Why skiing conditions are relevant for GPR surveys

GPR survey at Odberg on March 12th using the snow-MIRA. The wheels are replaced with snow tracks and several modifications of the antenna mounting allow for smooth gliding over the snow. Foto: NIKU

Publisert:

16.06.2021

Oppdatert:

16.06.2021 kl.14.13

How to prevent a 1.5t motorised georadar from acting as a very expensive snow plough AND getting usable GPR data. More about our winter surveys.

As mentioned in our last blog, this past winter was one with long periods of very cold weather and several heavy snowfalls which resulted in snow cover at all our monitoring sites. While the single channel antenna could be quipped with children’s skis to drive on snow, things are a bit more challenging when it comes to using motorized MIRA’s in wintry conditions: They are simply very heavy. A MIRA system as used in VEMOP weighs around 1.5 tons. So, buying a pair of used skis in a thrift store is not an option.

Luckily, our colleagues at NIKU have already come up with a solution to this problem during a previous research and development project in collaboration with Statens vegvesen. As with the single channel GX system by Malå, NIKU needed a set-up that would allow them to drive the heavy MIRA in deep snow while the GPR antennas glide smoothly on top of it.  To achieve this, the wheels on the vehicle were replaced with snow tracks and a new lightweight glass fiber antenna mounting was constructed. An elongated and upwards bend skid plate under the antenna box prevents the MIRA-box from acting as a very expensive snowplow.

Before being used in VEMOP, the modified equipment was tested under various snow conditions on several sites throughout Norway (https://forskning.no/arkeologi-niku-norsk-institutt-for-kulturminneforskning-partner/radarsignaler-i-sno-utvider-letesesongen-for-arkeologer/1592332). Results were mixed with very positive as well as some rather negative outcomes. One of the main goals of VEMOP is thus to apply and evaluate GPR surveys on snow under controlled conditions to study the effects of varying snow-depth and snow properties.

The past winter in Norway has offered us a range of different snow conditions to put to the test. A very cold period during January and February resulted in a thick layer of frozen soil covered only by a thin layer of snow. Several heavy snowfall events subsequently led to the build-up of a thick cover of dry snow. Finally, at the beginning of the spring, rising temperatures caused the snow to become wet and heavy and ultimately led to standing water on top of the still frozen soil. Luckily, we were able to capture all these different conditions with both the MIRA and the GX.

Screenshot from www.senorge.no. The displayed data show skiing conditions in Vestfold during the time of a GPR survey on March 12th. The snow had already melted at the areas close to the coast (Heimdal, Hovland and Lunde) while Odberg still had an approx. 20cm thick layer of wet snow. Foto: NIKU

So, how is this related to skiing? Some of the key factors necessary to study and understand GPR data sets acquired on snow are snow height and the snow properties as well as weather conditions during the time of survey – and these are also key factors when it comes to cross-country skiing. In a ski-crazy country like Norway it is rather easy to get access to these types of data, which could make it easier to look for certain, favorable conditions when planning future surveys.

But now enough of the winter. While we are writing this, spring has finally also come to Norway and with it one of the prime seasons for GPR surveys. Why? More about that in the next blog!