Fine-scale geomorphology through photogrammetric ...

Fine-scale geomorphology through photogrammetric analysis: comparison between Unmaned Aerial System and ground-based approach on Mt Batur and Altyn Tagh fault. Allan Derrien* Earth Observatory of Singapore

*

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Abstract

Altyn Tagh: 09/04/2014-10/06/2014 Supervisor: Pr. Paul Tapponnier* Gunung Batur: 05/21/2014-05/26/2014 Supervisor: Dr Kyle Bradley* Report written in 03/2015

Development of stereophotogrammetric software has enabled to process low resolution digital images into georeferenced 3D models, given a sufficient amount of photographs. The quality of the result highly relies on the size of the digital images in pixels, thus images from 12Mp give a reasonably sharp model after processing. In a first mission, we applied this technique to ground-based photographs on several sites along the Altyn Tagh Fault (ATF) in China. In a second mission, we took photographs from an Unmaned Aerial System on Gunung Batur volcano in Indonesia. Photographs were taken at regular time intervals and twenty georeferenced targets were placed on the ground beforehand. In both cases camera alignement was optimized using these targets and the residual geolocalisation errors used to quantify the reliability of the measurements. Using the models, we succesfully measured offsets ranging from 7.1 to 84m along Altyn Tagh fault, mapped volcanic geomorphology with a 5cm/ pixel precision and estimated volumes of emitted material during the 20th century eruptions between 3.2∙106 and 27.9∙106 m3. Even if this study remains preliminary, it gave encouraging results fo further work.

Keywords: UAS survey, Fine-scale photogrammetry, Gunung Batur, Altyn Tagh fault

1. Ground-based survey on Altyn Tagh Fault [1] Measurements of movement along strike-slip faults are important both as evidence of local tectonics and monitoring of earthquake hazard along known faults. On strike-slip faults, objects offset by terrain motion can provide relevant information on short and long term slip rate as well as on earthquake recurrence and magnitude. The Altyn Tagh fault, arguably the most important strike-slip fault in Asia, shows numerous evidence of ground motion and is well documented in its central segment. Left-lateral slip rates are assumed to range from 20-30mm/yr (Tapponnier et al. [2001]) to 10mm/yr (England and Molnar [2005]) in the quaternary to about 10 mm/yr in the present day (Zhang et al. [2007], Elliot et al. [2008]), according to the model used for quaternary slip rate restitution (i.e. rigid block extrusion or continuous deformation). [2] Photogrammetric processing uses subtle changes in the point of view of cameras on a scene to achieve correct restitution of the 3 spatial dimensions. Thus, it requires good visibility, and the scene to remain mostly unchanged while photographs are taken for restitution. Since cameras are the most available and

diverse data acquisition tool today, the photogrammetric approach fits any scale and most situations where other techniques would be unfit, either because they cannot be scaled for the object, or are too expensive, or are a political issue. [3] Here we investigate the potential of ground-based photogrammetry as a relevant tool for precise motion monitoring along strike-slip faults in the future. We used a photogrammetric approach to produce DEMs and orthophotos for measuring offsets from ground-based 12Mpixels pictures. Two different ways of acquiring the data were used. (a) A canon S110 was mounted on a retractable pole with an inclination of 45˚ below the horizontal. Images were acquired automatically every second as we walked the survey areas. Each area of about 100*100m was covered by 200 photographs. This was done for survey sites A, B, C (fig. 1). (b) Photographs were taken from a ridge overviewing the survey area using the same camera (sites A, B, C, D, E and F, fig. 1). The data was subsequently processed in chunks using Photoscan. Processing time varied from 24 or more hours on a field computer [8Gb RAM] to 6 hours on an observatory computer [64 Gb RAM] for a 1000 pictures dataset.

a.

b.

c.

d.

Fig.1 [a] Map illustrating the Tibetan plateau main faults and locating the study area on the Atlyn Tagh fault. [b] Map locating the survey (black) and investigation (white) zones. What is called investigation zones are the areas that were investigated but not surveyed for various reasons. [c] and [d] Maps locating the survey sites. From west to east: site D, C, E, B (Annanba basin), A (Yuleken Basin) and F (Aksay). 2. UAS survey on Gunung Batur [4] Gunung Batur (Mount Batur in Bahasa Indonesia) is a balinese stratovolcano located 8.25˚ south of the equator and 115.35˚ east of the prime meridian, part of the Sunda subduction system. It has produced several mildly explosive eruptions in the late twentieth century, accompagnied by basaltic lava deposits. The active cone rises 700m above the bottom of a double caldeira, respectively 7.5km and 11km in diameter, with the southeastern part of the outer caldeira filled by the Batur lake . Until today studies

