Analysis of the volcanic evolution of Nevado del Ruiz from the last
eruptive period to the current state, using multispectral images
provided by the group of satellites SENTINEL 1-2-3
Undergraduate thesis submitted to the department of Geosciences in partial
fulfillment of the requirements for the degree of
Geoscientist
Carlos Daniel Robayo Reyes
Director
Ph.D. Jillian Pearse
Universidad de los Andes
Science Faculty, Geoscience Department
Bogotá, Colombia
Analysis of the volcanic evolution of Nevado del Ruiz from the last eruptive
period to the current state, using multispectral images provided by the group of
satellites SENTINEL 1-2-3
Carlos Daniel Robayo Reyes
Submitted to the department of Geosciences in partial fulfillment of the
requirements for the degree of:
Geoscientist
Remote Sensor Research
Universidad de los Andes
Science Faculty, Geoscience Department
Bogotá, Colombia
2019-II
Director:
Ph.D. Jillian Pearse
Presenter:
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Abstract
The analysis of satellite images produced by remote sensors, are a great tool for the identification of a range of ground cover and topsoil to the identification of particular lithologies and rock configurations regionally. The main objective is to evaluate the ability of Landsat and Sentinel satellites to define lithologies and types of volcanic flow and with them to carry out a hazard analysis. The particular features that are going to be analyzed correspond to: lava flows, lahars, ash, volcanic gases, volcanic rocks, erosion, snow, vegetation cover as well as other features such as slopes and water sources of the Nevado del Ruiz volcano. The information has been collected through an analysis that includes a multitemporal period from the last eruptive period to the present. In addition, images of past years to this last eruptive period were analyzed, where the peaks with the highest volcanic activity were recorded, in order to complement the results obtained. The justification for the acquisition of these results lies in creating volcanic geology mapping of ash, lava flows, pyroclasts, snow and lahars flows that clearly demonstrate the dangers that the volcano represents to nearby populations and avoid tragedies such as the one that happened in the municipality of Armero in the year of 1985.
Resumen
El análisis de imágenes satelitales producidas por sensores remotos es una gran herramienta para la identificación de un rango de cobertura del suelo y capas superficiales del suelo para la identificación de litologías particulares y configuraciones de rocas a nivel regional. El objetivo principal es evaluar la capacidad de los satélites Landsat y Sentinel para definir litologías y tipos de flujo volcánico y con ellos realizar un análisis de peligros. Las características particulares que se analizarán corresponden a: flujos de lava, lahares, cenizas, gases volcánicos, rocas volcánicas, erosión, nieve, cubierta vegetal, así como otras características como pendientes y fuentes de agua del volcán Nevado del Ruiz. La información se ha recopilado mediante un análisis que incluye un período multitemporal desde el último período eruptivo hasta el presente. Además, se analizaron imágenes de años anteriores a este último período eruptivo, donde se registraron los picos con la mayor actividad volcánica, para complementar los resultados obtenidos. La justificación para la adquisición de estos resultados radica en crear mapas geológicos volcánicos de flujos de cenizas, flujos de lava, piroclastos, nieve y lahares que demuestren claramente los peligros que representa el volcán para las poblaciones cercanas y evitar tragedias como la que ocurrió en el municipio de Armero en el año de 1985.
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Acknowledgments
Quiero ofrecer mi gratitud profunda a mis padres Zulay y Carlos que me han acompañado en todo este proceso constructivo en mi vida, que me han acompañado en las buenas y en las malas y que me han tenido paciencia en todo momento. Quiero agradecer también a todos los profesores con los cuales tuve la fortuna de compartir. Ellos trasmitieron su conocimiento para que yo creciera un poco cada día más como persona y como profesional. Quiero agradecer especialmente a Jill, mi directora de Tesis, la cual estuvo muy atenta y me brindo toda la ayuda que pudiera servirme para poder lograr este proyecto. A mis amigos, con los cuales he compartido varias experiencias en mi paso por la Universidad.
“The people who are crazy enough to think they can change the world are the ones who do.”
Steve Jobs
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Objectives
General Objective
Historical analysis of lava flows, pyroclastic flows, lithologies since the last eruption of the Nevado del Ruiz Volcano until today using multispectral optical images of the SENTINEL 1-2-3 group.
Specific Objectives
1. To create volcanic geological cartographic that give evidence of the behavior that Nevado del Ruiz volcano has maintained.
2. To compare old multispectral optical images of the LANDSAT group together with recent images of the SENTINEL group 1-2-3. This in order to evaluate the improvement that the images of the new group of satellites can offer.
3. To process satellite images that provide information on factors that characterize the volcano as: lava flows, pyroclastic flows, lahars, volcanic rock, volcanic gases (past flow patterns and eruption product types) and thus be able to establish potential hazards in the populations.
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Table of Contents
1.Introduction ... 7 1.1 Preliminary Context ... 7 1.2 Motivation ... 9 2. Geological Framework ... 9 2.1 Localization ... 9 2.2 Geomorphology ... 102.3 Local Volcanic Geology ... 11
2.3.1 Glacial Ice ... 12
2.3.2 Glacier Deposits (Qg): ... 12
2.3.3 Pyroclastic Deposits (Qto):... 12
2.3.4 Volcanic Andesites (NgQa): ... 12
2.3.5 Batolito el Bosque (Pgbd): ... 12
2.3.6 Cajamarca Complex: ... 12
2.4 Regional Geology ... 13
2.4.1 Cajamarca Complex ... 13
2.4.2 Quebradagrande Complex ... 13
2.4.3 Granitic Milonite of the Guacaica ... 13
2.4.4 Stock of Manizales... 14
2.4.5 Regional Faults ... 14
3. Methodology ... 14
3.1 Bibliographic Review ... 15
3.2 Laboratory Methodology (Image processing) ... 15
3.3 Flow Chart ... 16
4. Results ... 16
4.1 Uniband Results ... 22
4.2 Composite Bands Results ... 23
4.3 Supervised Classification ... 25
5. Discussion ... 27
5.1 Uniband Classification ... 27
7 5.2 Multiband Classification ... 29 5.2.1 Composite Bands ... 29 5.2.2 Supervised Classification ... 32 5.2.3 Hazard Analysis ... 36 6. Conclusions ... 38
Appendix 1: Geological map of the studied area ... 42
Appendix 2: Spectral signatures of the different elements studied in this project ... 43
Appendix 3: Wavelength range for each band (LANDSAT 5 and SENTINEL) ... 45
Appendix 4: Volcanic threat map of the Nevado del Ruiz Volcano ... 46
Appendix 5: Volcanic threat map of the Nevado del Tolima Volcano ... 47
Appendix 6: Lagunilla River path before reaching Armero ... 48
1.Introduction
1.1 Preliminary Context
The monitoring of volcanic processes in areas of difficult access can be a complicated task without the use of tools such as remote sensing. Similarly, the permanent uncertainty that exists in the behavior of volcanic belts is a constant risk for the populations that may be close to these areas. Using passive and active remote sensors such as satellite images, it is possible to obtain a lot of information that can be analyzed and interpreted in order to understand in a more objective way the behavior that a volcano may have and thus take safety measures that can avoid imminent dangers in the population. Considering the above, historical data of the risky areas adjacent to a volcano in Colombia and the volcano itself will be analyzed to look for patterns of volcanic behavior. Thereby, to be able to a better understanding of the activity that this volcano has taken these last years. Maps that characterize the lithologies present in the volcano have already been made by field mapping. For example, in appendix 1, it is shown the geological map of the area. However, a multitemporal mapping using supervised classifications with SENTINEL images, that evidence the behavior that the volcano has led, has not been performed yet. Thus, in this project multitemporal dynamic maps are made (several maps in a given period) that show the behavior of the flows. Something that does not show the geological map itself since it is a representation of reality that has no temporality.
