Visual amplitude interpretations

64 5. Mapping lava flows of the 2012-13 Tolbachik, Kamchatka fissure eruption

flows extruded between 15 November 2012 and the corresponding time the syn- and post-eruption data pairs were acquired (Sec. 5.2.3). In addition, the lava flow volume can be estimated (Sec. 5.2.4) and lava extrusion rates for different time intervals of the eruption can be calculated (Sec. 5.2.4).

5.2. Mapping lava flows and measuring flow volume 65

decisively faster. However, amplitude results are explicit and cannot be disrupted by unwrapping errors. They can therefore be used to confirm the interferometric results.

15 Nov 12 10 May 13

07 Dec 12 01 Jun 13

18 Dec 12 23 Jun 13

09 Jan 13 15 Jul 13

22 Feb 13 17 Aug 13

16 Mar 13 11 Oct 13

d

Low Backscatter intensity High

Figure 5.11– Amplitude images: results for the pre-eruption data pair, for all syn-eruption data pairs, and for the first post-syn-eruption data pair.

Figure 5.11 shows the geocoded intensity images of the lava flow area from selected TanDEM-X data pairs. Displayed are the results of the pre-eruption data pair acquired on 15 November 2012, of all syn-eruption data pairs acquired between

66 5. Mapping lava flows of the 2012-13 Tolbachik, Kamchatka fissure eruption

Figure 5.12– Google Earth image showing the Tolbachik test site. The white rect-angle marks the TanDEM-X scene, the blue rectrect-angle the analyzed lava flow field.

The transition from a vegetated area in the west to a barren area in the east is clearly visible.

December 2012 and August 2013, and of the first post-eruption data pair acquired on 11 October 2013. The results of the remaining post-eruption data pairs are comparable with the first post-eruption data pair and are shown in Appendix A.1 for completeness.

Whereas only older flows are visible in the reflectivity image of the pre-eruption DEM acquired on 15 November 2012, the syn-eruption images and the post-erup-tion image clearly show the more intense reflectivity of the new lava flows of the 2012-13 eruption. The different images give visual information about the area cov-ered with new lava during a certain time of the eruption, i.e., which lava flow field (see Fig. 5.16) was formed when. The first selected syn-eruptive data pair was ac-quired on 07 December 2012 and only shows the flows effused to the south and west (Leningradskoe Lava Field and Vodopadnoe Lava Field), whereas the syn-eruptive data pair acquired on 22 February 2013 also shows the starting development of the Toludskoe Lava Field which later on effused to the east. This lava flow field is grow-ing over time in the subsequent syn-eruption data pairs. The post-eruption data pair acquired on 11 October 2013 finally shows the entire lava flow. A temporal analysis of the development of the lava flow fields is given in Section 5.2.3.3.

In addition to the brighter reflectivity of the new lava flows, it becomes further obvious that the backscatter intensity in the western part of all images is higher than in the eastern part, separated by a sharp transition running from north to

5.2. Mapping lava flows and measuring flow volume 67

07 Dec 12 01 Jun 13

18 Dec 12 23 Jun 13

09 Jan 13 15 Jul 13

22 Feb 13 17 Aug 13

16 Mar 13 11 Oct 13

10 May 13

d

Low Backscatter difference High

Figure 5.13– Amplitude difference images of each syn-eruption and the first post-eruption data pair with respect to the pre-post-eruption data pair from 15 November 2012.

south. Its sharpness varies with the time of year the data was acquired. That is the reason why this border could not be identified using the amplitude images only. With the aid of optical images (i.e., Google Earth, see Fig. 5.12), however, it appeared as a transition in vegetation. The area to the west seems to be vegetated while the area east of the line appears barren. It is therefore labeled as vegetation line in Figure 5.10. The ’barren’ area was covered by lava flows of the 1975-76 eruption which led to the absence of vegetation.

This vegetation line becomes obvious in all syn- and post-eruption data pairs and hinders the visual amplitude interpretations (Fig. 5.11). Whereas the flows that

68 5. Mapping lava flows of the 2012-13 Tolbachik, Kamchatka fissure eruption

were effused to the east are clearly distinguishable from the surrounding area, the backscatter intensity of the flows that were effused to the west resembles the backscatter of the surrounding surfaces. The flows to the west are only apparent in selected images, for instance in the 23 June 2013 amplitude image.

Since the general approach of this chapter is to map and measure lava flow extent and volume with DEM differencing, also the amplitude images were differenced.

Within this process, the pre-eruption difference image is subtracted from each syn-and post-eruption amplitude image. Figure 5.13 shows the difference images for all syn-eruption data pairs and for the first post-eruption data pair acquired on 11 October 2013. The results for the remaining post-eruption images are depicted in Appendix A.2 for completeness.

Like the single amplitude images (without differencing) discussed above (Fig. 5.11), the differenced amplitude images (Fig. 5.13) clearly show the development of the lava flows over time to the different directions. It becomes further obvious that whereas the lava flows were hardly distinguishable from the surrounding surface in the single amplitude images, they are better represented in the difference images. It becomes also clear that the backscatter characteristic of the westernmost lava flow is different compared to the rest of the lava flow field.

In contrast to the single amplitude images where the vegetation line was obvious in all images, the amplitude differences only show the sharp transition during the summer months from May to August. This results from the fact that the pre-erup-tion data pair was acquired in winter on 15 November 2012. The vegetapre-erup-tion in the western part therefore shows the same characteristic like in all winter images. In the summer images, however, the characteristics of the vegetation differ. Thus, the differencing of the summer images with the pre-eruption winter image leads to the sharp transition in the summer images, which also hinders to distinguish the western lava flows from the surrounding surface.