The Challenges of Measuring Geyser Deformation with LiDAR

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The Challenges of
Measuring Geyser
Deformation with LiDAR
I
n September 2010 a multi-disciplinary team of geoscientists set up camp
for four days at Yellowstone National
Park’s Lone Star Geyser. The objective
of the expedition was to use geophysical
methods to characterize eruption cycles
in the geyser system. The team deployed
several instruments around the geyser
cone, including infrared sensors,
visible and infrared video, and microphones for capturing acoustic signals.
Measurements of water velocity and
channel cross-sectional area for the
main streams flowing away from the
geyser were used to estimate discharge
and give an idea of the total water output
from the system in each cycle.
A Leica ScanStation C10 was also set
up on a hill above the geyser in order
to map hydrothermal ground deformation. The geophysical instruments
recorded distinct eruption cycles at
Lone Star, but the LiDAR scans did not
show clear cycles of ground inflation
and deflation. The question arises: why
was LiDAR unsuccessful here? In this
article we will describe the scanning
results and consider potential sources
of error that may have obscured the
deformation signal.
Lone Star Geyser
Geyser eruptions are driven by the
systematic filling and emptying of an
underground fluid storage system.
Geothermally-heated water becomes
superheated under pressure, eventually
breaking through rock constrictions
and flashing to steam, creating a jet of
boiling water. The regularity of geyser
eruptions is related to the geometry
of the plumbing system and the time
needed to fill and empty the system.
These cycles result in ground inflation
and deflation that can be sensed at the
earth’s surface.
Lone Star is an isolated cone geyser
that erupts regularly every three hours.
Major eruptions are preceded by smaller
water fountaining events starting about
one hour before the main eruption.
These pre-eruptions signify when the
plumbing system has become partly or
fully recharged. Following the 30 minute
eruption period, there is a relaxation
and recharge period of about 90 minutes
(Karlstrom et al., in review). We would
thus expect to see maximum inflation
just before the eruption and maximum
deflation about 30 minutes to an hour
after the start of the eruption.
LiDAR scanning at Lone Star
The scanner was placed on a hill above
the outwash plain, about 55 m from
the geyser cone. The first three scans of
By Susan Schnur & Adam Soule
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Figure 1. At top is the point cloud for the entire study area, colored by intensity. The blue lines indicate
outwash streams and the geyser cone (circled in white) is in green, with a black shadow behind it.
At bottom is a TIN model of the focus area used for the regular 20-minute scans.
Displayed with permission • LiDAR Magazine • Vol. 3 No. 3 • Copyright 2013 Spatial Media • www.lidarnews.com
the day were used to test the scan area
and subsequent scans focused on just
the region around the geyser (Figure
1). A complete scan was started every
20 minutes over the course of about 6
hours. The scanning continued through
two major eruptions and likely captured
two complete inflation-deflation cycles.
The stream flow measurements estimate
an average eruption volume of 21 ± 4 m3
of water, which should correspond to a
maximum of about 7 ± 1 cm of ground
deformation if we assume a simple
cylinder model with a 10 m radius
of influence around the geyser cone
(Karlstrom et al., in review).
Measuring deformation
Deformation is assessed by comparing
the vertical offsets between successive
scans and between scan pairs before
and after each eruption. Two major
eruptions were captured during the
scan period (Table 1). Scan 5 (SW5)
was made just before the first eruption
and Scan 7 (SW7) was made about 20
minutes after the end of the eruption.
Scan 15 (SW15) was made just before
the second eruption and scan 17 (SW17)
was made about 20 minutes after the end
of the eruption. Maximum offset should
be visible between these scan pairs.
Offset was measured by cropping each
point cloud to just the raised area around
the geyser, located within a 10 m radius
of the geyser cone, and not including
the geyser cone itself. These points were
used to create a TIN model of the surface
in Leica Cyclone. The vertical distances
between TINs for successive scans could
then be measured at 5 cm intervals in a
grid pattern to generate offset statistics.
