VALIDATION STUDY OF AVATECH’S RAPID SNOW PENETROMETER, SP1

Eric R. Lutz1,2*, Hans-Peter Marshall3

1Sawtooth Avalanche Center, Ketchum, ID, USA

2 Dartmouth College, Hanover, NH, USA

3Boise State University, Boise, ID, USA

ABSTRACT: We validate early prototypes of SP1, a light-weight rapid-push snow penetrometer devel-oped and produced by MIT-based start-up AvaTech. The device records snowpack hardness profiles and other information as an integral part of a snowpack observation system. Using SnowMicroPen (SMP) profiles, manual profiles and other observations, we evaluated the depth and force readings of SP1 profiles. We collected comparison data at sites in three mountain ranges in Idaho and from an ice cap in northern Greenland. Pits contained a broad range of hardness (including new snow and numerous crusts) and moisture content (dry, moist, saturated, and mixed conditions). Initial comparisons indicate that SP1 profiles recorded stratigraphic features also observed in SMP profiles and manual profiles. Force values appear to be consistent between profiles at a given field site. Truncation of near-surface measurements was fairly common, and errors in absolute and relative depths existed in some profiles and not in others. Possible contributing factors may include 1) the ranging unit’s accuracy when starting a test (~1.5 m above snow surface), 2) solar transmission through the upper snowpack delaying surface identification, 3) the ranging unit’s sampling rate may have been too slow for tests with high push rate variability caused by hard layers, and 4) the post-processing depth algorithm. Improved depth measure-ments will allow for its application in slab thickness estimates and possibly weak layer identification. The SP1 has the potential to become an accurate and efficient tool for snow professionals.

KEYWORDS: Probe, penetrometer, hardness, stratigraphy, SP1, SMP.

1. INTRODUCTION

Since the 1930’s, professionals have recognized the need to develop snow probes that can effi-ciently collect quantitative stratigraphic observa-tions that, in combination with other observations provide invaluable information for assessing slope stability. The fastest and most widespread probes used by avalanche professionals are their own ski poles, which can efficiently provide valuable strati-graphic info over large areas (Meiners et al. 2012). But thin and subtle weaknesses are difficult to de-tect, and impossible in deeper snowpacks or if covered by impenetrable layers. Ram penetrome-ters reliably quantify hardness of deep snowpacks, but they’re time-consuming and can’t resolve thin weaknesses (Pielmeier and Schneebeli, 2003). The SnowMircoPen (SMP) can consistently cap-ture stratigraphic and bond-scale information per-tinent to stability (Lutz et al., 2009; Pielmeier and Marshall, 2009) yet its operational use is limited because of its size, weight, and purchase- and

maintenance costs. Just like ski pole probing, technologies can have their limitations… AvaTech is developing the SP1, a light-weight rap-id-push snow penetrometer that records hardness profiles and additional information. We compare results from a “Version 0” prototype of the SP1 with manual profiles and SMP profiles. We evalu-ate the instrument’s depth and resistance meas-urements; an evaluation of the unit’s functionality (e.g., ease of operation, functionality of hardware or software, etc.) is beyond the scope of this work.

2. METHODS

2.1 Field Methods

We collected data at three sites in the central mountains of Idaho (Bogus Basin, Galena Summit, McConnel) in the spring of 2014 and at the summit of an ice cap located ~50 km north of Thule, Greenland (North Ice Cap) in June 2014. At each site, we collected SP1 profiles plumb and normal to the snow surface, 20 – 50 cm uphill of the planned snow pit. Unless otherwise noted, pushes were completed in about 2 seconds.

We collected SMP profiles at two of the four field sites, in both instances within 10 cm of corre-

* Corresponding author address: Eric R. Lutz, 1669 High Pastures Rd, Woodstock, VT 05091; tel: 406-599-2107; email: snowscience@gmail.com

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sponding SP1 measurements. At all sites we rec-orded a detailed manual snow profile (hardness, grain type and size) and then cut back into the snowpit face until we bisected the vertical holes created by the SP1 measurements; marking the holes with avy probes before digging facilitated this process. In the bisected SP1 holes we then measured the exact depth (precision of 0.2 cm) from the snow surface to stratigraphic features that we identified in the manual profile as promi-nent or consistent (Fig. 1).

Fig. 1: In Greenland, 14 vertical probe holes were

exposed by excavated back into the pit wall. The thin dark horizontal features are soft faceted layers that were etched out.

In addition, stability tests, density measurements and near infrared photography were also per-formed at some of the sites.

2.2 Comparison

First, we evaluated the overall character of the SP1 profiles: do they depict a plausible stratigra-phy, as observed in hand-hardness and SMP pro-files? Second, we assessed the depth accuracy of the SP1 for targeted stratigraphic features we manually observed and measured (depth and hardness) in the field.

3. RESULTS AND INTERPRETATION

3.1 North Ice Cap (Thule, Greenland)

At North Ice Cap two prototypes recorded a total of 14 profiles, alternately spaced at 10 cm inter-vals along a 1.3 m transect, set back ~20 cm from the snowpit. The presence and depth of several stratigraphic features, associated with facets, depth hoar and crusts, were manually recorded at all 14 excavated profiles (Fig. 1).

