Application of georadiolocation to assess the influence of the highway on the depth of permafrost in polygonal swamps of the North of Western Siberia

Introduction

The ecosystems of the Arctic and Subarctic are extremely vulnerable to anthropogenic and natural changes. Global warming, along with an increase in anthropogenic load, will in the near future contribute to a reduction in the area and capacity of permafrost rocks ^[1], with their complete thawing in the southern part of the cryolithozone, which will lead to significant landscape changes during the construction and operation of industrial and social infrastructure facilities at high latitudes ^[2]. The construction of highways in the North of Russia is one of the main factors of anthropogenic impact on the tundra geosystems of the cryolithozone ^{[3, 4]}. In this regard, studies of permafrost soils reflecting changes in climatic, geocryological and landscape conditions are of particular relevance ^[5].

One of the regions where subarctic geosystems are experiencing significant anthropogenic stress along with climatic warming is the North of Western Siberia. Against the background of warming in the last decade, thermokarst ^[6] and thermal erosion processes have intensified in the southern tundra of Western Siberia ^[7]. Polygonal swamps of the North of Western Siberia remain geosystems with relatively low MMP temperatures, which determines their maximum resistance to climatic changes ^[8]. However, this fact actualizes studies of the stability of swamp geosystems to anthropogenic disturbances, including those arising during the construction of such extended linear objects as highways.

Complex geocryological studies of polygonal marshes have been conducted in the northeastern part of the Pur-Taz interfluve since 2016 ^[9]. Within the framework of these works, the need for detailed geo-radar studies has been determined, allowing for a detailed assessment of spatial changes in the depth of seasonal thawing and the occurrence of the roof of permafrost rocks ^[10]. Geo-radar profiling has shown its effectiveness in studying the features of the occurrence of MMP in swamp geosystems ^[11]. In this regard, the purpose of this work is to determine, with the help of geo-radar profiling, the features of spatial differentiation of the depth of the permafrost roof in the polygonal swamps of the North of Western Siberia, disturbed during the laying and operation of the bulk highway.

Research objects

The object of the study is the intersection of the polygonal swamp with the bulk highway Tazovsky – Gaz-Sale (Tazovsky district of the Yamalo-Nenets Autonomous Okrug) with a cement-concrete coating within the Pur-Taz interfluve (Fig. 1). Coordinates of the research area: 67°20'26" s.w., 78°57'07" V.D., the width of the base of the road embankment is 20 m, the road is 12 m, the part of the road covered with concrete slabs is 6 m, the height of the embankment is up to 2.5 m. The temperature of the MMP for the research area is from -3 to -7 ° C ^[9].

Fig. 1. The location of the research site, the location of the 3GAZ transect is shown by a red punson on the large-scale cartographic diagram on the right.

Research methods

Geo-radar studies were conducted in September 2020 on the 3GAZ profile (transect) crossing the highway. The depth of the MMP roof was determined by the method of continuous profiling by the Zond-12E georadar (Radar Systems, Inc., Riga, Latvia) with a 300 MHz surface shielded antenna connected to it. The distance between the measurement points (tracks) ranged from 2 to 5 cm, the geometric binding of the profile was carried out using the Garmin GPSmap 64S GPS navigator.

Data processing was carried out in the software package Prizm 2.7 (primary processing) and RadExPro (OOO "Deco-Geophysics UK", Moscow, Russia). At the stage of primary processing, georadarograms were scaled by markers, the recording start time was adjusted. The subsequent processing stage included bandpass and two-dimensional filtering, amplitude correction, input of relief data predicting deconvolution, migration along the Horizon ^{[12, 13]}. After processing, the velocities of electromagnetic waves were determined, which within the seasonally small layer were calculated on the basis of direct measurements at the points of the profiles. Certain values of the velocity interpolated between points are calculated by linear interpolation.

The geo-location data before and after processing are shown in Figure 2 (Fig. 2).

