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| {{:mark:poland_and_lu.jpg}} | {{:mark:poland_and_lu.jpg}} | ||
| - | **Figure 1.** A stack of interferograms made by Poland and Lu (2008) reveals no apparent deformation. | + | |
| - | Coherence on the edifice and within the crater is limited due to the presence and disturbance caused by snow. | + | **Figure 1.** A stack of interferograms made by Poland and Lu (2008) reveals no apparent deformation.\\ Coherence on the edifice and within the crater is limited due to the presence and disturbance caused by snow. |
| Because of the wide spatial and temporal spacing, and to a lesser extent, the levels of noise, in data collected by trilateration and GPS, no conclusions could be made about any potential surface deformation that may have occurred locally on the edifice or within the crater of Helens. The ability to produce spatially continuous maps of surface displacements gives InSAR the ability to resolve the question of whether localized deformation may have occurred at Mt St Helens prior to its 2004 eruption. A study conducted by Poland and Lu in 2008 attempted to image both pre and post eruptive deformation at Mount St Helens using interferogram stacking. Because of decorrelation caused by the presence of snow and dense vegetation, even stacks of interferograms were unable to obtain signal within the crater or on the edifice prior to the eruption (Fig 1). While the results prior to the 2004 eruption were inconclusive, post eruptive results successfully imaged subsidence around and on parts of the edifice. | Because of the wide spatial and temporal spacing, and to a lesser extent, the levels of noise, in data collected by trilateration and GPS, no conclusions could be made about any potential surface deformation that may have occurred locally on the edifice or within the crater of Helens. The ability to produce spatially continuous maps of surface displacements gives InSAR the ability to resolve the question of whether localized deformation may have occurred at Mt St Helens prior to its 2004 eruption. A study conducted by Poland and Lu in 2008 attempted to image both pre and post eruptive deformation at Mount St Helens using interferogram stacking. Because of decorrelation caused by the presence of snow and dense vegetation, even stacks of interferograms were unable to obtain signal within the crater or on the edifice prior to the eruption (Fig 1). While the results prior to the 2004 eruption were inconclusive, post eruptive results successfully imaged subsidence around and on parts of the edifice. | ||
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| **Dataset Description** | **Dataset Description** | ||
| - | In this study, the Stanford Method for Persistent Scatterers developed by Hooper et al. (2012) was applied to a set of eight SAR scenes collected by the ERS-2 satellite covering Mount St Helens over the pre-eruptive period, from 1996 to 2002 (Figs ##, ##). Many SAR scenes from track 156, frame 2673 were available, but only those from the summer and fall months were chosen in order to minimize the effects of snow. During processing, the number of scenes was further reduced to eight after interferogram pairs having poor perpendicular baselines and high decorrelation were eliminated. Additional SAR datasets from different track –frame combinations exists over Mount St Helens, but are more limited in the time they span, and their number of scenes. | + | In this study, the Stanford Method for Persistent Scatterers developed by Hooper et al. (2012) was applied to a set of eight SAR scenes collected by the ERS-2 satellite covering Mount St Helens over the pre-eruptive period, from 1996 to 2002 (Figs 2a, 2b). Many SAR scenes from track 156, frame 2673 were available, but only those from the summer and fall months were chosen in order to minimize the effects of snow. During processing, the number of scenes was further reduced to eight after interferogram pairs having poor perpendicular baselines and high decorrelation were eliminated. Additional SAR datasets from different track –frame combinations exists over Mount St Helens, but are more limited in the time they span, and their number of scenes. |
| Results | Results | ||
| {{:mark:big_map.jpg}} {{:mark:table.jpg}} | {{:mark:big_map.jpg}} {{:mark:table.jpg}} | ||
| - | StaMPS processing was run successfully on the pre-eruptive ERS-2 data, yielding a decent density of stable pixels both on the edifice and within the crater. Refined interferograms were created alongside maps of average velocity over the timespan of 1996-2002. An example interferogram and average velocity map overlain on Google Earth imagery are shown below (Figs ##, ##). | + | **Figures 2(a,b)** The map of Washington state on the left shows the location of the SAR scene track 156, frame 2673. StaMPS processing was carried out on a small ~200 square kilometer patch within the frame centered on Mount St. Helens. The table to the right lists the dates of each SAR scene used in StaMPS processing and its perpendicular baseline relative to the master scene. |
| - | {{:mark:ex_int.jpg}} {{:mark:stamps.jpg}} | + | StaMPS processing was run successfully on the pre-eruptive ERS-2 data, yielding a decent density of stable pixels both on the edifice and within the crater. Refined interferograms were created alongside maps of average velocity over the timespan of 1996-2002. An example interferogram and average velocity map overlain on Google Earth imagery are shown below (Figs 3, 4). |
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| + | {{:mark:ex_int.jpg}} | ||
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| + | **Figure 3.** An example interferogram from StaMPS processing, spanning nearly one year from September 1997 to August 1998.\\ A clear relationship between phase or range change and elevation can be seen in this interferogram indicating contribution from\\ the atmosphere. Displacements are in the Line Of Sight with red (positive) moving towards the satellite and blue (negative) moving\\ away from the satellite. | ||
| **Discussion and Conclusions of StaMPS Processing** | **Discussion and Conclusions of StaMPS Processing** | ||
