Remote Sensing Volcanoes: Deformation Mapping

Having never seen one for real - active, dormant or extinct - the first thing that comes to my mind when I think of a Volcano is this popular depiction from Chacha Chaudhary comics-

Figure 1: Sabu's anger leading to Volcanic Eruption on Jupiter; Source: https://theothermeunfolded.com/blog/chacha-chaudhary-digest-4/

For those unfamiliar, Sabu was an alien from Jupiter who was residing on Earth with India's most famous comic book superhero - the diminutive yet witty old man - Chacha Chaudhary. Easily winning several strength-based duels with villainous thugs, Sabu sometimes faltered due to his lack of wit or when he encountered adversaries more powerful than himself. When the latter occurred, he would be subject to incessant needling from the antagonist which would prompt him to get into a Hulk-esque rage- symbolized by a volcano erupting on his home planet. In this beast mode, he was indomitable and pulverized the opposition. This is how comic books go, however, I can't help but draw parallels to the current war-like situation unfolding before us in real life.

Volcanic Eruptions have a mythical aura, a phenomenon which humans have always feared and revered at the same time. Unsurprisingly so, because even in my country India, which harbors only 2 active volcanoes and that too in the remote Andaman islands - we have an iconic name for this object of awe. In India's national language Hindi, a volcano is called 'Jwalamukhi' which can be translated as 'the Mouth that spurts Fire'.

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Given my limited knowledge about volcanoes or volcanic activity, I am grateful to RUS Copernicus & EO College whose learning resources have been tremendously useful in developing my understanding of this topic, like it has for many topics before this.

For instance, it was quite fascinating to know that volcanoes do not explode all of a sudden, generally. Rather, to put it in my own way, volcanoes behave like living organisms - breathing in, breathing out, expanding and contracting in size, shape and volume, metabolizing energy within and only when the internal pressure is too hard to contain, does it explode in a violent expression of fury.

It is this 'lively' behavior which captures the attention of researchers and they have devised ways to capture it and interpret it in the branch of science known as volcanology. To clarify, this 'liveliness' (the technical term for this is deformation) is not discernible to the naked eye, rather it can be observed using geodetic monitoring techniques such as Leveling, GPS & Interferometric Synthetic Aperture Radar (InSAR).

Figure 2: Mt. Nyiragongo eruption in Congo. Source: Captured from video by Raphael Kaliwavyo Raks-Brun

We will use the InSAR technique on satellite imagery to study three volcanoes - 1) Pico Do Fogo (Cape Verde) 2014, 2) Cumbre Vieja (Canary Islands) 2021 & 3) Mt. Nyiragongo (Congo), 2021.

Before we progress further, I'd recommend you to read this insightful article published by USGS on Volcano deformation.

Mapping on-surface deformation phenomenon is relatively easier. The recent devastating eruption near Tonga happened from the submarine volcano - Hunga Tonga–Hunga Haʻapai below the surface of the ocean which made it difficult to detect and observe before the devastating eruption occurred on 15th January 2022.

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The processing chain involved in extracting deformation (phase fringes) and precise displacement values (upswell and subsidence) from SAR Satellite Imagery is fairly extensive. Depiction below -

Figure 3: Deformation processing chain on SNAP Imagery Analytics Software

Below is a detailed video which will take you on a guided recording of performing each of these steps on a satellite imagery processing software (SNAP) to derive and visualize deformation and displacement map of Cumbre Vieja Eruption (Canary Islands) in 2021.

Do read on in case you do not wish to see the video.

Video 1: Mapping Deformation using Satellite Imagery - Cumbre Vieja volcano in 2021

To explain the above steps individually in a simple manner-

a) Read & Read(2)- Two sets of imagery are used - one prior to the eruption and second one post-eruption to do change detection.

Figure 4: You may observe that there are 27 Imagery Sections (3*9) in this SAR Imagery set over Cumbre Vieja on 16th September 2021 (pre-eruption)

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b) TOPSAR-Split- SAR Imagery is heavy (Each set is around 1 GB in size). To reduce computational load while performing analysis on it, invariably one has to restrict the image to the area of interest. TOPSAR-Split helps us to select specific imagery sections and data bands from the entire set, eliminating the rest, thereby making the imagery less taxing on the software.

Figure 5: TopSAR Splitted image (contains 2 sections only now) over Fogo, Cape Verde on 10th October 2014 (pre-eruption)

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c) Apply (Updated) Orbit Files- Orbit Files within SAR Imagery contains satellite velocity and position information. By default, this may not be the updated / corrected version in the downloaded imagery. We have to update the orbit files to allow for precise pixel overlays in the next steps involving interferometry.

