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Figure 2. Data collection using MALA 250 MHz GPR unit (left photo) and auger sampling for moisture content at depth (right photo).
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ID 1a 1b 1c 1d
% moist Depth (m) Notes 0.14 1.25 0.56 2 0.88 3 minor gravel at 3.15 0.89 3.5 bedrx
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0.25 9.24 15.57
1.5 1.75 2
gravel silty-clay sat‘d
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1.88 1.02 11.60
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clay color change bedrx @3.15m
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10.87 17.76
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1.087 12.00
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sat’d @1.9
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0.74 0.76 0.98 1.60
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14.10 17.68
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0.86 6.45 15.38 14.09
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RESEARCH QUESTIONS Working with Dr. Nathan Bridges at JPL, we have come up with two primary but different research questions. First, can the radar be used to identify activation or reactivation structures and bounding surfaces in dunes, as well as the interface between the dunes and the irregular Navajo Sandstone bedrock underlying the dunes? These internal stratigraphic features can be used to identify periods of stability or activity, possibly recognizing changes in local wind regimes that result in changes of sediment transport. Similarly, they could yield evidence of aeolian processes at work at different intensities and directions. A second research question we address is the ability of GPR to constrain water content (liquid and ice) in dunes. In the imaged areas, there are elevated water tables that provide good environments for examining this aspect of the radar. The shallow nature of the ground water (< 0.5 m in places) may also yield frozen dune water in winter, providing another opportunity to assess the technique in year two of the project. Water detection using GPR depends on the saturation of the sediments, the size of the sediments (fines such as clay result in severe signal attenuation), and the salinity of the water. Fresh water has a high dielectric constant, resulting in longer signal travel times. When ice is present, the dielectric constant is much lower than with water, resulting in increased travel times. Travel time in the presence of volatile water vapor (e.g., frost, such as observed in Proctor Crater; Fenton et al., 2003) would depend on the water content of the volatile.
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INTRODUCTION AND PROBLEM The absence of liquid water on the surface of Mars means that wind is the current active landscape forming process (Greeley et al., 1998). As a result, the surface of Mars contains abundant dune forms and other depositional features related to aeolian processes. These features include those associated with dust-sized sediments (200 MHz). The tradeoff is the loss of depth imaged as the signal is more easily attenuated at these higher frequencies. Lower frequencies can image to greater depths but with diminished resolution of features. The research question asked here is what is a reasonable balance between frequency and resolution? The answer may hinge on the planetary study objective (e.g., sedimentary rock or dune form investigation) behind any future deployment of GPR. An optimal frequency (or frequencies, depending on the goal of the imaging project) for imaging the subsurface may be identified by examining the same subsurface features imaged over a range of frequencies.
Figure 1. 1997 aerial photo of the Coral Pink Sand Dunes. The Coral Pink Sand Dunes is one of the larger active dunefields in the Colorado Plateau. The dune field is located at the head of the Sand Canyon wash about five kilometers north of the Utah-Arizona state line in southwestern Kane County, Utah, about 43 km west of Kanab, Utah. Most aeolian bedforms in the dune field are in the parabolic, transverse, or barchanoid ridge class of dunes. The dune sands accumulating in the modern valley are probably sourced from the Navajo Sandstone, which was a Jurassic erg. GPR transects were collected from inset area, shown in closeup view. Note the star dune W of the transect intersection. Repeat aerial photos indicate this star dune formed between 1960 and 1972 through the merger of multiple smaller transverse dunes.
Table 1. Moisture content of sediment samples collected at various depths. Orange highlights note bedrock contacts. Yellow highlights note water table or vadose zone.
Depth (m)
ABSTRACT On Mars, wind is the dominant active landscape forming process. The Martian surface contains abundant duneform and other depositional features related to aeolian processes. To investigate the effectiveness of GPR for Martian imaging, we tested GPR on terrestrial analogs for the Martian surface. We collected two, perpendicular, 250 and 500 MHz GPR lines across the active dunefield at the Coral Pink Sand Dunes State Park, Kanab, Utah. The transects crossed dunefield features including barchanoid dunes, interdune swales, the shallow water table, and interdune bedrock exposures. These features provide analogs for a range of potential targets on Mars. The collected data provide detailed images of internal features of the dune such as crossbedding, the water table, and the sand/bedrock contact. Shallow (< 4 m) hand-augered borings, coincident with the GPR transects, provide ground truth for the depth to the water table and shallow bedrock. These measured boundaries coincide with strong, continuous reflections in the GPR data. As expected, the 250 MHz GPR images penetrate more deeply into the dunes, but the 500 MHz images are more detailed. Shallow features (< 10 m) are evident at both frequencies.