remain focused mostly on caldera formation and the ubud ignimbrites deposit (Marinelli and Tazieff [1968], Wheller and Varne [1986], Sutawidjaja [1990], [2009], or petrological analysis of the lavas (O. Reubi, I.A. Nicholls [2004]). Volcano morphology and deformation still remain almost unstudied. 13 historical eruptions have been reported in the second half of the 20th century, moslty along a NE-SW fissure system including the three summit craters (Batur I, II and III) of the active cone (fig. 2) [5] The absence of vegetation on a substantial part of the volcano (fig. 2b), a rare fact in Indonesia, makes it

a.

suitable for photogrammetric rendering of the volcanic morphology. We used a 40x40cm quadcopter equiped with 2700mAh batteries and a Canon S110 camera to obtain aerial photographs of the lava flows, making 29 flights over 4 half days. The 3000 photos were used for reconstruction of the topography and orthoimage, and cover an area of 3.9km2. 3. Data, errors and uncertainties

b.

Batur I Batur II Batur III

Fig. 2. [a] Map of the Sunda subduction zone locating the main geological features and Mt Batur [b] Map showing the location of survey area on mount batur, as well as the area covered by barren lava flows (dark grey) and the NE-SW fissure system (red). a.

c.

[6] Digital photogrammetric reconstruction of an objet introduces errors, mostly due to lens deformation for non-flat lens objectives. Here the canon s110 we used had a small focal of 5.2mm so that the errors were minimized, although some still remain after camera calibration in the software (residuals, fig. 3). The second source of errors lies in the georeferencing and especially in the precision of ground control points location (1) in the photos and (2) in a geodetic reference system. The center of the targets materializing the ground control points on the photos can be e precisely identified (±1pixel) but the handheld GPS that were used for georeferencing dispersion lack the necessary precision required in order to match the photo precision. The maximum dispersion of the measure at the exact same spot on several distinct times was 3.2 b.

d.

Fig. 3. Photogrammetric survey results on Altyn Tagh (site F), after camera alignement and optimisation. [a] Orthophoto [b] Easting error on GCP, in meters. [c] Northing error [d] Z error. The GCPs are located by the red dots.

meters on vertical and 1.1 meter on horizontal, resulting in relatively important errors on GCP location with respect to the model (fig. 4b,c,d) This diminishes the quality of the geolocalisation of the models but not scaling and relative position of the points inside the map. In this report, we will call that the general geolocalization uncertainty. [7] The ground control points measured location and their estimated position on the model usually give close, but different, results (the maximum misfit was 2.8m). We will call that the punctual measure error here. That error was minimized on sites A, B ,C and F by doing a triple measure on each point, but the maximal error only went down to 2.4m. When using a handheld GPS, distances between measured ground control points location and model estimation of their coordinates are similar (0.1 to 2.8m) for models of the same size (200x200m) regardless of the fact that aerial view or ground-based view is used. On the other hand, though, the measured position of the ground control points for the lava flows model was more unfit with the model (5.2m max) when it was built full-scale, i.e. 2kmx2km. This could indicate an accumulation of distortion from lens geometry (even if a calibration based on 1˚) Camera specifications and 2˚) Optimisation of ground control points error is applied after alignement of the cameras) within the model, leading to less reliability of the measurements for long distances/big objects. In order to improve this default, a physical scaling object (tipically, that would be a retractable “cube” of 1x1x1m) should be placed on the terrain beforehand and be used as reference, or measurements should be made between identifiable points using laser distancemeters, instead of handheld GPS.

Efficiency index (pixel/picture)

Model precision (cm/pixel)

Fig. 4. Classification of the models following an efficiency index. Blue: high precision ground-based models. Red: high efficiency airborne models. Green: overviewing models. High precision models (1 to 2.5cm/pixel) were acquired with a ground-based approach using a 5m retractable pole, so that the pictures are close to the ground and need to be numerous to cover the desired area, hence yielding a low efficiency index (250), because less pictures are needed for the same area. This approach also limits the model edges distortion due to the lens properties. The last group presents the lowest precision and a comparatively low efficiency. These are the models created from an overlooking point of view on the site. As the efficiency index is similar as for the ground-based pole acquired models, it means that this approach gives away precision to cover a bigger area. Fig. 5 illustrates these three methods.