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The volcano studied in this project is Nevado del Ruiz. The Nevado del Ruiz volcano is one of the most active volcanoes in Colombia. This volcano is located in the west part of Colombia, on the border between the departments of Caldas and Tolima. The morphology and behavior of this volcano places it within the group of stratovolcanoes. This type of volcanos has periodic eruptions and considerable sizes together with steep slopes. The petrographic composition corresponds in majority to ash and pyroclastic rocks in addition to two pyroxene andesites, with variations to dacites and basaltic andesites (Ingeominas, 2001). The constant activity of this volcano represents a constant risk for the different populations that may be close to it. For this reason, the constant evaluation of the volcano to determine possible eruptive processes is necessary. The analysis of the historical eruptive events of Nevado del Ruiz volcano is also a source of information that can help to understand its behavior. Thus, an analysis using remote sensors of the last eruptions registered by the Colombian Geological Service will be carried out. These eruptions are registered since the year of 2016 to the present. Today the volcano is in yellow alert level III, this means constant changes in the behavior of volcanic activity.
Using spectroscopy, which analyzes the entire spectrum of light and its decomposition into different types of rays depending on the wavelength, valuable data can be obtained on the properties of materials (materials like rocks or lava flows) which are quite useful for differentiating between each type of material (Sabins, 2007). This is possible, due to reflectance, transmittance and absorbance properties, which behave differently for each rock, material or element being analyzed (Sabins, 2007). Thus, this information can be obtained using satellite images from modern satellites such as the SENTINEL project, as part of the Copernicus Program. In this program, 3 missions have been carried out so far, which began since 2014 with SENTINEL 1, passed through SENTINEL 2 in 2015 and finally SENTINEL 3 in 2016 (E.S.A, 2014). These satellites show the images of the studied area for each type of wavelength. Each image associated with a wavelength is what is called a band (see Appendix 3). Each band gives different information about the materials; therefore, the study lies in carrying out a single or multiband analysis, which reveals a lot of interesting information. Hence, the volcanic information that can be obtained includes elements such as lava flows, pyroclastic flows (volcanic ash and rocks), thermal emissions, sulfur deposits, gas emissions among others.
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1.2 Motivation
The analysis of large geological structures potentially dangerous to humans, is of great importance within the framework of prevention against threats and thus avoid vulnerabilities that may exist. The detection of these vulnerabilities is closely linked to the study of these structures and the surrounding human populations. The spatial recognition and mapping of superficial deposits related to recent volcanic activity allow the characterization of areas affected by lahar, pyroclastic and lava flow paths, which is useful in disaster management. Such mapping can be achieved by using optical sensors of different spatial and temporal resolution. There are several studies related to temporal and spatial variation of the frequency and distribution of lahars in active volcanoes. The Pinatubo (Philippines) and La Casita (Nicaragua) volcanoes are among the most monitored using remote sensing tools. Such monitoring is based on textural features, superficial sedimentology and morphology of lahar deposits (Davila et.al, 2011). In the case of this study, the interest lies particularly in the threat generated by the Nevado del Ruiz volcano. Therefore, its activity will be studied from the last eruptive periods to the present, performing a mineralogical and volcanic characterization using satellite image spectroscopy. Thus, the main objective is to study past flow patterns and eruption product types in order to analyze hazard. The current volcanic threat map of the Nevado del Ruiz Volcano, created by the Colombian Geological Service (Appendix 4), serves as a source to correlate the data obtained in this project and thus perform the hazard analysis.
2. Geological Framework
2.1 Localization
Nevado del Ruiz volcano is a stratum volcano located in one of the northernmost parts of the central Andes mountain range. This volcano is part of a large group of different volcanoes close to each other. Among them are the Nevado de Santa Isabel, El Cisne, Tolima and Quindío, which all together correspond to the large Ruiz-Tolima volcanic massif. Specifically, this volcano is located between the departments of Caldas and Tolima between the jurisdictions of the municipalities of Villamaría and Murillo (Gonzáles, 2001).
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2.2 Geomorphology
The characteristic shape of the stratovolcanoes is conical, which gives them a steep slope and a great height. This characteristic form is not very stable; therefore, it generates that the cone-crater system collapses periodically as greater height is acquired by the different eruptions of magma, pyroclasts and ashes. This periodic cone-crater system collapse causes the volcano to be in constant dynamism and therefore not to fall into passive activity. In addition to this, the great activity that this volcano presents is closely associated with the convergent limit in the subduction of the Nazca and Caribbean plate on the South American plate (oceanic crust subduction) and the Panama and Coiba microplates (Taboada et al., 2000, Bohorquez et al, 2005). This subduction generates large longitudinal chains of volcanoes that are located throughout the Andes. Nevado del Ruiz is formed in general by several layers of volcanic ash, pyroclasts of various kinds and hardened lava, which have a discordant contact with the other characteristic complexes of the central mountain range. In the same way, this volcano being of the stratovolcano type, generates magmas very rich in silica that usually cool very quickly and generate dacites, andesites and rhyolites (extrusive igneous rocks of fine grain). The volcano has a total of 3 volcanic cones: 2 parasites and 1 main. The main crater has the name of Arenas crater and is located on the highest summit of the volcano. This main crater has a diameter of 870 meters x 890 meters and a depth of 247 meters (Gonzáles, 2001). The two parasitic cones
Latitude: 4° 53' 43'' N Longitude: 75°19'21'' W Altitude: 5321 MASL
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have the names of La Olleta and La Piraña. The drainage is eccentric radial and the Güalí, Lagunillas and Recio rivers, tributaries of the Magdalena river, and the Chinchiná river, tributary of Cauca are born in the volcanic building; These are narrow, deep valleys and high slopes (Gonzáles, 2001).
2.3 Local Volcanic Geology
The basic information for the elaboration of the geological maps of the 206 Manizales and 225 Nevado del Ruiz quadrants(appendix 1), by Ingeominas in 2001, was taken from the map of quadrangle K-8 (Mosquera et al., 1977 ), and was complemented with some detailed surveys and surveys during the years 1990 and 1991 in areas considered critical for their geological, tectonic or location conditions, or where there was insufficient information for the publication scale (Gonzales, 2001).
The geological data were placed on preliminary letters of the IGAC at 1: 25,000 scale, but the scale of work in the field is smaller considering the separation between transfers for the entire area. The information was then collected at a scale of 1: 50,000 and subsequently 1: 100,000, a scale chosen for publication. (Gonzales, 2001). The resolution of the maps made by Ingeominas can be considered good due to a direct contact with the structures of the area and their lithologies, however the transects made in the field during the mapping of the quadrants 206 and 225 do not cover the entire geological map. This implies that there is uncertainty about the location of the numerous contacts between the different lithologies presented in the geological map (appendix 1). The goal of comparing geological maps with the results of this project is to verify the information obtained from the satellite images and determine if there is a greater precision in the maps of the flows produced with these images.
Nevado del Ruiz volcano is built on the same basement of Cerro Bravo, in the complex intersection of four groups of faults, where the most significant are Palestine and Termales- Villamaría. There have been three stadiums called Ruiz Ancestral, Ruiz Viejo and Ruiz that include the construction and alternate destruction of three buildings. This construction and destruction generate lavas, deposits of pyroclastic flows, deposits of debris avalanches, deposits of falling pyroclasts and deposits of lahars and domes. Its products are essentially two-pyroxene andesites, with variations to dacites and basaltic
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andesites (Gonzáles, 2001). Geological characterization locally includes from the top of the volcano to the bottom:
2.3.1 Glacial Ice
Quaternary age glacial ice, which is located above the craters and top of the volcano.
