Detecting the development of active lava flow fields with a very-long-range terrestrial laser scanner and thermal imagery

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L22305, doi: /2009gl040701, 2009 Detecting the development of active lava flow fields with a very-long-range terrestrial laser scanner and thermal imagery M. R. James, 1 H. Pinkerton, 1 and L. J. Applegarth 1 Received 26 August 2009; revised 21 October 2009; accepted 23 October 2009; published 25 November [1] Regular topographic surveys of active lava flows could provide significant insight into the development of flow fields, but data of sufficient accuracy, spatial extent and repeat frequency to quantify the processes involved have yet to be acquired. Here, we report results from the use of a new very-long-range terrestrial laser scanner (TLS) on active lavas at Mount Etna, Sicily. The scanner proved capable of providing useful topographic data from volcanic terrain at ranges up to 3500 m, with laser returns from ash-covered slopes as well as from lava. Despite very low effusion rates (<1 m 3 s 1 ), topographic changes associated with the emplacement and inflation of new flows and the inflation of a tumulus were detected. Irregular data spacing resulting from oblique views makes the interpretation of laser-derived digital elevation models alone difficult, but fusing topographic data with thermal images allows active flow features to be clearly visualized. Citation: James, M. R., H. Pinkerton, and L. J. Applegarth (2009), Detecting the development of active lava flow fields with a very-long-range terrestrial laser scanner and thermal imagery, Geophys. Res. Lett., 36, L22305, doi: /2009gl Introduction 1 Lancaster Environment Centre, Lancaster University, Lancaster, UK. Copyright 2009 by the American Geophysical Union /09/2009GL [2] Despite significant advances in our understanding of lava flow emplacement processes, many challenges remain. Initial emplacement can be reproduced relatively well by numerical flow models [e.g., Crisci et al., 2004; Hidaka et al., 2005; Vicari et al., 2007], but processes of importance during sustained eruptions, such as channel switching, flow inflation and the production of ephemeral vents, cannot currently be forecast. In order to increase our understanding of these processes, and to improve flow models, detailed and frequent assessments of the activity and topographic evolution of flow fields are required. [3] Topographic data of sufficient accuracy and resolution can now be acquired by the laser scanning techniques regularly used for a wide range of environmental applications including geohazard assessment [e.g., Glenn et al., 2006; Jones, 2006; Mazzarini et al., 2005; Rosser et al., 2005]. From airborne platforms, dense, accurate and extensive digital elevation models (DEMs) of inaccessible and hazardous terrain can be obtained. However, the logistics and cost of airborne surveys prohibits repeat at the daily (or more frequent) intervals necessary to assess flow processes and define the characteristics of flow field development. More frequent surveys can be carried out using groundbased terrestrial laser scanners (TLSs) but, until recently, the use of such instruments for volcanological applications has been hampered by quoted instrument ranges being generally limited to hundreds of metres, with long range variants extending to 2000 m. On volcanic terrain, which has low reflectivity values (e.g., <30% [Pesci et al., 2007]), practical maximum ranges are usually reduced to 1100 m [Hunter et al., 2003]. Here, we report the use of a new Riegl LPM-321 instrument with a quoted maximum range of 6000 m, which was successfully used to image lavas and ash-covered slopes over distances up to 3500 m. [4] Although a ground-based approach can enable frequent data acquisition, in rugged terrain the oblique fields of view can be subject to multiple occlusions that restrict overall data coverage and, consequently, very-long-range terrestrial laser scanning on volcanoes is not straightforward. Nevertheless, the potential of TLS data for monitoring the stability of volcanic edifices, craters and active lava domes has been previously illustrated on Mount Etna, Vesuvius and Soufrière Hills volcano [Hunter et al., 2003; Jones, 2006; Pesci et al., 2007]. For detecting change associated with active lavas, repeated topographic surveys have been previously carried out using a mm-wave radar (AVTIS) [Macfarlane et al., 2006] and ground-based photogrammetry [James et al., 2007]. AVTIS can image through cloud, which is not possible with a TLS, but is less portable and has a beamwidth greater than five times that of the LPM-321. The equipment used for photogrammetric surveys is significantly more portable than a TLS, but data processing is generally more complex and the technique is less suitable for measurements over ranges greater than a few hundred metres. [5] Very-long-range laser scanners thus offer a promising solution for obtaining topographic data suitable for recording lava flow field development, and, to assess instrument capabilities and limitations in an active volcanic environment, an LPM-321 was deployed at Mount Etna during June Here, we present and discuss the results of the first TLS measurements of active lavas and inflation processes and the benefits of combining these data with thermal and visible imagery in order to visualize active volcanic regions. 2. The Eruption of Etna [6] The eruption of Etna is documented in reports from the Instituto Nazionale di Geofisica e Vulcanologia, Catania, ( It began on 13 May, 2008 when a fissure opened to the East of the South East Crater between 2900 and 2700 m asl, above the headwall of the Valle del Bove. After the first week of degassing, Strombolian explosions, and ash-rich L of5