Fig. 2. Data on the 3 GAZ profile before (a) and after (b) processing

During field work on separate pickets of geo-radar profiles, the power of the seasonally thin layer (STS) was measured, measurements were carried out with a graduated metal probe with a circular cross section with a diameter of 10 mm and a length of 150 cm ^[14]. The number of pickets in the profiles varied from 8 to 18 depending on the landscape diversity. When describing vegetation, the type of tundra plant communities and the maximum height of the tiers of vegetation cover were determined.

Research results

Anthropogenic disturbances caused by the construction of the highway have a significant impact on the geosystems of polygonal swamps, which is reflected in the change in the occurrence of the roof of permafrost, determined by geo-radar data. Fig. 3 shows a fragment of data with interpretation obtained on the geo-radar profile crossing the polygonal swamp and the Tazovsky – Gaz-Sale highway. The roof of the MMP (the sole of the seasonally thin layer) is distinguished by the high-amplitude reflection of the radio signal, the depth of its occurrence is confirmed by the results of measurements with a metal probe. According to geo-radar data, the following elements are clearly differentiated within the profile: the highway in the center, bounded by swampy relief depressions, at the beginning and at the end of the fragment, the profile passes through a polygonal swamp. These reflections from the roof of the MMP within the polygonal swamp are in the time interval of 20-30 ns, increasing under the road up to 50 ns and especially under the roadside drop (up to 70-80 ns).

Fig. 3. 3GAZ geo–radar profile according to the profile "polygonal swamp - bulk highway – polygonal swamp".

In addition to the reflection from the sole of the seasonally thin layer on the profile crossing the highway, an axis of common phase is additionally allocated, presumably corresponding to the sole of the road embankment (Fig. 4). These two axes of common phase differ from each other by the polarity of the signal: if the reflection coefficient is positive on the sole of the seasonally thin layer (the speed of electromagnetic waves in frozen soils is several times higher than in thawed water-saturated), and the common-mode axis has a positive polarity, then the common-mode axis of the wave reflected from the bottom of the embankment has a negative polarity. In Figure 4, one georadarogram passing through the thickness of the embankment is shown in amplitude form. The maximum absolute amplitude of reflection from the roof of the MMP is positive, from the sole of the embankment is negative. This indicates a decrease in the speed of electromagnetic waves at the boundary of the mound sole and may mean that this boundary coincides with the groundwater level (UGV) under the bulk sediments ^[15].

Below the roof of the MMP, multiple reflections are observed on radar images, masking possible useful waves (see Fig. 2). It is not possible to distinguish any boundaries below the sole of the seasonally distant layer according to georadar data.

To calculate the time of arrival of a radio signal to the depth of the defined boundary, it is necessary to know the propagation velocity of electromagnetic waves in the medium. There are several ways to determine the wave velocity: conducting geo-radar studies with a source and receiver separated, determining the velocity by diffraction hyperbolas, binding to direct observation data, using a priori information ^[13]. The propagation velocity of the radio signal within the seasonally distant layer was calculated as double the power of the layer divided by the arrival time of the reflected wave and was about 5 cm/ns. The speed of the radio signal in the body of the road embankment was determined by the hyperbolas of diffraction, averaging about 10 cm/ns. There are no diffraction hyperbolas in peat horizons lying under the thickness of the road embankment. The value of the velocity of electromagnetic waves here was assumed to be 7.5 cm/ns, since the velocity of these waves in soils is inversely proportional to their humidity ^{[12, 16]}. Under the condition of constant values of other physical characteristics of peat, in comparison with undisturbed areas, the peat horizons underlying the embankment become dense when moisture is squeezed into the sand fill. As a result, the volume humidity of peat directly under the road embankment becomes lower than outside it, and the speed of electromagnetic waves is higher. The buried peat layer is located below the groundwater level and the speed of the radio signal in it is inversely proportional to the depth of the UGV. Based on the above, the speed of the radio signal in this layer is defined as the arithmetic mean between the values of such in wet peat and dry sand of the embankment.

The analysis of geo-radiation data, taking into account the calculated radio signal speeds, shows that the main areas of lowering of the roof of the MMP are localized in roadside depressions (Fig. 5).