Mean offset between TINs shows a
slow deflation of about 8 mm occurring
Scan
Time
SW4
SW5
SW6
SW7
SW8
SW9
SW10
SW11
SW12
SW13
SW14
SW15
SW16
SW17
SW18
SW19
SW20
10:15
10:35
10:55
11:15
11:35
11:55
12:15
12:35
12:55
13:15
13:35
13:55
14:19
14:35
14:55
15:15
15:35
Mean Vertical
Offset [mm]
Geyser activity
Minor eruption
Main eruption throughout
Eruption ended
-5
-2
-1
More wind
Minor eruption beginning
Minor eruption ongoing
0
11
-26
Main eruption
27
Table 1. Summary of scans, geyser activity, and measured mean vertical offset based on
comparison of TINs for the isolated geyser mound.
between SW5, just before the first eruption and SW11, about 2 hours after the
eruption (Table 1). The second eruption
shows a much stronger offset. Beginning
at SW13, where minor eruptions indicate
the system has reached saturation, there
is inflation of about 11 mm up to just
before the eruption. In the 40 minutes
during and after the eruption there is a 26
mm drop in the surface. This is followed
by 27 mm of inflation in the 40 minutes
following the eruption, and SW19
returns to the same location as SW15.
This pattern seems to match the expected
30-60 minute recharge period measured
using the other geophysical methods.
Spatially, offset in the TINs seems
focused on the geyser mound and
decreases at the edge of the 10 m radius
(Figure 2). The spatial distribution of
offset between points in the whole scan
area was also visualized using the open
source CloudCompare package. Figure
3 shows areas of the scan where offset is
minimal (< 5 mm), and reveals a strange
cyclic pattern. If offset was affected by
scanner angular accuracy, we would
expect error to be smallest at the scanner
“0” azimuth, in the center of the image.
If offset was affected by wind, we would
expect offset to increase from left to
right across the image, as the scanner
is increasingly shifted away from its set
rotation track (Olsen, 2012). Neither of
these patterns is evident here.
Correcting for potential
scanner error
An attempt was made to correct for scanner error by realigning point clouds using
a cloud-to-cloud approach. One potential
problem with re-alignment is that the
process may actually remove nearuniform deformation by trying to correct
for it. To address this, the re-alignment
was done using the whole scan area,
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rather than just the cropped region. The
hope here is that most of the scan area
used for the alignment is not deforming
and will provide a good baseline.
Cloud-to-cloud alignments for
SW5-SW7 and SW12-SW17 removed
most noticeable offset. The mean vertical offset for SW5-SW7 is -5 mm for the
unaligned scans and only -2 mm for the
aligned scans. For the SW12-SW17 pair,
mean offset for the unaligned scans is
-16 mm and only +2 mm for the aligned
scans. Offsets of 2 mm are both within
scanner error, so there is no longer a
significant offset after either eruption.
Statistics for the cloud-to-cloud
alignment give some insight into how
the scanner would need to be tilted to
align the scan pairs. For the first eruption there is little significant rotation
in either the X, Y, or Z directions. For
the second eruption, the observed large
offset is created by significant X and
Z rotation. That is, the scanner would
need to tilt forward and rotate from side
to side. It is possible that a combination
of horizontal wind motion and forward
slumping of the scanner could have
created this offset. Alternately, the cloud
alignment may need to be done on a
much larger scan area so that it can
include points from features that remain
stable throughout the eruption.
Signal or Error?
From this data set it is not immediately
clear if the scanning captured ground
deformation or not. Many of the offset
measurements are below scanner error
budget for the ScanStation C10, which
has angular accuracy of 60 μrad (about 3
mm at a distance of 50 m) and distance
accuracy of about 4 mm. The second
eruption shows a significant drop of 26
mm, followed by inflation back to 27
Figure 2. Offset between scan pairs directly before and after each eruption. For the first
eruption, deflation seems more pronounced closer to the geyser. The second eruption has a
much greater offset that is nearly uniform throughout the cropped area.
mm, which seems to match the geyser
cycle and is reasonable, considering we
expect up to 7 cm of ground deformation. However, in reality, the system
is probably much broader than 10 m
in radius, so the actual deformation is
likely to be much less. Deformation may
even be on the mm to sub-mm scale, in
which case it would be indistinguishable
from total scanner error.
It is strange, however, that
offset should be lacking during the first
eruption and so clear for the second
eruption. Additionally, recharge seems
to occur very slowly after the first
eruption (2 hours) and very quickly after
the second eruption (40 minutes). The
geophysical methods indicate a standard
deviation of 13 minutes for the length of
the recharge period, which is too short
to account for the difference between
the first and second eruption. Estimates
of ground deformation from stream
discharge measurements range from
about 5 cm to 9 cm for the nine eruption
cycles where these measurements could
be made (Karlstrom et al., in review).
