Acoustic waves are pressure waves that travel through a fluid (liquid or gas), such as air. Science and CRREL research about acoustic wave interactions with snow are discussed on this page.

Why is it quiet when there is snow on the ground?

     Many people have noticed that it’s very quiet outdoors when there is snow on the ground, and especially after a recent snowfall.

What causes this?

     The pores in the snow cover are responsible for the quiet conditions. When acoustic waves travel horizontally above the snow, the increased pressure of the wave momentarily pushes some air into the pores. This air returns to the atmosphere after the wave passes, but some energy has been lost from friction and thermal effects. Over a short distance, this mechanism can significantly reduce the sound energy in the acoustic wave. This situation is in contrast to propagation over acoustically hard surfaces, like concrete or water, where sound attenuation can be much less. People on shore can often hear people in a boat speaking softly, even over a few hundred meters, so watch your language the next time that fish gets away!

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     At CRREL, researchers have conducted simple tests of this effect using a loud blank pistol:

Experimental set up to measuring the noise from a blank pistol over snow and no snow.

     The image above shows how the measurements were made, and the plots to the right show the typical results. These plots show an acoustic pressure wave recorded by a microphone 60 m (a little less than 200 feet) from the shot. The loud “bang” recorded in the summer without snow is reduced to a quieter “whoomp” in the winter by the snow cover. These recordings show that not only is the sound quieter, it is also longer in duration, or distorted, when a snow cover is present.

Why is the Army interested in snow acoustics?

     Any type of sensor system that listens for certain sounds will perform differently when snow is present. The figure to the right shows that the frequency content of the waveforms measured with the blank pistol. This display shows that frequencies above about 100 Hz (bass frequencies) are strongly attenuated. Any sensor system that tries to identify sounds will have to take this distortion into account if it is to perform well under winter conditions.

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Can acoustic waves be used to learn about the snow?

     Yes. As a result of research conducted at CRREL, we now understand enough to be able to use acoustic waves to probe the snow and measure parameters of interest in snow cover dynamics. Dr. Jerry Johnson (CRREL-Fairbanks) was the first to apply a detailed theoretical model known as Biot’s theory to the propagation of waves in snow (Johnson, 1982). Since then, Dr. Don Albert and colleagues have confirmed that a simplified version of Biot’s theory can be used to determine the snow cover depth and average permeability (the resistance of the snow to air flow, an important but hard to measure parameter). The theoretical matching was done using an automatic computer program, and the theoretical results agree with the measured snow depth to about +/- 2 cm (or 2 inches). We are currently verifying the permeability agreement by acoustic and direct measurements. As the snow cover metamorphoses or melts, its pore structure and depth undergo changes, and these changes can be monitored acoustically.

Determining Snow Depth from Wave Form

     An example of what acoustic waves can tell us about the snow is shown above. Here, measurements were conducted in Norway at the edge of a forest. Because of radiation from the dark tree trunks, the snow cover was much shallower just inside the edge of the forest, and acoustic waveform measurements were able to track these changes in snow depth. (Albert, 1998).

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What other snow acoustic research investigations have been done at CRREL?

     CRREL researchers have conducted acoustic measurements on seasonal snow in New England, Michigan, Alaska, and Norway. In addition, measurements on polar firn have been conducted in Greenland. These measurements are supplemented with laboratory tests, theoretical work, and computer modeling (see references for examples.)

     In cooperative work with Norwegian researchers (Albert and Hole, 2001), we have shown that large amplitude blast waves are also affected by the pores in a seasonal snow cover (see image below). These effects occur even for very short propagation distances (Albert, 2002).

Effects of Snow on Blast waves.

     A snow cover also provides an ideal situation for the detection of a wave known as an acoustic surface wave. CRREL researchers have published clear observations of this type of wave (Albert, 2003).

     Theoretical modeling has shown that an air-filled porous material like snow has a very different behavior compared to a water-filled porous material like saturated soil or undersea sediments. (Albert, 1993).

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Research Successes

CRREL has a unique capability in seismic and acoustic snow research. Research highlights include:

• First to apply Biot theory to snow
• First to study acoustics for snow depth & permeability
• Fist determination of impact of snow cover on blast waves
• Theoretical determination of subsurface “quiet zone” for emplacement of seismic sensors at South Pole station.
  

