The developed data processing approach shows that TanDEM-X is a suitable sensor to generate high-resolution DEMs to study the lava flows of the 2012-13 Tolbachik eruption. The final estimated lava flow area of about 36 km2 as well as the extruded lava flow volume of 0.53 km3±0.01 km3 is reasonable, especially compared to the values calculated by Dvigalo et al. (2014) and Belousov et al. (2015). This was also shown for the derived lava extrusion rates.
In addition to the mentioned error sources, the uncertainty estimation was able to clearly prove the feasibility of the developed differential DEM approach. The analysis of the four areas with different characteristics showed that independently of the time of year and location, all elevation differences were centered around zero. The mean over all elevation difference maps of all areas µ is with -0.21 m very low, as is the mean standard deviation over the mean of all elevation differences σµ with 1.63 m.
Only a few data pairs in reference area B showed non-reliable elevation values, which are due to the geometric decorrelation effects along the flanks of Plosky Tolbachik.
Since the lava flow area itself was comparably flat, it is expected that geometrical decorrelation did not influence the lava flow volume estimates. The accuracy as well as the precision of the results prove that TanDEM-X enables to map lava flow extent and measure lava flow volume with the differential DEM approach with a high accuracy.
The accurate measurements of important volcanological parameters are very promis-ing. The differencing algorithm between a pre-eruption and several syn-eruption and post-eruption data pairs demonstrates the potential for using bistatic TanDEM-X satellite data in volcano research and monitoring procedures. The test of using the bistatic TanDEM-X data in near real-time to monitor ongoing lava extrusion dur-ing eruptions showed promisdur-ing results. However, the time delay until the data is available will be an issue.
The DEMs themselves as well as the derived volcanological parameters provide valu-able information and can be used – among others – to model lava or pyroclastic flows, to assess hazard and risk of an eruption, or can be used as input for repeat-pass de-formation analysis. The parameters derived in this study were already successfully used in a lava flow simulation program to reproduce the flow path of the 2012-13 Tolbachik eruption (Kubanek et al., 2015a) which again corroborated the estimated lava flow volume calculated in this study.
Since the presented results are more reliable in winter than in summer, different approaches are suggested in the following to further analyze the effects the vegetation has on the DEM differences and on the volume calculation of the lava flow:
5.4. Conclusions and perspectives 91
Improve the deramping. The deramping could be improved while using only the non-vegetated area to generate the planar function to correct the phase information after unwrapping. This ramp can then be linearly filled to cover the whole scene.
This will lead to better results regarding the volume calculation and calculation of area extension of the flows.
Processing of the continuously acquired TanDEM-X data of the region.
The TanDEM-X time series for the Tolbachik region is long. Starting in March 2014, a data pair was acquired almost every 11 days, resulting in 17 bistatic data pairs until October 2014, where the satellite mission started acquiring data in pursuit monostatic mode. The effective baselines of the bistatic data are up to 143 m in March 2014 and decrease successively until September 2014 to about 33 m. Analyz-ing the data pairs throughout the year should reveal comparable volumes calculated for the so far used post-eruption data pairs. If the summer data pairs show the same trend like the syn-eruption II ones used in this study, the effect can only be explained by vegetation.
Field trip to Tolbachik volcano. The photography shown in Figure 5.30 already gives an impression about the vegetation structure around parts of the Tolbachik flow field. This indicates the importance to study the lava flow during fieldwork. A field trip to the Tolbachik region would provide a more detailed insight into the structure of the vegetation. Of major interest is, of course, the transition zone, the interaction of the lava flow with the vegetation as well as the influence of snow cover. Also multispectral images could be used additionally to classify the vegetation. The best case would obviously be to acquire a photogrammetric DEM of this post-eruptive stage to compare with the TanDEM-X data.
6. Quantification of small
volumetric changes at Volc´ an de Colima, Mexico
Chapter 6 deals with the application of the bistatic TanDEM-X data to study small topographic changes at Volc´an de Colima in Mexico. Like Merapi, Volc´an de Colima is a stratovolcano with varying activity. Long terms of dome growth are intermitted by small explosions and culminate in complete dome destructions. A small explosion in June 2011 marked the end of magma ascending into the summit and stopped the ongoing building of the lava dome. Nine bistatic data pairs (two before and seven after the explosion) are analyzed to reveal the topographic and volumetric change on the western crater rim. The results are further validated using aerophotogrammetric DEMs generated by James and Varley (2012). To analyze the repeatability of the TanDEM-X-based DEMs and the derived volcanological results, further data pairs acquired between June 2011 and December 2012 are evaluated. Altogether, 26 data pairs, including the seven post-explosion data pairs mentioned above, are analyzed.
As the volcano remained quiet until January 2013, it is assumed that no topographic changes occurred during this time. The results therefore give an insight into the precision of repeatedly derived TanDEM-X DEMs in different terrain types. Parts of this chapter have already been published in Kubanek et al. (2013b, 2014, 2015b).
6.1 Geological setting
Volc´an de Colima (19.513◦N, 103.587◦W, summit 3860 m above sea level) is located approximately 30 km north of the city of Colima in western Mexico (Fig. 6.1) and composes, together with its older neighbor Nevado de Colima (19.563◦N, 103.609◦W, summit 4270 m above sea level), the Colima Volcanic Complex (CVC, see Fig. 6.2).
While being a stratovolcano, Volc´an de Colima is one of the most active volcanoes in North America.
94 6. Quantification of small volumetric changes at Volc´an de Colima, Mexico
Figure 6.1– Location of Volc´an de Colima in Mexico. The upper right image shows the state of Mexico. The lower left image shows a DEM generated from TanDEM-X data. The lower right part shows an amplitude image of the summit area. The red rectangle in the lower right image indicates the section that was used for the present analysis to reveal changes of the lava dome. It has an extension of about 0.037 km2.