Testing electrical resistivity tomography (ERT) on ice and debris

Exciting! A guest blog post from Dr Sarah Thompson who spent 6 months working with us here during summer 2016. Although it wasn’t even her main project while in Innsbruck, discussions about field data and confusing results meant that we cooked up a plan to use the cold room here in ACINN to do some laboratory tests of geophysical measurements. This meant that Sarah ended up spending a lot of time battling technical difficulties in a walk-in freezer, which is never that much fun. Here is what she was up to in there:

We wanted to test the difference in resistivity signal between temperate and cold ice beneath debris layers of varying characteristics to allow us to interpret field data collected from the debris-covered Ngozumpa Glacier in Nepal.

Electrical resistivity tomography (ERT) is a geophysical tool used to image subsurface properties by measuring the electrical conductance properties of different materials. Electrical resistivity is largely controlled by the presence of water held in fractures and pores. As a result, a marked increase in resistivity occurs at freezing point. ERT techniques have been used successfully for hydrological and permafrost investigations, detecting and areas of ice and frozen ground and to identify the presence (or absence) of ice in glacial moraines. The mechanisms of electrical conduction in natural ice, such as that contained in polar ice sheets, mountain glaciers or frozen ground, are still not fully understood but measurements typically vary over several orders of magnitude, with temperate ice having a much higher resistivity than cold ice.

In late autumn of 2010 and 2014, ERT surveys were carried out on Ngozumpa Glacier to locate the ice margin at the debris-covered glacier terminus. Inversion and interpretation of ERT data collected in 2010 gave resistivity values more commonly associated with cold glacier ice. Very little is known about the thermal regime of large Himalayan debris-covered glaciers but isolated studies have suggest some glacier in the region may be polythermal. While it is feasible that the ice imaged may be cold, the ERT data are a 2-D representation of an actually 3-D distribution of electrical subsurface properties, this leads to an uncertainty in the data which is not generated by data error or noise. Also, the high contrast in resistivity within the profiles can cause the equivalence problem, specifically, the signal of the highly resistive ice could be suppressed by the thin, more conductive surface layer.

We set out to test the hypothesis a ‘debris layer of sufficient thickness and conductivity masks the highly resistive signal of glacier ice beneath reducing the resistivity to values commonly associated with cold ice or frozen sediments’.

To do this we froze a block of ice (0.8 x 1.2 m) in a plastic water butt and installed a string of temperature sensors (encased in green tubing) through the middle and edge of the block to monitor ice temperature during a series of miniaturized geophysical surveys:

A scaled electrode array was created using stainless steel nails, complete with saltwater sponges and attached to the electrode points of the cable used in the full-size field measurements:

We carried out surveys using surface debris layers of different characteristics, including coarse (left) and fine (right) grained debris:
Surveys were also conducted over cold (< -15º C) and temperate ~ 0º C bare ice, for which holes were drilled into the ice and a very small amount of salt water added to allow elected connection:

As far a possible all surveys were carried out using both cold and the temperate ice. A combination of different debris characteristics were added to the ice surface to test the effect of debris thickness, grain size, saturation and freezing on the resistivity signal from underlying temperate or cold ice.

Sarah did a phenomenal job in tedious and testing conditions and we will use the results of these hard-won laboratory studies to hopefully unravel the meaning of the field data, but as its not a side project for both of us it may take some time!

About lindsey

Environmental scientist. I am glaciologist specialising in glacier-climate interactions to better understand the climate system. The point of this is to understand how glaciated envionments might change in the future - how the glaciers will respond and what the impact on associated water resources and hazard potential will be.
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