Nicolas Villeneuve

Rapid Urban Mapping Using SAR/Optical Imagery Synergy

This paper highlights the potential of combining Synthetic Aperture Radar (SAR) and optical data for operational rapid urban mapping. An algorithm consisting of a completely unsupervised procedure for processing pairs of co-registered SAR/optical images is proposed. In a first stage, a texture analysis is conducted independently on the two images using eight different chain-based Gaussian models. In a second stage, the resulting texture images are partitioned by an unsupervised fuzzy K-means approach. Finally, a fuzzy decision rule is used to aggregate the results provided by the classification of texture images obtained from the pair of SAR and optical images. The method was tested and validated on images of Bucharest (Romania) and Cayenne (French Guiana). These two study areas are of different terrain relief, urban settlement structure and land cover complexity. The data set included Radarsat-1/ENVISAT and SPOT-4/5 images. The developed SAR/optical information fusion scheme improved the capabilities of urban areas extraction when compared with the separate use of SAR and optical sensors. It also proved to be suitable for monitoring urbanization development. The encouraging results thus confirm the potential of combining information from SAR and optical sensors for timely urban area analysis, as required in cases of disaster management and planning in urban sprawl areas.

Pāhoehoe, ‘a‘ā, and block lava: an illustrated history of the nomenclature

Lava flows occur worldwide, and throughout history, various cultures (and geologists) have described flows based on their surface textures. As a result, surface morphology-based nomenclature schemes have been proposed in most languages to aid in the classification and distinction of lava surface types. One of the first to be published was likely the nine-class, Italian-language description-based classification proposed by Mario Gemmellaro in 1858. By far, the most commonly used terms to describe lava surfaces today are not descriptive but, instead, are merely words, specifically the Hawaiian words ‘a‘ā (rough brecciated basalt lava) and pāhoehoe (smooth glassy basalt lava), plus block lava (thick brecciated lavas that are typically more silicic than basalt). ‘A‘ā and pāhoehoe were introduced into the Western geological vocabulary by American geologists working in Hawai‘i during the 1800s. They and other nineteenth century geologists proposed formal lava-type classification schemes for scientific use, and most of them used the Hawaiian words. In 1933, Ruy Finch added the third lava type, block lava, to the classification scheme, with the tripartite system being formalized in 1953 by Gordon Macdonald. More recently, particularly since the 1980s and based largely on studies of lava flow interiors, a number of sub-types and transitional forms of all three major lava types have been defined. This paper reviews the early history of the development of the pāhoehoe, ‘a‘ā, and block lava-naming system and presents a new descriptive classification so as to break out the three parental lava types into their many morphological sub-types.

Long‐term mass transfer at Piton de la Fournaise volcano evidenced by strain distribution derived from GNSS network

Basaltic volcanoes are among the largest volcanic edifices on the Earth. These huge volcanoes exhibit rift zones and mobile flanks, revealing specific stress field conditions. In this paper, we present new deformation data issued from the Global Navigation Satellite Systems (GNSS) network installed on Piton de la Fournaise. Density of the GNSS stations allowed us to reach a sufficient resolution to perform a spatially significant analysis of strain at the scale of the active part of the volcano. Since 2007, summit inflation during preeruptive/eruptive sequences (summit extension/cone flanks contraction) alternates with summit deflation during posteruptive/rest periods (summit contraction/cone flanks extension) and generates a “pulsation” of the volcano. This volcano “pulsation” increases rock fracturing and damage, decreases the rock stiffness, and increases the medium permeability. The deformation regime of the mobile eastern flank evidences mass transfer in depth from the summit to the east. During the long-term summit deflation recorded between 2011 and 2014, the upper eastern flank extended steadily eastward whereas the lower eastern flank contracted. Simultaneous extension and eastward displacement of the upper eastern flank and eastward contraction of the middle and lower eastern flank contributes to build the Grandes Pentes relief, steeping the topographic slope. We relate the eastern flank topographic slope spatial variations to rock or basal friction angle changes. The lower flank contraction process is an evidence of its progressive loading by the upper eastern flank, which brings this flank closer to an eventual instability.

