Patent Description:
Snow is a natural or man-made porous medium consisting of ice, air, and at <NUM>, water. The ice matrix forms a continuous connected structure defining its physical properties. The basic physical properties describing this porous medium is the content of air and ice per volume, expressed as specific density or porosity, and the size of the connected ice structures, expressed as specific surface area.

The specific surface area of ice, expressed also as optical equivalent diameter (OED) of porous ices and its specific density in porous ice structures are the dominating parameters determining many physical properties of snow.

The current field methods to determine the specific density are by gravimetry, by measuring the weight of a known volume of snow and indirectly by measuring its dielectric properties and neutron probe experiments. In the laboratory, density can be measured X-ray tomography. These methods have different shortcomings.

Gravimetry requires the extraction of a known volume. The precision of this method is shown to be precise at best to about ±<NUM>%. An important limitation is that the layers of the natural snowpack are often thinner than the minimal mechanically possible dimension of <NUM>-<NUM>. However, these layers are often important for the mechanical, physical and chemical properties of the snowpack. According to <NPL>, and <NPL>.

Dielectric measurements at different electromagnetic frequencies (often at <NUM>-<NUM>, or around <NUM>) are based on the frequency dependent adsorption of electromagnetic radiation. As with the gravimetric samples, spatial resolution is limited by the size of the antenna. In addition, the insertion of the antenna compacts the snow. The evaluation of the measurement has to take into account the geometrical shape of the ice structure, for example discussed in <NPL> and <NPL>.

Micro- X-ray computed tomography (micro-CT) resolves the internal ice and air structures with a resolution of <NUM>-<NUM> micrometers. This is sufficient to measure density changes with mm-resolution. The measurements require the extraction of snow samples, transport in a cold laboratory, and analysis in a micro-CT.

The specific surface area of snow can be measured by different direct and indirect methods. With the exception of the near-infrared photography and micro-CT, these methods are point measurements.

A wide range of optical methods exist for OED retrievals, whereas only one in-situ optical method based on near-infrared transmittance measurements exists for snow density retrievals in the field.

Because there are still debates about the magnitude of future climate warming, it is essential to better understand climate change where the impacts are strongest: Arctic, Antarctic and alpine regions. For evaluating a climate change induced snow cover variability in these regions, it is important to have accurate, high-resolution, low-cost and user-friendly field measurement systems that allow for a comprehensive monitoring of the most important parameters of a snow cover, the snow density and the optical equivalent diameter.

The drawback of the optical transmittance measurement methods is that a fragile thin snowpack wall has to be prepared and the entire snow pit has to be sheltered from natural surrounding radiation. Even though many density retrieval tools exist, none of them allows for a fast, spatially highly resolved in-situ determination of the snow density in the field at a reasonable cost.

The object of the present invention is to create a snow density measurement device for non-invasive measurements with improved resolution, easy to use by avoiding tedious preparation work before measurements. Also an improved method for measuring snow densities is achieved.

This invention solves some inherent problems of the known methods to measure density. First, the vertical spatial resolution of the proposed method results in a continuous density measurement with a resolution of better than <NUM>. The spatial resolution compared to gravimetry measurements improves <NUM>-<NUM>-fold. Second, the vertical image allows immediate quantification of the spatial variability. Multiple measurements across the imaged size allow to quantify spatial variability. Wherein no time-consuming preparatory work needs to be done on the snow before the measurements.

Vertical resolution is in the mm-range, the spatial coverage of approx. <NUM> x <NUM> is sufficient to detect many stratigraphic features (wind deposition, weak layers, melt crusts, infiltration channels), is independent of observer bias, is applicable to hard-to-sample fragile snow types (new snow, depth hoar) and measures two essential properties with one instrument.

Further understanding of various aspects of the invention can be obtained by reference to the following detailed description in conjunction with the associated drawings, which are described briefly below.

It should be noted that in the differently described embodiments, the same parts are provided with the same reference symbols or the same component names, the disclosures contained in the entire description being able to be applied analogously to the same parts with the same reference symbols or the same component symbols.

A preferred exemplary embodiment of the subject matter of the invention is described below in conjunction with the attached drawings.

A snow density measurement instrument B, comprising at least one illumination device <NUM>, at least one light detector <NUM>, a controlling computational unit CU is shown in <FIG>. The snow density measurement instrument B is box-shaped and comprises an instrument coverplate <NUM>, which can cover an interior of the new density measurement instrument B. In the instrument coverplate <NUM> at least one aperture <NUM> or slit <NUM> is left out, which is used to measure snow density of a snow side S non-invasively by taking a digital image in the near-infrared electromagnetic spectrum. Such box-shaped instrument B can use one or several digital cameras <NUM>, <NUM>' as detectors, to take images of the snow surface, with and without the instrument coverplate <NUM> or apertures/slits <NUM>.

