The application of a long-range laser scanner for monitoringvolcanic activity on Mount Etna

Graham Hunter a, Harry Pinkerton b;�, Rob Airey a, Sonia Calvari c

a 3D Laser Mapping Ltd., Sherbrook Enterprise, 100 Sherbrook Road, Nottingham NG5 6AB, UKb Environmental Science Department, Lancaster University, Lancaster LA1 4YQ, UK

c Instituto Internazionale di Vulcanologia, Consiglio Nazionale delle Ricerche, Piazza Roma, 2, 95123 Catania, Italy

Received 24 January 2002; received in revised form 1 September 2002; accepted 1 September 2002

Abstract

There is a requirement for rapid remote monitoring of ground deformation of volcanoes, especially duringeruptions. A long-range reflector-less laser scanning instrument, manufactured by Riegl Laser Measurement Systems,was used on Mount Etna in October 2000, and its potential for monitoring ground surface changes associated withvolcanic activity was evaluated. The 3DLM LR1 scanner system can measure distances up to 2 km with an accuracyof / 25 mm in typical conditions. Measurement rates range from 1 point per second at 2 km to 4 points per second at10 m. The system also records reflective intensity, and this proved useful in distinguishing between different types ofvolcanic material. Cloud and gas obscured some measurements, but the scanning nature of the instrument, coupledwith effective data filtering software, allowed robust digital terrain models to be created.1 2003 Elsevier Science B.V. All rights reserved.

1. Introduction

The need for detailed surveys of volcanoes isnow accepted and well established. These provideinvaluable information on magma movement inthe upper parts of volcanoes (e.g. Murray et al.,2000a), volumetric data on lava £ow volumes (e.g.Stevens et al., 1997, 1999; Murray and Stevens,2000b) and volumetric changes after emplacement(Murray, 1988) as well as information on changesin the summit region during periods of growthand collapse (Murray, 1980; van Wyk de Vrieset al., 2000; Calvari and Pinkerton, 2002). Tradi-tional methods involve high-precision total station

surveys and, more recently, GPS surveys. How-ever, there are many situations during eruptionswhen these methods are of limited usefulness, ei-ther for safety reasons or because of rapid rates ofchanges in the topography. This is particularlytrue during summit eruptions of volcanoes. Con-sequently, there is a need for alternative, rapid,remote surveying methods.The ideal eruption surveying system should be

capable of measuring the following from a safedistance:(1) Volumetric changes of lava £ow ¢elds.

These will provide essential data on e¡usion ratesand will lead to improvements in our knowledgeof volcano plumbing systems.(2) Proximal pyroclastic accumulation rates.

This can be a signi¢cant component of magmatic

0377-0273 / 03 / $ ^ see front matter 1 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0377-0273(03)00036-2

* Corresponding author.

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R

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production during eruptions and, because they arenotoriously di⁄cult to measure safely duringeruptions using conventional surveying methods,there are commonly large errors associated withthese measurements.(3) Cone and dome growth rates and dimen-

sions. During many eruptions, there is concernover the stability of growing domes or cones.Conventional surveying methods are di⁄cult andfrequently impossible to use. Consequently, re-mote sensing methods need to be employed.Recent applications of Synthetic Aperture Ra-

dar methods to volcanoes have shown the useful-ness of data from satellites such as the EuropeanSpace Agency satellite ERS-2, launched in 1995.These have con¢rmed that the surface elevation oflava £ows changes after emplacement through acombination of cooling, compaction and defor-mation of the underlying ground surface (Stevenset al., 2001). Relative elevation changes as smallas 28 mm are detectable using this method. Themain limitations of this technique are low coher-ence (and hence inability to analyse SAR data) ofrecent or active lava £ows, low spatial resolution(30 m), and a satellite return period of 35 days. It

is clear that, while satellite data have considerablepotential for monitoring volcanoes (e.g. Francis etal., 1996; Wadge et al., 2002), there is a need foran alternative remote method of surveying duringeruptions.In mid-1999, the Austrian company Riegl Laser

Measurement Systems manufactured a laser scan-ning system. The 3DLM LR1 scanner system(Fig. 1) can measure to a maximum range of2000 m with an accuracy of / 25 mm in typicalconditions. This instrument was considered tohave considerable potential for remote surveyson many volcanoes.3D Laser Mapping Ltd., which develops soft-

ware and provides applications support for thissystem, participated in trials on Mount Etna be-tween 22 and 25 October 2000. These were de-signed to establish the practical operating limita-tions on volcanoes and to evaluate the value ofthe scienti¢c data collected. During the trial peri-od, there was no e¡usive activity on the volcano,though there was degassing from the NE crater,and there were frequent strombolian eruptionsfrom two vents within the Bocca Nuova. Somestrombolian eruptions were accompanied bydense, yellowish or white gas plumes containing¢ne ash, and others ejected large bombs up to150 m above the rim of the Bocca Nuova.

