Colombia Trip Report Five USF scientists (Tim Dixon, Rocco Malservisi, Nick Voss, Surui Xie, and Jacob Richardson) and Mel Rodgers from Oxford University visited Colombia from Feb 23 to March 2, 2015 to work with scientists at the Servicio Geologico Colombiano (SGC; formerly INGEOMINAS). SGC is the government agency in Colombia charged with geologic hazard assessment. We operated USF’s Terrestrial Radar Interferometer (TRI) at several sites selected by SGC around Nevado del Ruiz volcano near the city of Manizales to investigate the use of TRI for volcano hazard studies. We also operated a camera-‐equipped drone supplied by USF’s AIST (Alliance for Imaging and Spatial Technology) as well as several ground-‐based cameras. Nevado del Ruiz is a large strato-‐volcano in the central Cordillera of Colombia. This volcano had a moderate to large (VEI 3) eruption in 1985, but one that was responsible for a large number of fatalities, approximately 23,000. A lahar (mud and ash flow) inundated the town of Armero, and many smaller communities on the main drainages from the volcano. The consequences of this eruption were exacerbated by partial melting of the glacier at the top of the volcano, which sent large volumes of water downstream, mixed with volcanic ash and rock. Previous eruptions in 1595 and 1845 also caused major lahars and loss of life at or near Armero. The volcano has shown recent signs of unrest [Ordonez et al., 2015] and was emitting a large gas plume during our observations. There have been frequent small ash emissions since December 2014, coating the glacier, so there is obvious concern that a lahar similar to the 1985 event could re-‐occur in the future if a larger eruption occurs. Sophisticated lahar flow models are now available to predict regions of high hazard. However, these require detailed knowledge of the topography (such as high precision Digital Elevation Models, or DEMs) which are difficult to generate on high relief volcanoes, especially in tropical regions, where near-‐constant cloud cover makes the use of optical satellite techniques challenging. The utility of the TRI for volcano studies falls principally in three areas: generation of high precision Digital Elevation Models (DEMs); imaging of relatively rapid events such as lahars, dome growth and changes in capping glaciers; and measurement of slow deformation via repeat measurements (many volcanoes experience measurable surface deformation prior to major eruption). In the last category, a special monument must be constructed that will be stable over several years and can hold a custom-‐designed mount for the TRI. The camera-‐equipped drone as well as ground-‐based photographs can also be used to construct DEMs using the Structure from Motion (SfM) technique. The SfM approach involves taking large numbers of photographs from a range of locations and exploiting parallax and feature identification to construct a 3-‐D surface that can be scaled to a DEM with a small number of control points. The drone is a Phantom 2, four rotor helicopter model with gyro stabilization, GPS tracking, and high resolution video camera. The drone allows photographs from 10-‐20 meters above ground level, useful for generating the right amount of parallax in the vertical dimension. The TRI and SfM techniques are highly complimentary: the upper slopes of the volcano tend to be cloud covered, but can be imaged by the TRI. On the other hand, some of the lower canyons tend to be in radar shadow, but can be imaged with SfM. We made observations at three locations. The first site (“Azufrado”) was located on the edge of Rio Azufrado on the northeast flank of the volcano, at an altitude of about 4,000 meters (Figure 1). Rio Azufrado is a major drainage off the volcano and a potential future lahar channel (it was one of the main lahar channels that contributed to the Armero disaster). The upper flanks of the Azufrado drainage are of particular interest as the currently active Arenas crater is very close to the upper reaches of this drainage and there is concern that any potential new volcanic activity would occur in this area. Detailed DEMs of this drainage would enable better modeling of lahars. The higher slopes of the volcano were cloud covered during most of our observations, but were clearly imaged by the TRI (Figure 2). The optical images observed the lower canyons which were in radar shadow (Figures 3 and 4). The two data sets will be combined to produce a high fidelity DEM of the Rio Azufrado lahar channel (to our knowledge these two data types