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     Acoustic Waves are pressure waves that travel through a fluid (liquid or gas), such as air, while Seismic Waves are defined as waves that travel in a solid material. Because solids can support shear as well as compressional motion, there are two general types of seismic body waves in a bulk solid. In snow, a porous material, the seismic waves travel in the ice frame rather than in the air-filled pores.

     Seismic waves have been used for a long time to investigate polar ice sheets. Before the development of radar they were the only way to measure ice thickness, and they are still used to investigate polar firn properties, glacier bed dynamics, and to measure water depths below floating ice shelves.

     Because of the increasing difficulty of using explosives in seismic exploration, CRREL researchers have developed rapid techniques of conducting these measurements using a commercially-available shotgun source. They were able to measure waves propagating 70 m deep in the Greenland ice cap, allowing a firn density vs depth curve to be empirically constructed in less than one day on site.

     A Global Seismic Network station (SPA, “South Pole A”) has been in operation for many years at the South Pole, Antarctica, but the data quality from this station has often been poor because of Station activities (aircraft and bulldozer traffic and electrical generators). When the Station operators conducted a noise survey to see if moving the instruments away from the Station would improve performance, they found that there was very little noise reduction even many km away.

     Clearly, the sensors needed to be installed in a borehole, but it was unclear how deep and how far away the borehole should be. Using archived seismic measurements from the 1960’s, CRREL researchers constructed a computer model of the seismic noise propagation at the South Pole (Figure to the right). When the sensors were installed at the recommended location, 8 km from South Pole Station, 280 m deep (Albert, 1998), this site became one of the quietest measuring locations in the world, and the new installation is now designated as QSPA (“Quiet South Pole A”) station.

     While seismic waves can travel through a seasonal snow cover, it is difficult to measure them because the snow cover is so thin compared to the seismic wavelength (which is typically meters to tens or hundreds of meters at low frequencies compared to snow layers often much less than a meter thick). However, seismic sensor systems can often perform very well under such conditions (Moran and Albert, 1996), and overcome some of the high wave attenuation problems and signal distortion that acoustic waves encounter (see Snow Acoustics details).

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Acoustic References

Albert, D.G. (2003). Observations of acoustic surface waves in outdoor sound propagation. Journal of the Acoustical Society of America 113, 2495-2500.

Albert, D.G. (2002). Reduction of blast noise by a snow cover. Noise Control Engineering Journal 50, 200-203.

Albert, D.G. (2001). Acoustic waveform inversion with application to seasonal snow. Journal of the Acoustical Society of America 109, 91-101.

Albert, D.G., and L.R. Hole (2001). Blast wave propagation above a snow cover. Journal of the Acoustical Society of America109, 2675-2681.

Albert, D.G. (1998). Snow cover effects on impulsive noise propagation in a forest. Noise Control Engineering Journal 46, 208-214.

Albert, D.G. (1993). A comparison between wave propagation in water- saturated and air-saturated porous materials. Journal of Applied Physics 73, 28-36.

Johnson, J. (1982). On the application of Biot's theory to acoustic wave propagation in snow. Cold Regions Science and Technology 6, 49-60.

Seismic wave references

Albert, D.G. (1998). Theoretical modeling of seismic noise propagation in firn at the South Pole, Antarctica. Geophysical Research Letters 25, 4257-4260.

Moran, M., and D.G. Albert (1996). Source location and tracking capability of a small seismic array. USA CRREL Report 96-8.

The SNOW Interest Group is sponsored by Cold Regions Research and Engineering Laboratory, part of the USACE Engineer Research and Development Center
For more information about the SNOW Interest Group and its members, contact:
Janet Hardy Phone: 603-646-4306 Fax: 603-646-4644 E-mail: Janet.P.Hardy@erdc.usace.army.mil Cold Regions Research and Engineering Laboratory 72 Lyme Road, Hanover, New Hampshire, USA 03755-1290 Pagemaster: kclaffey@crrel.usace.army.mil Page last updated: 12 June 2006(kjc)	Matthew Sturm Phone: 907-353-5183 Fax: 907-353-5142 E-mail: Matthew.Sturm@erdc.usace.army.mil USA-CRREL-Alaska (Building 4070) P.O. Box 35170 Ft. Wainwright, Alaska 99703-0170 Security and Privacy Notice