It is thus possible that the contrast
in offset between eruptions is due to
natural variations in eruption strength.
The cyclic distribution of error as
shown in Figure 3 suggests that other
forces, such as wind may have moved
the scanner. However, wind picked up
before the second eruption, and it is the
first eruption that does not show a clear
inflation-deflation signal. The regularity
of the vertical offset for the second
eruption also does not seem indicative
of wind effects. For these reasons the
scanner movement estimated in the
cloud-to-cloud alignments seems
a more likely explanation for the
significant offset following the second
Displayed with permission • LiDAR Magazine • Vol. 3 No. 3 • Copyright 2013 Spatial Media • www.lidarnews.com
may help indicate when movement has
happened and allow us to correct for it.
Figure 3. Comparison of offset between SW5 and SW7, before cloud-to-cloud alignment.
Blue areas are points with less than 5 mm of total offset. The cyclic pattern located to either
side of the scan azimuth seems to indicate some recurring vertical motion. The white circle
indicates the location of the geyser.
eruption. This movement may be due
to slumping of the scanner due to an
unstable setup (e.g. the tripod legs were
not securely fixed in the ground).
A significant unknown in this study
is the spatial distribution of the underground fluid storage system and the
nature of ground deformation on short
timescales. Over what radius surrounding the geyser would we expect to see
deformation? Was the scanner itself
located on deforming ground? Does
porosity in the ground take up deformation so that only a small amount is
expressed at the surface? Is a 20-minute
interval sufficient to capture deformation
if deflation occurs rapidly after inflation?
These unknowns are mostly related to
our understanding of geyser systems.
Suggestions for the Future
The lesson learned here is that when
working in outdoor environments,
scanner movement can affect point
cloud accuracy when measuring very
small changes in a surface. It is therefore
important to have a way to verify
whether or not the scanner moved
during scanning, particularly when
doing repeat scans from one position.
Without this information it may be hard
to tell if measured offsets actually reflect
geologic processes.
Several actions can be taken to deal
with possible scanner motion. The easiest
of these is to take thorough field notes
that indicate when abnormal events, such
as gusts of wind, may have happened.
The scanner tripod also needs to be
stabilized as much as possible to avoid
translation of the scanner origin. A shield
can be set up around the scanner to
protect it from wind. Level compensator
readings can be used to correct scanner
rotation in real time. A wind station
can be set up as close to the scanner as
possible to give a sense of wind speed
and direction. Targets can be placed in
the scan area to allow better registration
of scans. Complete stabilization is hard to
do when working in protected areas such
as national parks, but these other options
Acknowledgements: We would like to
thank Dr. Michael Olsen for his guidance
in analyzing this data set and his helpful
suggestions for preparing this article.
Thanks also go to Chuck Meertens
and Jim Normandeau of UNAVCO
for organizing data collection and
initial processing, and Shaul Hurwitz for
organizing the logistics of the Lone Star
Geyser experiment. We are also grateful
to Leica Geosystems for providing Oregon
State University with the software used
in data analysis and to the developers
of CloudCompare for providing their
excellent open source software.
Susan Schnur is a 2nd year Ph.D. student
in marine geology at the College of Earth,
Ocean and Atmospheric Sciences at Oregon
State University. She has an M.Sc. in Geography from the University of Zürich and a
B.A. in Geology from Carleton College. Her
research focuses on submarine volcanism,
seamount chains and geospatial methods
for analyzing seafloor topography.
Adam Soule is an Associate Scientist in the
Department of Geology & Geophysics at
Woods Hole Oceanographic Institute. He
has a Ph.D. from the University of Oregon.
His research focuses on physical volcanology in terrestrial and submarine volcanic
systems with an emphasis on magmatictectonic interactions at mid-ocean ridges.
References
Karlstrom, L., Hurwitz, S., Sohn, R.,
Vandemeulebrouck, J., Murphy, F.,
Rudolph, M.L., Johnston, M.J.S., Manga,
M., and McCleskey, R.B. Eruptions at
Lone Star Geyser, Yellowstone National
Park, USA, Part 1: Energetics and
Eruption Dynamics (in review).
Olsen, M.J. (2010). Scannin’ in the Wind.
LIDAR News eMagazine, 2(6).
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