2.3.2 Glacier Deposits (Qg):
Detrital deposits of quaternary age formed by recent glacial action. They consist of fragments and blocks of lava inside a sandy-clay matrix. (Mosquera et al., 2010)
2.3.3 Pyroclastic Deposits (Qto):
Unbound deposits composed of ashes, lapilli and pumice edges. Locally they present sandy lenses of glacial origin. They soften the pre-existing morphology. (Mosquera et al., 2010)
2.3.4 Volcanic Andesites (NgQa):
Andesitic flows of Neogenic age, which constitute most of the outcropping rocks. (Mosquera et al., 2010)
2.3.5 Batolito el Bosque (Pgbd):
Batolito el Bosque is mostly made up of medium-grain leucocratic biotitic granodiorite of age 49.1 ± 1.7 Ma (Paleogene) K / Ar in biotite. (Mosquera et al., 2010)
2.3.6 Cajamarca Complex:
Cajamarca Complex (Metamorphic Basement): Rocks of the Paleozoic age basement, constituted mainly by polymetamorphic sequences located east of the San Jerónimo Fault, locally affected by diaphorsis and dynamic metamorphism. (Mosquera et al., 2010). Within the area, a differentiation of the complex can be made in 3 different lithologies:
Pq: Quartzites and myceous quartzites with local transition to quartz schists and quartz-feldspathic neises. (Quadrant 225)
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Pev: Actinolytic-chloritic schists of green color, locally with intercalations of quartz-sericitic schists. (Quadrant 225)
Pes: Quartz-sericitic, micaceous and quartzous schists and phyllite and quartz-phyllite locally with intercalations of chloritic and actinolytic schists. (Mosquera et al., 2010).
2.4 Regional Geology
2.4.1 Cajamarca Complex
Constituted mainly by regional metamorphism of low grade and paleozoic age, which constitute the central nucleus or basement of the central mountain range of the Andes (López, et al., 2009). The complex is limited to the west by the San Jerónimo Fault. Facies of green schist are presented up to amphibolites of rocks such as slates, phyllites, quartzitic schist, green schist, quartzites, gneisses, granulites, amphibolites and marble belts (Moreno, et al., 2008). In this way, the colors that appear most are the green, black and gray. All of them product of the different minerals such as chlorite-albite-epidote, actinolite, graphite, quartz, cericite and biotite. The Cajamarca complex limits the west with the Quebradagrande complex. It was initially defined by Maya and González in 1995.
2.4.2 Quebradagrande Complex
This complex is younger than the Cajamarca complex since it appears with cretaceous ages and was initially defined by Maya and González in 1995. Its main characteristic is the intercalation of sedimentary rocks with metamorphic rocks. It is presented as a discontinuous belt along the east flank of the Romeral fault system. It consists of conglomerates, sandstones, chert, shales and slates with a low degree of metamorphism. Sediments are associated with basalt and intrusions of basic to intermediate composition, among which andesitic porphyry stand out (Echeverri, 2009).
2.4.3 Granitic Milonite of the Guacaica
The Granitic Milonite of the Guacaica corresponds to a predominantly peralumine intrusive syntectonic body, of calcoalkaline affinity and granitic composition with
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andalusite, sillimanite and cordierite (Montenegro, 2017). The mineralogical, textural and structural characteristics of this body allow it to be classified as a granitic rock with minor dioritic-quartzodioritic milonitized components. It mainly corresponds to granitic gneises (López, et al., 2007).
2.4.4 Stock of Manizales
The stock of Manizales, of Paleocene age, is an intrusive felsic body, characterized by three compositional variations: biotitic tonalite with hornblende, biotitic tonalite and granodiorite. The most characteristic minerals of this complex are biotites, epidotes, amphibols, plagioclase, quartz, potassium feldspar and hornblende. This complex outcrop north of the Nevado del Ruiz volcano, on the western flank of the central mountain range. In general, it is constituted by leucocratic rocks, phaneritic of medium to fine grain size (Montenegro, 2017).
2.4.5 Regional Faults
Within the study area, several structural faults can be found, which determine limits and geological contacts in the previously described complexes. Some of the most important have already been mentioned in the descriptions of the regional complexes. Among the most important are the Palestina Fault, the Villamaria Termales Fault, the Santa Rosa Fault, the Nereidas Fault and the Rioclaro Fault. The Palestina fault crosses the Nevado del Ruiz volcano from southwest to northeast, so it determines an area of high tectonic and volcanic activity. Other faults that are part of the region but are not close to the volcano are: the Campoalegrito Fault, the Campo Alegre Fault, the San Ramon Fault, the San Eugenio Fault and the San Jeronimo Fault (Gonzáles, 2001).
3. Methodology
The analysis will be carried out through the use of multispectral optical images provided by the SENTINEL 1-2-3 group of satellites, which present greater number of bands compared to the older LANDSAT group. In this way, the use of this new group of satellites will allow a better identification and analysis of the studied volcano. The images will be obtained by platforms such as Copernicus, USGS, Libra Development among others.
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After being obtained the different groups of images, a processing of these will be made using specialized software such as Geomatica, ArcGis and others to identify lithologies, lava flows, pyroclastic flows, changes in humidity and biomass. With this, to have a better understanding of the behavior of the studied volcano and thus understand possible risks that may exist in the surrounding populations.
3.1 Bibliographic Review
The study methodology for this project consists initially of a bibliographic review of the studies carried out in the Nevado del Ruiz volcano area. The literature review includes studies on volcanism through the use of SENTINEL and LANDSAT images. In the same way, bibliography that shows information on identification and prevention of volcanic hazards and risks will be analyzed, this in order to establish the parameters and criteria necessary to identify in the analysis of satellite images. These parameters will generate the necessary information to make the hazard analysis of the sector. Similarly, petrographic, geochemical and stratigraphic studies will be taken into account to be able to make a complete characterization of the sector.
3.2 Laboratory Methodology (Image processing)
After carrying out the bibliographical research on the theoretical framework of the studied area, a laboratory analysis using ArcGIS and PSI Geomatics software will be performed. The satellite images were downloaded from the United States Geological Survey website and subsequently processed. The software will allow spectroscopy discrimination in terms of reflectance and absorptance. An uniband and multiband analysis will be carried out for the different SENTINEL and LANDSAT images. The processing of the images includes a preprocessing that corresponds to the correction of the images by different factors. Within the corrections we have: geometric correction, orthorectification, radiometric correction and atmospheric correction. After this preprocessing, the uniband processing will consist of identifying the range of digital numbers (pixel) corresponding to each element analyzed. With the identification of these ranges, a cartographic discrimination of each element can be made by assigning a particular color. To create this classification, the reclassify function is used in the case of ArcGis and PCT editing in PSI Geomatica. Multiband processing is better than uniband processing and consists of combining 3 bands to create a false color. The combination of these 3 bands allows to highlight certain elements and attenuate
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others. Similarly, the foregoing will allow to identify in a more accurate way, compared to the uniband analysis, the presence or absence of a certain analyzed element. To create this classification, the Image Classification Supervised function is used in the case of PSI Geomatica. This function allows to create a sample, picked by the user, that allows to generate an interpolation in the whole image. It is important to use this function since arbitrary results are not created by the program.
3.3 Flow Chart
4. Results
The elements to be studied correspond to ash, young lava flows, old lava flows, lahars, pyroclastic flows, volcanic gases (sulfur dioxide, hydrogen sulfide and carbon dioxide), wasteland, volcanic rock and vegetation. Each of these elements has a reflectance and absorptive pattern for each specific wavelength. These patterns were investigated for each element and can be consulted in the appendix 2 of this project. Taking this information into account, the common reflectance pattern was constructed for all the elements, which allows an easy comparison of absorption or reflection patterns between these studied elements.