Unfortunately data recorded by one of the proto-types was corrupted and could not be recovered by the time of this publication. Figure 2 depicts the hand-hardness profile with seven SP1 profiles, as well as colored lines that demark the actual depth of targeted stratigraphy

Fig. 2: Hand-hardness profile (left) with seven resistance profiles from the SP1 prototype. For each pro-file, colored lines mark the depth of manually observed stratigraphic features, marked A – G in the co-located hand-hardness profile (S = surface). These features included facets, depth hoar, and crusts.

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The range in force values appears consistent be-tween the profiles. Measurements are missing from the upper 20 cm of the snowpack in a couple of these profiles. It is difficult to ascertain if the upper portion of these profiles is missing (truncat-ed) or if the post-processing algorithm displaced downward and/or compressed/warped the values in the middle part of the signal.

Profile 3 and 5 appear to have skewed or elongat-ed stratigraphy in the mid pack. This may be the result of the range unit under-sampling depths (ranges) during the dynamic push, possibly exac-erbated in snowpacks containing multiple large steps in hardness (which affect the push rate).

3.2 Bogus Basin (Idaho)

Figure 3 shows four SMP profiles that were paired with SP1 profiles within 10 cm. The last profile lo-

cation was also coupled with slow-push (~5 se-conds) and a fast push (~1 second) SP1 profiles to test the influence of push speed (Fig. 4).

The SMP recorded consistent stratigraphy, includ-ing high textural variability between 30-60 cm depth, and slightly harder layers with less textural variability between 60-80 cm. The first two SP1 profiles recorded very similar features, albeit with a ~10 cm offset closer to the surface. The third SP1 profile appears dampened and the fourth pro-file is highly compromised. Three SP1 profiles have truncated profiles that start ~15-30 cm below the surface.

Extensive melt occurred in the upper snowpack during this field day. The progression from a clean signal to one which is dampened and compro-mised could have been caused by wetting of the prototype sensor mechanism, however more work is required to verify the cause.

Fig. 3: Comparison of SMP and SP1 prototype profiles at Bogus Basin, ID. SMP profiles (light blue) were paired with SP1 profiles (gray). The SMP recorded consistent stratigraphy as did the first two SP1 profiles, albeit with a ~10 cm bias (red arrows). Because the SMP is driven by a highly accurate step-ping-motor system, SMP depths have an uncertainty of less than 1 mm. The third and fourth SP1 profiles appear dampened and compromised, possibly due to wet surface snow. These SP1 profiles appear trun-cated by up to 35 cm or compressed and offset.

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Fig. 3: Comparison of profiles from the SMP (left) and the SP1 prototype at three push speeds (from left to right): ~2-3 seconds,5 seconds, and 1 se-cond.

Profiles performed at different push rates ap-peared to contain different amounts of information, recorded over a different depth range. The rapid push profile (Fig. 4, right) appears to contain clus-ters of information separated by data-light sec-tions. The moderate push profile contained the highest force values.

CONCLUSION

Our field tests revealed that the SP1 prototype recorded a consistent range of hardness values when pushed at moderate rates. Some profiles accurately captured the stratigraphic character as recorded by the SMP and manual profiles. As with the SMP, collecting at least three profiles at a giv-en location will likely improve the accuracy of strat-igraphic observations.

Our tests also indicated that the depth measure-ment system and algorithm, as applied at the time of this publication, need improvement for certain applications, such as consistently estimating slab thickness and detecting the presence/absence of weak layers. In our datasets, data was frequently missing from the upper snowpack, and depth er-rors of 5-10 cm were not uncommon in some pro-files. Possible contributing factors may include 1) the ranging unit’s accuracy at the start of a field test (1.5 m above snow surface), 2) solar trans-mission through the upper snowpack delaying sur-face identification, 3) the ranging unit’s sampling rate may have been too low for tests with high

push rate variability caused by hard layers, and 4) the post-processing depth algorithm. The SP1 has the potential to become a highly efficient and accurate snow penetrometer, and the progress AvaTech has made with development of this new snow instrument in such a short time is impres-sive.

CONFLICT OF INTEREST

The authors of this study were not supported financially or materially by AvaTech, the producers of SP1.

ACKNOWLEDGEMENTS

We would like to thank AvaTech for giving us the oppor-tunity to test the SP1 prototype.

REFERENCES Lutz, E., K. Birkeland, and H.P. Marshall. 2009. Quantifying

changes in weak layer microstructure associated with artifi-cial load changes. Cold Regions Science and Technology 59(2-3), 202-209.

Pielmeier, C., and M. Schneebeli, 2003: Stratigraphy and changes in hardness of snow measured by hand, ram-sonde and snow micro penetrometer: A comparison with planar sections, Cold Regions Science and Technology, 37, 393–405, doi:10.1016/S0165-232X(03)00079-X.

Pielmeier, C., Marshall, H.P., 2009. Rutschblock-scale snow-pack stability derived from multiple quality-controlled SnowMicroPen measurements. Cold Reg. Sci. and Tech-nol. 59, 178–184 (this issue).

Schweizer, J. and J. B. Jamieson, 2003: Snowpack properties for snow profile analysis. Cold Regions Science and Tech-nology, 37, 233-241.

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