2 headwall of the Valle del Bove above Monte Centenari (Figure 1a). [7] During the time of fieldwork (5 12 June, 2009), the eruption was in its final decline, with activity restricted to slow effusion from a region of tumuli near the top of the lava delta. Multiple short flows, with lengths of several hundreds of metres were active on both the northern and southern sides of the delta. The eruption finally ceased on 6 July Figure 1. (a) View of the lava delta taken during February 2009 from the southern wall of the Valle del Bove. The developing lava delta on the headwall of the valley is clearly visible due to its lack of snow cover. The upper and lower thick arrows show the position of the fissure and the top of the lava delta respectively, and the NorthEastCrater(NEC)andSouthEastCrater(SEC)are indicated. (b) Overview map (UTM coordinates) of the eastern side of Mount Etna overlain with areas of TLS data acquisition. The scanner locations are represented by the three symbols given in the key. The background shaded relief map was constructed from SRTM data (collected in 2000 and available from U.S. Geological Survey, EROS Data Center, Sioux Falls, SD), and the inset shows the scanner mounted on its tripod at Belvedere. Box region illustrates the position of data shown in Figure 3. eruptions at multiple vents on the fissure, activity stabilized to a relatively steady effusion of lava from the lowest vent. The ensuing activity was then characterized by generally slow effusion, producing short 0 a 0 a lava flows (1 2m 3 s 1, flow lengths <3 km), with a short period of elevated activity (mid June to mid July 2008) temporarily extending flow lengths up to 6 km. As the eruption progressed, the limited length of the flows, combined with regular channel switching, resulted in the steady construction of a lava delta on the 3. Data Acquisition [8] Data were collected using a Riegl LPM-321 laser scanner comprising a laser head, co-mounted digital camera, and scanning mount (measuring approximately cm and weighing 16 kg). For field deployment, the instrument was powered by an external 24 V battery, mounted on a sturdy tripod and controlled by a laptop computer (Figure 1b, inset). Within the laser head, the near- IR (905 nm) time-of-flight distance meter has a pulse rate of 20 khz and provides measurements at 1000, 100 or 10 Hz, with the lowest rate corresponding to a very-long-range capability resulting from extensive line-of-sight averaging. The range accuracy is 25 mm and, for a flat natural target of a reflectivity 80%, nearly orthogonal to the beam and larger than the beam footprint, the maximum measurement range is 6000 m. With a beam divergence of 0.046, the laser footprint is 80 mm, 0.8 m and 3.2 m at ranges of 100, 1000 and 4000 m respectively, and the scanning mount allows data to be collected in an angular raster with minimum azimuth and elevation steps of [9] At Etna in 2009, the scanner was used from three different locations (Figure 1b): Schiena dell Àsino (5 June), Pizzi Deneri (6 June) and Belvedere (9 and 12 June). At each site, data collection comprised scanning the scene of interest, as well as scanning retroreflective targets (whose positions with respect to the scanner were obtained by dgps) for georeferencing. All data presented here were collected using the very-long-range mode at 10 points/ second, with a scene typically being acquired, in several sections, within a few hours. With the instrument leveled, the target scans were used to orient in azimuth and transform data to the UTM coordinate system (Zone 33N). Note that due to the trial nature of the deployment, precise georeferencing was not required, so absolute accuracy was limited to the metre-scale accuracy provided by the GPS base station observations. [10] To facilitate visualization and interpretation of the laser data, images were simultaneously captured using a FLIR Thermacam S40 thermal camera and a digital SLR camera (Canon Eos 500D, with a 200 mm lens). The cameras acquired time-lapse imagery from positions on stable ground within a few metres of the scanner. Here, the thermal images were processed to enhance visualization at the expense of their radiometric scaling, so temperature scales are not provided. 4. Results [11] Figure 1b shows an overview of the areas covered by the TLS surveys. Due to the low reflectivities and oblique 2of5

