InSAR Mapping of displacements and lava

flows related to recent eruptions (2010-2014)

at Piton de la Fournaise

1,3M.G. Bato, 1J.L. Froger, 1M. Tridon, 1V. Cayol, 1A. Harris, 2N. Villeneuve

1Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, Clermont-Ferrand, France 2Laboratoire GéoSciences Réunion, Université de La Réunion, Saint-Denis Messag, France

3ISTerre, Université Savoie Mont Blanc, Le Bourget-du-Lac, France

Grace.Bato@univ-savoie.fr

Displacement maps from the co-eruptive TSX interferograms of the October 2010 eruption. (a). Horizontal displacement map showing a maximum eastward dis-placement of 40 cm and a 10 cm westward movement. (b). Vertical displacement map showing the in-flation of the volcano at a maxi-mum upward movement of 30 cm. Both (a) and (b) are signatures of the opening of a dyke.

OCTOBER 2010

DATA:

In this study, 41 TerraSAR-X—TanDEM-X (TDM) pairs of Stripmap bistatic images were used for which we produced 41 bistatic in-

terferograms. These were acquired from ascending and descending orbital passes between September 06, 2011 and November 12, 2012.

σ)

Displacement maps from the co-eruptive CSK interferograms of the June 2014 eruption. (a). Horizontal displacement map showing a maximum eastward dis-placement of 19 cm and an 8 cm westward movement. (b). Vertical displacement map showing the infla-tion of the volcano at a maximum upward movement of 26 cm. Similar to October 2010 eruption, both (a) and (b) show signatures of a dyke.

JUNE 2014

MONOSTATIC CASE: Displacement maps were not possible to produce using the MONOSTATIC dataset for the June 2014 lava flow due

to the low signal-to-noise ratio present in the image. Similarly, as shown in the left figure, our method to characterize the thickness has

failed for this case because in reality, we should expect a thin flow deposit for the June 2014 lava flow.

BISTATIC CASE: Because of the unique acquisition geometry of the bistatic case, we can only obtain the thickness information of the

lava flow (right figure). This has a mean thickness of about 2 m.

MONOSTATIC DATASET

CSK: 09 July – 19 Sept (8 interferograms, 4 Asc, 4 Desc)

1st image: 18 days after the eruption

BISTATIC DATASET

TDM: 29 June – 10 July (2 Desc)

1st image: 9 days after the eruption

CONCLUSIONS

Co-eruptive data Ground displacement related to the eruption Immediate mapping of the lava flow emplacement using the

coherence

Post-eruptive data Horizontal and vertical displacement of the lava flow Accurate information on thickness, volume and effusion rate

Monostatic data Ground displacements + topographic change Volume + effusion rate ! Requires long period of time to have a good database ! Not very helpful for near-real time monitoring Bistatic data Topographic change High accuracy: Volume + Effusion rate !Good for near-real time monitoring !Good for thin flow deposits

ACKNOWLEDGEMENTS

This research was financed by the French National Program of Spatial Remote Sensing (PNTS, AO2011- 649924), the French Government Laboratory of Excellence initiative n°ANR-10-LABX-0006, the Région Auvergne and the

European Regional Development Fund. This study will not also be possible without the generosity of the Agence Nationale de la Recherche (ANR) through the framework of GEOSUD, ANR-10-EQPX-20 project, ASI through the CSK

2080 project for our CSK images, and DLR for our TSX and TDM images through the LAN 0237 and XTI_GEOL_NR0224 projects, respectively.

The emplacement of lava flows can cause changes in the backscattering properties of the ground surface be-tween the two radar acquisitions so that it appears black in a coherence map. This idea can be exploited to perform an early mapping of the lava flow regardless of any weather and time conditions. This will also gave an initial estimate of the area covered by the lava flow : (a). Coherence map calculated from ascending TSX in-terferogram spanning 33 days (September 19, 2010 to October 22, 2010) gave a 0.71 km2 area estimate. (b). Coherence map calculated from TSX descending orbital pass interferogram that spans 55 days (September 1, 2010 to October 26, 2010) obtained a 0.75 km2 area estimate. The difference in the estimated area is the time cov-ered by each of the coherence maps. While the ascend-ing one only covered the first 9 days of the eruption, the descending covered 13 days, implying that there is a 0.04 km2 of new ground formed after the 9th day.

The results of the CSK-derived characteristic maps in UTM coordinates.

(a). The isopach or thickness map presenting a maximum thickness of 15 ± 0.3 m and an average flow thickness of 5 ± 3.2 m. Negative

thickness values are depressions found outside the lava flow. From this thickness map, a DRE volume of 3.00 ± 0.17 x 106 m3 was obtained.

(b). The horizontal displacement rate, maximum at 4 ± 0.08 cm yr-1 towards the east. (c). The vertical displacement rate, maximum at

14 ± 0.17 cm yr-1. (d). The goodness-of-fit or r2 map, indicating a "good fit" between the actual and the model data if the color is closer to

red (i.e. r2 = 1). (e). The residue or ε map after the inversion.

SUMMARY

DATA:

We used 54 Stripmap Cosmo SkyMed (CSK) images acquired during ascending (swaths HI09: incidence angle, αi=40.13° and HI15:

αi=48.85°) and descending (swath HI18: αi=52.40°) orbital passes between February 15, 2011 and January 01, 2012. From these images

we computed 435 interferograms.

The Piton de la Fournaise, in La Reunion island, is one of the most active basaltic shield volcanoes of the world. Thanks to the initial effort of the Observatoire de Physique du Globe de Clermont-

Ferrand, France, Piton de la Fournaise is also one of the few volcanoes in the world that is monitored with InSAR data on a regular basis. In this work, we present maps of the co-eruptive displacements

related to the October 2010 and the June 2014 eruptions. These maps were produced using whichever of the following satellite data is/are available: Cosmo-SkyMed (CSK), TerraSAR-X (TSX) or Tan-

DEM-X (TDX). The results gave us a clear idea about the mechanism of the eruption (i.e., propagating dyke towards the surface). Then, by computing the coherence of the co-eruptive interferograms,

we have also been able to map the extent of the lava flows and to estimate their areas.

In addition, we have derived accurate thickness map of each lava flows using two alternative techniques. The first one involves a large database of interferograms produced from monostatic CSK im-

ages. The second one is based on the use of few bistatic TSX-TDX interferograms. Results show that CSK database does not only deliver thickness, volume and effusion rate values but it can also provide

displacement rate information after the lava flows were emplaced. However, we emphasize that for this case, accuracy is highly dependent on the number of interferograms used. Evidently, a large da-

tabase such as the one that we have used will require a long time to be built and may not be very compatible with near-real time monitoring. The bistatic dataset, on the other hand, can only provide

thickness, volume and effusion rate measurements but with accuracy even with just two interferograms. Furthermore, even for thin flow deposit such as the June 2014 lava flow, the bistatic dataset

constantly showed exceptional results.

THE PITON DE LA FOURNAISE