One of the key factors for successfully processing georadar data is to know the speed at which the electromagnetic pulse sent out by the georadar travels through the subsurface. Through time-depth-conversion, we can estimate the depth in which the archaeological structures we are looking for are located. The so-called signal velocity depends on a range of factors, among them soil and sediment types as well as soil moisture content present at the site being surveyed. In areas with colder climate such as Norway, where temperatures can drop below zero for a portion of the year, ground frost becomes relevant. This is because the signal velocity is higher in frozen ground than it is in a ‘normal’, unfrozen matrix. It is therefore crucial for us to know at what depth the frostline, which marks the depth to which the ground is frozen, runs at our test sites when we survey there.
So how can we measure the frost line?
Ironically, it is the georadar that today is most-used to do that. For us, however, this indirect method is not a viable solution as we intend to study and compare georadar data by relating them to external environmental factors – using the georadar would be somewhat compromising our data integrity. We have therefore chosen to use frost depth meters, also called frost tubes to allow for a direct and quick monitoring of the frozen ground.
Frost tubes have a simple mode of operation, consisting of an outer protective pipe and an inner water-filled tube. The outer pipe has a diameter of 2 cm, is sealed at the bottom but can be opened on top. The inner, tightly fitting tube is filled with colored water and sealed at both ends. The colour comes from methylthioninium chloride or methylene blue, a dye that turns water dark blue when it is liquid but becomes clear when frozen. The inner tube’s clear plastic material makes it easy to observe the difference in colour. Whenever we conduct a survey at any of the sites and need to know the amount of ground frost, we can simply open the top of the outer pipe and pull up the inner tube. The amount of clear ice at the top of the inner tube corresponds to the depth of the frostline in the ground.
We installed the frost tubes next to the monitoring station of each of the four test sites in November 2020, when the temperatures started to drop. We drilled ca. 1.5 m deep boreholes, in which we then inserted the tubes.
Much as we would have liked to try the frost tubes right away, large amounts of precipitation and warmer temperatures prevented that. It was not until January, which brought a marked drop in temperatures and finally snow, that we could first test our system. So far, everything works fine - only some minor modifications were necessary to allow for an easier access of the inner tubes – and we are happy to report that during the monitoring surveys at the beginning of this year, test site Odberg showed ca. 5 cm frozen ground, while a survey two weeks later conducted during a very cold period indicated the frost line being at 22 cm below surface.
With Winter in full swing here in Norway, we are currently preparing to teach our GX 450 how to ski – more on that in the next blog.