Matthieu Kervyn

Mapping volcano topography with remote sensing: ASTER vs. SRTM

High spatial resolution topographic data are crucial as a prerequisite for evaluating and modelling volcanic hazards. Remote sensing now provides some of the best methods of retrieving digital elevation models (DEMs) over extensive volcanic regions. Here we compare the advantages and limitations of Shuttle Radar Topography Mission (SRTM) DEMs, derived from radar interferometry, and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) DEMs, derived from digital photogrammetry. Applications are presented for Mauna Kea (MK), Hawaii and Oldoinyo Lengai (OL), Tanzania. We quantitatively assess the accuracy of the respective DEMs to document the size of moderate‐sized volcanic features. ASTER DEM accuracy depends on spectral contrast within the image and on the availability of high‐quality ground control points. Unlike the ASTER DEMs, which are user‐derived, processed SRTM DEMs are provided without estimations of the vertical accuracy for each scene, and the end‐user has no control over the processing method. From comparison with a 10‐m spatial resolution DEM derived from 1 : 24 000 scale topographic maps of MK, we estimate root mean square errors at 8, 10 and 13 m for SRTM 30‐m, SRTM 90‐m and ASTER 30‐m DEMs, respectively. For scoria cones (<200 m high, <2 km basal diameter), SRTM 30‐m, SRTM 90‐m and ASTER 30‐m DEMs underestimate cone height by 9.5, 27 and 14%, respectively, mostly because of the averaging effect of decreasing spatial resolution. For height estimations of volcanic features higher than ∼100 m in the OL region, all of the DEMs tested were found to be consistent.

A new approach to assess long-term lava flow hazard and risk using GIS and low-cost remote sensing: the case of Mount Cameroon, West Africa

Mount Cameroon (MC), West Africas most active volcano, is a typical example of a hazardous volcano in a densely populated area, often inflicting damage by lava flows. The spatial variation of the lava flow hazard has not been mapped systematically at MC and at many other volcanoes in developing countries because they are insufficiently documented and not continuously, systematically monitored with integrated state‐of‐the‐art methods. Sophisticated lava flow models requiring the specification of numerous physical properties cannot yet be applied in such cases. Hence, a low‐cost approach is proposed to assess the medium‐ to long‐term spatial variation of lava flow hazard. The ‘lava flow zonation system using low‐cost methods’ (LAZSLO) approach takes advantage of freely available topographic data and moderate‐resolution (10–100 m) satellite imagery. For any eruption, the formulation used here expresses the probability of lava flow invasion at any location as a product of the spatial variation of eruption probability and exposure of low‐lying terrain to lava flow reach. The last term is derived by determining the upslope area from where lava flows can invade a given site. A standard UNESCO formula is then used to estimate the lava flow risk. Hazard assessment results at MC indicate that the southwest and northeast flanks are the most hazardous from lava flows, whereas the risk of damage is greatest in the area between Limbe, Mapanja and Batoke, where industrial, urban and tourism infrastructures are developing rapidly. The presented approach can be applied to assist local authorities with disaster preparedness and emergency planning at many poorly studied volcanoes, particularly in developing countries with limited financial resources.

Thermal remote sensing of the low-intensity carbonatite volcanism of Oldoinyo Lengai, Tanzania

Although Tanzania, Kenya and Ethiopia contain a number of active and potentially hazardous volcanoes, none of them are routinely or continuously monitored. Of these, Tanzanias Oldoinyo Lengai (OL) has been active almost continuously over the past two decades (since 1983). Recent activity has been confined to small‐scale effusive and explosive eruptions of natrocarbonatite within the summit crater, with lava flows occasionally overflowing the crater rim and extending onto the volcano flanks. The automated MODVOLC algorithm falls short of detecting all thermal anomalies within OLs crater. The sensitivity of the algorithm is insufficient to detect anomalies of the size and magnitude presented by those at OL. We explore how Moderate Resolution Imaging Spectroradiometer (MODIS) infrared (IR) bands can still be used to monitor activity. We cross‐verify our observations against field reports and higher resolution satellite images (ASTER, Landsat ETM+). Despite the limited extent and low temperature (∼585°C) of natrocarbonatite lavas, relative variations in eruption intensity and periods of increased activity alternating with periods of reduced or no detectable activity can be observed using the MODLEN algorithm. Although activity in the past two decades has been moderate at OL, a more intense explosive eruption is overdue and there is a need for routine monitoring in the future. Our work makes a case for low‐cost thermal IR monitoring as an essential component of such a monitoring programme at several Tanzanian, Kenyan and Ethiopian volcanoes. The approach presented here is already available for routine use.

