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Detecting terrestrial caves by applying topographic analysis techniques to thermal imagery

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Abstract and Figures

Introduction: Since Rinker’s [1] groundbreaking work on terrestrial cave detection nearly 50 years ago, our ability to find caves using airborne and spacecraft acquired imagery has improved considerably. Due to superior analytical techniques, improved instrument optics, and high resolution imagery, we have furthered terrestrial cave detection capabilities [2,3,4] and confirmed cave-like features on the Moon [5,6,7] and Mars [8,9,10]. Lunar caves may serve as the best locations for human habitation [2,4,6], while Martian caves are of great interest as astrobiological targets, accessing potential water-ice reserves, as well as astronaut bases [2,4]. Further, geothermal vents associated with vapor plumes identified on Saturnian [11], Jovian [12,13] and Neptunian moons [14] hint at additional planetary subterranean access points, and represent high priority targets for future habitability studies. Cave detection is typically most successful when multiple thermal images are acquired during both the warmest (mid-afternoon) and coolest (predawn) times of day [3]. Although data acquisition is logistically easiest on Earth, repeat thermal imagery over short temporal periods is lacking for most terrestrial locations. When searching for caves on other planetary bodies, obtaining multiple images for regions of interest within a limited window of time is challenging. Accordingly, researchers must rely on imagery sporadically captured from spacecraft platforms where acquisition is dictated by fly-by or orbiter mission schedules and tempered by other mission objectives. For extraterrestrial cave detection, we explored two targeting categories using terrestrial analogs. (1) Deep caves, which have sufficient linear length and/or depth to adequately buffer interior environments from harsh surface conditions. On Mars, these caves would be among the best candidates to search for evidence of life. (2) Shallow caves, which extend tens of meters in length, may represent suitable sites for establishing astronaut bases on the Moon and Mars. Within these less protected, but more easily accessable sites, habitat pods may be constructed or inserted a small distance within the cave entrance.
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DETECTING TERRESTRIAL CAVES BY APPLYING TOPOGRAPHIC ANALYSIS TECHNIQUES TO
THERMAL IMAGERY. J.J. Wynne1, J. Jenness2, M.D. Jhabvala3, T.N. Titus4 and D. Billings5. 1The SETI Institute, Carl
Sagan Center, Mountain View, CA and Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ,
jut.wynne@nau.edu; 2Jenness Enterprises, GIS Analysis & Application Design, Flagstaff, AZ; 3NASA Goddard Space Flight
Center, Instrument Systems and Technology Division, Code 550, Greenbelt, MD; 4U.S. Geological Survey, Astrogeology Sci-
ence Center, Flagstaff, AZ; and, 5National Speleological Society, Desert Dog Troglodytes, Los Angeles, CA.
Introduction: Since Rinker’s [1] groundbreaking
work on terrestrial cave detection nearly 50 years ago,
our ability to find caves using airborne and spacecraft
acquired imagery has improved considerably. Due to
superior analytical techniques, improved instrument
optics, and high resolution imagery, we have furthered
terrestrial cave detection capabilities [2,3,4] and con-
firmed cave-like features on the Moon [5,6,7] and
Mars [8,9,10]. Lunar caves may serve as the best loca-
tions for human habitation [2,4,6], while Martian caves
are of great interest as astrobiological targets, access-
ing potential water-ice reserves, as well as astronaut
bases [2,4]. Further, geothermal vents associated with
vapor plumes identified on Saturnian [11], Jovian
[12,13] and Neptunian moons [14] hint at additional
planetary subterranean access points, and represent
high priority targets for future habitability studies.
Cave detection is typically most successful when
multiple thermal images are acquired during both the
warmest (mid-afternoon) and coolest (predawn) times
of day [3]. Although data acquisition is logistically
easiest on Earth, repeat thermal imagery over short
temporal periods is lacking for most terrestrial loca-
tions. When searching for caves on other planetary
bodies, obtaining multiple images for regions of inter-
est within a limited window of time is challenging.
Accordingly, researchers must rely on imagery sporad-
ically captured from spacecraft platforms where acqui-
sition is dictated by fly-by or orbiter mission schedules
and tempered by other mission objectives.