2. Speci¢cations of the LR1 laser scanner

The long-range 3DLM LR1 scanner system(which is based on the Riegl LPM98-2K instru-ment; Riegl, 2002) has the following speci¢ca-tions:

bMaximum measurement range (in typical con-ditions): 2000 m.

bMinimum measurement range: 10 m.bMeasurement accuracy: typically / 25 mm.bMeasurement resolution: 4 mm.bBeam divergence: approximately 1.2 mrad (i.e.

120 mm beam width per 100 m range).bScanning range: 150‡ vertical angle by 360‡

horizontal angle.bScanning rate: between 1 and 4 points per

second (depending on re£ectance and range).The LR1 is powered by a 12-V battery and data

Fig. 1. 3DLM LR1 long-range scanner.

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are transferred to a data logger or PC using astandard RS232 serial link. The LR1 weighs 31kg, including its case.

3. Mount Etna test area

Between February 1999 and October 2000,there were several periods of intense activity inthe summit area of Mount Etna (Calvari et al.,2002). Between February and November 1999,two large lava £ow ¢elds were erupted at rela-tively low e¡usion rates from three primary ventsat the base of the SE Cone, one of the four erup-tive centres on the summit region of Etna. Be-tween mid-October and early November, a fan-shaped £ow ¢eld was erupted from the westernrim of the Bocca Nuova, currently the largestsummit crater on Mount Etna. Major periods of£ow advance in the Western £ow ¢eld took placeduring phases of relatively high £ow rate thatlasted a few hours to days. In addition, three col-lapse events occurred on the outer £anks of theBocca Nuova, the Chasm and the SE Cone. Someof these were hot mass £ows, while others in-volved major rock falls (Calvari and Pinkerton,2002). On 25 October, part of the western rimof the Bocca Nuova crater collapsed outward fol-lowing rapid rise of magma within the crater. Thiscollapse took place during another period of in-tense strombolian activity, and lava subsequentlyemerged at a high e¡usion rate through thebreached crater wall. A major concern during

this eruption was the potential instability of theSE Cone. The cone did not collapse during thiseruption. However, there is concern (Calvari andPinkerton, 2002) that it could follow a patternsimilar to that of the NE Cone in 1978 (Murray,1980) and collapse unexpectedly. There is there-fore a need to survey the SE Cone during the nextsummit eruption, and in particular, to look forany pre-failure movement.

4. Field measurements

A large number of sites were surveyed aroundMount Etna during the trial period. These weredesigned to test the instrument under a range ofoperating conditions. Three of the sites are pre-sented here as case studies to illustrate both thestrengths and limitations of the 3DLM LR1 scan-ner system.A laptop was used for data collection, and this

provided real-time images of the survey data. Aspecialist software application, TerraScan (Terra-solid, 2002), was used to classify laser points andto ¢lter the data to remove clouds, gas or vegeta-tion. The package also permitted rapid post-pro-cessing of data for plotting, contouring, and forviewing cross sections or 3-D perspective views.

4.1. Case study 1: Tre Levelli

Fifteen metres uphill from the main roadsideentrance to Tre Levelli, a classic lava tube in the

Fig. 2. Photograph of a small quarry above Tre Levelli on the southern £ank of Etna showing three thin £ows and fallen blocksresting on an unexposed lower £ow. The quarry is 8 m high.

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1792 lava £ow ¢eld (see Calvari and Pinkerton,1999), there is a small quarry which contains threethin lava £ow units. These overly a larger lava£ow which is partially covered by fallen blocks(Fig. 2). A survey of this quarry was undertakenat a viewing distance of 20 m and this generatedvery precise topographic data (Fig. 3A) and anintensity image (Fig. 3B) that closely resemblesthe photographic image of the quarry (Fig. 2).A 3-D perspective view of the quarry (Fig. 4A)and cross section (Fig. 4B) show the detailedstructure very clearly.In Fig. 3A, a colour is assigned to each of the

11 070 individual data points based on its range.

The scanner uses infrared light to measure rangeand also records the intensity of the re£ected lightfor each point. This re£ectivity will change withmaterial type, although it is also a¡ected by am-bient light conditions. Consequently, it is di⁄cultto use these data in rigorous quantitative analy-ses. Nevertheless, the intensity image gives an ex-cellent representation of material type within anindividual scan.The raw data captured by the scanner are

range, intensity, vertical angle and horizontal an-gle. These are converted to 3-D points using thefollowing equations:

x ¼ range sina cosP ð1Þ

y ¼ range sina sinP ð2Þ

z ¼ range cosa ð3Þ

Fig. 3. (A) Scan area of the Tre Levelli quarry coloured byrange. Minimum distance 15 m, maximum distance 978 m.The scan window of this area contains 90 vertical lines of123 points each. (B) Scan area of the Tre Levelli quarry col-oured by re£ective intensity (relative measure of re£ected in-frared light). White is highest re£ectivity.

Fig. 4. (A) 3-D perspective view of the Tre Levelli quarry.(B) Cross section view through the centre of Fig. 3B. Themaximum height of the quarry is 8 m. Vertical and horizon-tal scales are identical.

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where a is the vertical angle and P is the hori-zontal angle.The scan data were then transferred to a 3-D

CAD software package in which it was possible tomake perspective and section plots. These plots(Fig. 4A,B) clearly show the three upper lava£ows and the fallen blocks.