3 oblique nature of the views highlights individual channels by clustering returns along levees, but, in daylight and at long range, active flows are difficult to distinguish from other recent lavas in individual images from standard digital Figure 2. Relationships between laser return amplitude, measurement range and surface type in TLS data acquired from Schiena dell Àsino. (a) The view from the scanner location and (b) corresponding classification of surface types. (c) Return amplitude plotted against range and classified by surface type. observation angles the longest measurement recorded is 3978 m. Previous work on volcanic terrain has shown that the return strength depends on surface type, texture and incidence as well as on range [Mazzarini et al., 2007; Pesci and Teza, 2008; Pesci et al., 2008] and, for data collected from Schiena dell Àsino, the effect of range and surface type on amplitude is illustrated in Figure 2. In common with data from other TLS instruments, return strength is given by a normalized amplitude value, in Figure 2c, on a scale of 0 1. A significant scatter exists within the amplitudes for any particular range, with vegetation providing generally elevated values with respect to areas of bare surface. The envelope of the data suggests that the maximum expected return from volcanic terrain could be up to 4500 m but, practically, useful point cloud densities were generally restricted to distances <3500 m. Thus, although the active flow region was visible at a range of 4500 m from Schiena dell Àsino, it could not be effectively imaged with the TLS from this location. [12] From the other two sites used for surveys, Belvedere and Pizzi Deneri, the distance to the active flow region was 1300 and 1900 m respectively, and data covering the top of the lava delta were successfully acquired. The individual point returns are shown in Figure 3a where, for illustrative purposes only, the outline of the lava flow field has been highlighted by shading recent topographic change. The Figure 3. (a) Topographic data of the head of lava delta. TLS point cloud data acquired from Belvedere and P. Deneri are shown by their constituent individual data points. The flowfield is highlighted by an illustrative shading of the difference between a 5-m DEM calculated from the TLS data and an equivalent derived from SRTM data (see Figure 1b). The region expanded in Figure 4c is boxed. (b) To identify the positions of currently active flows in the TLS data, the DEM has been draped with the thermal image shown in the inset, taken from P. Deneri. TLS data covered by the thermal image was collected on July 6, between 09:16 and 12:56. 3of5

4 Figure 4. Changes in the lava flow field as observed from Belvedere. (a) A thermal image taken looking NE from Belvedere of the top of the lava delta on 9 June, with white indicating the active flow area. (b) Excerpts of visible images of the delta top (covering the extent labeled in Figure 4a) taken on 9 and 12 June, show the change resulting from inflation (1) and a new flow (2). Local horizons (top) traced in the image are (bottom) reproduced to aid comparison. (c) Topographic height change, determined by differencing 5-m resolution DEMs constructed from the laser scanner data acquired on 9 and 12 June, overlain by the positions of the individual laser returns, given by circles and dots respectively. The long-dashed lines show the lateral extents of the images (Figure 4b) and the inflation (1) and new flow (2) areas are indicated. The region labeled 3 shows the position of the flow active on 9 June, as shown in Figure 4a. The short-dashed box outlines the region of static topography used for DEM comparison (see text). cameras. However, ongoing activity is clear in thermal images that can be incorporated with topographic data [James et al., 2006] to help visualize flow positions. In Figure 3b, the inset thermal image taken from Pizzi Deneri (6 June) has been orthorectified and draped over a DEM of the laser data to illustrate the locations of the three active flows. [13] The repeated scans from Belvedere (9 and 12 June) allow the very-long-range TLS data to be analyzed for changes due to the emplacement of small lava flows. Over this period, the activity observed from Belvedere is illustrated in Figures 4a and 4b. The thermal image (Figure 4a) shows an active lava flow at the head of the lava delta on 9 June and, in Figure 4b, the evolution of the local horizon at the top of this region is shown by excerpts from digital images. The complete change in the appearance of the local horizon mostly reflects resurfacing by new flows providing a new horizon location, but also tumulus inflation, giving an elevated horizon with little location change (arrow 1, Figure 4b). [14] To accurately merge or compare different TLS surveys, an iterative closest point (ICP) [Besl and McKay, 1992] algorithm is commonly used to refine the relative registration of point cloud data. For the repeat surveys from Belvedere, suitable areas of static topography