Here, the controlling computational unit CU, at least one illumination device <NUM> and light detector <NUM> are located in the interior of the box-shaped snow density measurement instrument B, facing in direction of the snow side S, while covered by the box. The snow side S comprises ice particles <NUM> and pore spaces <NUM> between the ice particles <NUM>, which are deflecting an incident radiation <NUM>, which was radiated from inside the box-shaped snow density measurement instrument B, with a defined distance d between detector <NUM> and slit <NUM> surface, through the at least one slit <NUM> with a given slit width I. For stability reasons the illumination device <NUM>, preferred at least one near-infrared light source and the at least one detector <NUM> are connected to a base plate of the box-shaped snow density measurement instrument B.

Due to the density of the snow and its grain size <NUM>, slit width I, wavelength of the incident beam <NUM>, a penetration depth <NUM> is resulting and a reflected radiation and reflected radiation distribution can be measured. The penetration depth <NUM> defines the degree of lateral propagation of the radiation on the snow side S. The reflected radiation over the slit <NUM> or more general the aperture <NUM>, is measured with the at least one optical detector <NUM>.

The method presented here extends the method for optical determining the OED by the instrument coverplate <NUM>, with the at least one aperture <NUM> or the multiplicity of local slits or apertures <NUM>, through which a density dependence of the reflection in the apertures/slits <NUM> is induced: On a free, uncovered snow surface the diffuse reflectance in the near infrared depends only on the ratio of scattering and absorption coefficients of snow, which, by inversion, only determines the OED. The invention exploits the independent influences of absorption and scattering if the diffuse reflectance is measured over apertures (illumination and detection areas) of varying sizes <NUM> on an otherwise covered surface. This, by inversion, allows to estimate OED and density simultaneously.

The box-shaped snow density measurement instrument B is to be designed to measure snow surfaces with an area between <NUM><NUM> to <NUM><NUM>. Therefore the whole box B respectively the area of the instrument coverplate <NUM> should be coordinated with it. The instrument coverplate <NUM> should be formend between <NUM><NUM> to <NUM><NUM>, depending on the application in the field, rather square or rectangular, larger or smaller depending on the snow cover. The snow cover in the taiga / tundra is often only <NUM> high, in oceanic areas several metres high.

The shape of the apertures <NUM> in the instrument coverplate <NUM> is slit-like, wherein the slits <NUM> preferably running with constant slit width I along more than half of the width of the instrument coverplate <NUM>. Due to the different types of snow, the lower limit of a reasonable slit width I is approx. <NUM>, the upper limit approx. The slit <NUM> opening is rectangular, running perpendicular to the general layering of the snow cover. In principle, the aperture could also be round, e.g. for application on ski slopes.

A snow side S respectively the surface of the snow side S is first imaged in near-infrared, then partially covered with the at least one aperture <NUM> in the instrument coverplate <NUM> and again illuminated and imaged. The change in reflectance determines the snow density. The specific surface area of the snow is determined via the homogenous scattering and adsorption of light. The density of snow can be determined by combining the known specific surface area with the reflection of a partially covered surface. The scattering is almost independent of the wavelength, while the absorption is strongly dependent on wavelength.

A wavelength range between about <NUM> - <NUM>, near-infrared, is optimal for the application due to the following reasons:.

At <NUM>, the change in reflectivity is greater (better signal-to-noise ratio), but since small particles are produced during the preparation of the profile (snow dust), the reflectivity is often distorted. For surface applications (slopes), this spectral range could be interesting at best. <FIG> shows a schematic representation of the ratio r /r∞ of the local reflection r, with instrument coverplate <NUM>, relative to the reflection r∞, without coverplate <NUM>, across the opening as a function of the penetration depth L. The penetration depth L is depending on both the size of the ice grains (OED) and the density of the porous ice structure. For very high densities (low penetration depth L0, <FIG>), the lateral dispersion of the radiation is very limited, with the result that the reflection within the opening is identical to that without the coverplate <NUM> (r/r∞ = <NUM>). With decreasing density (increasing penetration depth at identical OED, L1 to L3 in <FIG>,) the lateral dispersion of the radiation increases and thus the reflected radiation within the apertures <NUM> decreases (r /r∞ < <NUM>). From the reflectance distribution across the apertures <NUM> together with the knowledge of the OED, the density of the porous ice structure can be inferred.