Mapping volcanic terrain using high-resolution and 3D satellite remote sensing

Abstract Most of the hazardous volcanoes, especially those in developing countries, have not been studied or regularly monitored. Moderate-to-high spatial resolution and 3D satellite remote sensing offers a low-cost route to mapping and assessing hazards at volcanoes worldwide. The capabilities of remote sensing techniques are reviewed and an update of recent developments is provided, with emphasis on low-cost data, including optical (Landsat, ASTER, SPOT, CORONA), topographic (3D ASTER, SRTM) and synthetic aperture radar data. Applications developed here illustrate capabilities of relevant remote sensing data to map hazardous volcanic terrain and derive quantitative data, focusing on mapping and monitoring of volcanic morphology. Limitations of the methods, assessment of errors and planned new sensors are also discussed.

Remote sensing study of sector collapses and debris avalanche deposits at Oldoinyo Lengai and Kerimasi volcanoes, Tanzania

Evidence for volcano collapse and debris avalanche deposits (DADs) at Oldoinyo Lengai (OL), Tanzania, has been obtained from mapping and fieldwork. Three major DADs have been identified, named Zebra, Cheetah and Oryx DADs. Field evidence indicates geologically young ages. On this basis a remote sensing (RS) study of the active carbonatite volcano OL and the surrounding rift plain was carried out, using Shuttle Radar Topography Mission (SRTM) digital elevation data, Landsat and ASTER imagery, geological maps and aerial photographs. The SRTM digital elevation model (DEM) allowed morphological characterization of OL and reassessment of the volcano volume to 41±5 km3. This enabled the identification of collapse scars, fields of large hummocks (>300 m across), sharp deposit edges typical of DADs, and estimation of the minimum thickness of the DADs. Multispectral and topographic RS data interpretation allowed mapping of the extent and estimation of the volume of two sector‐collapse scars and three DADs. The DADs extend up to 24 km from OL and have volumes ranging from 0.1 to ∼5 km3. Striking radial ridges and grooves were identified in some parts of the DADs. The morphological variability for ridges and grooves in different DADs is attributed to contrasting flow dynamics and avalanching material. A volcano collapse and the corresponding DAD, ∼1 km3 in volume, were also characterized by RS at the nearby Kerimasi volcano. The presence of young DADs highlights the need for routine monitoring of ground deformation and seismicity at OL to anticipate hazardous events.

The direct impact of landslides on household income in tropical regions: a case study from the Rwenzori Mountains in Uganda

Landslides affect millions of people worldwide, but theoretical and empirical studies on the impact of landslides remain scarce, especially in Sub-Saharan Africa. This study proposes and applies a method to estimate the direct impact of landslides on household income and to investigate the presence of specific risk sharing and mitigation strategies towards landslides in a tropical and rural environment. An original cross-sectional household survey is used in combination with geographical data to acquire detailed information on livelihoods and on hazards in the Rwenzori mountains, Uganda. Ordinary least square regressions and probit estimations with village fixed effects are used to estimate the impact of landslides and the presence of mitigation strategies. Geographical information at household level allows to disentangle the direct impact from the indirect effects of landslides. We show that the income of affected households is substantially reduced during the first years after a landslide has occurred. We find that members of recently affected households participate more in wage-employment or in self-employed activities, presumably to address income losses following a landslide. Yet, we see that these jobs do not provide sufficient revenue to compensate for the loss of income from agriculture. Given that landslides cause localized shocks, finding a significant direct impact in our study indicates that no adequate risk sharing mechanisms are in place in the Rwenzori sub-region. These insights are used to derive policy recommendations for alleviating the impact of landslides in the region. By quantifying the direct impact of landslides on household income in an agricultural context in Africa this study draws the attention towards a problem that has been broadly underestimated so far and provides a sound scientific base for disaster risk reduction in the region. Both the methodology and the findings of this research are applicable to other tropical regions with high landslide densities.

Advances in the remote sensing of volcanic activity and hazards, with special consideration to applications in developing countries

Applications of remote sensing for studies of volcanic activity and hazards have developed rapidly in the past 40 years. This has facilitated the observation of volcanic processes, such as ground deformation and thermal emission changes, lava flows, eruption clouds, ash and gas emissions, as well as mapping of volcanic structures and hazardous terrain, even for volcanoes in remote regions. Advances in the remote sensing of volcanoes, from ground‐based sensors to sensors onboard airborne and spaceborne platforms, are reviewed. A key point made in this review is that volcanic remote sensing could have a much broader impact if the techniques and data were readily available to scientists studying/monitoring potentially hazardous volcanoes in developing countries. Perspectives on particular needs, with regard to sensor types, data availability and training, required to take volcanic remote sensing further in coming years are highlighted.

