1. Introduction to InSAR
Anthropogenic subsidence due to coalbed methane production in the Powder River Basin
Kelly Devlin1,2, Adrian Borsa2, Wesley Neely2 1Temple University, Phila., PA 2SIO UCSD, La Jolla, CA
This work was funded with a 2016 Scripps Undergraduate Research
Fellowship (SURF) REU site Grant at the Scripps Institution of Oceanography
(NSF-OCE 1359222). Satellite date was obtained through ESA, JAXA, and ASF.
For further information contact kellydevlin@temple.edu
Powder River Basin
Interferometric synthetic aperture radar (InSAR) is a form of remote
sensing that relies on Earth observation satellites (EOS). In this study
we utilized data from the ALOS and Sentinel-1A satellites. These
satellites send out a burst of microwave radiation towards the Earth’s
surface. The waves are reflected back with a different phase towards
the satellite, which in turn uses the waves to create a composite of the
surface. An EOS can make multiple passes over the same area (Figure
4). I used a computer program called GMTSAR and differenced two
images to create an interferogram, which shows the change in phase
of the reflected waves. I then umwrapped the interferogram and used
it to create an image that shows surface deformation that corrects the
color scale used to indicate phase difference. Finally, I corrected the
interferograms to show change in elevation.
Figure 5: Diagram of EOS using repeat passes for InSAR. The process illustrated
describes the process prior to unwrapping. Courtesy of Geoscience Australia
The Powder River Basin (PRB) is a geological formation located in
northeastern Wyoming and southeastern Montana (Figure 2).
Historically it has been a major producer of coal in Wyoming.
Improvements in energy production and technology led many
companies to turn to pumping coalbed methane (CBM) starting in the
early twenty-first century. This led to a boom and subsequent bust in
CBM production in the PRB. The extraction of CBM requires the drilling
of wells thousands of meters deep. Groundwater is usually extracted
from these wells before the CBM is brought to the surface (Figures 1
and 3). While the CBM is channeled to a compressor, the common
practice was to release the untreated groundwater as runoff. This
untreated groundwater has been found to have high levels of salinity
and sodicity [1]. This poses a threat to the local environment and
economy. High levels of salinity and sodicity leads to soil degradation
and shock to local flora. Many Wyoming residents that rely on the PRB
as grazing land have been affected by degradation [2].
Figures 1-3: (left)
Diagram of CBM
well with
groundwater
extraction.
(above) Map of
PRB in Wyoming
and Montana.
(below) Timeline
of CBM well.
Courtesy of
Wyoming State
Geological Survey,
ClimateWest, and
Black Diamond
Energy, Inc.
Figures 4a and 4b : (a) A usually dry field is covered with untreated ground
water from nearby well. This land is typically used for grazing. (b) Release
of untreated groundwater from well. Large amounts were released to the
surface during the CBM boom and bust. Both courtesy of Dustin Bleizeffer
Observations
Analysis
Implications
References
Figures 6a-d: (a) map of Wyoming with area of interest highlighted in black box (b) zoomed in area with outlines of ALOS and Sentinel interferograms (c) ALOS interferogram scene overlapping
Sentinel interferogram (d) Sentinel interferogram overlapping ALOS interferogram
ALOS
Sentinel
ALOS
Sentinel Sentinel
ALOS
A
A’
B
B’
Figures 7 and 8: (left) ALOS scene spanning 231 days from 2007-10-04 to 2008-05-21 with appropriate color bar. Area of
interest is highlighted in black box and shown in Figure. (right) Sentinel scene spanning 361 days from 2015-06-11 and
2016-06-05. Area of interest is highlighted in black box and shown in Figure. Subsidence is denoted with a negative
change in elevation, which appears blue on the color scale. Uplift is denoted with a positive change in elevation, which
appears red on the color scale. Note that the scales for ALOS and Sentinel are not the same.
Figures 9 and 10: (top) Zoomed in ALOS scene with profile track from A
to A’ shown in red. This scene covers the time of peak CBM production
in the PRB. (below) Zoomed in Sentinel scene with profile track from B
to B’ shown in red. The time in the scene occurred after most CBM
production had ceased. Profiles are shown in Figures 10a and 10b.
• The InSAR scenes obtained from the ALOS and Sentinel
satellite data provide evidence of anthropogenic subsidence
due to CBM pumping in the PRB over two different time
periods. The isolated areas of subsidence denoted in blue on
the interferograms appear do not appear to be caused by
natural processes.
• Given that it can be difficult to detect disruptions in natural
processes due to human activity. This difficulty makes these
findings compelling.
• The ALOS scene occurred at a time of significant CBM
pumping, while the Sentinel scene occurred after the bust.
Surprisingly, the Sentinel showed much more subsidence
than the ALOS scene. This raises questions on the exact
nature of the subsidence. There is also increased fracking in
the area as shown in Figure 12, which could account for
increased subsidence.
• Using known CBM well depth available online, we will be able
to run computer models to attempt to estimate the volume
of groundwater pumped in an interferogram scene. The PRB
is a historically arid place, so any waste of groundwater is a
significant event
• There is little to no literature on using InSAR to study CBM
production. This technique has the potential to provide new
insights into past and current processes in the PRB.
Figures 11a and 11b: (a) Elevation profile across red line shown in Figure 8. The graph shows the calculated change in elevation from the ALOS interferogram without the average. The area
of interest highlighted with the black box shows an area of subsidence overlap between ALOS and Sentinel scenes. (b) Elevation profile across red line in Figure 9. The graph shows the
calculated change in elevation from the Sentinel interferogram without the average. The area of interest highlighted with the black box shows an area of subsidence overlap between ALOS
and Sentinel scenes Even though the ALOS scene covers a time of greater CBM production, the subsidence shown in the Sentinel scene is much more significant. This is surprising given that
almost all CBM production has ceased in the CBM.
Future Work
• I plan to expand the scope of both the ALOS and Sentinel
satellite data. The next step will be to examine more paths
covered by ALOS. This study currently only includes one path
covered by ALOS.
• I will also use Sentinel data to track the expected recovery of
the PRB from CBM production. Fracking is a current practice
in the PRB. InSAR has the potential to offer insight into
fracking as well as CBM pumping.
• In addition to ALOS, the European Space Agency (ESA) also
has the Envisat satellite. This satellite covers a similar time
period as ALOS. I plan to use Envisat to create InSAR scenes
and compare the findings to ALOS scenes.
• I will also be able to use the InSAR scenes from the various
satellites to create computer models to estimate the volume
of groundwater pumped over a certain period of time. This
can also be compared to available statistics concerning the
amount of CBM produced and groundwater pumped. My
findings could be used a reference for the accuracy of these
statistics.
[1] Frost C., Mailloux J. 2011. Establishing appropriate water quality numeric
standards under the Clean Water Act: Lessons from a case study of coalbed
methane produced water discharge to the Powder River, Wyoming and
Montana. Wyo. Law Rev. 11(1):1-23
[2] Bleizeffer, D. Coalbed Methane: Boom, Bust and Hard Lessons. Wyoming
State Historical Society. 2016 Jul 13.; [accessed 2016 Aug 19].
http://www.wyohistory.org/essays/coalbed-methane-boom-bust-and-hard-
lessons.
Figure 12: Graph of oil
production in the PRB
from 2000-2014.
Production is divided by
geographic regions. This is
a possible explanation for
increased subsidence seen
in the Sentinel scene.
Courtesy of EIA
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Changeinelevation(cm)
Distance from B (m)
Sentinel Elevation Profile from B to B'
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Changeinelevation(cm)
Distance from A (cm)
ALOS Elevation Profile from A to A'