Modeling of Explosive Pingo-like Structures and Fluid-Dynamic Processes in the Arctic Permafrost: Workflow Based on Integrated Geophysical, Geocryological, and Analytical Data

Abstract

Understanding the mechanisms responsible for the origin, evolution, and failure of pingos with explosive gas emissions and the formation of craters in the Arctic permafrost requires comprehensive studies in the context of fluid dynamic processes. Properly choosing modeling methods for the joint interpretation of geophysical results and analytical data on core samples from suitable sites are prerequisites for predicting pending pingo failure hazards. We suggest an optimal theoretically grounded workflow for such studies, in a site where pingo collapse induced gas blowout and crater formation in the Yamal Peninsula. The site was chosen with reference to the classification of periglacial landforms and their relation to the local deformation pattern, according to deciphered satellite images and reconnaissance geophysical surveys. The deciphered satellite images and combined geophysical data from the site reveal a pattern of periglacial landforms matching the structural framework with uplifted stable permafrost blocks (polygons) bounded by eroded fractured zones (lineaments). Greater percentages of landforms associated with permafrost degradation fall within the lineaments. Resistivity anomalies beneath pingo-like mounds presumably trace deeply rooted fluid conduits. This distribution can be explained in terms of fluid dynamics. N–E and W–E faults, and especially their junctions with N–W structures, are potentially the most widely open conduits for gas and water which migrate into shallow sediments in the modern stress field of N–S (or rather NEN) extension and cause a warming effect on permafrost. The results obtained with a new workflow and joint interpretation of remote sensing, geophysical, and analytical data from the site of explosive gas emission in the Yamal Peninsula confirm the advantages of the suggested approach and its applicability for future integrated fluid dynamics research.

1. Introduction

Periglacial processes in the Arctic include frost heaving with the formation of pingos as a result of freezing of water-saturated sediments and gas-bearing pingo-like mounds. Some mounds can grow rapidly and then collapse with explosive gas emissions that produce large craters on the surface. A number of such craters were found in northern West Siberia in the period of 2014 through 2023 [1,2,3,4]. The available models attribute the origin of craters in the Yamal Peninsula to an explosion of pressurized gas resulting from the dissociation of gas hydrates in shallow permafrost [5,6] or rather focus mostly on the morphology of craters [7]. Reliable constraints on the gas emission and crater formation processes can be obtained from the composition of gas and other fluids in the respective sites. Meanwhile, it is problematic to analyze the chemistry of gas and other fluids soon after the blowout event as they volatilize rapidly and become mixed with air. The chemistry of the liquid components released by a gas explosion can very rarely be measured. For instance, in the case of a water-filled crater still releasing some gas found in early 2015 on the Seyakha River [2], no gas composition data were obtained. The researchers who worked inside the first crater discovered in Yamal [1] only mentioned methane measurements but did not report the results. All known gas emission craters are located within gas fields, thus indicating that permafrost may store free or clathrate (hydrate) gas in relatively shallow traps. Gas traps in permafrost can be detected in several successive steps, by integrated analysis of satellite imagery, lineament analysis, and geophysical survey of subsurface structures [8]. The same routine was applied to discriminate between morphologically similar classical pingos and pingo-like frost mounds as precursors to gas blow events [9,10,11].

Pingos are typical elements of periglacial landscapes which grow either in closed systems of freezing closed taliks or in open systems receiving groundwater inputs in zones of both continuous or discontinuous permafrost, respectively. Structural, geophysical [12], and geochemical [13] data reveal several common features in at least 30% of frost mounds, whether or not they are precursors to gas blow craters:

• intra- and sub-permafrost quasi-vertical chimney-like features most often associated with tectonic fracturing [11,14];
• high concentrations of methane (orders of magnitude above the background [13]) in soil and precipitation of autogenous minerals from migrating fluids;
• signatures of high pressure (fountains of groundwater, possibly gas-bearing) [12,13].

