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ANNALS OF GEOPHYSICS, 62, 2, VO220, 2019; doi: 10.4401/ag-7875

“3D LAVA FLOW MAPPING OF THE 17–25 MAY 2016 ETNA ERUPTION
USING TRI-STEREO OPTICAL SATELLITE DATA„
Gaetana Ganci*,1, Annalisa Cappello1, Vito Zago1,2, Giuseppe Bilotta1, Alexis Hérault1,3,
Ciro Del Negro1
(1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Catania, Italy
(2) Dipartimento di Ingegneria Elettrica Elettronica e Informatica, Università di Catania, Catania, Italy
(3) Conservatoire National des Arts et Métiers, Paris, France

1. INTRODUCTION

Mount Etna is one of the most active and hazardous
volcanoes in the world, well known for the persistent
activity from the summit craters and frequent lava
flow-forming eruptions from vents situated on the
flanks of the volcano [Cappello et al., 2013; Acocella et
al., 2016]. From 2011 to 2017, numerous eruptive
episodes occurred; most of them characterized by the
emission of lava fountains, pyroclastic material, and
short-lasting lava flows that mostly spread within the
Valle del Bove [Cappello et al., 2018; Ganci et al., 2018].

On the evening of May 17, 2016 an intense Strom-
bolian activity started at the North-East Crater (NEC),
followed by the emission of volcanic ash. This eruptive
activity ended the following day, when the Voragine
(VOR) began to erupt, producing a pulsating lava
fountain and a small lava flow from the western rim
of the Voragine-Bocca Nuova depression (Figure 1).
On 18 May, a new eruptive vent opened at the eastern
base of the NEC, near the two cones that formed dur-
ing the July-August 2014 activity, which fed a lava
flow that expanded towards the northern wall of Valle
del Bove. This lava flow remained active until the early

Article history
Receveid August 8, 2018; accepted September 7, 2018.
Subject classification:
Etna volcano; Satellite remote sensing; Pléiades imagery; Digital elevation model.

ABSTRACT
During basaltic eruptions, the average rate at which lava is erupted (effusion rate) is one of the most important factors controlling the evo-

lution, growth and extent of the flow field. This has implications both for forecasting purposes, highlighting the importance of the effu-

sion rate as input parameter of physics-based numerical models, and to advance knowledge on the shallow feeder system by constrain-

ing the supplied mass. Satellite remote sensing provides a mean to estimate the average effusion rate by applying a direct conversion from

the measured radiant heat loss by an active lava flow. This conversion relies on a set of parameters of lava (e.g. rock density, heat capac-

ity, vesicularity, emissivity, etc.) and suffers of multiple sources of uncertainties and measurements errors, whose quantification is still an

open problem. Here we constrain the volume of lava emitted at Mt Etna on 17-25 May 2016 and emplaced out of the summit craters, by

using pre-eruptive and post eruptive digital elevation models (DEMs) obtained processing satellite images acquired by the Pléiades con-

stellation, which provides images at 50 cm resolution in stereo and tri-stereo mode. The 3D processing of the tri-stereo Pléiades imagery

(acquired on 24 December 2015 and 18 July 2016), performed using the free and open source MicMac photogrammetric library, provides

estimations of the distribution of thickness and the bulk volume emitted. The integration of multi-platform remote sensing products rep-

resents a new potential of merging capabilities to enable a more comprehensive response to effusive crises.

morning of 19 May. During the morning of 19 May a
new paroxysm occurred at the VOR, with the emission
of a new lava overflow toward west, which descended
over the lava of the previous evening. The sequence
of summit eruptions at Etna in May 2016 ended with
two episodes of intense Strombolian activity at the
NEC during the night of 22-23 May, and at the VOR
between 24 and 25 May, which fed multiple flows that
covered the lavas emplaced during the 18-19 May
events.

Mapping these volcanic deposits constitutes a crit-
ical component for constraining the average rate at
which lava is erupted, controlling lava flow morpho-
logical parameters, and giving insights into emplace-
ment processes [Del Negro et al., 2016]. However, the
calculation of the amounts of lava erupted is still an
open issue, and problems arise when trying to quan-
tify the associated uncertainties [Ganci et al., 2018].
Photogrammetry, using both aerial- and satellite-im-

agery, has been proven to be useful in monitoring
volcanic activity and mapping volcanic hazards [Baldi
et al., 2000; Gwinner et al., 2000; James et al., 2014;
Neri et al., 2017; Smets et al., 2017; Carr et al., 2018].
The recent availability of high spatial resolution data
acquired in stereo or tri-stereo configuration (e.g.,
Pléiades) gives new opportunities for frequently up-
dating the topography and modeling the 3D shape of
volcanic products by differencing successive to-
pographies.

