IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A1_3G-O6

Room A4

Date/Time: July 21 10:15-10:30

Ground Penetrating Radar and Terrestrial Laser Scanner survey on cross-stratified
overbank deposits from the 2006 eruption of Tungurahua volcano, Ecuador.
Guilhem A Douillet1 , Jean R Dujardin2 , Amir Abolghasem1 , Ulrich Kueppers1 , Jonathan Hall3 , Patricia
Mothes3 , Maksim Bano2 , Donald B Dingwell1
1

Department of Earth and Environmental Sciences, University of Munich (LMU), Germany, 2 Ecole etObservatoire des Sciences de la Terre, UniversitÃl’ de Strasbourg, France, 3 Instituto GeofÃ sico,Escuela PolitÃl’cnica Nacional Quito, Ecuador
E-mail: g.douillet@min.uni-muenchen.de

The deposits of the August 2006 pyroclastic density currents (PDCs) at Tungurahua, Ecuador exhibit two main
depositional characteristics. Topography confined deposits show coarse, unsorted, massive, meter thick layers.
Ash dominated, cross stratified deposits outcrop on the overbanks of valleys, organized as spatially isolated andlimited bodies. Dune bedforms shape the surface of these deposits. We combined a terrestrial laser scanner (TLS)
and a ground penetrating radar (GPR) survey on the overbank deposits.
TLS provides a dense data cloud (measurements at ca. 5cm steps) with cm precision. The GPR survey permits tolook at the internal cross stratification patterns in a non destructive way. Three antennae with frequencies of 250,
500 and 800MHz, permitted to image the deposits down to 10, 7 and 3m depth, resp. The GPR data profits fromthe TLS results, which are integrated in the processing.
The TLS results permitted to compare previous eyewitness measurements with quantitative parameters derived
from the numerical data. We picked the crests of the dune bedforms by defining them as local maxima. Theirorientations, average slope angles, length and thickness are defined from two successive local extrema. We
applied a Fourier and wavelet analysis in all directions to characterize wavelengths and pseudo amplitudes.
A large gridding with the 250MHz antenna permitted to recognize and follow the major flow units and the preeruptive surface. Up to 6 units are recognized, the 3 basal ones interpreted as dense pyroclastic flows deposits
and the top ones as dilute PDCs deposits, based on the GPR signal. This may indicate that the valleys were filledby the time of deposition of the dune bedforms, a result not inferred in previous studies. The deposited volume canalso be derived.
From a dense array (profiles at 10cm spacing) over dune bedforms with the 800 and 500MHz antennae, weimaged the 3D internal patterns. This confirmed previous 2D, outcrop based observations and constrained thelateral evolution of stratigraphic features. Monotone lateral profiles rule out genesis from currents with different
orientations. However, a single dune bedform can show both downstream and upstream migrating crests during
the same stage of the flow. The root of the structures is located much deeper than previously expected and record
the initial conditions for initiation of a bedform. The striking spatial stability during the whole deposition stage
indicates that these bedforms are triggered by basal topographic disturbances. Several structures cannibalized bylarger ones proof that the bed rapidly accommodates temporal changes in the dynamics of the currents.
This innovative combination of field methods brought unexpectedly good results. In particular, they permitted toimage individual flow events and how deposits from successive pulses interacted with each other.

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