Simulating the thermorheological evolution of channel-contained lava: FLOWGO and its implementation in EXCEL

Abstract FLOWGO is a one-dimensional model that tracks the thermorheological evolution of lava flowing down a channel. The model does not spread the lava but, instead, follows a control volume as it descends a line of steepest descent centred on the channel axis. The model basis is the Jeffreys equation for Newtonian flow, modified for a Bingham fluid, and a series of heat loss equations. Adjustable relationships are used to calculate cooling, crystallization and down-channel increases in viscosity and yield strength, as well as the resultant decrease in velocity. Here we provide a guide that allows FLOWGO to be set up in Excel. In doing so, we show how the model can be executed using a slope profile derived from Google™ Earth. Model simplicity and ease of source-term input from Google™ Earth means that this exercise allows (i) easy access to the model, (ii) quick, global application and (iii) use in a teaching role. Output is tested using measurements made for the 2010 eruption of Piton de la Fournaise (La Réunion Island). The model is also set up for rapid syneruptive hazard assessment at Piton de la Fournaise, as we show using the example of the response to the June 2014 eruption.

Comparative Study on the Performance of Multiparameter SAR Data for Operational Urban Areas Extraction Using Textural Features

The advent of a new generation of synthetic aperture radar (SAR) satellites, such as Advanced SAR/Environmental Satellite (C-band), Phased Array Type L-band Synthetic Aperture Radar/Advanced Land Observing Satellite (L-band), and TerraSAR-X (X-band), offers advanced potentials for the detection of urban tissue. In this letter, we analyze and compare the performance of multiple types of SAR images in terms of band frequency, polarization, incidence angle, and spatial resolution for the purpose of operational urban areas delineation. As a reference for comparison, we use a proven method for extracting textural features based on a Gaussian Markov Random Field (GMRF) model. The results of urban areas delineation are quantitatively analyzed allowing performing intrasensor and intersensors comparisons. Sensitivity of the GMRF model with respect to texture window size and to spatial resolutions of SAR images is also investigated. Intrasensor comparison shows that polarization and incidence angle play a significant role in the potential of the GMRF model for the extraction of urban areas from SAR images. Intersensors comparison evidences the better performances of X-band images, acquired at 1-m spatial resolution, when resampled to resolutions of 5 and 10 m.

Retrieving 65 years of volcano summit deformation from multitemporal structure from motion: The case of Piton de la Fournaise (La Réunion Island)

The structure-from-Motion photogrammetry technique enables use of historical airborne photography to achieve high-resolution 3D terrain models. We apply this method on Piton de la Fournaise volcano (La Reunion), which allows a unique opportunity to retrieve high-resolution (1.5-0.11 m) Digital Elevation Models and precise deformation maps of the volcano since 1950. Our results provide evidence that the summit volume increased throughout the study period, at a stable rate of 2.2 Mm3/yr between 1950 and 2000, increasing by a factor of four (to 8.0Mm3/yr) prior to the major 2007 eruption which was accompanied by summit caldera collapse. At the same time, summit deformation was asymmetric, with 9.2±2.5 m of eastward seaward displacement, and 1.3±2.5 m to the west during 1950-2015. Our results reveal a temporal evolution in the volcano magma influx rate and deformation. Tracking these fluxes and the long-lived preferential eastern motion is crucial to mitigate risks associated to flank destabilization.

Modeling the lava heat flux during severe effusive volcanic eruption: An important impact on surface air quality

The April 2007 eruption of Piton de la Fournaise, one of the most active volcanoes in the world, was the strongest eruption in recent decades with 230Mm3 of lava emitted and more 300KT of SO2 degased. The surface concentrations of SO2 have been measured by the ORA (Air Observatory of the Reunion Island) and showed that many stations exceeded the critic threshold for health. These high concentrations led to important health issues, accompanied by environmental and infrastructure degradations. Realized with MesoNH atmospheric model, our simulations show the transport of sulfur and his component between 2 April and 6 April 2007, with a focus on the influence of heat flow from lava. For this purpose, we have implemented ForeFire, a surface model initially realized to simulate forest fire, by adapting it to reproduce the dynamic of a lava flow. Thus, all flows (SO2, heat, vapor, CO2, CO) are triggered depending on its dynamic. With this first approach, our simulations reproduce quite faithfully the surface field observation of SO2 provides by ORA. Various sensitivity analyzes exhibit that volcano sulfur distribution was mainly controlled by the lava heat flow. Without heat flow parameterization, the surface concentrations are multiplied by a factor 30 compared to the reference simulation. Simulations also put in evidence that the 5 April, during the height of the eruption, changes in meteorological conditions, especially weakening of atmospheric boundary layer stability, led to various pollutants to be transported in higher altitude (8000m). The main consequence is the volcanic pollutants are transported off the east coast of Reunion Island.

