Kaisa Halkola

A student at the University of Helsinki

Department of Geophysics

EU GLACIOLOLOGY FIELD COURSE AT TARFALA RESEARCH STATION 6-13^TH SEPTEMBER 2000

11.9.2000

The snow core was drilled without a motor. The auger consists of a lever part and pipes that are added in parts of about one meter during the drilling. At the end is a blade.

Nine centimeters of snow was taken away on the surface before drilling. After about every one meter the core was lifted up and a new part of tube was added to the auger. In fact, the core was lifted up more often because of the harder layers. The hole was reached until the depth of six meters.

As a core was lifted up the lenght and the weight of it were measured for the calculation of density (r =m/(A*h)). To measure the weight the core was put in a plastic bag and lifted with a spring scale. The area of the tube was known to be 1 dm. Samples were packed in plastic bags and marked for the further measurement at the station. At the station the samples were stored in a cold room.

The dielectric profiling (DEP) was made at the station. DEP-method measures the capacitance and conductivity along the core. The core samples are placed on a cradle in order and direction that corresponds the original placement. The cradle presents an electrode and the other electrode is moved manually along the core. The measurements are directed by a computer. The values are measured at a fixed time interval that corresponds two centimeters along the core. A signal from the computer helps to move the electrode in a correct speed. Capacitance and conductivity values are stored on computer and can be seen as a graphic immediately.

It was hard to drill trough the harder layers with arm strength. Because of that the drill had to be lifted up many times during the drilling. That made some soft snow to fall to the bottom from the walls of the hole. Though, this caused no error to the measurement, because the core was hard, easy to recognise from the loose snow.

For the conclusion it would have been interesting to have measurements also of the temperature in the borehole. Such measurements were not made in the hole, but in the snow pit ten meters further. In this report I will check if there would be some interesting relations between the temperatures and the results of the snow core, if I suppose that the temperatures corresponds those in the borehole.

The behaviour of density with depth in the snow core is shown in Figure 1. Because the density was measured of the samples of the length of even half an meter it doesn´t present the real variation so well. To get better results for the density the core should have been cut in sections of for example five centimeters.

Densities of the snow core vary from about 0.5kg/dm to 0.7kg/dm. At about these densities properties of snow changes remarkably in a process called firnication. These densities are the maximums that can be obtained by packing. In the process high pressure and melt-freeze cycles make the density to increase more rapidly. As the process keeps going more and more firn will deform and consolidation occurs. When the density reaches about 0.8kg/dm the air space between grains closes off. The air remaining is present only as bubbles. Ice has formed.

It has been observed that the spring, summer and autumn layers transform more quickly to firn and ice than the winter layers. One reason for that is the bigger water content . Percolation rate (trough the pressure) and temperature are the main actors for the formation of firn and ice.

So, the densities of the samples indicates that firnication takes place quite a lot. If the density would have been measured for thinner layers you could do more exact conclusions about the layers.

Temperature effects a lot on deformation in snow layers. The grains grow in the direction of the temperature gradient and is correlated in the strength of it. This often leads to formation of very loosely bonded crystals called depth hoar. This phenomenon is called temperature gradient deformation and occurs at temperatures below 0° C. As the temperature is around 0° C the melt-freeze-cycle forms clusters of large grains. Smaller grains melt in lower temperatures and form bondings between larger grains. When the temperature drops again these will freeze and large melt-freeze grains form. The density will not the limit of firnication.

The strong wind breaks the grains. Smaller grains will form layers in which the density is higher. The layer will harden more rapidly, because bondings are more likely to form among small grains. Melting and rain water may form ice lenses between snow layers.

Conductivity and capacitance values are shown in Figure 2 and Figure 3. Conductivity and capacitance seems to behave similarly. Increase in those values may indicate impurities, air bubbles, higher liquid water content etc. in the sample. It was clearly noticed that the breaks between samples caused a peak in conductivity and capacitance. That´s because at the points of breaks there is lots of air that causes error. Also cheese and bread pieces left in the plastic bags (lunch bags) caused error, but that happened in the case of ice core on an other day. In that case we noticed also that the ice core had melt during the storing which would have caused error in results.

COMPARING THE VALUES AT THE FIXED DEPTH

The result of the density measurements are very much averaged, but at the depth of 3-4 meters you can notice higher values. Higher values are also seen in conductivity and capacitance values around these depths. Below the depth of four meters and around 2.5 meters the density seems to be lower, and so do the dielectric values. Anyway, by looking just these values I would be very sceptic to do any strong conclusions of the relations.

Ice layers were marked in notebook in the depths of 38, 48, 73, 89, 98, 107, 142, 232, 244, 250, 323, 345, 492, 495, 576 centimeters. When comparing them with DEP data it´s difficult to find clear correlation. DEP values varies a lot anyway.

Temperatures in the snow pit, ten meters away from the borehole, are presented in Figure 3. It seems to be warm layers in the depth of 110 centimeter and getting colder when going deeper until two meters, the bottom of the hole. In DEP-data you might notice higher values in warm layers and lower in the colder ones. The temperatures are around 0° , so the higher dielectric values might indicate free water in the layer. In lower temperatures the water is stronger bounded around the grains, which make the dielectric values, which are a lot higher for the water than for ice (80/1), drop.

W.S.B. PATERSON, The Physics of Glaciers, Second edition, Pergamon press