For extraterrestrial cave detection, we explored two
targeting categories using terrestrial analogs. (1) Deep
caves, which have sufficient linear length and/or depth
to adequately buffer interior environments from harsh
surface conditions. On Mars, these caves would be
among the best candidates to search for evidence of
life. (2) Shallow caves, which extend tens of meters in
length, may represent suitable sites for establishing
astronaut bases on the Moon and Mars. Within these
less protected sites, habitat pods may be constructed or
inserted a small distance within the cave entrance.
Hypotheses: H1: Thermal imagery may be exam-
ined using techniques for analyzing digital elevation
models. All cave entrances (deep and shallow) will
appear as distinct “topographic” features on a 3-D rep-
resentation of a thermal image. We tested this idea
using: (1) topographic position index (TPI), (2) ther-
mal gradient (or slope), and (3) curvature (Fig. 1A).
TPI is the relative value of a specific pixel compared to
the average thermal value of a neighborhood of pixels.
Thermal gradient is the rate of change of thermal val-
ues indicating where temperature values change quick-
ly. Curvature represents the rate of change in slope on
the “thermal surface” depicted as regions of concavity
(dips or valleys) and convexity (hills and peaks).
Figure 1. (A) Example of image layers developed for topo-
graphic analysis. (B) Method for extracting pixels values for
deep cave, shallow cave and control points (random surface
locations) used to test H1 and H2. Resolution ~0.5 m2.
H2: Thermal signatures associated with deep cave
entrances are discernable from shallow caves (e.g.,
alcoves and caves 50 m in length) and control points
(i.e., random areas of surface) when imagery is cap-
tured at the appropriate time for a single acquisitional
period (either predawn or midday).
9029.pdf2nd International Planetary Caves Conference (2015)
Data and Analysis: We collected thermal imagery
of Pisgah Lava Beds, located ~175 miles northeast of
Los Angeles, CA. Using a Quantum Well Infrared
Photodetector thermal imaging camera [15] mounted
in the forward camera port of a Beechcraft King Air
B200 (operated by NASA Armstrong Flight Research
Center), we acquired imagery at nadir. Two missions
were flown one from 1222 to 1457h on 11 April
2011 (4,445 midday images), another from 0426 to
0649h on 13 April 2011 (3,675 predawn images; [16]).
For our study sites (nine deep caves, four shallow
caves and 13 control sites), a seven point grid was cen-
tered over each feature and the pixel values (i.e., digi-
tal number, DN) extracted (Fig. 1B). We analyzed
DNs of these three groups using Welch’s t-tests [16]
for all paired comparisons of mean actual thermal, TPI,
gradient and curvature values.
Results: H1: Barring shallow caves vs. control sites
for curvature in the predawn imagery (p = 0.07), deep
and shallow cave entrances were significantly different
from control sites for TPI, gradient, and curvature (p
0.02). Deep caves were not discernable from shallow
caves for all topographic techniques (p 0.35).
Figure 2. Scatter plot of diurnal difference and diurnal
mean for midday and predawn of actual thermal DN values
of deep cave (red), shallow cave (green) and surface control
sites (blue). DNs = digital numbers.
H2: Most paired tests of actual thermal imagery were
significantly different (p 0.042), except for deep cave
vs. control sites in the predawn imagery (p = 0.118)
and deep vs. shallow caves in the midday imagery (p =
0.117). In most cases, topographic analyses techniques
differentiated deep and shallow caves from control
sites (H1 above).
Discussion: Applying topographic analytical tech-
niques shows promise as a valid method for analyzing
thermal imagery. Using these techniques, we were able
to differentiate deep and shallow caves from control
sites except for one case (shallow cave vs. control sites
for curvature of predawn imagery). In one case, we
were able to discern deep from shallow caves using the
actual thermal of predawn imagery. Because this result
occurred only once, we interpret it cautiously. Our
limited ability to detect differences between deep and
shallow features may be due to a small sample of shal-
low caves (n = 4). Based upon this limited sample,
these features exhibit thermal behaviors similar to deep
caves (Fig. 2). Further, this result may be due to limita-
tions in our seven point DN extraction method. While
this systematic approach appears successful overall, it
may be affecting the results by limiting the DN values
analyzed for each feature. Most of the cave entrances
were either crescent (lateral entrances viewed at nadir)
or oblong in shape. Thus, our method of selecting
points in a circle may be extracting DN values in loca-
tions where surface temperatures are interacting with
cave temperatures resulting in subpixel mixing [17].
