ERS1/2 interferometry on Erta Ale volcano: the study of a proto-ocean ridge using SAR

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1 ERS1/2 interferometry on Erta Ale volcano: the study of a proto-ocean ridge using SAR Atzori S. (1), Coltelli M. (2), Marsella M. (1), Puglisi G. (2) 1) Dip. Idraulica, Trasporti e Strade, Università di Roma "La Sapienza. 2) Istituto Internazionale di Vulcanologia - CNR, Catania Introduction Erta Ale volcano is the only example of a persistently active subaerial volcano above a mantle plume forming an oceanic spreading axis. The volcano is located in the Afar region of Ethiopia, an area representing the northernmost end of the Great Rift Valley of East Africa. Despite its unique characteristics, this volcano is also one of the less studied in the world. The reasons for this are both the harsh climatic conditions and its location in a war zone during the last two decades, making it impossible to organise research missions. The last systematic studies of this volcano were carried out in a framework of a collaboration between CNRS (France) and CNR (Italy) at the beginning of the 70 s. At that time mapping products at scales larger than 1: were not available for this area, and so the geological map of the Erta Ale volcano was based on aerial photographs approximately referenced. The desert climate and the gentle slopes of Erta Ale, allow to apply SAR-based techniques in ideal conditions to extract a Digital Elevation Model (DEM) useful for future research missions and remote sensing studies. In this work the first DEM of Erta Ale volcano obtained by SAR interferometry, by processing ERS1/2 descending pass images is presented. Furthermore, a plan of research activities that might be carried out in this area using SAR data will be presented through examples. Geodynamic Framework Erta Ale volcanic range stands above a mantle plume forming an oceanic spreading axis in the northernmost end of the Great Rift Valley of East Africa. The coexistence of an oceanic type spreading axis and a well developed continental rift zone in a relatively small area (about 100 per 100 km large) represent a very peculiar geodynamics framework in which mantle upwelling and continent rifting find a dynamic equilibrium (Tazief, 1973). In this area exist a theoretical chance to measure both rifting divergent movements and crust uplift due to the intense volcanism. Geophysical data on the dynamic of this area will be useful to model the process of opening of an ocean and will give a new insight into the plate tectonic. Erta Ale range is located into the Danakil depression between the Etiopian Scarp on the western side and the Danakil Alps on the eastern side. Ethiopian Scarp represent the abrupt eastern slope of the westward tilted Ethiopian plateau, the inland portion of Africa continent. In a relatively narrow scarp zone Ethiopian plateau step down to Danakil depression from more than 2000 m of altitude to below the sea level. Danakil Alps limit the depression to the east. They appear as an asymmetric mountain massif reaching over 1000 m of altitude with its gentle slop facing Red Sea. Danakil depression is a trench about 2000 m deep and between 30 and 60 km wide. It is the continuation of Great Rift Valley of East Africa the cut the continent up to Red Sea. Its floor is occupied by a salt plane, in some places overlain by present-day alluvium, situated below sea level (about 100 m at the Erta Ale range northern termination). In the depression center between 14 and 13.5 of latitude north rise the Erta Ale volcanic range. Erta Ale volcanic range is a huge volcanic edifice developed along a N335 axis, approximately 90 km long per 40 km wide. It is made up of several volcanic centers some of these still active and one, Erta Ale caldera, certainly in persistent eruptive activity since the beginning of this century. Its shape is intermediate between a fissure volcanoes and a typical shield volcano and confirm that the oceanic spreading it is in early state of evolution. The volcanic range is built of widespread Pahoehoe lava flows with minor Aa flows generally basaltic in composition. Large calderas, including the present active, and a long volcanic rift zone characterize the range axis where the most of lava is emitted. However, some small and steep stratocones produced by more silicic and explosive eruptions are present along