| - | Considering the map of average velocities, it can be seen that pixels on the edifice and in the crater are being selected as stable. The phases of the pixels selected over Mount St Helens are spatially correlated to a good degree, indicating that reliable and low noise phase information can in fact be pulled from areas which were decorrelated in previous studies. However, there is still much uncertainty about what physical features on the edifice the persistent scatterers correspond to. While it may appear that there is a distinct signal of uplift just off-center of the volcano, there is good reason to believe that the presented results are heavily influenced by atmospheric effects. In several of the interferograms created through StaMPS processing, a strong correlation between phase and elevation was present (Figure ##), indicating influence from atmospheric changes. | + | Considering the map of average velocities, it can be seen that pixels on the edifice and in the crater are being selected as stable. The phases of the pixels selected over Mount St Helens are spatially correlated to a good degree, indicating that reliable and low noise phase information can in fact be pulled from areas which were decorrelated in previous studies. |
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| + | {{:mark:stamps.jpg}} | ||
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| + | **Figure 4.** Final StaMPS result showing average velocities over the time period of 1996-2002. Apparent uplift signal just\\ west of the crater is likely an artifact of the atmosphere removal process. Velocities are in the Line Of Sight with red \\ (positive) moving towards the satellite and blue (negative) moving away from the satellite. | ||
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| + | However, there is still much uncertainty about what physical features on the edifice the persistent scatterers correspond to. While it may appear that there is a distinct signal of uplift just off-center of the volcano, there is good reason to believe that the presented results are heavily influenced by atmospheric effects. In several of the interferograms created through StaMPS processing, a strong correlation between phase and elevation was present (Figure 5), indicating influence from atmospheric changes. | ||
| {{:mark:ph_v_elev.jpg}} | {{:mark:ph_v_elev.jpg}} | ||
| - | In generating the velocity map shown in Figure ##, a tool within StaMPS was used to try and estimate the atmospheric contribution to phase. This tool takes advantage of the fact that the atmospheric contribution to Interferometric phase, is often correlated with terrain elevation. Plots of phase versus elevation are displayed for each interferogram, and the user decides whether and how to fit a line to the data (Figure ##). The linear fit to the data is used to create an atmospheric phase mask which is subtracted from the interferogram after unwrapping phase. In some interferograms, however, the relationship between phase and elevation is less clear (Figure ##), and deciding how or whether to fit a line at all can be subjective, difficult, and substantially impact the final results. | + | **Figures 5 (left) and 6 (right).** On the left is an example plot of unwrapped phase vs elevation for an interferogram heavily influenced by the atmosphere. The red line fit to the data is used to create and remove an atmospheric phase screen. On the right is an example of when fitting a line to the phase-elevation data may be subjective or not fully representative of the atmosphere in the interferogram. |
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| + | In generating the velocity map shown in Figure 4, a tool within StaMPS was used to try and estimate the atmospheric contribution to phase. This tool takes advantage of the fact that the atmospheric contribution to Interferometric phase, is often correlated with terrain elevation. Plots of phase versus elevation are displayed for each interferogram, and the user decides whether and how to fit a line to the data (Figure 5). The linear fit to the data is used to create an atmospheric phase mask which is subtracted from the interferogram after unwrapping phase. In some interferograms, however, the relationship between phase and elevation is less clear (Figure 6), and deciding how or whether to fit a line at all can be subjective, difficult, and substantially impact the final results. | ||
| StaMPS processing of SAR data over Mount St Helens identifies pixels with low phase noise on the edifice and within the crater. This indicates that there is promise for Persistent Scatterers processing techniques like StaMPS to overcome decorrelation due to snow and trees and potentially image pre-2004 eruptive deformation. However, because of the possibility that StaMPS results are heavily influenced by atmospheric changes which are difficult to remove using the phase - elevation correlation alone, more work must be done before real signal can be differentiated from artifacts of the atmosphere removal process. It is this fact which motivates the second part of this study: an investigation of the effects of atmosphere on StaMPS processing at Mount St Helens. | StaMPS processing of SAR data over Mount St Helens identifies pixels with low phase noise on the edifice and within the crater. This indicates that there is promise for Persistent Scatterers processing techniques like StaMPS to overcome decorrelation due to snow and trees and potentially image pre-2004 eruptive deformation. However, because of the possibility that StaMPS results are heavily influenced by atmospheric changes which are difficult to remove using the phase - elevation correlation alone, more work must be done before real signal can be differentiated from artifacts of the atmosphere removal process. It is this fact which motivates the second part of this study: an investigation of the effects of atmosphere on StaMPS processing at Mount St Helens. | ||
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| {{:mark:profs.jpg}} | {{:mark:profs.jpg}} | ||
| - | The altitude of each pressure level is estimated for each climate data point. Example profiles are shown in figure ##. The Digital Elevation Model (DEM) used is from the NASA’s Shuttle Radar Topography Mission (SRTM). | + | **Figure 7.** Example MODIS profiles of Pressure, Temperature, Water Vapor Pressure, and Refractivity with respect to Altitude are shown. The refractivity is integrated from the DEM height to the top of the profile in the calculation of phase delay. |
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| + | The altitude of each pressure level is estimated for each climate data point. Example profiles are shown in figure 7. The Digital Elevation Model (DEM) used is from the NASA’s Shuttle Radar Topography Mission (SRTM). | ||
| **Methods** | **Methods** | ||