4. Modelling terrain geometry: accuracy, shadow zones and feature identification. [8] One of the major drawbacks of the ground-based approach is the omnipresence of shadow zones on the model, since the camera has to be oriented at an angle from vertical so that the operator does not appear on the photographs (fig. 5a). That is a problem for the cleanliness of the model, though it does not prevent from precise feature identification as shadows occur mainly behind rocks and boulders, smaller in scale than the features (terraces, ridges, streams). Results were cleaner for the aerial view models (fig 5b). [9] We classified the models computing a model efficiency index i=area(in square meters)*precision(in pixel/m2)/number of pictures. This way, the models requiring less pictures for the same quality have a better efficiency index (Fig. 4). We identify three clusters of points reflecting three acquisition methods.

Fig. 5. Schematic illustration showing the acquisition methods. [a] and [c] (oblique) were used on Altyn Tagh and [b] (vertical) on Mount Batur

[9] Note that the local terrain geometry influences the choice of the method. On Altyn Tagh fault surveys studies areas consist of offset alluvial fans, with a relatively smooth surface and a gentle northward slope so that the angle between the camera view and the mean ground level is optimized when the camera is looking uphill (south), thus minimizing the shadow zones. Secondly, these areas often have a topographic height nearby from where method (c) can be applied (fig. 6a). In contrast, the volcanic field shows a rough terrain geometry, with lots of crevasses, levees and cinder cones that makes poor quality ground-based models. In any case though, airborne survey always overcome the shadow zone problem and enhances the overall efficiency. [10] The models were interpreted so that terraces and stream offsets could be measured on Altyn Tagh fault (fig. 6b and fig. 7a,b,c,d). Four clusters can be evidenced around 7.45±1.3m, 28±2.6m, 41±5.2m, and 82.5±10.5m (table 1), and other values are found at 11.13±0.23m, 17.01±2.22m and 54.76±5.27m, which is coherent with the coseismic displacements, slip rates

Location Type SiteA riser riser SiteB stream stream stream stream SiteC riser riser

Offset(m) 7.12±2.13 7.73±4.18 28.03±2.41 54.76±6.27 26.4±4.09 39.81±10.47 17.01±6.22 11.13±1.23

SiteD SiteE SiteF

stream stream stream riser stream riser riser stream

41±4.54 81±6.27 84±9.32 29.19±2.15 41.71±4.13 41.92±3.53 7.26±1.45 7.2±1.11

Table 1. Measurements of offsets from the models. a.

meters

a.

b.

meters

b. c. T2

T1

meters

T0 T4

T3 d.

Fig. 6. [a] Landsat image of Site C (Altyn Tagh) locating the area of interest with offset terraces at the apex of the fan [b] interpretation on top of the DEM from the model.

Offset, in meters

Fig. 7. [a] to [c] Interpretation on top of the DEM from the models for sites F, B and A (Altyn Tagh). [d] measured offsets.

and recurrence intervals for seismic events evidenced in previous studies (Mériaux et al. [2005], Xu et al. [2005], Cowgil et al. [2009], Chen et al. [2012]) (table 2). [11] Although data acquired on mount Batur is not covering the entire lava flows area (fig. 2), a geomorphic map could be established by completing the missing areas with satellite imagery. The map shows some of the major 20th century lava flows (1904, 1905, 1926, 1964, 1968, 1974) and the associated ash deposits (fig. 9c). Some of the eruptive fissures could be mapped, although the oldest are covered under newer features. Use of several blend values between the final orthoimage, the elevation contour map and a hillshaded image extracted from the DEM enabled to precisely identify contacts between features. The observed morphology was interpreted using historical record from the Smithsonian Institute global volcanism program (table 3). Chronologically: (1) the 1904 eruption products are still visible on the northwest part of the lava field, near the inner caldeira rim, and present a sparse vegetagion cover.Measurement of its thickness is not possible from this data gathered here (Fig. 9a). (2) The lava field of the major 1905 eruption (Batur I, II and III) is still visible in some places including some of its borders but is covered by recent events and sparse vegetation . (3) The 1926 flow lies west of Batur III, still half visible, half covered by the 1974 flow and an ash and tephras layer. That layer increases in thickness as it gets closer to the 1963 source, indicating that the 1963 eruption is the potential cause for the biggest part of the ash deposits SW of Batur III. Moreover, this layer has probably been deposited in an early stage of the 1963 eruption, since the lava flow of 1963 shows no sign of heavy ash cover (fig. 9d,e,f,g). (4) Contemporary to the 1963-64 Mt Agung major eruption (the other volcano part of Batur volcanic field) (Zen and Hadikusumo [1964]) this event at Mt Batur can be linked to a source 100m SW of Batur III. (5) The 1974 eruption produced a lava flow going downhill west from the

