bechor, PhD Thesis (2006)

• SLCs formed from forward and backward squinted beams

• Beam filtered in Doppler• Interferograms formed from each

beam

• MAI phase gives along-track displacement from

differencing forward/backward interferograms €

φforward = −4πxλ

sin(θ sq + β )

€

φMAI = −2πλx 1 m ≈ 0.6 rad

Average north displacement using MAI

North component of displacement averaged over all InSAR acquisitions

Coverage•ALOS L-band yields high

correlation over challenging volcanoes

•South America data show comprehensive coverage possible

•46 day repeat is too long- misses many signals

•PALSAR data rate/volume too low to monitor 600 volcanoes

From: Fournier et al., 2010

NASA DESDynI-R Mission• Launch in 20XX• L-band, potential 2 m

resolution• Free and open data policy• Specifically designed for InSAR• Volcano hazards one of the

major science objectives

Artist’s concept from JPL

Additional exciting missions• ALOS-2 (Japan): L-band follow-on to ALOS,

launch 2013?, 1-10 m resolution, 14 day repeat• But likely will be commercially oriented with data

hard to get• Tandem-L (Germany): Similar in philosophy to

Tandem-X, but companion to DESDynI, no money yet

• Sentinel-1 (ESA): C-band heir to Envisat, 12 day repeat, 5 m resolution, 201w? Launch

• There are others…

Summary and looking ahead

• InSAR continues to evolve better accuracy and temporal/spatial coverage

• Volcano hazard applications benefit• Future satellites converging on ~12 day

repeats and m-scale resolution• Limiting factor is probably data policy-

agencies still don’t get the science message and pursue commercialization

• If data are acquired, volcanologists will come