Pingos and pingo-like mounds commonly occur at fluid venting sites [12], but they do not necessarily contain gas. Therefore, it would be wrong and costly to drill all such features in the Yamal area to check their relation with a gas source and thus estimate the potential gas blow hazard. The mounds that collapse and cause explosive gas emissions to form craters were hypothesized to be morphologically distinct; however, no relevant information was provided [9,10,11]. Additionally, their height, width, and aspect ratio (length/width) showed poor correlation with gas occurrence. In this respect, the approach of detecting lineaments by remote sensing and geophysical methods appears to be a workable alternative to drilling: soil methane concentrations measured along lineaments were considerably higher than the background (up to 300%) [8].

According to our experience from oil and gas fields of the Yamal-Nenets Autonomous District, geophysical surveys can detect chimney-like features in permafrost, such as those found beneath some pingos and thermokarst lakes in the Taz Peninsula [15]. The patterns of chimneys we revealed, along with published evidence [16,17], indicate that the location and alignment of pingos and pingo-like mounds can be controlled by tectonic fracturing and related inputs of gas and/or groundwater.

Numerous known cases of hazardous pingo failure can be due to the expansion of gas upon cracking and its explosive release together with ejected rock fragments, while blowout may be triggered by minor sparkling or simple abrupt expansion. Some models [1,5,6,18,19] explain the presence of gas prone to explosive release by the dissociation of gas hydrates in shallow permafrost degrading under climate warming. Relic metastable gas hydrates self-preserved in permafrost above the zone of hydrate stability [20,21] dissociate as permafrost temperature increases, especially after anomalously warm summers [1]. However, despite a wealth of published evidence [18,19,20,21,22,23], the contents of gas hydrates, a depot for deep fluids in northern and Arctic West Siberia, remain poorly constrained, as well as the role of their generation and dissociation. Gas hydrates in northern West Siberia, especially, within the Yamal-Nenets Autonomous District, are as widespread as in physiographically similar petroleum provinces of Canada and Alaska. They were found in the Bovanenkovo gas field in the Yamal Peninsula [22] and in other petroleum fields southeast of Bovanenkovo (Nadym area): in above-Cenomanian sediments of the Medvezhiye gas field [24] and in the Pestsovoye oil-gas-condensate field known for active frost heaving [25,26].

Subsurface fluid dynamic processes may be active heaving agents [25,26]. Pressurized gas–water flows that rise to the Earth’s surface and entrain permafrost fragments can provide additional driving force for surface uplift similar to mud volcanism [14]. Considering periglacial processes, including frost heaving and thermokarst, in terms of fluid dynamics and linkage between surface and subsurface structures can provide insights into the role of groundwater, gaseous fluids, and subsurface heat flow in the formation of periglacial landforms.

The subsurface features connected with periglacial landforms can be resolved by geophysical surveys with reference to data from remote sensing and lineament analysis of satellite images. The geophysical methods should be suitable for the conditions of heterogeneous warm permafrost comprising isolated frozen blocks separated by unfrozen zones lying over a 3 to 11 m thick active layer [27,28,29,30,31,32,33,34,35,36,37,38].

Furthermore, the origin of pingos and pingo-like mounds can be better understood due to constraints on their composition. Gas chemistry data for pingo ice have implications for heaving conditions and ice genesis. The limited available data [26] indicate that the ice core of some pingos is rich in gas (up to 10 vol.% of methane and hydrogen in some layers) and has an ultra-fresh composition differing from the host saline ground, which is evidence for the condensation origin of water vapor carried with deep gases. The gas phase in the soil at pingo sites can contain helium, as in Quaternary soil core samples from the Pestsovoye hydrovolcanic field where He contents reached 0.179–0.167 vol.%, or an order of magnitude higher than in other samples from northern West Siberia. However, pingo ice in the Pestsovoye samples lacks He but may contain gas hydrates [26].

Gas-chemical data from wells of the Medvezhiye field revealed high contents of methane in drilling mud within intervals bearing free gas and low methane in the above-Cenomanian hydrate-bearing strata, where the (C2-C4)/(C5-C6) ratio in the gas phase is <1 and CH4/(C5-C6) < 500 [24]. Since the C5-C6 homologs exist in rocks as vapor rather than pore hydrates, they enrich the free gas released into the well. The iC4/nC4 < 1 ratio is also informative, as only isobutene has a hydrate form, while free gas is enriched with normal butane in the gas hydrate intervals.