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Initially, the reflectance percentages of each element were identified for a defined range of wavelengths. Table 1 shows the results obtained.
After identifying the reflectance ranges for each material (minimum and maximum values), these percentages were transformed into digital number values. Digital numbers are usually attributed to a pixel and have a range that varies between 0 and 255. Values close to 0 are associated with reflectance values close to 0%, while values close to 255 are associated with close reflectance values at 100% A pixel can be associated with a single digital number or several digital numbers depending on the number of bands being used. If a pixel is associated with only one digital number, an uniband analysis is being performed, where the only maximum colors presented are white or black. On the other hand, if a pixel is associated with more than 1 a digital number, a multiband analysis is being carried out, where there are colors that may be false or true. Table 2 shows the transformation of reflectance percentages of each element studied to digital number values: Finally, an average of the maximum values and minimum values of the digital numbers is made (see Table 3) and the data obtained is plotted (see Figure 1).
After obtaining the general spectral profile of reflectance of all the elements to be analyzed, a color convention was made for each combination of bands performed. This color discrimination facilitates the identification of each element studied at the time of conducting the supervised composite band analysis since the color convention will indicate how each element will look in each specific band combination. Table 4 shows the color convention.
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0.45 - 0.52 Micrometers
Type Enhanced Data
Min Max
1. Ash 37,36% 42,05% 2. Young Lava Flows 21,57% 21,57% 3. Old Lava Flows 41,06% 53,81% 4. Lahars 14,52% 15,69% 5. Pyroclastic flows 30% 31,37% 6. Sulfide Dioxide (Volcanic Gases) 1% 1% 7. Erosion (Wastelands) 10,61% 14,28% 8. Snow 100% 100%
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Volcanic Rock (Basaltic
Andesites and dacites) 21,42% 21,43% 10 Vegetation 3,750% 5%
0.52 - 0.60 Micrometers
Type Enhanced Data
Min Max
1. Ash 42,05% 49,54% 2. Young Lava Flows 22,35% 22,36% 3. Old Lava Flows 53,81% 57,17% 4. Lahars 14,12% 14,250% 5. Pyroclastic flows 12,55% 13,70% 6. Sulfide Dioxide (Volcanic Gases) 1% 1% 7. Erosion (Wastelands) 14,28% 20,82% 8. Snow 100% 100% 9
Volcanic Rock (Basaltic
Andesites and dacites) 22,10% 22,14% 10 Vegetation 5% 10%
0.63 - 0.69 Micrometers
Type Enhanced Data
Min Max
1. Ash 51,82% 55,90% 2. Young Lava Flows 27,45% 27,46% 3. Old Lava Flows 56,46 58,76% 4. Lahars 15,25% 16,63% 5. Pyroclastic flows 16,54% 21,30% 6. Sulfide Dioxide (Volcanic Gases) 1% 1% 7. Erosion (Wastelands) 22,12% 26,12% 8. Snow 100% 100%
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Volcanic Rock (Basaltic
Andesites and dacites) 22,10% 22,14% 10 Vegetation 6,25% 11,25%
0.76 - 0.9 Micrometers
Type Enhanced Data
Min Max
1. Ash 59,54% 62,27% 2. Young Lava Flows 17,25% 17,26% 3. Old Lava Flows 68,14% 90,62% 4. Lahars 17,50% 18,50% 5. Pyroclastic flows 23,03% 25% 6.
Sulfide Dioxide (Volcanic
Gases) 1% 1% 7. Erosion (Wastelands) 31,02% 37,14% 8. Snow 100,00 100%
9 Volcanic Rock (Basaltic Andesites and dacites) 21,43% 21,79% 10 Vegetation 57,50% 70%
1.55 - 1.75 Micrometers
Type Enhanced Data
Min Max
1. Ash 62,27% 62,30% 2. Young Lava Flows 91,37% 91,38% 3. Old Lava Flows 58,23% 63,89% 4. Lahars 9,19% 10,81% 5. Pyroclastic flows 9,19% 11,59% 6. Sulfide Dioxide (Volcanic Gases) 1% 1% 7. Erosion (Wastelands) 53,47% 55,51% 8. Snow 30% 50%
9 Volcanic Rock (Basaltic Andesites and dacites) 30,80% 34,80% 10 Vegetation 18,750% 35%
2.08 - 2.35 Micrometers
Type Enhanced Data
Min Max
1. Ash 62,30% 62,40% 2. Young Lava Flows 95,69% 98% 3. Old Lava Flows 56,81% 63,19% 4. Lahars 7,75% 8,63% 5. Pyroclastic flows 8,03% 8,65% 6. Sulfide Dioxide (Volcanic Gases) 86% 86% 7. Erosion (Wastelands) 51,43% 54,29% 8. Snow 15% 45,60% 9
Volcanic Rock (Basaltic
Andesites and dacites) 23,93% 33,60% 10 Vegetation 10,63% 17,50%
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0.45 - 0.52 0.52 - 0.60 0.63 - 0.69 0.76 - 0.9 1.55 - 1.75 10.40 - 12.15 2.08 - 2.35 Type Min Max Min Max Min Max Min Max Min Max Min Max Min Max 1. Ash 95,268 107 107,21 126,327 132,141 142,5 151,83 158,79 158,79 158,87 0 0 159 159 2. Young Lava Flows 54,998 55 56,993 57,018 69,9975 70,02 43,988 44,013 232,99 233,02 0 0 244 250 3. Old Lava Flows 104,7 137 137,2 145,778 143,973 149,8 173,76 231,08 148,49 162,93 0 0 145 161 4. Lahars 37,026 40 36 36,3375 38,8875 42,39 44,625 47,175 23,428 27,572 0 0 20 22 5. Pyroclastic flows 76,5 80 32 34,9401 42,1719 54,31 58,723 63,75 23,428 29,546 0 0 20 22,1 6. (Volcanic Gases) Sulfide Dioxide 2,55 2,6 2,55 2,55 2,55 2,55 2,55 2,55 2,55 2,55 228,5 233,25 219 219 7. Erosion (Wastelands) 27,056 36 36,414 53,091 56,406 66,61 79,101 94,707 136,35 141,55 0 0 131 138 8. Snow 255 255 255 255 255 255 242,25 255 76,5 127,5 0 0 38 116 9 Volcanic Rock (Basaltic Andesites and dacites) 54,621 55 56,355 56,457 56,355 56,46 54,643 55,554 78,54 88,74 0 0 61 85,7 10 Vegetation 9,5625 13 12,75 25,5 15,9375 28,69 146,63 178,5 47,813 89,25 0 0 27 44,6
Table 2. Reflectance in DN of each element for ranges of wavelengths
0.45 - 0.52 0.52 - 0.60 0.63 - 0.69 0.76 - 0.9 1.55 - 1.75 2.08 - 2.35 10.40 - 12.15 13.2 - 16,25 1. Ash 101,24 117 137,34 155,308 158,827 159
2. Young Lava Flows 55 57 70,01 44,0003 233,006 246,9 3. Old Lava Flows 120,95 141 146,91 202,42 155,708 153 4. Lahars 38,513 36 40,641 45,9 25,5 20,88 5. Pyroclastic flows 78,25 33 48,241 61,2367 26,4869 21,27
6. Sulfide Dioxide (Volcanic Gases) 2,55 2,6 2,55 2,55 2,55 219,3 230,86 229,5 7. Erosion (Wastelands) 31,735 45 61,506 86,904 138,95 134,8
8. Snow 255 255 255 248,625 102 77,27
9 Volcanic Rock (Basaltic Andesites and dacites) 54,634 56 56,406 55,0981 83,64 73,35 10 Vegetation 11,156 19 22,313 162,563 68,5313 35,86
20 0 100 200 300 0.45 - 0.52 0.52 - 0.60 0.63 - 0.69 0.76 - 0.9 1.55 - 1.75 2.08 - 2.35 10.40 - 12.15 13.2 - 16,25 R ef le ct an ce ( D N ) Wavelength (Nm)
Spectral Profile
Figure 1. Spectral signature of the elements studied
Color Range for each Element Studied (RGB) LANDSAT (4,5,3) LANDSAT (7,4,1) LANDSAT (1,3,5) LANDSAT (7,3,1) LANDSAT (7,5,2) SENTINEL (8A,11,4) SENTINEL (12,8,2) SENTINEL (2,4,11) SENTINEL (12,4,2) SENTINEL (12,11,3) Type Min Max Min Max Min Max Min Max Max 1. Ash 2. Young Lava Flows 3. Old Lava Flows 4. Lahars 5. Pyroclastic flows 6. Sulfide Dioxide (Volcanic Gases) 7. Erosion (Wastelands) 8. Snow 9 Volcanic Rock (Basaltic Andesites and dacites) 10 Vegetation
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An example of the snow color convention will be shown below, for the SENTINEL 8A-11-4 band combination, presented in table 4:
First, it is important to identify from appendix 2 the wavelength range for bands 8A, 11 and 4 of the SENTINEL 2A satellite. For the 8A band that corresponds to Near Infrared Narrow (NIR Narrow), it is possible to observe that the light wavelength range is