Here we employed Pléiades data, which provide
images at 50 cm in stereo and tri-stereo mode, to de-
rive digital elevation models (DEMs) and map the
products emitted during the eruptive episodes oc-
curred at Mt Etna between 17 and 25 May, 2016. We
also computed some morphometric parameters (area,
maximum length, thickness, and volume) from the
three-dimensional distribution of the volcanic de-
posits obtained by DEM difference.

GANCI ET AL.

FIGURE 1. Elevation change obtained by differencing the two DEMs derived from Pleiades images acquired before (24 December
2015) and after (18 July 2016) the May 2016 Etna eruptions. Colors indicate the flow thickness in meters inside the lava
flow fields. Flow a was emitted during the 18-19 and 22-25 May eruptive episodes occurred at BN and VOR. Flow b is
the lava flow fed on 18-19 May by the eruptive vent opened at the eastern base of the NEC, near the July-August 2014
cones. The inset shows the zero-peaked histogram of the terrain residuals, proving the two DEMs are properly aligned.
By Nuth and Kääb co-registration, the standard deviation of elevation changes was reduced from 5.3 to 1.6 m.

2. METHODS
We produced two DEMs at a spatial resolution of 2

meters, starting from Pléiades primary images acquired
on 24 December 2015 (pre-eruptive) and 18 June 2016
(post-eruptive) in tri-stereo mode containing RPC (Ratio-
nal Polynomial Coefficient) files. Both images have been
acquired with minimal cloud percentage (i.e. less than
5%) and cover an area of about 15 km × 15 km, which
includes the summit area of Etna volcano and almost all
the Valle del Bove.

The processing of the tri-stereo Pléiades imagery was
performed using the free and open source MicMac soft-
ware (Multi-images Correspondances, Méthodes Au-
tomatiques de Corrélation) [Rupnik et al., 2017] that
performs: (i) tie points recognition and matching between
images; (ii) calibration and orientation, recognizing rela-
tionships between viewpoints and objects; (iii) correla-
tion, producing dense matching for 3D scene
reconstruction. To retrieve areas, volumes and thickness
distribution of the lava flows emplaced out of the sum-
mit craters on 17-25 May 2016, the difference between
post eruptive and pre eruptive DEMs was computed.

A crucial step for deriving this difference is the co-
registration between the two DEM datasets, since mis-

aligned DEMs could result in a wrong estimation of
elevation change.

We applied the co-registration method introduced by
Nuth and Kääb [2011]. This methodology firstly finds it-
eratively the horizontal shift between two DEMs, based
on a slope-aspect method, and removes it. Then, a po-
tential elevation-depended error is checked and removed.
Finally, higher-order biases are checked and corrected, by
rotating the coordinate axis, if necessary (e.g. along/cross
track corrections). Corresponding corrections, which re-
sult in a polynomial function, can be then applied.

3. RESULTS

By subtracting the 2015 DEM from the 2016 one,

we obtained the topographic changes due to the vol-
canic deposits emplaced during the eruptive activity
of Etna occurred on May 2016 (Figure 1). In this way,
we estimated the distribution of thickness, areas and
volumes (Table 1) in correspondence of the flows em-
placed during the 18-19 and 22-25 May eruptive
episodes occurred at BN and VOR (flow a), and the
flow emitted on 18-19 May by the eruptive vent
opened at the eastern base of the NEC, near the July-
August 2014 cones (flow b).

We found that flow a exhibits an aerial extension
of 1.14 km2 with a volume of 4.25 × 106 m3, while
flow b has an area of 0.24 km2 with a volume of 0.4
× 106 m3 (Table 1). Considering both lava flow fields,
we found a cumulative area of 1.38 km2.

In order to quantify errors associated with volume
estimation, we computed the histogram of the eleva-
tion differences outside the lava flow fields (inset of
Figure 1) finding a standard deviation of 1.6 m, which
results in an error of 2.2 × 106 m3. The total volume is
thus 4.65 ± 2.2 × 106 m3.

4. DISCUSSION AND CONCLUSIONS

We quantified the topographic-derived volume and

associated uncertainties of the lava flows erupted out
of the summit craters of Mount Etna on May 2016.