Our research is proceeding in the following areas: (1)
examining different pixel extraction methods used for
dataset development; (2) increasing sample size by
obtaining representative samples for analysis of differ-
ent entrance shapes (crescent, oblong and oval) for
both deep and shallow caves through fieldwork verifi-
cation; (3) analyzing signal strength of deep cave en-
trances by comparing deep cave entrance size and
length against thermal pixel values of these deep cave
entrances; and, (4) identifying regions on the landscape
most similar to known cave locations using Mahalano-
bis distances [18] (which describe the distance in mul-
tivariate space of the mean actual thermal, TPI, gradi-
ent, and curvature values) and confirm their identity
(deep or shallow caves) through a field effort.
Acknowledgments: This project was supported by
NASA Exobiology Program grant #EXOB07-0040 and
the NASA Goddard Space Flight Center. Glen Cush-
ing, Jasmeet Dahliwal, Laszlo Kestay and Cecilia
Leung provided valuable comments that improved this
abstract.
References: [1] Rinker J. N. (1975), Photogram. Eng.
Remote Sens. 41, 13911400. [2] Wynne J.J. et al. (2009),
Abstract 2451, 40th LPSC, Houston, TX; [3] Titus, T.N. et al.
(2011), Abstract 8024; 1st Internatl. Planet. Cave Workshop,
Carlesbad, NM; [4] Wynne, J.J. et al. (2008), Earth Planet.
Sci. Lett. 272: 240250; [5] Haruyama, J. et al. (2010), Ab-
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(2012), J. Cave Karst Stud. 74: 3347; [10] Cushing, G.E. et
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Hansen, C.J. et al. (2011), Geophys. Res. Lett. 38, L11202;
[12] Geissler, P.E. & M.T. McMillan (2008), Icarus 197:
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Duxbury, N.S. & R.H. Brown (1997), Icarus 125: 8393;
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[16] Welch, B. L. (1947), Biometrika 34: 2835; [17] Foody,
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9029.pdf2nd International Planetary Caves Conference (2015)
... The first groundbreaking work on terrestrial cave detection using airborne and spacecraft acquired imagery was made by Rinker [7] nearly 50 years ago. Wynne et al. [8] presented analytical techniques, improved instrument and high resolution imagery of terrestrial cave detection and confirmed cave-like features on other planets. Cave detection is typically most successful when multiple thermal images are acquired during both the warmest (mid-afternoon) and coolest (predawn) times of the day. ...
... The main method used in this research is detecting cave entrances by thermal cameras. This includes thermal ground imaging with high thermal resolution of long distances to detect caves (figure 2,7,8,9). This technology is important because it is the core of the research. ...
... All the necessary technologies for the first part of the research are currently evaluated with excellent results. 8 We surveyed the location of new unknown caves using this technique. For this purpose, we studied three pilot regions with great potential for locating new caves: ...
Article
Full-text available
This work aims to create new scientific knowledge by developing new innovative technology for remote detection of unknown underground cavities and deep-seated rockslides. To achieve the goal, we intend to develop new innovative technology for remote localization of unknown underground cavities and deep-seated rockslides using a thermal camera mounted on a unmanned aerial vehicle (UAV). Technology is a defined sequence of operations and procedures under optimal conditions, varying within certain allowable limits, resulting in obtaining a particular result or product that meets certain requirements. Therefore, the development of new technology involves determining the allowable limits of deviation of the optimal conditions of operations and procedures for producing a good result that meets the established requirements. The development of new technology also involves determining the limits of its applicability under different external conditions. This paper includes determination of the allowable limits and the limits of applicability of the new technology being developed. Until now, no technology has been developed for remote detection of unknown underground cavities or deep-seated rockslides. The goal of this work is the development of such innovative technology with numerous applications in construction, environmental studies and protection, security, defence and infrastructure.