Intersection with South-North Transect
Using Ground Penetrating Radar to Image Terrestrial Analogs of Martian Aeolian Deposition: The Coral Pink Sand Dunes, Kanab, Utah
Clement, W P (
[email protected]) CGISS, Boise State University,1910 University Drive, Boise, ID 83725 United States Wilkins, D E (
[email protected]) Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States Ford, R L (
[email protected]) Department of Geosciences, Weber State University, 2507 University Circle, Ogden, UT 84408 United States
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ACKNOWLEDGMENTS A NASA Idaho Space Grant Consortium Research Initiation Grant provided funding for this research. We thank Dr. Nathan Bridges of the NASA Jet Propulsion Laboratory for his cooperation and comments. Thanks also goes to Dean Anderson, Superintendent for the Coral Pink Sand Dunes State Park, Utah, and his staff for their cooperation and assistance.
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GPR Processing: We dewowed the data to remove low frequency noise, then we corrected the data for the topography along the survey. Next, we bandpass filtered the data between 100 to 600 MHz to increase the signal-to-noise ratio of the radar data. For plotting, we normalized each trace to the maximum value in that trace. We used a velocity of 0.125 m/ns to convert from time to depth. This value is from a walkaway profile acquired at the Coral Pinks Sand Dunes in an earlier experiment.
DISCUSSION We started this study by asking two research questions. First, can the radar be used to identify activation or reactivation structures and bounding surfaces in dunes, as well as the interface between the dunes and the irregular Navajo Sandstone bedrock underlying the dunes? Generally, the 500 MHz ground penetrating radar provides high resolution imagery to about 5 meters depth. This imagery allows us to interpret sedimentary structures and bounding surfaces within the dunes. Cross-section images of the transverse dunes shows the active and buried slipfaces dipping in the direction of dominant transport and dune movement. Previous studies of repeat aerial photography indicate dunes in the proximity of the star dune reached their present configuration after 1960, and have moved little since then. What may be the bounding surfaces from the most recent movement are visible in the long W-E profile (inset). Even more recent activity, most likely from off-highway vehicle (OHV) disturbance of the surface, can be seen in the southern dune along the S-N profile.
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GPR Acquisition: We used a Mala Ramac GPR system using 250 and 500 MHz shielded antennas. The radar signal was triggered using a wheel triggering system. To find the average trace spacing for the surveys, we divided the total profile length by the number of traces in that profile. For the west-east line, the trace spacing is 0.1684 meter; for the south-north line, the spacing is 0.043 meter.
The second research question we address is the ability of GPR to constrain water content (liquid and ice) in dunes. The water table reflection aligns with water table depths recorded in the boreholes. At some places along the profile, (e.g. W-E 190), the water table reflection merges with the bedrock reflection. At these locations, the water table may coincide with the bedrock/sand contact. Another explanation is that the bedrock reflection may mask the water table reflection, especially if the water table is close to the bedrock/sand contact. Similarities between the dune sand and the underlying sandstone exacerbate the difficulty in discriminating between the two.
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Sediment Analysis The sediment samples are extremely well-sorted, having been processed by wind in Jurassic and recent times. The color of the dunes, rather than coral, is more orange, with intensity increasing with moisture. Analysis of the orange coating indicates it is a thin veneer of iron oxide (K. Nicoll, per. comm.). Above the water table the water content is typically less than one percent except in Borehole 2 where a thin silty clay layer is present. Below the water table, moisture content increases to greater than 15 percent by weight. Water content was also elevated at the bedrock contact in Borehole 3; however, the sediment was dry at the other bedrock contact in Boreholes 1 and 10, suggesting that the water table may be structurally controlled.
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Figure 3. Intersection of the two transects with location of boreholes indicated. Note the moist region in the swale; the water table was reached within a meter, with water flowing in and caving in the auger hole wall.
shallow bedrock
Sediment Processing We weighed the moist sample fraction, and then dried the samples in a 100°C oven for 96 hours. We reweighed the dry samples and calculated the moisture moisture content by weight. The moisture contents of the samples are shown in Table 1.
The bedrock-sand contact is an obvious event in the reflection profile (e.g., beneath the transverse dune at 125 m on S-N line). At 230 meters (S-N line), the bedrock (visible at the surface) contains steeply dipping reflectors indicating the bedding within the Navajo Sandstone.
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Sediment Sampling We used a three-inch auger to collect sediment samples at approximately 1 meter depth intervals. While sediments were incredibly uniform, subtle changes in color or moisture were noted. We augered until the water table or bedrock were reached, or to a maximum of four meters. Samples were sealed in Ziploc® plastic bags to retain moisture.
This preliminary analysis shows that 500 MHz GPR adequately penetrates sand to image reflectors to about 5 m depth. The water table and the bedrock/sand contact are strong, continuous reflectors. Equipping future Mars rovers with high frequency GPR will greatly enhance the chances of finding Martian water. REFERENCES CITED Fenton, L., Bradfield, J., and Ward, A. (2003) Aeolian processes in Proctor Crater on Mars: Sedimentary history as analyzed from multiple data sets. Journal of Geophysical Research, v. 108, No. E12, 5129, doi:10.1029/2002JE002015. Greeley, R. Kraft, M., Wilson, G., Sullivan, R., Kuzmin, R., Malin, M., Bridges, N., Herkenhoff, K., Golombek, M., and Smith, P. (1998). Aeolian geology of the Mars Pathfinder site. In 29th Lunar and Planetary Science Conference Proceedings and Abstracts.