could be identified using thermal images, allowing ICP-based adjustment within the Riegl RiProfile (v.1.5.0) processing software. In Figure 4c, individual laser returns from both surveys are marked by circles (9 June) and dots (12 June), and are superimposed over a DEM difference map calculated from these datasets. Areas of static topography to the south east and to the west are illustrated by co-located returns associated with little height change, reflecting the repeatability of the measurements and the relative registration of the datasets. For the region enclosed in the finedashed box, the mean DEM difference value is 0.30 m, with an rms of 0.37 m, values that are much greater than the millimetric to centimetric values usually associated with TLS data acquired over much shorter ranges [e.g., Pesci et al., 2007]. This reflects co-registration difficulties that result from very-long-range observations of extremely rough topography giving an irregular and in some places sparse, data spacing. Nevertheless, the values indicate that the data and registration are of sufficient quality to permit analysis of metre-and-larger changes appropriate to lava flows. [15] In the areas that had experienced activity, inflation of the tumulus (1) did not significantly change the planimetric position of returns, but increased height values as indicated by the underlying DEM difference map. However, changes to the flow shown in Figure 4a (labeled 3), and the 4of5

5 emplacement of a new flow with associated inflation (2), are indicated by laser returns in new planimetric positions. The irregular and varying data spacing resulting from the oblique view and complex topography means that, for these small changes, interpretation of the DEM analysis alone could be misleading. For example, the two areas of height decrease around (501520, ) are not associated with data points and result only from differing interpolation over occluded areas. For larger magnitude changes, DEM analysis would be sufficient but, to elucidate changes such as the emplacement of small flows, inspection of individual returns is advisable. 5. Conclusions [16] A preliminary study aimed at assessing the use a new, very-long-range TLS for monitoring the development of lava flow fields has shown that the instrument is capable of delivering topographic data for volcanic terrains over distances up to 3500 m. The generally rugged nature of active volcanoes means that instrument site selection is important to minimize occlusions and to permit observation of the interest area as well as of areas of static topography for accurate co-registration of the data. Compared to data collection with standard TLS instruments, very-long-range measurements are significantly slower to acquire and the resulting point cloud data are less dense. For very rough topography such as 0 a 0 a channels, this can decrease coregistration accuracies of datasets to decimetric. Nevertheless, and despite low lava effusion rates, topographic changes associated with the emplacement and inflation of new flows and the inflation of a tumulus were detected. For small changes, interpretation of the topographic data alone is difficult and a process-based understanding of the changes is facilitated by combined analysis of laser scanner data, thermal and visible images. Consequently, very-long-range TLS methodology has significant potential for providing frequent topographic data of volcanic features, including lava flows, during future eruptions at Etna and other volcanoes. [17] Acknowledgments. This work was funded by NERC (NE/ F018010/1). We thank two anonymous reviewers for their constructive comments. References Besl, P. J., and N. D. McKay (1992), A method for registration of 3-D shapes, IEEE Trans. Pattern Anal. Mach. Intell., 14, , doi: / Crisci, G. M., R. Rongo, S. Di Gregorio, and W. Spataro (2004), The simulation model SCIARA: The 1991 and 2001 lava flows at Mount Etna, J. Volcanol. Geotherm. Res., 132, , doi: /s (03) Glenn, N. F., D. R. Streutker, D. J. Chadwick, G. D. Thackray, and S. J. Dorsch (2006), Analysis of LiDAR-derived topographic information for characterizing and differentiating landslide morphology and activity, Geomorphology, 73, , doi: /j.geomorph Hidaka, M., A. Goto, S. Umino, and E. 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Ventura (2008), Remote sensing of volcanic terrains by terrestrial laser scanner: Preliminary reflectance and RGB implications for studying Vesuvius crater (Italy), Ann. Geophys., 51, Rosser, N. J., D. N. Petley, M. Lim, S. A. Dunning, and R. J. Allison (2005), Terrestrial laser scanning for monitoring the process of hard rock coastal cliff erosion, Q. J. Eng. Geol. Hydrogeol., 38, , doi: / / Vicari, A., H. Alexis, C. Del Negro, M. Coltelli, M. Marsella, and C. Proietti (2007), Modeling of the 2001 lava flow at Etna volcano by a cellular automata approach, Environ. Modell. Software, 22, , doi: /j.envsoft L. J. Applegarth, M. R. James, and H. Pinkerton, Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK. (m.james@lancaster. ac.uk) 5of5