main fissure zone (fig. 8), although the top of the flow is covered by ash and tephras from a posterior event. (6) Very mild eruptions in the end of the 20th century deposited tephras northwest of Batur III. On 23/05/2014, fumarolic activity was present in Batur III central and northwest craters, as evidenced by sulfur deposits visible on the orthoimage in fig. 8b. [12] The encompassing review on volcanic fields morphometric analysis from Grosse et al [2012] gives a basis for a systematic analysis of Batur surface morphology, although the classification work realised from 30m DEMs had to be adapted to fine-scale mapping. An approximation of Batur lava flows volumes could be given by measuring the mean thickness of the lava flows edges when this data was available on the model and multiplying by the flows area measured on Landsat imagery (fig. 8, fig. 11). Note that (1) only edge thickness was used so that mean lava thickness may be underestimated and (2) only the visible surface area of each flow was used, so that fig. 10 actually gives minimal erupted volumes for all eruptions exept maybe 1974 and 1968, which are not covered by any subsequent lava flow. In this context, regular elevation data would lead to precise and accurate measurement of erupted volumes. In complement, the volume of the visible cinder cones was estimated by substraction of the obtained DEM with the mean local slope surrounding the feature (fig. 10, fig. 11) Previous studies exist on photogrammetric monitoring/study of volcanic morphology, wether it

Fig. 8. Cross-section showing the method used to compute cinder cones volumes from the model by integration of the eleveation difference between the measured topography and a base level interpolated from the perimeter of the feature (mean local elevation).

Table 2. Summary of published figures on Altyn Tagh fault slip rate (Aksay segment) in 2012, from Chen et al. [2013]. Sa, T refer to the nature of the feature displaying the offset (stream or riser).

a.

b.

100m

100m

c.

1974 1968 1963/64

a.

1926

b.

1905

d.

e.

f.

1904

g.

Collapse Ash deposit Fissure

d.

e.

f.

g.

10m

Increasing ash thickness

Fig. 9. [a] and [b] Topo/Orthoimages showing the contact between 1904 and 1974 flows (note the 1904 cinder cone) and the Batur III crater zone. [c] Geomorpological map showing the main visible features, the extent of the survey area (black polygon) and location of inlets. [d], [e], [f], and [g]: blend between hillshaded image from the DEM and contour layer showing increasing smoothness of the ground texture from west to east on the 1905 and 1926 flows covered by ash from the 1963-64 eruption. Note that the 1963 flow [g] somehow covers the ash, indicating that the 1963 eruption might have had an early explosive phase.

Start Date

Stop Date

Certainty

VEI

Evidence

Activity Area or Unit

1999/03/15 2000/06 (?) Confirmed 1 Historical Observations 1998/07/04 Unknown Confirmed 1 Historical Observations 1994/08/07 1994/08/14 Confirmed 1 Historical Observations Batur III 1976/03/26 1976 /03/26 Uncertain 1 1974/03/12 1974/04 Confirmed 2 Historical Observations Batur III 1973 Unknown Confirmed Historical Observations Batur III 1972/01/19 1972/03 Confirmed 2 Historical Observations Batur III 1971/03/11 1971/08/25 Confirmed 1 Historical Observations 1970/01/05 1970/01/15 Confirmed 1 Historical Observations SW flank (1963 vent) 1968/01/23 1968/02/15 Confirmed 2 Historical Observations SW flank (Batur III) 1966/04/28 Unknown Confirmed 1 Historical Observations SW flank (west of 1965 vent) 1965/08/18 1965/12 Confirmed 1 Historical Observations SW flank (near Batur III) 1963/11/05 1964/05/10 Confirmed 2 Historical Observations SW, W flanks (near Batur III, Butus) 1926/08/02 1926/11/21 Confirmed 2 Historical Observations SW flank below Batur III 1925/01/05 1925/01/05 Confirmed 2 Historical Observations Batur II 1924/03 1924/03(?) Confirmed 2 Historical Observations Batur II 1923 Unknown Confirmed 2 Historical Observations Batur II 1922/08/30 Unknown Confirmed 2 Historical Observations 1921/01/29 1921/04/17 Confirmed 2 Historical Observations SW flank (Batur II) 1905 Unknown Confirmed 2 Historical Observations Batur I, Batur II, Batur III 1904 Unknown Confirmed 2 Historical Observations West caldera floor (Gunung Anti) 1897 Unknown Confirmed 2 Historical Observations Batur I 1888/05/30 1888 May 31 Confirmed 2 Historical Observations SE flank of Batur I 1854/04/28 Unknown Confirmed 1 Historical Observations Batur I 1849 Unknown Confirmed 2 Historical Observations 1847 Unknown Uncertain 1821/03/16 Unknown Confirmed 2 Historical Observations Batur I 1804 Unknown Confirmed 2 Historical Observations Batur I

