MEASURING THE RATE OF LAVA EFFUSION BY INSAR

G.Wadge

Environmental Systems Science CentreHarry Pitt Building

3 Earley Gate, University of ReadingReading RG6 6AL, UK

Email:gw@mail.nerc-essc.ac.uk

ABSTRACT

The rate at which lava emerges from a volcano is a fundamental property of the dynamics of the eruption. Intensive fieldmeasurements can capture this. However, for many, often cloud-covered, volcanoes with long-lived eruptions, spaceborneInSAR provides a potentially useful source of information. Repeated DEM creation at intervals allows the changingthickness of the lava flow field to be measured and incremental changes to calculate the volumetric lava flux rate. ERS datafrom (i) an andesitic lava dome eruption at Soufri re Hills, Montserrat , and (ii) a basaltic andesite lava flow-field at Arenalvolcano, Costa Rica illustrate the method. There are two main limitations. Firstly, flowing or otherwise thermo-mechanically unstable surfaces that are active between interferogram pair acquisitions leads to decorrelation. This effect isparticularly difficult on lava domes where the surface is extremely dynamic. Compound lava flow-fields are more tractable.Secondly, very slight motions on flows that have "stopped" can be confused with topography in repeat-pass interferograms.The InSAR-measured rate of lava effusion at Arenal fits well with rates calculated by other methods over the last 30 years.Radar systems best suited to this task should be L-band, have short orbit repeat intervals and moderate perpendicularbaselines.

1 INTRODUCTION

The mean duration of volcanic eruptions globally is about 50 days. Apart from the geostationary satellites, spaceborneremote sensors generally have repeat cycles that are too long to measure the dynamics of processes within eruptions in anysystematic way. However, there are a small number of volcanoes that are in eruption for years or decades. These volcanoesare important for the opportunity they provide to study the dynamics of magmatic systems. In particular, the record of massflux through the volcano provides the first order measure of eruptive dynamics. At some volcanoes we can measure thismass flux by calculating the incremental addition of lava flow volumes from changes to the topography of the volcano. Thispaper is concerned with the potential of radar interferometry (InSAR) to make such measurements.

Traditionally, ground-based measurements of lava flow volumes are made from flux estimates at the vent, field mapping offlow margin thickness and area and instrumental field survey [1]. Remote survey by photogrammetry [2], airborne InSARand lidar can be used but are expensive for frequent surveys. The value of repeat-pass spaceborne and single-pass airborneInSAR for measuring new lava flow volumes by creating digital elevation models (DEMs) before and after the eruption hasbeen demonstrated [3]. The measurement context expored here in long-lived eruptions is more challenging. The new DEMwill represent a surface that is changing even as the measurement is being made.

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Proc. of FRINGE 2003 Workshop, Frascati, Italy,1 – 5 December 2003 (ESA SP-550, June 2004) 112_wadge

2 LAVA FLOWS FROM LONG-TERM ERUPTIONS

Fig.1 demonstrates the spatial character of new lava flows growing at different rates of magma flux. There are twomorphological end-members. Lava domes are rarely more than 1 km2 in area but may be many hundred of metres thick andare supplied from a vent hidden beneath the dome itself. Lava flowfields are inherently compound bodies up to 10 km2 inextent consisting of numerous discrete flows. The time interval that must elapse before a given volumetric flux of lavaproduces a sufficient increment in the height of the land surface that can be measured with confidence depends on the arealdistribution (Fig.1). For an InSAR measurement capability that cannot detect height changes below, say, 10m, then changesover 35 days, the normal time interval between ERS images, will not be detectable except at lava domes at flux rates ofabout 10 m3s-1 or more. Most long-lived eruptions have volumetric flux rates between about 0.1 and 15 m3s-1.

1

10

100

1000

0.11 105

Area (km 2)

MeanThickness (m)

10 m 3/s

1 m 3/s

0.1 m 3/s

10 m 3/s

1 m 3/s

Accumulation over 1 year

Accumulation over 35 days

Domes

Flowfields

Fig.1 Area-thickness variation plot for lava flow accumulation at two rate of volumetric flux over two periods.

3. INSAR MEASUREMENT OF CONTINUOUS TOPOGRAPHIC CHANGE

3.1 Lava Dome: Soufri re Hills Volcano, Montserrat

Ascending pass ERS tandem interferograms (there are no decending pass interferograms possible) of this lava dome werecollected between 1997 and 2000 [4]. Coherence is strong over the flanks of the volcano covered by new pyroclastic flowsand other deposits, but is weak or non-existent over the forested areas and most of the lava dome for most of theinterferograms. This applies even for the period between March 1998 and November 1999 when lava flow extrusionstopped. Thus it is possible to unwrap the interferogram successfully over the new deposits everywhere except for the domeitself (Fig.2). The agents that decorrelate the surface of the dome over 24 hours are various forms of instability includingrockfall, effusion of new lava, thermal contraction and block rotation. We do not know at what time interval of repeatimagery the dome could become coherent at C-band. The presumption then is that single-pass InSAR is required to mapconsistently the changing shape of lava domes.

Fig.2 Topography of southern Montserrat in early 1999. The left panel shows the topography derived fromphotogrammetry. The right panel shows the topography derived from an interferogram using a pair of tandem ERS images(39914-20241, 4/5 March 1999, B⊥ = 253m). The black circle marks the position of the lava dome.