2 SAR Interferometry processing The selection of the images was performed by inspecting the ERS archives with the DESCW software. In table 1 are listed all the image that were scheduled to be acquired over the Erta Ale area. Unfortunately the image dated 9/10/1997, which was virtually forming a good inteferometric tandem pair with the image of the 10/10/97 (baseline lenght of 360 meters), resulted not actually available. Furthermore, the image acquired on the 29/6/1993 that, for baseline constraints, may be combined only with the image acquired on the 10/10/97, resulted to be affected by major faults in the raw data, probably due to undetected missing lines. As a consequence, the results presented in this work were obtained by processing the 3 SLC full-scene evidenced in Table 1. These images were coupled forming the three interferometric pairs (IP1, IP2, IP3) listed in Table 2 with the associated values for the temporal baseline, perpendicular baseline and height of ambiguity. the two full-scene images the same subscene. - Coregistration: this step is performed to compute the orbit/earth geometry by establishing a 10x10 grid of image control points over the area. At the same time, a grid of patches is set up to be used by the cross correlation algorithm which perform the coregistration of the Master and the Slave image. - Interferogram generation: as a first step the Slave image is resampled in the Master image space. In order to avoid the decorrelation between the images, an azimuth spectral filter and a baseline decorrelation filter are used. Before generating the raw interferogram, the flat earth phase is subtracted. By means of a phase coherence map it is also possible to perform an enhancement of the raw interferogram by eliminating noiser pixels; the adopted adaptive filter works by expanding or contracting the averaging window depending on the local estimate of the phase standard deviation. - Phase unwrapping: two different method for unwrapping the enhanced interferogram were applied in sequence: after the first one, based on a Multi-grid Date of acquisition 29/06/1993 2/05/1996 3/05/1996 9/10/1997 9/10/1997 Sensor ERS1 ERS1 ERS2 ERS1 ERS2 Orbit Frame Table 1 Image Pair IP1 IP2 IP3 Date 2/5/1996-3/5/1996 2/5/ /10/1997 3/5/ /10/1996 Temporal baseline 24 hours 17 months 17 months Perpendicular Baseline length (m) 115,98 204,52 338,99 Height of ambiguity (m/cy) 81,46 20,65 26,79 Table 2 The data processing was performed using the Image processing tools developed by Atlantis (EarthView InSar V ). The procedure used for the generation of interferometric products relevant to the seleceted image pairs can be summarized in the following steps: - Generation of Master and Slave images: the main purpose of this processing step is to reformat CEOS SLC image into an internal format for master and slave data entities and to extract from least-square algorithm, unwrapping errors may still be present in low coherent data, or in correspondence of the terrain discontinuities. In order to refine the results, a second algorithm, called Iterative Disk Masking, was used. This algorithm allows for a local manual correction of discontinuities by setting the correct path for the unwrapping. Areas where the residue density remain too high (and the coherence too low) are masked out.

3 FRINGE 99 Workshop - Liege (B), October DEM generation: the first product is the Slant Range height map for the areas where the phase unwrapping has been concluded successfully. From the satellite acquisitions. Coherence maps were produced for the all the processed image pairs: in Figure 1 and Figure 2, the results obtained for IP1 and IP2 are displayed Figure 1 Figure 2 previously performed orbit/earth geometry analysis, it is possible to calculate the slant range/height change, correct foreshortening and layover effects and resample the image in a map projection (geocoded DEM). Analysis of the coherence and amplitude maps The coherence parameter indicates the degree of similarity between two complex SAR images and, as a consequence, it measures the capability of extracting the topographic information from the interferometric data. The level of coherence strongly depends on the different backscattering surfaces and on the temporal baseline between the Figure 3 Test Area 1 in a gray scale representation As expected, the map relevant to the tandem pair (IP1) provides very high level of coherence over the whole area due to the lack of vegetation and to the dry soil. Nevertheless, a good level of coherence is also observed for both pairs with 1year time resolution, as shown in Figure 2. Low coherence appears in correspondence of the flat areas covered by silts, clays and sands and of the steepest surfaces around the major volcanic cones (Rorom Plain). In order to experiment the usefulness of SAR products for geological interpretation, a preliminary study has been accomplished by comparing coherence maps and the relative amplitude images to a geological map. For this aim, two test areas were selected and analyzed. In Figure 3 and 4 three different products are compared: the geological map (CNR-CNRS, 1972), the coherence maps (at 1-