As depicted in <FIG>, additional detectors <NUM>' are placed in recesses in the instrument coverplate <NUM> and also such detectors <NUM>' are connected to the computational unit CU. The detectors <NUM>' are facing the snow side S and can detect reflected radiation. The distribution of the back reflected radiation at detectors <NUM>', on the snow facing side of the coverplate <NUM>, can be detected and evaluated.

A series of preliminary experiments was performed using a grid with vertical slits <NUM> as apertures <NUM> to test the proposed method, as depicted in <FIG>. In <FIG> a schematics of an instrument coverplate <NUM> with four slits <NUM>, <NUM>', <NUM>", <NUM>‴ with different slit widths I, I', I", I‴ is shown. Essentially, the instrument coverplate <NUM> represents a frame with four vertical slits <NUM>, <NUM>', <NUM>", <NUM>‴, through which the snow side S can be seen, when lying on a snow area.

The number of vertical slits <NUM>, <NUM>', <NUM>", <NUM>‴ is chosen so that they do not interfere with each other, i.e. at least about twice the slit width I, I', I", I‴. The adjacent slits <NUM>, <NUM>', <NUM>", <NUM>‴ can all be the same width, or they can be in a regular pattern (e.g. <NUM>, <NUM>, <NUM>). All slits <NUM>, <NUM>', <NUM>", <NUM>'" are running parallel to each other. The length of the slits <NUM>, <NUM>', <NUM>", <NUM>'" runs over more than half of the length of the instrument coverplate <NUM>, here about <NUM>% of the length of the instrument coverplate <NUM>.

The reflectance without the coverplate <NUM> depends only on the grain size or OED of the ice grains, though the reflectance within the slits <NUM>, <NUM>', <NUM>", <NUM>'" depends additionally on the snow density.

<FIG> shows the dependency of the normalized reflectance r /r∞ across three slits <NUM>, <NUM>', <NUM>" of different slit width I, I', I". The narrower the slit <NUM>, the stronger the effect of the coverplate <NUM> and thus the reduction of the local reflectance at similar penetration depth L. Furthermore, the symmetric decrease in r /r∞ towards both sides of the slit <NUM>, <NUM>', <NUM>" is shown.

The box-shaped snow density measurement instrument B can be used in a surface mode and in a vertical mode. The surface mode works by placing the box on the snow surface. The vertical mode works on the wall of the snow profile.

The digital images taken are transferred to the computational unit CU and optionally to another computer or similar device, e.g. mobile phone. The images are evaluated by a software and converted to density profiles.

The measured snow surface is illuminated by artificial light in the spectral range of <NUM>-<NUM>. Additional spectra can be added (e.g. green light) to measure the amount of snow impurities.

External light sources on the images are eliminated by taking a dark image at the beginning and end of an imaging sequence.

Larger areas of snow on a flat or vertical surface than covered by the box-shaped snow density measurement instrument B are combined by making a mosaic of images and digitally combining them into one density and specific surface area result.

The instrument coverplate <NUM> can be easily interchangeable and/or movably arranged on the box-shaped snow density measurement instrument B, for a quick exchange or replacement.

Claim 1:
Snow density measurement instrument (B), comprising at least one illumination device (<NUM>) generating an incident beam (<NUM>), at least one light detector (<NUM>) based on silicon CCD- or CMOS-sensors measuring the reflected light and a controlling computational unit (CU), wherein the snow density measurement instrument (B) is formed box-shaped, the at least one illumination device (<NUM>) and the at least one light detector (<NUM>) are connected to a base plate of the box-shaped snow density measurement instrument (B) and arranged in the interior of the snow density measurement instrument (B), which is covered with an instrument coverplate (<NUM>) facing a snow side (S) and covering the box-shaped snow density measurement instrument (B), while from ice particles (<NUM>) reflected radiation can be measured in the at least one light detector (<NUM>) with digital images controlled and processed by the controlling computational unit (CU) in the interior of the snow density measurement instrument (B) in a non-invasively way, wherein the digital images are evaluated by a software and converted to density profiles, characterized in that
a multiplicity of apertures (<NUM>) in the form of slits, in particular running parallel to each other, are left out in the instrument coverplate (<NUM>), the slits having different slit widths (I) , allowing passage of incident near-infrared radiation (<NUM>) radiated by the at least one illumination device (<NUM>) in a wavelength range between <NUM> and <NUM> through the multiplicity of apertures (<NUM>) to the snow side (S).