At the base of the embankment, there is a sharp (up to 2 m) lowering of the roof of the MMP, which is due to the runoff of moisture into the roadside depression, as well as the pulling out of polygonal vein ice, which actively occurred in abnormally warm years ^[9]. In addition, when the surface of the slopes of the road embankment is heated, the maximum warming of the soils occurs at the foot of the embankments, where the capacity of the bulk soils is minimal ^[17]. The maximum depth of thawing in the foundations of the slopes of the road embankment increases the risk of reducing the stability of the roadbed and causes its uneven deformations ^[18].

Fig. 5. Change in the depth of the roof of the MMP according to georadiolactional data on the 3Gaz transect crossing the Gaz-Sale –Tazovsky highway.

Nevertheless, the lowering of the permafrost roof on the studied sections of the highway is significantly less than those (8-10 m) observed at the intersection of the bumpy swamp in the European North, located in the subzone of the massively insular distribution of the MMP ^[10]. This is due to the relatively low temperatures of permafrost rocks in the swamp geosystems of the Taz-Purovsky interfluve.

Directly under the road embankment, the roof of the MMP lies at a considerable depth (up to 3 meters or more). However, if we do not take into account the height of the bulk sand column, in the buried peat deposits, the permafrost roof is located within the natural depths of occurrence, only in some places decreasing by 0.3 m (Fig. 6). Probably, this specificity of the occurrence of the MMP roof is due to significant cooling of the road embankment cleared of snow in winter, as well as increased thermal conductivity sandy soils in summer. Similar results were obtained when studying the configuration of the MMP roof under a highway crossing a bumpy swamp in the European North ^[10].

Figure 6. Results of georadiolocation on the 3 GAZ profile.

According to mathematical calculations of the temperature fields of road embankments ^[17], a certain configuration of the MMP roof change under the road at the research site is more consistent with that for the soils of central Yakutia. In the model given for this territory, the depth of the MMP under the embankment is close to the level of that of undisturbed areas, and the depth of the permafrost roof at the base of the road does not exceed 3 m ^[17].

The method of manual probing makes it possible to determine variations in the depth of seasonal thawing up to 1.3 m within polygonal swamps and roadside depressions intersecting them. The results of manual sounding of the roof of the MMP confirm the nature of the spatial differentiation of the depth of its occurrence revealed during georadolocation studies (Figures 5-7). The minimum depth of the MMP roof, as well as the capacity of the STS, are characteristic of the main surface of polygonal peat bogs and inter-polygonal cracks (Fig. 7).

Fig. 7. The depth of the MMP roof, determined by manual probing at the research site.

In the wells, the power of the STS increases only by 20%, and the depth of seasonal thawing increases even more in the marginal parts of peat polygons. In roadside depressions , the roof of the MMP lies deeper than 1 m . The main warming factors in the depressions are excessive volume soil moisture (up to 100%), high height of shrub vegetation (up to 2 meters or more), which causes increased snow accumulation.

Conclusion

Georadiolactic profiling makes it possible to determine significant changes in the depth of the permafrost roof both in natural and anthropogenic disturbed areas of polygonal swamps, where manual sensing methods are not available.

In the zone of influence of the highway, the maximum lowering of the roof of the MMP (up to 2 m) is observed at the base of the road embankment. Despite the deep occurrence of the roof of the MMP (3 m) under the road embankment, the thickness of the thawed buried peat horizons here is similar to that of the seasonally shallow layer of undisturbed peat polygons. The features of spatial differentiation of the depth of the permafrost roof in the polygonal swamps intersected by the bulk highway in the North of Western Siberia are similar to those characteristic of regions with a continuous low-temperature cryolithozone.

Thanks

The work was carried out with the financial support of the RFBR and the Yamalo-Nenets Autonomous Okrug within the framework of the scientific project No. 19-45-890011 "Assessment of the resistance of polygonal peatlands of the northern part of the Pur-Taz interfluve to anthropogenic impact against the background of climate change" and within the framework of the state assignment of the Institute of Biology of Komi NC UrO RAS No. 122040600023-8 "Cryogenesis as a factor in the formation and evolution of Arctic and boreal ecosystems of the European Northeast in the conditions of modern anthropogenic impacts, global and regional climatic trends".