between 0.855 and 0.875 micrometers, for the band 11 corresponding to Short-Wave Infrared 1 (SWIR- 1) it is between 1.56 and 1.65 micrometers and for band 4, which corresponds to red within the visible light, it is between 0.65 and 0.68 micrometers. Considering these ranges in the combination of bands, it is possible to search in figure 1 and in table 2, the digital numbers associated with the snow for these 3 different ranges or bands. Thus, for the NIR band, the minimum digital snow number is 242.25 and the maximum is 255. For the SWIR-1 band, the minimum digital number is 76.5 and the maximum is 127.5. And for the Red band, the minimum and maximum digital number is 255. Finally, after having the digital numbers associated with the pixel, it is already possible to determine their respective color. This color is a false color that is not
observable in reality. To be able to observe the true color of the pixel in reality, it would be necessary to combine the SENTINEL2 bands: 4,3,2 corresponding to red, green and blue (visible light). In this particular case, as the combination of bands is SENTINEL2: 8A, 11.4; in order to create a false color it is necessary to put in the red channel, the digital numbers of the band 8A (NIR-Narrow), in the green channel, the digital numbers of the band 11 (SWIR-1), and in the Blue channel, the digital numbers of the band 4 (NIR). Therefore, by putting the combinations R: 242 G: 76 B: 255 (minimum) and R: 255 G: 127 B: 255 (maximum), a range of lilac colors is obtained, as shown in table 4 of color convention. The previous procedure was performed for each element studied and for each combination of bands analyzed.
After obtaining the reflectance profile and the color convention, the processing was carried out. For all types of analysis performed, either uniband or multiband an improvement was made for each of the images. For this, the mean and variance of all digital numbers of all pixels was increased, this to obtain images with greater brightness and greater contrast respectively. To perform the contrast enhancement, a frequency histogram of the digital numbers present in each channel was performed, either for a single channel (uniband) or for multichannel: red, green, blue (multiband). Stretching will allow new values of digital numbers to be included in the frequency histogram in such a way that the range of values is increased and therefore that these new values that
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are being included affect the variance of the entire image. Specifically, the type of stretching performed was equalization. This type of stretching consists of matching the same number of digital numbers for each histogram displayed. To achieve this, an increase must be made to those histograms that have a smaller amount of digital numbers, where the histogram that has the largest digital numbers present in its data remains constant.
Finally, the uniband and multiband processing was performed. In the uniband processing, a reclassification of pixels was performed in the analyzed image with the ranges shown in table 2. Multi-band processing includes several supervised classifications and different band compositions. In the supervised classification results are obtained from a sample. The results obtained are shown below.
4.1 Uniband Results
LANDSAT 5: Image of September 8, 1986: Band 1
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4.2 Composite Bands Results
Fig 3: LANDSAT 5: Image of December 29, 1989 (7,4,1) Fig 4: SENTINEL 2A: Image of January 20, 2016 (12,11,3)
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Fig 7: SENTINEL 2A: Image of August 2, 2017 (12,4,2) Fig 8: SENTINEL 2A: Image of December 25, 2018 (12,11,8)
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4.3 Supervised Classification
Fig 11: LANDSAT 5: Image of September 8, 1986 (7.4,1) Fig 12: SENTINEL 2A: Image of January 20, 2016 (12,8,2)
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Fig 15: SENTINEL 2A: Image of August 2, 2017 (12,8,2) Fig 16: SENTINEL 2A: Image of December 25, 2018 (12,8,2)
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5. Discussion
5.1 Uniband Classification
The uniband classification consists in using a particular band for the identification of a particular feature studied. For example, to distinguish soil from vegetation, biomass content, shoreline, bathymetry, among others. Therefore, when making an uniband classification, certain elements that are being studied cannot be identified if the band is not specialized for the study that is being carried out.
At the beginning of the processing, the classification of the Blue band of an image belonging to the year 1986 was performed. The blue band, in this case corresponds to the number 1 of the LANDSAT 5 satellite. It is possible to observe this classification in figure 2.
The result obtained in the classification of the blue band shows information about the soil, however there are certain aspects to consider. For example, it is possible to observe that there are certain parts that are gray, and this happens because all the features analyzed together do not cover 100% of the reflectivity range. That is, by looking at table 2, which shows the range of reflectance as the digital number of the pixel of each element studied, it can be evidenced that it is not possible to cover the entire range from 0 to 255, which implies that there are empty spaces which correspond to ranges of reflectivity of other elements that are not being studied in this project. It is for this reason that these gaps in the digital range were treated gray.
It may also occur that there is an overlap in the range of reflectance of digital numbers between the same elements studied. For example, it is possible to observe in table 2 that there is an overlap in the range 0.45 - 0.52 micrometers between volcanic ash and the flow of ancient lavas between digital numbers 104 and 107. Likewise, it is possible to observe in table 2 that for the range 0.76 - 0.9 micrometers, there is a much greater overlap between the vegetation and the volcanic ash between the digital numbers 151 and 158, leaving the entire range of ash reflectance included in the range of vegetation and therefore being very difficult to discriminate both elements studied in an uniband analysis.
5.1.1 Blue Band Image
However, after considering the above considerations, it is pertinent to analyze the results obtained in figure 2. In this figure, most of the highlighted areas correspond to vegetation,
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which is green. Vegetation can include paramo and sub paramo vegetation very close to the Nevado del Ruiz and the other Nevados and are in an altitude of about 3200 meters above sea level. Within the vegetation of paramo and sub paramo, it can be found frailejones, chuscales, grasslands, reeds and lichens. Similarly, within the area defined as vegetation, it can be found areas of high Andean forest, located approximately 2500 meters above sea level. However, this Andean forest is further away from the snowy mountains because this type of forest is generated at heights below those of the paramo. Within the Andean forest vegetation, it can be found shrubs, ferns, trees such as raque, encenillo, mortiño, canelo, romero, aliso among others (Armenteras, 2003).