have not previously been combined). SGC constructed a stable monument at this site, so we will also be able to do repeat observations here with the TRI to look at longer term deformation of the volcano flank. The tentative plan is to perform repeat observations at this site in about 6 months. The monument at Azufrado consists of a rebar-‐reinforced concrete block 1 meter square and 60 cm in depth. Three threaded bolts five inches in length were drilled into the monument and secured with concrete wedge anchors. The standard Gamma permanent radar mount (60 cm height) was secured to this monument for our observations (Figure 1). Measurements were made at three separate days during our visit, on February 25, 27 and 28. Each measurement session was about an hour in length in the middle of the day, and collected at least 25 separate images at two minute intervals. Antenna angle was set to zero degrees (imaging from zero degrees elevation up to 35 degrees elevation, sufficiently high to capture the crest of the volcano and capping glacier. The multiple observations collected here will allow us to: 1. generate a high quality DEM through averaging; 2. assess noise in the DEM; 3. collect statistics on noise in the displacement estimates (over a one hour period, there is essentially no deformation of the solid rock surfaces). It is likely that the main source of noise in the displacement estimates will be variable atmospheric water vapor. Some of our observations were made with a clear atmosphere, at other times most of the region was covered with dense fog. 4. Define a precise “before” image, for comparison to an “after” image acquired in about 6 months, for measurement of longer-‐term rock displacement. 5. Finally, it is possible that over the three day span of our observations, displacements can be observed on the glacier. The second site (“La Llorona,” or weeping woman) is located on the same road, at approximately the same elevation, about 2 km south of the first site. Observations were acquired here during a single observation session on February 28, primarily for DEM generation. Observations at the first (Azufrado) site could not image a large valley to the south due to shadowing (Figure 1). Most of this valley is well imaged from the Llorona site. This site did not have a permanent mark. Observations were conducted with a tripod on a small hillock with the top of the tripod located about 33 cm from ground level (Figure 5). SGC intends to construct a permanent mark at this location for future deformation observations. A preliminary DEM from combining single scans at Azufrado and Llorona is shown in Figure 6. This DEM has not yet been corrected for atmospheric errors or other known artifacts. The third site (“Refugio”) is located close to and west of the crater, at an altitude of approximately 4,800 meters. It was occupied for a short time (30 minutes) on February 25 near the end of the day (Figure 7-‐10). This site is quite close to a clearly defined glacier front (Figures 7 and 8). It was not possible to construct a new monument at this location due to time and logistics constraints (site access is quite difficult). SGC has several existing geodetic marks at this location, steel pins set in concrete that protrude about one inch above the concrete surface. We used the mark closest to the glacier (the easternmost site, opposite the wooden building (the Refuge; see Figure 10). Note that the previous Refuge was destroyed in the 1985 eruption, although its foundation is still visible. We used a laser centering device to ensure that the phase center of the radar antenna was directly over the mark; the radar was approximately 1 meter above the mark (top of tripod at approximately 35 cm above ground level; see Figure 10). Because of the large topographic relief at this site, and the need to image the glacier, the radar antennas were aimed above their normal setting (basal elevation zero degrees); instead, the basal elevation was set to +10 degrees This setting gave clear returns from the glacier and adequately images the lower slopes (see attached image). Unfortunately, the radar failed after only three image acquisitions at this site, possible due to high elevation, cold, or dust. The three observations are sufficient to produce a DEM. However, for displacement estimates of the glacier, it is recommended that a dedicated monument for the radar be constructed. Drone observations were also made here (Figure 9), so it should be possible to compare results from the two data sets. Lessons Learned Radar: the