Endogenous and exogenous growth of the monogenetic Lemptégy volcano, Chaîne des Puys, France

The monogenetic Lemptegy volcano in the Chaine des Puys (Auvergne, France) was quarried from 1946 to 2007 and offers the possibility to study scoria cone architecture and evolution. This volcano was originally 50–80 m high, but scoria excavation has resulted in a 50-m-deep hole. Beginning in the 1980s, extraction was carried out with the advice of volcanologists so that Lemptegy’s shallow plumbing system and three-dimensional stratigraphy have been preserved. Detailed mapping enabled key stratigraphic units to be distinguished and the constructional phases to be reconstructed. The emplacement and evolution of the shallow plumbing system have also been unraveled. The growth of this monogenetic scoria cone included two temporally well-separated eruptions from closely spaced vents. The activity included Hawaiian, Strombolian and Vulcanian explosions, lava effusion, cryptodome and dome formation, partial collapse, satellite vent formation, eruptive pauses, and intrusion emplacement with consequent uplift. The cone shape, structure, and hence the local stress field, plumbing system, and thermal state were continuously changing, which in turn influenced the eruptive style and location. The plumbing system morphology and microtectonic structures both record local stress field and magmatic flow direction changes. Lemptegy volcano’s internal architecture, stratigraphy, and evolution show how complex a monogenetic volcano can be.

Dynamics of Land Cover/Land Use Changes in the Mekong Delta, 1973–2011: A Remote Sensing Analysis of the Tran Van Thoi District, Ca Mau Province, Vietnam

The main objective of this study is to assess the spatio-temporal dynamics of land cover/land use changes in the lower Mekong Delta over the last 40 years with the coastal Tran Van Thoi District of Ca Mau Province, Vietnam as a case study. Land cover/land use change dynamics are derived from moderate to high spatial resolution (Landsat and SPOT) satellite imagery in six time intervals ranging from 1973 to 2011. Multi-temporal satellite images were collected, georeferenced, classified using per-pixel method, validated, and compared in post classification for the land use/land cover change detection in decades. Seven major land cover/land use classes were obtained, including cultivated lands, aquaculture ponds, mangrove forest, melaleuca forest, built up areas, bare lands, and natural water bodies. The accuracies of the land cover/land use maps for 1973, 1979, 1989, 1995, 2004, and 2011 were 81%, 82%, 86%, 87%, 89%, and 89%, respectively. The results show that the area of cultivated lands reduced over the period 1973–2011, however, it still represents the dominant land use in the case study. Aquaculture ponds were almost absent in 1973 but greatly increased from 1995 to 2004, to represent 20% of the land surface in 2011. Overall, from 1973 to 2011, bare lands, cultivated lands, mangrove forest, and melaleuca forest decreased by 104 km2, 77 km2, 61 km2, and 5 km2, respectively. In contrast, aquaculture lands and built up areas increased by 123 km2 and 120 km2, respectively. Temporal analysis highlights that these changes took place mostly between 1995 and 2004. This study is a first step to identify the main drivers of land use changes in this delta region, which include economical policies as well as demographic, socio-economic, and environmental changes.

3D imaging of volcano gravitational deformation by computerized X-ray micro-tomography

Analogue models are commonly used to gain insights into large-scale volcano-tectonic processes. Documenting model surface topography and the three-dimensional (3D) aspect of deformation structures remains the greatest challenge in understanding the simulated processes. Here we present the results of volcano analogue models imaged with an X-ray computerized micro-tomography (μCT) system developed at the Ghent University Centre for Tomography (UGCT). Experiments simulate volcano deformation due to gravitational loading over a ductile layer, a process affecting many natural volcanoes built over a sedimentary substratum. Results show that μCT is able to provide a 3D reconstruction of the model topography with unprecedented resolution. Virtual cross sections through reconstructed models enable us to map the main structures at depth and to document the deformation of the brittle-ductile interface due to contrasting X-ray attenuation. Results for lateral spreading and vertical sagging into thin and thick ductile layers, respectively, are illustrated for circular cones and elongated ridges. Results highlight structural patterns not seen in previous models, such as: 1) the 3D form of a polygonal brecciated zone at the center of spreading cones; 2) the complete lack of such a zone in sagging cones; and 3) relay structures between graben-bounding faults in spreading cones. In addition, detailed imaging of tension gashes and of the flexure surface below sagging cones enables the 3D strain distribution to be explored. Experiments with non-cohesive and low cohesion granular materials present striking differences in surface topography and fault characteristics. Despite limitations associated with the scan duration, μCT reconstruction of analogue models appears a powerful tool for better understanding the complex 3D deformation associated with volcano-tectonic processes.