... On Earth, caves are found by a combination of geological and geomorphological map analyses and their direct reconnaissance in the field (e.g., Ground Penetrating Radar). More advanced, albeit still somewhat experimental techniques, include combining thermal and visible imaging (e.g., Rinker, 1975;Cushing & Okubo, 2017;Wynne et al. 2015). These techniques should be fully developed and expanded upon to detect caves on other planetary bodies. ...
... Consequently, cave entrances can show a thermal contrast with respect to surface temperature, thus becoming potentially detectable by means of thermal infrared (TIR) cameras, especially when the temperature contrast between cave entrance and surface is greater (Wynne et al., 2008). This thermal contrast has been used to detect large caves through thermal remote sensing techniques (Rinker, 1975, Wynne et al., 2015 even as far away as Mars (Cushing et al., 2007(Cushing et al., , 2015Wynne et al., 2008). Nevertheless, airborne and satellite thermal remote sensing has typical spatial resolution of meters to hundreds of meters (Calvin et al., 2005;Küenzer et al., 2013) due to their flying height, being therefore unsuitable to spot most karstic cave entrances. ...
Article
Caves are buried geological features of great scientific and engineering interest. Based on the well-known thermal inertia of cave settings with respect to the surface, we have tested the use of thermal infrared (TIR) cameras carried by unmanned aerial vehicles (UAVs) to identify and characterize shallow karstic conduits. A pilot area was selected in a limestone rock massif from the Betic Cordillera (Southern Spain). At surface, this area appears as a doline field with several explored shaft entrances. The main and deepest shaft was thermally monitored at different depths, showing two well defined seasonal stages: (i) air and temperature stratification in summer and (ii) upward air flow and temperature homogenization in winter. We carried out UAV surveys at the expected maximum thermal contrast hours, finding out that winter dawns yielded the most distinctive images. These images show new warm air exits (warm spots) apart from the known cave entrances, and emphasize some of the main entrances depicting large warmed areas around them, which, in turn, can be related to shallow voids and conduits in the cave system. Furthermore, TIR images have been georeferenced using a network of identifiable points that can be transferred from visible orthoimages. The resulting TIR orthoimage mosaics have allowed us to define a number of quantitative parameters to characterize the warm spots. The most important of these parameters are: (i) the characteristic temperature, which is the maximum significant temperature of the air escaping from cave openings; (ii) the warmed area, which is the area that appears on the TIR orthoimage with a temperature greater than the surface temperature mode; and (iii) the average temperature weighted to the affected area. The georeferenced TIR orthoimages and the derived parameters are a new and very valuable tool for both speleological exploration and engineering purposes.
... Conference participants also discussed novel techniques for detecting planetary caves. Colleagues and I presented on how to apply terrain analysis techniques used on visible spectrum imagery to identify caves within thermal imagery [22]. We also proposed a roadmap for planetary cave detection using remote sensing. ...
Abstract 8024; 1 st Internatl
  • J N Rinker
  • J J Wynne
Rinker J. N. (1975), Photogram. Eng. Remote Sens. 41, 1391-1400. [2] Wynne J.J. et al. (2009), Abstract 2451, 40 th LPSC, Houston, TX; [3] Titus, T.N. et al. (2011), Abstract 8024; 1 st Internatl. Planet. Cave Workshop, Carlesbad, NM; [4] Wynne, J.J. et al. (2008), Earth Planet.
  • C J Hansen
Hansen, C.J. et al. (2011), Geophys. Res. Lett. 38, L11202;
  • N S R H Duxbury
  • Brown
Duxbury, N.S. & R.H. Brown (1997), Icarus 125: 83-93;
  • B L Welch
  • G M D P Foody
  • Cox
Welch, B. L. (1947), Biometrika 34: 28-35; [17] Foody, G.M. & D.P. Cox (1994), Int. J. Remote Sens. 15: 619-631.
  • P Mahalanobis
Mahalanobis, P. (1930), J. Proc. Asiatic Soc. Bengal 36: 541-588.
  • P E M T Geissler
  • Mcmillan
  • Roth
Geissler, P.E. & M.T. McMillan (2008), Icarus 197: 505-518; [13] Roth et al. (2014), Science 343: 171-174; [14]