radar needs to be upgraded to improve reliability (see Recommendations, below). Also, first time occupation of a new monument would be simpler if threaded bolts were already set into the concrete and sized exactly to fit the radar mount (see Recommendations, below) SfM: this technique is promising, although data reduction is challenging for large data sets. Also, the drone will need to be upgraded to enable more reliable high altitude operation. Recommendations 1. Compile a new DEM for the upper part of Rio Azufrado using combined TRI and SFM data; 2. Compare to best existing DEM (30 m SRTM); 3. Assess the influence of improved DEM accuracy on lahar flow models; 4. Design and construct up to 20 stainless steel radar mounts (4-‐bolt planar mounts to be set into concrete ground monuments) to match the Gamma radar pillar mount. This would enable faster set-‐up the first time radar measurements are made from a new monument. USF will do this in the next three months. 5. SGC to use the new radar mount to construct new monuments at this and other volcanoes (e.g., Purace; Galeras); 6. Upgrade the radar: replace the current hard drive/operating system with a new flash drive, and enable operation of the radar from an external hard drive that can be backed up; add forward/back scanning capability to speed up data acquisition and reduce power consumption. 7. Make additional radar measurements in 6 months to look for deformation. 8. Train SGC personnel to make radar measurements independent of the USF group (the radar can be shipped to SGC from Tampa). References Ordonez, M., C. Lopez, J. Alpala, L. Navaez, D. Arcox, M. Battaglia (2015) Keeping watch over Colombia’s slumbering volcanoes. EOS: Earth and Space Science News, vol. 96, 14-‐17. Figures Figure 1. USF and SGC scientists install the TRI at the Azufrado site. The radar is visible on the right hand side, with three antennas: the top antenna transmits the Ku band (1.7 cm wavelength) signal, while the bottom two antennas receive the signal. Phase differences between the middle and bottom receive antennas are used to generate the DEM. Phase differences at one or both of the receive antennas from successive scans of the radar are used to estimate displacements of the surface. The radar is mounted to an aluminum pillar, which is in turn bolted to the concrete pad. Figure 2, Top: Google Earth perspective view of Nevado del Ruiz, looking south (Santa Isabel volcano is in the background, about 10 km away). Base: radar image intensity draped over Google Earth image, showing data acquisitions at Azufrado (east of crater, on the left) and Refugio (west of crater, on the right). The edge of the capping glacier was imaged in both data acquisitions. Note the large central valley (black area) in the Azufrado image that is not imaged from this location due to shadowing. It is imaged from the Llorona site (see Figure 5). Figure 3. Images from the ground (left) and an aerial image from the Phantom 2 drone
(right) enable SfM software to create a shape model of the Azufrado drainage.
Correlating points are indicated with orange lines. Points such as these show the parallax
effect exploited by SfM software.
Figure 4. A preliminary shape model of the Azufrado drainage is created with
AutoDesk's ReCap 360 using photos from a ground-based cell phone and the drone. This
shape model, essentially a three-dimensional triangulated irregular network (TIN) has
~200,000 triangles. The left side shows a shaded relief view of the model, while the right
side (a "painted" view) includes the color and albedo information from the photographs.
Note the water fall in the background (vertical drop approximately 75 meters).
Figure 5. Radar set-up on tripod at Llorona
5700
6000
5400
5000
4500
4500
Elevation(m)
4800
(m)
5500
5100
4000
4200
3900
'24"
4˚53
4˚55
'12"
−75
8"
4
˚19'
0"
0
˚18'
−75
˚1
−75
"
6'12
Figure 6. Preliminary DEM of east side of Nevado del Ruiz made from combining the
single scan TRI data at Azufrado and Llorona. More than 75 scans were collected at
these locations, and can be averaged to reduce noise. The DEM shown here has not been
averaged and has not yet been corrected for atmospheric and other known artifacts.
Figure 7. Radar set-up on tripod at Refugio. Note glacier in background.
Figure 8. Close-up of glacier at Refugio. The glacier covers most of the top portion of
the photograph, but is ash-covered on the left hand side.
Figure 9: Drone (upper right, in front of cloud) deployed at Refugio (upper right). Figure 10. Location of TRI relative to warming hut at Refugio.