Table 3. Historical record of eruptions at Mt Batur, starting in 1804. Data from the Smithsonian institute global volcanism program. Eruptions that have produced features represented on Fig. 9 are in bold. is for research purposes as lava dome growth in Alaska (Mt Redoubt : Diefenbach et al. [2011]), morphometric studies (Nicaragua: Grosse et al. [2012], Vesuvio: Pesci et al. [2007]), Guatemala: Bemis et al. [2010] ) or operational purposes for risk assessment and eruption monitoring (Stromboli and Vulcano: Baldi et al. [2000],[2005]). For Batur volcano though, we shall remain humble in the interpretation of data as the survey does not cover completely the volcano and very little is known about its behaviour.

Fig. 10. Edge thickness values for the 1905, 1926, 1963, 1968 and 1974 lava flows. 42 points were regularly measured on the visible edges of these flows.

5. Conclusions and insights These two studies evaluated the potential of cheap photogrammetry using off-the-shelf cameras and UAS for geomorphologic studies (the canon S110 is a 300$ camera and systems similar to the quadrotor used for aerial photography on Mt Batur, suiting scientific studies, can easily be found ready-to-fly from the mainstream drone industry for less than 1000$). Orthoimages and DEMs of 1 to 5cm/pixels were obtained on 200*200m to 3.5km2 aeras. This work confirmed measurements realized in separate studies on Altyn Tagh fault and showed the validity of the method for small-scale measurements. Besides, the aerial material that could be gathered on mount Batur brings new morphologic information which, although limited to contemporary surface observations, remain nevertheless precious and enables first order

Fig. 11. Minimal estimation of erupted volumes for the 1905, 1926, 1963, 1968 and 1974 eruptions against duration.

estimations of the volumes emitted during eruptive events in the twentieth century. Photogrammetry has proved a valuable alternative to Lidar scanning, for it avoided the usual drawbacks of Lidar surveys (cost, difficulty to survey complicated lava fields, data post processing time and efforts) although the results are inferior in quality, as the hardware used for the surveys was of standard commercial quality. Advanced airborne photogrammetry was proved highly competitive with Lidar scanning in terms of result quality in several 2014 geoscience studies (John Howell [Aberdeen University], Ryan Perroy [Hawai’i university]) or produce DEMs with an even better accuracy (Ruff et al. [2014 Swiss geoscience meeting]) at reduced costs and post processing involvement. Airborne photogrammetry surveys have been shown preferable to ground-based, for they do not display the usual shadow zones encountered on results from the latter nor the difficulties to catch the terrain geometry. Overall it yields a better efficiency, coming out with results of the same quality with less photographs and survey time, giving room to extend the survey area. However, sometimes ground-based photogrammetry is the best choice, when terrain geometry is adapted (rather flat, entirely walkable or displaying an overvlooking ridge that enables parralax from a line of pictures.) and airborne survey cannot be carried out for any reason (weather, law). As the drone technology develops fast under a new pression of airborne systems industry, affordable and powerful systems become available, representing an option for geomorphologic surveys not to be overlooked. Finally, photogrammetry crossed with image autocorrelation analysis seems a promising tool for real-time 3D monitoring of motion/deformation on geological live structures.

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Aknowledgements I would like to express my gratitude and respect to Kyle Bradley, for his wise leadership during Mt Batur survey and my project at the Earth Observatory of Singapore, and Paul Tapponnier, for sharing his immense knowledge and understanding of continental dynamics and tectonics. My thanks go as well to Yann Klinger, Jérôme Van der Woerd, Xu Xiwei and Mingxing Gao who preciously helped me at various points during the Altyn Tagh field work.