On the other hand, it is possible to observe that the young lava flows are located in this image in the volcanic cone, and they appear light blue. This result agrees really good with the volcanic morphology of a stratovolcano such as that of the Nevado del Ruiz. Generally, volcanic cones are formed by the accumulation of solidified lava and pyroclasts outside the volcano, as a result of eruptions or explosions caused over time. The size of the volcanic cone, as well as its thickness, show variations depending on the amount of eruptions presented in the volcano's history. These variations contribute to the crater increasing or decreasing in size. It is also possible to observe in figure 2, that the ashes and volcanic rocks that are red and lilac respectively appear in the lower part of the volcanic cone. This is consistent with the idea of the high slopes that occur in the volcano since all the material emitted by the volcano ends in the outermost parts of the volcanic cone where the slope tends to stabilize. Despite the aforementioned, pyroclastic flows should also occur throughout the volcano's cone, as can be seen in any of the supervised analysis figures (fig 11 to fig 17), since this gives the signs of a constant flow around the entire volcanic slope, which is the case of the behavior that occurs in reality.
Pyroclastic flows and lahars are not discriminated or highlighted in figure 2, however, an association can be made that is useful for the analysis. In one hand, in this image the ashes can be grouped together with the pyroclastic flows which are composed of ash, lapilli, fragments of volcanic rocks and pumice edges, this because the ashes are a component of the flows. On the other hand, following the same idea, pyroclastic flows can be grouped together with lahars for the analysis of this image since lahars are composed of flows plus melted ice. Thus, although there are not so many lahars and pyroclastic flows discriminated in this figure, the aforementioned association can be made. For example, it is possible to observe in figure 2 that different drains are highlighted as if they were ashes and flows of young lavas, which is not entirely incorrect considering the association
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existing between the pyroclastic flows and lahars described above. From this analysis it can be affirmed that, although the lahars in figure 2 are not explicitly highlighted, the drains marked as ash or ancient lavas would then correspond to the lahar flows existing around the volcano.
Once Lahar flows have been identified, these flows follow a direction of preference towards the northeast. This preference is closely linked to the high slope presented on the eastern flank of the Nevado del Ruiz volcano (Gonzales, 2001). The lahars tend to follow the channels of the rivers, therefore, the accumulation of thaw along with all the volcanic material such as ash, lapilli, volcanic rock and pumice edges, tend to mix and flow through these channels until reach a tipping point. The large volume of transported material generates an increase in the flow rate (White, 2017). In this way, the constant increase in speeds due to increases in volume of transported material will at some point incur an overflow of all this material.
Finally, after performing the uniband analysis, it is possible to observe that the information obtained is good and serves to perform the analysis of little specific elements such as vegetation, humidity, soil discrimination among others. Nevertheless, to analyze more specific characteristics such as lava flows, pyroclastic flows and lahars it is pertinent to use a multiband analysis which provides 3 channels that make the identification of the elements studied more specific and precise.
5.2 Multiband Classification
First, an analysis was made of the different results obtained by making the different band compositions described in table 4. Then, the multitemporal analysis will be carried out using the results obtained when doing the supervised classification by the Maximum Likelihood method. In the composition of bands, for each year a composition of different bands was used in such a way that it could be evaluated what each one shows, and which ones can be effective to identify the elements studied in this project.
5.2.1 Composite Bands
In figure 4, at the beginning of 2016, it is possible to observe a combination of the SENTINEL bands 12, 11 and 3 corresponding to SWIR2, SWIR1 and Green respectively. In this combination features such as types of rocks and vegetation are highlighted. It is
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possible to observe the delimitation of the volcanic cone, which has a light brown color and corresponds to the volcanic rocks (basaltic andesites and dacites). Similarly, ash flows can be clearly seen throughout the volcanic cone and are presented as elongated lines or light green curves that indicate a high slope movement. In this combination it is also possible to observe the presence of wasteland, in olive green, just after the volcanic cone ends. These wastelands occur due to recent activities on the volcano, such as flows or tectonic activity, which significantly reduce vegetation growth. All the colors mentioned above can be checked in the color chart (Table 4) for the combination 12,11,3.
Continuing with the analysis, in figure 5 of the year 2016, it is possible to observe a combination of the SENTINEL 2,4 and 11 bands corresponding to Blue, Red and SWIR 1 respectively. In this combination, the snow is quite prominent, which has a yellow color. In the same way in this band the flows of ash, pyroclastic and lahars stand out very well. Lahars are observed in darker blue color unlike ash and pyroclastic flows since lahars are the accumulation of pyroclasts plus water. Water by absorbing more light and reflecting less, will generate a darker color. The aforementioned can be observed in the spectral signature of the different elements studied (figure 1), where the signature of the lahars is below the signature of the ash and pyroclast flows, which implies less reflectance. On the other hand, the vegetation is observed of a quite marked dark blue color while the volcanic rocks are observed of a bluish gray color which allows to make a contrast and therefore delimit the volcanic cone of the vegetation. In this combination, features such as wastelands, lava flows, and volcanic gases cannot be so easily evidenced. All the colors mentioned above can be checked in the color chart (Table 4) for the combination 2,4,11. In figure 6, at the end of 2016, it is possible to observe a combination of the SENTINEL 12, 8 and 2 bands corresponding to SWIR 2, NIR and Blue respectively. The combination of bands of this figure is widely used in geology because it uses the three less correlated bands. Band 12, in red, covers the segment of the electromagnetic spectrum in which clay minerals absorb, rather than reflect, energy; band 8, in green, covers the segment in which the vegetation strongly reflects; and band 2, in blue, covers the segment in which minerals with iron oxides absorb energy (Rayo, 2012). Figure 6 corresponds to an image of the year 2016, which corresponds to the last eruptive period recorded by the Colombian geological service. In figure 6, information of volcanic rocks could be obtained: The turquoise color corresponds to andesites and basaltic dacites, the red color corresponds to lava flows, the dark brown color corresponds to sand and clays, the black color corresponds to volcanic gases and the yellow delineation corresponds to lahars. It is possible to observe that the
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lava flows are located at the top and adjacent to the Arenas crater and just above them in a northwestern direction are volcanic gases that are shown in black. The sand-clay matrix with solidified lava components presented in dark brown color, is easily differentiated from other types of rocks. That because for this particular combination of bands, the clay minerals tend to absorb all the light and not reflect too much, generating consequently that dark hue.
The lahars have a majority composition of water, product of the ice layer. Also, they are composed by sand, mud, clays and pyroclasts. All these components mixed together. It is for that reason that in figure 6, lahars can be identified with brown colors like that of clays, but unlike they have longitudinal shapes resulting from the movement in high slope. Thus, in figure 6, the lahars are shown delineated with yellow color for better differentiation. It is important to mention that this delineation was made after the realization of the composite image. All the colors mentioned above can be checked in the color chart (Table 4) for the 12,8,2 combination.
In figure 7, at the end of 2017, it is possible to observe a combination of the SENTINEL 12, 4 and 2 bands corresponding to SWIR 2, Red and Blue respectively. In this combination glaciers are highlighted with a turquoise color. In the same way, pyroclastic and ash flows can be evidenced, which have a grayish blue and greenish brown color respectively. Likewise, the flows are presented as curved or straight lines that follow the high slope movement behavior. The vegetation and the wastelands are both red and the difference is that the vegetation appears in a darker red, while the wastelands in a lighter red. Volcanic rocks can also be observed and have a light brown color. All the colors mentioned above can be checked in the color chart (Table 4) for the combination 12,4,2. In figure 8, at the end of 2018, it is possible to observe a combination of the SENTINEL bands 12,11 and 8 that correspond to SWIR 2, SWIR 1 and NIR respectively. This combination of bands allows to easily identify Andean forest vegetation, paramo and sub paramo vegetation, wastelands, volcanic rock and snow. The Andean forest vegetation has a range of light blue and dark blue and this type of vegetation is approximately 5 km far away from the volcanic cone. The vegetation of paramo and subparamo has a dark green color and is adjacent to the volcanic cone. The wastelands can be seen in an olive-green color and are right in the lower part of the volcano. The snow is partially covered by some clouds; however, it has a dark blue color. The lahars and pyroclastic flows also have a dark blue color quite similar to the Andean forest vegetation, which makes it difficult to
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discriminate in this band composition. All the colors mentioned above can be checked in the color chart (Table 4) for the combination 12,11,8.