Previously published results related to this eruption
provided an estimation of 5.6 – 8.9 × 106 m3 for the
cumulative volume infilling the BN-VOR depression
and the western flow [Edwards et al., 2018]. Our esti-
mation for the western flow (flow a in Figure 1) falls
within this range, permitting to infer a volume of 3 ±

3D LAVA FLOW MAPPING AT ETNA VOLCANO

Lava flow Area (km2)

Thickness (m)

Lava volume (m3)

Min Average Median Max

flow a 1.14 1.0 3.7 3.3 16.4 4.25 × 106

flow b 0.24 0.5 1.7 1.4 7.7 0.4 × 106

TABLE 1. Areas, thicknesses and volumes estimated for the two lava flows emplaced during the 18-19 and 22-25 May eruptive
episodes occurred at BN and VOR (flow a), and the flow emitted on 18-19 May by the eruptive vent opened at the east-
ern base of the NEC (flow b).

GANCI ET AL.

1.6 × 106 m3 for the BN-VOR depression.
Moreover, we found that the thickness distribution

of flow b is in good agreement with typical field mea-
surements related to the small lava flows emplaced on
Valle del Bove [Behncke et al., 2014]. Indeed, by di-
viding flow b in five sectors based on the topography
average slope, thickness ranges between: 0.9 and 2.3
m in the upper part of the flow (average slope: 7.4°),
0.5 and 1.5 m in the second sector (average slope:
11.3°), 0.9 and 2.9 in the third sector (average slope:
7.5°), 0.5 and 2.9 m in the fourth section (average
slope: 13°), 1.1 and 3.1 in the last sector, which is the
flattest area (average slope: 5.5°).

The total volume estimated from DEM differencing
independently bounds the one derived from thermal
satellite data processed via the HOTSAT system [Ganci
et al., 2011b] (Figure 2). A cloud index has been com-
puted considering the percentage of cloudy pixel in
the volcanic area [Ganci et al., 2011a; 2016] since SE-
VIRI- and MODIS-derived radiant heat flux curves (re-
spectively blue squares and red dots in Figure 2), are
heavily affected by clouds (dashed grey curve). By ap-

plying the cooling curve method introduced by Ganci
et al. [2012], we inferred a total volume of 2.12×106
m3 expressed as dense-rock equivalent (DRE). Apply-
ing a correction of 25% for vesicularity to the topo-
graphic-derived bulk volume [Swainston, 2017], it
yields 3.48×106 m3 of DRE volume. The volume dif-
ference of 1.36×106 m3 is mainly due to cloud cover-
age on IR satellite data.

The presented methodology provides new merging
capabilities of multi-platform remote sensing products
to monitor effusive eruptions, enabling a rapid re-
sponse during effusive crises. In particular, the com-
parison with HOTSAT derived volumes provides a first
order validation to the eruption rate derived from ther-
mal satellite data, opening new perspectives in study-
ing the conversion between radiant heat flux and time
averaged effusion rate, which is still an open problem
[Garel et al., 2012]. The 18-25 May eruption at Mt Etna
was characterized by a small lava flow field, involving
quite high relative errors associated with volume esti-
mation. Nevertheless, since relative error decreases
when increasing the thickness of the lava flow, this

FIGURE 2. Radiant Heat Flux derived from SEVIRI (blue squares) and MODIS (red diamonds) data, and cloud coverage in percentage
(dashed grey line) at Mt Etna during 18-23 May 2016.

3D LAVA FLOW MAPPING AT ETNA VOLCANO

methodology would be particularly efficient for long-
lasting eruptions. Further research will include new
study cases involving both Pléiades and infrared satel-
lite data, in order to find new constraints to the radi-
ant heat flux - eruption rate conversion, as eruptive
conditions change.

Acknowledgements. Thanks are due to EUMETSAT for SEVIRI
data. Pléiades data were available through the SVOP project.
This work was developed within the framework of Tecnolab,
the Laboratory for Technological Advance in Volcano Geo-
physics of the INGV in Catania (Italy) and was partially sup-
ported by the DPC-INGV 2012–2021 agreement. This paper
benefited from the comments and suggestions of two anony-
mous reviewers.

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*CORRESPONDING AUTHOR: Gaetana GANCI,

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania,

Osservatorio Etneo

Catania, Italy

email: gaetana.ganci@ingv.it

GANCI ET AL.