3.2 Lava Flowfields: Arenal Volcano, Costa Rica

Arenal volcano in Costa Rica has been producing a compound lava flowfield (and subordinate explosive deposits)continuously since 1968 [2]. At any time one or two lava flows with typical dimensions of 2000 x 200 x 20 m are usuallyflowing from the summit crater down the slopes of the western half of the volcano over a height range of about 900 m.In general the available descending pass tandem pair interferograms of Arenal from 1993 to 1999 show good coherenceover the flowfield for pairs with perpendicular baselines up to about 500m. These interferograms can be unwrapped withvarying degrees of success. Fig.3 shows the tandem ERS interferogram for 5/6 October 1997. When unwrapped theresulting surface, after registration and ground calibration can be subtracted from a DEM of the volcano representing thepre-eruption topography to give an image of thickness of the flowfield between 1968 and 1997. The volume and thicknessdistribution of this compares well with a similar exercise for the SRTM-derived DEM for the period 1968-2000. Theincremental volumes from these sources adds important information to the long-term magma budget calculations begun in1983 [2].

Fig.3 Top panel: wrapped interferogram of the Arenal volcano flowfield using an ERS tandem pair (32564-12891, 6/7 Oct.1997, B⊥ = 376m). Lower panel: flowfield thickness map representing the difference between the topography of the tandempair and the pre-eruption surface shown in shaded relief. The colour scale is from 0 to 270m and the width of the lowerpanel is 8.5 km.

24 hour separation repeat-pass ERS InSAR works well for the lava flowfield of Arenal. Some 35-day ERS interferogramsalso contain useful information. However flow instability leading to decorrelation becomes apparent over this period(Fig.4). Also anomalous fringes are apparent in some parts of the flowfield where residual differential (flow) motion hasoccurred that is not yet sufficient to decorrelate the signal (Fig.4). If not isolated from the parts of the interferogram used tocalculate the DEM then this could be mistaken for topographic change [5]. This issue is of considerable importance for thecapability of any repeat-pass SAR system to capture the topography of continuously evolving flow fields as at Arenal.

Fig.4 35-day interval ERS unwrapped interferogram (11679-12180, 4 Sept. - 9 Oct. 1993, B⊥ = 123m) of the Arenal lavaflowfield from 1993. An incoherent active lava flow is evident (a) whilst to the south (b) another flow, now "stopped" ismoving sufficiently to modify the phase pattern.

4 SUITABILITY OF SAR SYSTEMS

Which existing and planned satellite-based InSAR systems are most able to overcome the problem of decorrelation andflow-induced errors? We ignore the issue of single-pass InSAR and very rapid repeat cartwheel InSAR and variableincidence angles. The main relevant system variables are radar wavelength, perpendicular baseline and orbit repeat interval.Longer wavelength reduces the decorrelation due to phase ambiguity in proportion to wavelength. If baselines are too smallthen the topographic signal will be too weak. Finally, the interval between images controls directly how much motion isrecorded. Thus systems with large wavelength, short repeat intervals and larger baselines are best. The combined effect ofthese three factors are expressed in Table1 as a normalised index of robustness of DEM measurement to motion error. Thevalues for average perpendicular baseline are indicative only. The Palsar radar on ALOS and TerraSAR-L are the mostpromising for this application.

Table 1. Approximate InSAR system variables relevant to the robustness of DEM values to lava flow motion for severalSAR systems normalised to the ERS/ENVISAT value.

ERS/ENVISAT

RADARSAT PALSAR TERRASAR-X

TERRASAR-L

EVINSAR

λλλλ (cm) 6 6 24 3 24 24Av. B⊥⊥⊥⊥ (m) 300 500 300 100 100 10T (days) 35 24 46 11 16 11Robustness tomotion error

1 1.1 3 0.5 2.9 0.4

a

b

5 CONCLUSIONS

• InSAR has a role to play in measuring magma flux rates of long-lived eruptions• Single-pass InSAR is needed to measure lava dome growth• ERS tandem DEMs of the active lava flowfield at Arenal Volcano yield useful information on magma flux rates• The main problem for repeat-pass InSAR at long-lived lava flows is surface decorrelation and between-image flow

motion being mistaken for topography.• The best suited current and future SAR systems for this application are L-band systems with short orbit repeat

intervals and moderate baselines, such as Palsar and TerraSAR-L.

6 ACKNOWLEDGEMENTS

The Montserrat data were supplied via an ERS AO-3 data grant whilst the Arenal work was supported by a Category-1grant to GW and Dr Paul Cole. I would like to acknowledge the help of Dr. Nicki Stevens.

7 REFERENCES

1. Stevens, N.F., Murray, J.B. and Wadge, G., The volume and shape of the 1991-93 lava flow field at Mount Etna, Sicily.Bull. Volcanol., Vol.58, 449-454, 1997.2. Wadge, G.. The magma budget of Volcan Arenal, Costa Rica from 1968-1980. J. Volcanol. Geotherm. Res. Vol.19,218-302, 1983.3. Lu, Z., Fielding, E., Patrick, M.R. and Trautwein, C.M., Estimating lava volume by precision combination of multiplebaseline spaceborne and airborne interferometric synthetic aperture radar: the 1997 eruption of Okmok Volcano, Alaska.IEEE Trans. Geosc. Rem. Sens., Vol..41, 1428-1436, 2003.4. Wadge, G., Scheuchl, B., Cabey, L., Palmer, M.D., Riley. C., Smith, A., and Stevens, N.F. , Operational use of InSARfor volcano observatories: experience from Montserrat. Proc. FRINGE99. ESA Publication SP-478, 1999.5. Stevens, N.F., Wadge, G. and Williams, C.A. , Post-emplacement lava subsidence and the accuracy of ERS IfSARdigital elevation models of volcanoes. Int. J. Rem. Sensing, Vol., 22, 819-828, 2001.