4 Figure 4 Test Area 2 day and 1-year distance) and the amplitude image. At a first look it is clear that the information conveyed by the coherence maps are different and often complementary to the ones derived from the amplitude images. By inspecting the coherence maps of the first test area (Figure 3), centered around the Ale Bagu cone, it is possible to detect the discontinuity between the different type of terrain which is also detectable in the amplitude image. The area includes both sands and volcanic rocks that give rise to different back-scattering behavior, being lower for sand (black or dark gray areas) than for the lava (white areas). Both coherence and amplitude maps highlight surfaces with very high roughness, like the Aa lava flow outpoured from the northern flank of the Ale Bagu cone. Conversely, only the amplitude image emphasize some Earth surface features, like the wadies crossing the Rorom Plain, that disappear in 1 year coherence map, because its time-life is very short. In the second test area (Figure 4) a part of the fracture system cutting the southern flank of the Erta Ale volcanic range is shown. The fractures are well visible in the 1 year coherence map as features with very low coherence value. This is probably due to the local accumulation of the aeolian deposits (sands) filling the fault steps. As already recognized in the previous figure, the different surfaces and/or terrain (sands, pahoehoe or aa lava flows) are again easily detected. Erta Ale DEM generation All the analyzed image pairs exhibit a very wellshaped interferometric fringes, as shown in Figure 5 and Figure 6 for the dataset IP1 and IP2. Unfortunately, the most coherent Tandem interferogram and the relative DEM resulted affected by a disturbance which caused the appearance of not existing ground features. This effect can be explained either by the presence of atmospheric artifacts or by the occurrence of a data. The investigations aimed at understanding the atmospheric origin of this disturbances are unfortunately limited by the absence of any standard meteorological information, both from meteo satellites and ground based measurements. Owing to the presence of this disturbances in the Tandem interferogram, a merging of the most coherent data from IP2 and IP3 was necessary in order to extract a DEM covering the whole area. As a first step, the quality of the earth/orbit geometric correction applied on the basis of the orbital parameters was roughly estimated by comparisons with the only available Ground Control Point measured through an undifferenced pseudorange GPS solution. The horizontal offset observed between the GPS coordinates and the one measured on the image is of about 9 along the North direction and about 1 05 in the East component. These inconsistency can be addressed to the inaccuracy of the orbital data and to the positioning errors of a not-differential only C/A code GPS receiver. In order to improve the procedure for image georeferencing, the use of precise orbits and the measurements of a number of identifiable ground control points should be considered. The coregistration of the three DEM products was performed by using the Tandem derived DEM (IP1) as a reference for the IP2 and IP3 DEMs: the resampling was made on the basis of more than 20 homologous points corresponding to very well outlined ground features. The merging of the three DEMs was produced by averaging the height values of the IP2 and IP3 DEM (Figure 7) and by using the less reliable values from IP1 (Figure 8) for low coherent areas in IP2 and IP3. The generated DEM for the Erta Ale area is shown in 3D perspective in Figure 9. In consideration of the poor available data and to the lack of any satellite and terrestrial data over this area, the produced DEM can be considered up to now the most accurate tool for geological and geodynamical investigation.