Hot Spots and Large Igneous Provinces

This chapter examines the tectonic geomorphology of hot spot volcanism and specifically oceanic hot spot volcanism. The best way to explain and understand volcanic edifices development above hot spots is by using nested scales: starting from a large overview to progressively more and more detailed scales. The construction of hot spot volcanic chains and the identification of morphologies from the deep sea floor to volcano summits are considered in detail. Mechanisms of construction, evolution, and dismantling of volcanic edifices are examined with descriptions using well-known examples and models. Limitations and imperfections within models for hot spot volcanism are mainly related to the specific context and scale, and to magmatology. At the oceanic scale, the models are influenced by the geodynamic context (plate motion and proximity of a subduction or an accretion zone). At the volcanic edifice scale, the models are constrained by the history of the volcano development, for example, cones superimpositions, proximity of fractures, zone of weakness, stress field, major mass wasting, and rift zones.

La Réunion Island: A Typical Example of a Basaltic Shield Volcano with Rapid Evolution

La Reunion Island is a volcanic area located in a tropical climate context. Volcanism has built up a 7 km high cone over the last five million years. For the last two or three million years, magmatic, tectonic and weathering processes have been engaged in a gigantic race to shape the island’s morphologies, with dramatic consequences in terms of landforms such as volcanic peaks (the dormant Piton des Neiges and active Piton de la Fournaise), cliffs or “ramparts”, deep gorges and waterfalls which, together with endemic flora and fauna, helped the central part of La Reunion to be recently recognised by UNESCO as a World Heritage Site. Yet, induced natural hazards such as volcanic eruptions and landslides remain a threat for the population.

Erosion and Volcaniclastic Sedimentation at Piton de la Fournaise: From Source to Deep Marine Environment

With very high rainfall, steep slopes and young reliefs, La Reunion Island is renowned for its highest erosion rates worldwide. The erosion mainly occurs by rockfalls and landslides in the steep sides of the river canyons. Most sediment is transported from inland to the sea via rivers and streams. River channels are formed by incision into the basaltic pile of lava and breccia constituting the original volcanic shield. Erosion and sediment products in the largest rivers are mainly driven by the recurrence of tropical cyclones. Cyclonic conditions induce heavy rainfalls and torrential floods, causing land erosion and hyperconcentrated sediment loads in the main river mouths. The absence of coastal platform induces no sediment storage in shallow marine environment. Erosion processes onland and sediment transport to the coast, via river systems, induce direct sediment feeding of the upper submarine slope and canyon heads. The morphology of the submarine flanks of Piton de la Fournaise is rough and steep. Different submarine features were identified as debris avalanches deposits, erosive canyons and volcanic constructions. This chapter proposes a synthesis of morphological and sedimentological data on Piton de la Fournaise volcano, onland and offshore, at different scales. It describes the relationships between the main erosional structures onland and major submarine sedimentary systems. Combining onland erosion, sediment transport on the submarine slopes and sediment deposition in the deep-sea allow a better understanding of the transfer of volcaniclastic material on oceanic shield-volcanoes. The morphology of onland and submarine slopes of Piton de la Fournaise suggests two domains, distinguished by the maturity of land-to-sea sediment transfers, directly related to the maturity of hydrological networks onland. The first domain corresponds to limited land-to-sea transfer, with no hydrographic network, as observed on the volcanic rift-zones. The coastal area forms a rocky promontory and only coastal erosion of recent fragmented volcanic rocks, produces sediments in the upper submarine slope. Submarine instabilities, scarps, and local erosional canyons are superficial and discontinuous on the slope. The second domain corresponds to high-efficient land-to-sea sediment transfer, characterised by a direct connection between river mouths and submarine canyon heads (as for “Riviere des Remparts”). The submarine sedimentary systems are well-developed with continuous canyons, feeding deep-sea fans at the base of the slope. These volcaniclastic deep-sea fans are characterised by sandy lobe accumulation fed by small channels and are typically interpreted as the result of turbidite processes and deposits.