Finally, in figure 9, at the beginning of the year 2019, it is possible to observe a combination of the SENTINEL 8A, 11 and 4 bands corresponding to NIR narrow, SWIR 1 and Red respectively. This combination of bands is perfect to discriminate between vegetation types. The vegetation of paramo and subparamo is presented in green color while the vegetation of the Andean forest is presented in a range of teracotta colors. All the colors mentioned above can be checked in the color chart (Table 4) for the combination 8A,11,4.
5.2.2 Supervised Classification
Now, after analyzing each of the possible compositions for composite bands, the composition SENTINEL 12, 8, 2 was chosen to perform the supervised classification. That is because this composition is one of those that use the least correlated bands, being able to easily identify all the elements that are being studied. It is important that there is an easy identification of each of the elements since it will make the work easier when taking the sample that will allow the classification. Therefore, the analysis that will be performed in this section will correspond to the temporal space analysis. It is important to mention that the last eruptive period of the Nevado del Ruiz volcano corresponds to the year 2016 (Servicio Geológico Colombiano, 2016) therefore the analysis will be carried out within the range 2016 to 2019. However, the eruption occurred on November 13, 1985, which affected to the municipality of Armero Tolima, serves as a point of comparison to analyze the current behavior of the volcano. This eruption was a catastrophic event that threw large amounts of pyroclastic flows, ashes and mud that ended up becoming several lahars that destroyed the entire town (Mojica, 1985). It is for that reason that LANDSAT images from the late 80s will also be analyzed, which serve as a comparison with current images. The color conventions used for this classification can be seen in table 4.
September 8, 1986:
In figure 11 it is possible to observe 4 large lahar flows present in blue. The first flow goes in a northeastern direction. The second and largest is in the northeastern part of the volcano. It is possible to observe a large deposit of these lahar flows in this northeastern part, which begins to give indications that this was one of the main directions taken by the lahar that affected Armero in 1985. Moreover, this is a strong assumption since this is
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an image almost a year after the tragedy. The third lahar flow is in the eastern part of the volcano and does not present large deposits adjacent to the volcano, however, it is possible to observe in figure 11, that this flow merges with the first flow described above in the north-eastern part of the map. The fourth and last great flow is in the southeast part of the volcano and goes in the same direction. All flows tend to follow channels of very high slope which facilitates reaching speeds of more than 60 km/h (Mojica, 1985). In the easternmost part of the map you can see the city of Libano, located about 25 km from the volcano. It should be noted that, although this municipality is partially adjacent to these drains where lahar flows travel, they do not pass through it as if it were with the municipality of Armero. This is a very important factor that prevented the 1985 Lahar from affecting this municipality as well.
In this image it can be also seen the volcanic cone, which corresponds to the volcanic rocks highlighted in purple, whose diameter is approximately 8 km. Pyroclastic flows, gather all the material issued by the volcano including volcanic ash, and are highlighted in orange. Most of these flows occur in the southern part, in the western part and in the northwestern part of the volcano, which suggests a preferred direction of the volcano and/or the presence of greater slopes presented in these parts of the volcano.
The snow cap located in the highest part of the volcano had an approximate diameter of 4.5 km by that date. This corresponds to a large amount of ice knowing that the 1985 lahar took approximately 10% (Mojica, 1985) of the total volume present before the tragedy. In the southwestern part of the map it can be seen the ice cap of the Nevado Santa Isabel. Similarly, it should be noted that the activity of the volcano was still quite high in that year, considering the large cloud of volcanic gases highlighted in gray in this image. Volcanic gases include: sulfur dioxide, hydrogen sulfide and carbon dioxide.
Finally, in this image the wasteland is not highlighted as if it occurred in the composition of SENTINEL 12,11,8 bands (see figure 8). On the contrary, what stands out most is the vegetation that can be both paramo or subparamo and Andean forest.
December 29, 1989
Figure 3 corresponds to a LANDSAT 7,4,1 composition and not a supervised classification, however many factors can be analyzed in it. In this image it can be seen the cloud of volcanic gases above the volcano which indicates a period of great activity. Similarly, it can be seen in the main crater of the volcano (Arenas crater), flows of young red lavas that respond to this period of activity. The period of volcanic activity is also congruent
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with the amount of ice present since it is possible to appreciate a small decrease in the amount of ice of this period compared to the image of 1986. The volcanic activity causes the ice to melt and therefore that mixes with the pyroclastic flows, generating small lahars that flow through the 4 main drains described for the 1986 image. Similarly, an increase in pyroclastic flows can be observed, which cover the entire eastern flank of the volcano. January 20, 2016:
In figure 12, 3 of the 4 main lahars flows that could be observed in figure 11 of 1986 can be observed. However, it is appreciable that these flows are not as highlighted as if it happened in the 1986 image, except for the flow which is in the northernmost part of the volcano and goes north-east. This indicates that, the greater activity of lahar flows occurs in this direction of preference. It is possible to verify this activity of the flows since by comparing the 1986 ice layer (figure 11) with the ice layer of this figure, the 2016 ice layer has a smaller volume. This gives indications of a thaw that was combined with the pyroclastic flows forming the lahars that are presented in the direction of preference shown in this figure.
Additionally, this image shows a moment of high volcanic activity since a large volume of volcanic gases is observed. These gases do not usually occur in periods of low activity as can be seen in figure 15, which corresponds to a time of low activity. Moreover, the image shows many pyroclastic flows highlighted in orange that have a direction of preference towards the southeast flank of the volcano. There are also some lava flows highlighted in red and located in the highest part of the volcano, exactly in the Arenas crater. All the above is consistent with the information presented by the Colombian Geological Service, which suggests a period of high activity associated with the last eruptive period recorded (Servicio Geológico Colombiano, 2016).
January 30, 2016:
In figure 13, a high level of volcanic activity can still be seen. This relationship is clear because the acquisition difference of both images is 10 days. A large number of pyroclastic flows remain welling by the slope of the volcano; however, they have a slightly larger volume than in figure 12. Similarly, a considerable increase in wasteland can be observed in the lower part of the volcanic cone, which indicates recent activity by the volcano, as well as an increase in lava flows in the crater of the volcano.
35 September 16, 2016:
Figure 14 continues to correspond to the same year where the last eruptive period occurred. Here the large number of pyroclastic flows emanating from the volcano stand out. Most of the volcanic cone, composed of volcanic rocks, is covered by these flows. In the same way it can be seen a large cloud of gases, along with young lava flows over the crater of the volcano. This date corresponds to the peak of the highest volcanic activity in the whole year (Servicio Geológico Colombiano, 2016).
August 2, 2017:
In figure 15, a more peaceful activity period can be observed in comparison to the images of the year 2016. The main lahar flows appear less highlighted compared to the images of past years. However, pyroclastic flows continue to have the eastern flank of the volcano as the direction of preference. The ice sheet in this image has greater volume compared to the images of January 2016, which indicates a growth of this ice sheet product of a low activity stage by the volcano.
December 25, 2018:
In figure 16, 2 of the main lahar flows are highlighted again. These correspond to those in the north-east direction. The ice sheet of this image has a smaller volume than the image of the year 2017, which may indicate a period of relative volcanic activity during the year 2018. Volcanic activity causes some of the ice to melt and flow through the drains that born from the volcano. Similarly, pyroclastic flows increased throughout the cone compared to the image of 2017. This increase in flows is indicative of increased volcanic activity. Although there is an increase in activity during 2018, it is not of the same magnitude as in 2016.