Please note that the visual output remains the same as step b).

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d) Coregistration - Back Geocoding - This is a vital step in the Interferometric processing chain. It involves the factoring in of 'Digital Elevation Model' or DEM into the imagery. Interpreting radar reflectance / backscatter is only useful if we were to know the elevation values of the topographic surface that is our Area of Interest (AoI). This step also allows us to combine the two imagery sets into a single stack allowing for pixel to pixel comparison.

Figure 6: Back-Geocoded Stack of imagery - contains both pre (19 May 2021) and post eruption (12 June 2021) imagery within over Mt. Nyiragongo. The former is the master image while the latter has been designated as the slave.

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e) Coregistration - Enhanced Spectral Diversity - This optimization tool helps improve the precision of our geometrically-derived Coregistered Stack thereby minimizing pixel overlay errors.

Figure 7: ESD Coregistered image of the output from d) Backgeocoding

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f) Interferogram - A visual depiction which allow us to detect and quantify the ground

displacement that occurred between the pre and post volcanic imagery acquisitions with centimeter-level accuracy.

Figure 8: The Interferogram output over Fogo contains three outputs - a) intensity (uncalibrated energy which returns to the sensor) b) phase (distance between the satellite antenna and the ground targets) and c) coherence (similarity between the base and overlay pixels). The fringes in the phase diagram are indicative of deformation.

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g) TOPSAR Deburst - Involves removing the black lines (data discontinuities) between the imagery sections (bursts) thereby making the entire imagery seamless.

Figure 9: Notice that the black line (burst) has been removed from the bottom 'debursted' image of the phase over Fogo.

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h) Topographic Phase Removal - There are two types of phase information captured in the Interferogram - a) due to the topography and b) due to displacement. This step allows us to eliminate the topo-phase so that we are only left with displacement phase data values.

Figure 10: Notice that after Topo Phase removal the phase diagram significantly changes in the image on the bottom. This is the more accurate representation of phase.

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i) Multilooking - This step reduces the inherent speckle noise in the imagery (think of an Analog TVs dotty-grey screen error) and makes the pixels 'square' in shape allowing for easy interpretability.

Figure 11: The phase fringes (deformation) are easy to visualize after Multilooking has been applied, allowing for cleaner, square pixels over Mt. Nyiragongo.

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j) Goldstein Phase Filtering - The next step is to apply Goldstein Phase Filtering. Like Multilooking, this tool is also applied to reduce the 'noise' in the image and make the data ready for the later steps which involve the calculation of precise displacement (deformation) values.

Figure 12: Notice that this phase depiction is even more vivid than the one prior (multilooked) over Mt. Nyiragongo.

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k) Snaphu Export - Snaphu (Phase) Unwrapping - Snaphu Import - Phase to Displacement - These four steps can be clubbed into one for the purpose of our understanding - let us call the combined step as Phase to Displacement. While the interferometric phase gives us an indication where land subsidence (shrinkage) and upswell (growth) has happened, phase unwrapping using Snaphu tool allows us to calculate precise displacement values between the two sets of imagery (pre & post).

Figure 13: While the displacement depiction is not easy on the eyes, notice that the min and max displacement values are computed on the chart to your bottom left.

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l) (Range-Doppler) Terrain Correction - Alongside eliminating non-Land displacement values, 'Terrain Correction' also helps in making the imagery geometrically correct as the tilt of satellite among other aspects have inbuilt distortions. The correction helps the end product look proportionally as close as possible as it does in the real world.

Figure 14: Terrain-corrected displacement values over Fogo volcano. The blue area has displacement of about 10 cm. This is the cone of the volcano which erupted.

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m) Write (& Export) - the output for further analysis and visualization purposes

Phase Visualization on Google Earth

Video 2: The Foga phase layer is overlayed on Google Earth basemap. Strong deformations (phase fringes) are seen surrounding the volcano indicating loss of surface around the cone as well upswell due to magma accumulation in certain areas.

Video 3: The Mt. Nyiragongo phase layer is overlayed on Google Earth basemap. Strong deformations (phase fringes) are seen, not immediately around the volcano though. These can be attributed to persistent seismic (earthquakes) activity following the eruption - one such as validated here.

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Displacement Visualization on Google Earth

Video 4: Fogo displacement values are displayed towards the top left in the depiction overlayed on Google Earth basemap. We can clearly visualize the areas most affected by land deformation (blue and dark green).

With this, we come to the conclusion of this article. Hope you found the process of mapping deformation from SAR imagery to study volcanic activity to be interesting!

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