4.2. Case study 2: Fissure located SE of SE Cone

The SE Cone has been one of the main centresof activity since September 1998. At that time, itwas 50 m high, but by February 1999, its heightwas 120 m high and it had a basal diameter of200^250 m. The summit of the SE Cone collapsedseveral times during the 1999 eruption (Calvariand Pinkerton, 2002). These events, while rela-tively minor, resulted in blocks up to 4 m in sizetravelling 1 km from the summit of the cone. Thelargest instabilities occurred during periods whenmagma was at a high level in the SE Cone, andthere was concern, during the 1999 eruption, thatthere could be a large-scale collapse of this cone.Conventional surveying methods could not beused to assess stability during the 1999 eruption,and they are likely to be inappropriate duringfuture periods of activity on Etna because of thehigh frequency of intense strombolian activity atthis centre.A survey of the SE Cone (Fig. 5) was under-

taken from the roof of the Torre del Philosophy,1150 m from the summit of the SE Cone, during

stable weather conditions with no clouds, al-though there was occasional obscuration by gasfrom the Bocca Nuova. The data collected duringthis survey were used to construct a terrain mapof the ground surface (Fig. 6A), an intensity im-age (Fig. 6B), a detailed contour map (Fig. 7) anda cross section (Fig. 8) through the centre of theSE Cone. This survey was undertaken from a

Fig. 6. (A) Range image of the SE Cone from the Torre delFilosofo. Missing data correspond with periods when therewas obscuration by volcanic gas. (B) Intensity image of theSE Cone from the Torre del Filosofo.

Fig. 5. Photograph of the SE Cone on Etna from the roofof the Torre del Filosofo.

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viewing distance that would be safe during mosteruptions on Etna. The scan size in Fig. 6A was70U70 points making a total of 4900 point mea-surements. Distances shown in Fig. 6A rangefrom 155 m in the foreground to 1151 m at thecrest of the cone. If this area was scanned regu-larly during a period of volcanic activity, thenthese data have the potential to detect precursorymovement that may take place before a majorcollapse event.

4.3. Case study 3: NE Cone partially obscured bygas

A survey of the NE Cone (Fig. 9) from thetrack at its western base illustrates the e¡ect ofvolcanic gas (mostly water vapour and carbondioxide) on the operation of the scanner. The ver-tical lines on the range (Fig. 10A) and intensity

(Fig. 10B) images are caused by re£ections fromthe gas cloud. This e¡ect is caused by the scannermoving up and down capturing vertical linesfairly slowly, whereas the gas was moving fairlyrapidly from left to right.Approximately 20% of the 17U66 data points

in Fig. 10A,B were a¡ected by gas. In the crosssection shown in Fig. 11 the e¡ects of gas areremoved to allow the ground surface data to beplotted using post-processing ¢ltering software.The original re£ection data points are included,and they can be used to locate the position ofthe edge of the gas cloud.

Fig. 9. Photograph of the NE Cone from the track on thenorthern side of the cone.

Fig. 10. (A) Range image of the NE Cone; distance rangesfrom 2 to 701 m. Pixels to the left of the centre of the imageare caused by re£ection from a gas cloud (not present inFig. 9). (B) Intensity image of the NE Cone. Values are inthe range 3 (black, low re£ectivity) to 142 (white, intermedi-ate re£ectivity).

Fig. 8. Cross section of the SE Cone. Identical vertical andhorizontal scales.

Fig. 7. Contoured plan of the upper part of the SE Coneshowing individual points measured by scanning and calcu-lated contour lines. The ¢eld of view is approximately160U100 m. Contour interval is 5 m.

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5. Conclusions

Overall the Riegl 3DLM LR1 long-range scan-ner was shown to be a useful system for rapid,safe surveying and monitoring of topographicchanges on active volcanoes. The main limitationof the equipment was its inability to penetratethrough mist and volcanic gas. However, this isalso true of other conventional surveying methodsand we have shown that post-processing softwarecan remove re£ections from mist and volcanic gasfrom the scan data. An additional limitation is theweight of the current system. While it was possi-ble to carry the equipment several hundred metresacross lava £ow ¢elds during ¢eldwork on Etna,transporting it over longer distances would bechallenging.The instrument can scan at a rate of 1 point per

second at a maximum range of 2 km, so an imagecontaining 1800 data points can be captured in 30min. The instrument has already been used in theAlps for monitoring landslips in real time. In thisapplication, the 3DLM LR1 continuously mea-sures selected areas of terrain and triggers analarm if the terrain changes by a certain amount(Scheikl et al., 2000; Paar et al., 2000). This hasapplications in a number of volcanic situations,for example during the growth of volcanic domes,or as a precursor to major slope collapse eventson rapidly constructed pyroclastic cones, such asthe SE Cone of Mount Etna during the next erup-tion from this vent. This can readily be achievedusing di¡erential surveying methods undertaken

with the 3DLM LR1 long-range laser scanner sys-tem.

Acknowledgements

The authors would like to thank Hazel Rymer,Steve Self and Lionel Wilson for their commentson earlier drafts of this paper.

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