5 Remarks and plans for future works The processing of the available ERS1/2 data highlighted several problems (e.g. missing line in the data file) that actually reduced the possibility views for future works: on the basis of the orbital parameters provided by ESRIN, it results that the interferometric pairs, obtained coupling this image with the already available images, is characterized by a good geometry (height of ambiguity ranging from 10 to 40 m). Thus, major Figure 5 Interferogram IP1 Figure 6 Inteferogram IP2 to obtain an optimal DEM for the investigated area, in spite of the ideal environmental conditions (desert area) for the application of the SAR. This problem can be overwhelmed if other images will be acquired during future passes over the Erta Ale volcano area. For this aim, a request of data collection has been submitted to the ERS mission manager. Due to a conflict with the Wind Scatterometer planning, ESR-2 data have been requested at least for the next descending orbits, until the end of The preliminary evaluation of the orbit of a recent descending pass occurred on 15 October 1999, allows for an optimistic improvements of the results obtained in the present work can be foreseen, particularly for those areas where elevation model was obtained only using the noiser IP1 data (e.g. Rorom Plain). Another critical aspect that has to be mentioned concerns the lack of other altimetric data in the area. In the present work the height values of the obtained DEM have been referred to coordinates of few points determined with undifferenced pseudorange GPS solutions. Nominally, the height accuracy of these kind of GPS measurements is in the order of 300 m. Figure 7 Average DEM (IP2 and IP3) Figure 8 IP1 DEM

6 Unfortunately, due to operational constraints, it is very likely that these measurements will remain the only available for long time. In such a situation, a possible way to connect the DEM to a reference height system may be based on altimetric satellite information collected over the two lakes situated at both the ends of the Erta Ale volcano. Since the surfaces of the lakes remain relatively stable (at least in the order of a few meters) for long time and have comparable dimensions to the footprint of the ERS Radar Altimeter, it is possible to try to use the measurements carried out by these instrument above the land. This approach, once verified and applied over the investigated area, would provide at least an independent validation of the altitude estimation performed in the present work. Since the problem to obtain good quality DEM is a main issue for future works, much emphasize will be given to the improvement of its accuracy through the integration of the InSAR results data obtained from other techniques. It should be envisaged the use of radargrammetry method (Rosenfield, 1968) applied to the same ESR1/2 data. Provided that in this area exist several geological features easily recognisable in the SAR images, results from other methods may be used for comparison and validation of the INSAR DEM. Furthermore, if the project will have the needed financial support, also SPOT images might be take into account. Obviously, the DEM is not the principal aim from the geophysical point of view. Provided that a DEM will be suitable, the next step will be to investigate the ground deformations that occurs in this area. These could be related both to the peculiar tectonic framework and/or to the volcanic activity of the Erta Ale. In the first case, the movements are expected along the fault system that bound the volcano at NE and SW. In the second case, the deformations are related to the movements of the magma within the volcano plumbing system and usually appear in the form of inflation (or deflation) of the flanks. In both cases, the displacements in the vertical components, which are more easily detectable using InSAR, are expected to be larger than the horizontal one. The comparison between multi-temporal coherence maps and the geological map of Erta Ale range clearly showed that the analysis of the coherence can be fruitful in order to distinguish between volcanic and sedimentary (mainly aeolian sands) terrain and within different volcanic terrains (Pahoehoe and Aa lava flow, tuff and spatter cone, etc). The coherence map locally fits very well with the geological map of Erta Ale rang, in particular for the lava flows of different surface and age, as well as some structural features (e.g. fault system and crater rims). Even if some restrictions for its use still remains on low coherency regions, the use of coherence maps as a tool for geological interpretation has been already tested and positively evaluated over other test areas (Coltelli et al. 1995). In order to finely establish the performance of this tecnique on the teh Erta Ale area it is necessary to collect more detailed geological information. Furthermore, it would be helpful to apply the coherence analysis on images acquired over a longer time inteval than the one considered in this paper. References Barberi F., Varet J. (1970). The Erta Ale volcanic range (Danakil depression, Northern Afar, Ethiopia). Bull. Volcanol., 34, CNR-CNRS (1972). Geological map of the Erta Ale volcanic range. Coltelli M., Fornaro G., Franceschetti G., Lanari R., Migliaccio M., Moreira J. R., Papathanassiou P., Puglisi G., Riccio D., Schwabish M., SIR-C/X-SAR Multifrequency Multipass Interferometry: a new Tool for Geological Interpretation. J. Geoph. Res., E12, Rosenfield G.B. (1968). Stereo radar technique. Photogrammetry Engineering. 34, Tazieff H. (1973). La signification tectonique de l Afar. Re Géograph. Phys. Géologie Dynam., Paris, Vol. XV (2), 4, Figure 8 3D Perspective DEM