February 3, 2019:
Figure 17 shows a behavior very similar to that of 2018. The ice sheet seems to maintain the same volume and pyroclastic flows are distributed throughout the volcanic cone. Compared to the image of the year 2018, in this image it can be seen more wasteland near the volcano, which indicates a recent volcanic activity. Gases are still observed above the volcano, as well as lava flows.
36 September 30, 2019 (LANDSAT 8):
Figure 10 is the most recent of all images and corresponds to a composition of bands 7, 5 and 2 of the LANSAT 8 satellite. It shows all the drains through which the lahars from the volcano can flow. Moreover, it can be seen the drains where the lahar that affected Armero in 1985 traveled. Additionally, it can be observed that, during the year 2019, there was a growth of the ice sheet when making a comparison with the images of 2018 and early 2019. Despite this, some volcanic gases and young lava remain on top of the volcano. In this way, the activity presented by the volcano is currently associated with a yellow alert level III, recorded by the Colombian Geological Service.
5.2.3 Hazard Analysis
Within the analysis of risk, threats and vulnerabilities, considering the elements analyzed in this project, important ideas can be obtained. The threat generated by the volcano can be analyzed by discriminating the different threats that the volcano generates, such as: pyroclastic flows, lahars flows, volcanic ash, lava flows and snow, which were the factors analyzed in this project but nevertheless there may be many more. Each of these elements represents a threat to populations near the volcano. To assess the potential of these threats to affect these populations, it must be evaluated the state of vulnerability to each of these elements.
Vulnerability will therefore correspond to the weakness presented by each of these populations. These weaknesses would facilitate that some or all threats mentioned above affect these populations without difficulty. Therefore, the risk will correspond to the probability that a threat affects a population. Considering their vulnerability status, the probability of affectation may be greater or lower.
Thus, the greatest state of vulnerability is going to occur if any of the populations near the volcano are located nearby (less than 1 km) to some drainage, coming directly from the volcano, where lahar flows can flow. The lahar flows will bring all the material released by the volcano including pyroclastic flows, ash and melted ice, considerably increasing the flow of all that material flow. A population being close to these drains becomes weak since all the material that can flow through here has the potential to overflow and directly affect the population.
However, the risk is further increased if the drains where the lahars can flow have a fairly steep slope. This occurs since the high slopes increase the flow rate of the flow and therefore
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increase the probability that all this flow will overflow. This case was what happened in Armero in 1985, where there was a state of vulnerability and an increase in risk. The increase in vulnerability is due to the fact that the town was less than 1 km from the Lagunilla River while the increase in risk was due to the high slope that this river presents during its entire journey from the moment it emerges in the Nevado del Ruiz until it meets with the municipality of Armero (see appendix 6).
It is possible to analyze other cities and municipalities near the Nevado del Ruiz such as Manizales, Chinchiná, Santa Rosa de Cabal, Pereira, Armenia and Ibagué, and assess their vulnerability and risk status. For all these populations there is a great state of vulnerability since they are all close to a river that is a tributary of someone else that is rising in the Nevado del Ruiz or in one of the other adjacent Nevados (See figure 18). For example, Ibagué with the Combeima river, Armenia with the Quindío river, Pereira with the Otún river, Santa Rosa de Cabal with the Campoalegrito river, Chinchiná with the Claro river and Manizales with the La María ravine. However, the level of risk in all these populations Fig 18: 2019 LANDSAT 5 Map of towns near the Ruiz, Santa Isabel, Quindio and Tolima
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is not so high since the slopes of the rivers are not so steep except for the Combeima river and the Claro river. The Combeima River is born in the Nevado del Tolima and descends with a high slope to the city of Ibagué. In the case of a very violent eruption of the Nevado del Tolima, the snow-capped ice sheet would melt and combine with all the material of the pyroclastic flows, ash and lapilli to form a lahar that would go directly to the city of Ibagué. Thus, the city of Ibagué is in a high threat zone, which is consistent with the volcanic threat map of the Nevado del Tolima provided by the SGC (Colombian Geological Service) (see apendix 5). Similarly, the Claro river has a high slope in most of its trajectory until it reaches the municipality of Chinchiná. This high slope represents a high risk for this population and places it in a high threat zone, which is consistent with the Nevado del Ruiz volcanic threat map provided by the SGC (Colombian Geological Service) (see appendix 4).
On the other hand, other smaller populations that present vulnerability due to the proximity to a river and high risk due to high slopes are Mariquita and Honda. Both populations belonging to the municipality of Tolima and threatened by the activity of the Nevado del Ruiz volcano. This is consistent with the Nevado del Ruiz volcanic threat map and provided by the SGC (Colombian Geological Survey) (see appendix 4).
The historical records also agree with this analysis considering that the municipalities most affected, apart from Armero, by the eruption of the Nevado del Ruiz Volcano in November 1995 were Chinchiná, in its urban area, Villamaría, Manizales, Palestine and Neira in the rural area (Williams, 1990).
6. Conclusions
-The best combination of bands for the volcanic analysis performed in this project corresponded to the combination of the SWIR 2, NIR and Blue bands. This combination brings together the bands less correlated with each other, which highlights really good the elements studied in this project. These specific elements analyzed are volcanic rocks, lahar flows, pyroclastic flows, and lava flows.
- The evolution of the volcanic features analyzed in this project results in a behavior of points with high activity and other points with low activity. Throughout the year 2016, a high volcanic activity behavior was analyzed corresponding to the last eruptive period recorded by the SGC (Colombian Geological Service). In 2017, a period with low volcanic activity was analyzed, with only peaks of pyroclastic flows observed on the eastern flank
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of the volcano. In 2018 and early 2019, an increase in the activity of the Nevado del Ruiz was observed again. This volcanic activity was associated with an increase, observed throughout the volcanic cone, of pyroclastic flows and lava flow as well as a decrease in the volume of the ice level. Finally, by the end of 2019, a small decrease in volcanic activity was analyzed, however elements such as volcanic gases, lava flows, and pyroclastic flows are still presented in quantity. This indicates that the volcano is at a yellow alert level III activity level, which corresponds to an average activity level that is slightly lower than that recorded in 2016 (last eruptive period).
-When comparing the volcanic threat map of the Nevado del Ruiz (see appendix 4), together with the different results obtained in this project (see figure 11), it can be seen that in both cases the 4 large drainages of lahar identified are presented with supervised classifications. There is one more large drainage of lahar that goes in a northwestern direction (totally contrary to the directions identified in this project). This fifth was not identified in this project due to cloudiness in the study area.
-The 4 main drainages of lahar identified in this project, have a preferred direction to the east, which agrees with the different directions of pyroclastic flows obtained, which also have as preference the eastern flank of the Nevado del Ruiz. This preference of flows and lahars indicates that there is much greater susceptibility to transport combined volcanic material towards the east and therefore that there is greater threat and volcanic risk in these areas.
- The vulnerability presented by surrounding populations to the different Nevados corresponds to the stay in the vicinity of rivers which come from any of these Nevados, however, this vulnerability does not directly imply a risk. On the contrary, the risk that the threats, generated by the different Nevados, such as Ruiz, Santa Isabel, Quindío and Tolima, affect these populations depends on the high slope that these rivers may have (Rodríguez ,2017). Similarly, all the areas surrounding the volcano, within a maximum radius of approximately 75 km (see appendix 4), present a risk of falling ash and lapilli, where the closer to the volcano, the greater the risk.
- Finally, after obtaining all the results using the SENTINEL satellite group and the LANDSAT satellite group it is possible to conclude that SENTINEL shows an improvement in spatial resolution, which is 20 meters per pixel, while for LANDSAT it is 30 meters per pixel. Moreover, SENTINEL presents a higher spectral resolution which implies greater bands in favor of the analysis. In this way, this increase in spectral