Gas chemistry is also informative as to the conditions for the formation and dissociation of pingo-related gas hydrates because different gases (lighter than propane) form hydrates at different temperatures and pressures. Bubbling-free gas entrapped in ice from the Pestsovoye field (iC4/nC4 = 0.25) is apparently the remnant gas not converted to hydrate and migrated from stratigraphically lower hydrate-bearing intervals.

Thus, potentially hazardous periglacial features precursory to gas blow events with damage zones up to 400 m away from the epicenter [2] can be detected and discriminated from non-hazardous pingos by integrated geophysical, geocryological, and chemical analytical studies. The causes of pingo formation, as well as the possible effects of subsurface fluid-dynamic processes and the presence of metastable self-preserved gas hydrates [39], can be revealed by geological–geophysical modeling to depths of 500 m. Geophysical surveys, in turn, should be planned appropriately. The project can be successful on the conditions that (i) the combination and sequence of surveys are properly selected; (ii) the specific objectives for the pingo studies are clearly formulated; (iii) the revealed geophysical anomalies are checked by drilling, with carefully chosen hole parameters and logging techniques; and (iv) the field work at the site begins after the whole workflow has been designed in a due way.

The work requires joint efforts by a team of experts in different fields to combine geophysical results with laboratory analyses of stable isotopes and element contents in fluids that build the ice core. Meanwhile, there is no theoretically grounded approach to the integrated interpretation of geophysical, isotope, and chemical data from pingo sites. The aim of this paper is to develop and theoretically justify the optimal strategy of such studies.

2. Study Area

Most gas emission craters are known from reports of local people who either heard an explosion (as in the case of the Yamal event of 2014) or saw smoke and fire [40]. Local hunters also witnessed continuous gas ebullition that looked like boiling water around frost mounds in the study area. Those reports motivated studies of gas emissions from the Arctic permafrost [9,10,11,41,42], with a focus on fluid dynamic processes.

The site for the field studies was chosen in the Yamal Peninsula, 45 km southeast of Salekhard, the administrative center of the Yamal-Nenets Autonomous District (Figure 1A). Unlike many previously studied pingo-like structures, the site is located away from known oil and gas fields, at least within 300 km, which rules out their effect on the processes of interest. The site choice is logistically advantageous due to a ground road in the northern part (Figure 1B).

The site accommodates several mounds of different sizes and shapes which were detected in drone photographs and then found in the field (Figure 1C,D). Thus, the chosen site fits the requirements of the envisaged studies due to the presence of mapped pingo structures, the feasibility of geophysical surveys and drilling, as well as having good accessibility in the summer season.

3. Materials and Methods

Geophysical surveys and drilling at frost-heaving sites aim at (1) geological–geophysical modeling of frost heaving; (2) mapping possible conduits for fluids migrating from deeper reservoirs into permafrost as a prerequisite to pingo formation; (3) using geophysical and core sample data to identify pingo origin and estimating the possible contribution of fluid dynamics.

It is crucial to properly choose the best suitable site, according to the following criteria:

• the presence of mapped frost-heaving features as evidence of possible fluid dynamic activity;
• the presence of faults as potential fluid conduits;
• the feasibility of geophysical surveys;
• the possibility for drilling immediately within pingo structures;
• logistic accessibility in the warm season (short distance to residential communities).

In our case, the site choice was performed with reference to high-resolution satellite images via Jilin-1 shot in 2022, at different bandwidths: 0.61–0.79 μm (panchromatic), and 0.46–0.53 μm (blue), 0.54–0.59 μm (green), 0.63–0.69 μm (red), and 0.70–0.80 μm (near infrared) for the multispectral mode. Other parameters were as follows: radiometric resolution 10 bit/pixel; spatial resolution 0.75 m (panchromatic) and 3.0 m (multispectral); survey swath 12 km; sampling rate every 3.3 days. A larger area transcending the limits of the chosen site was studied using Landsat-8 images shot on 2 November 2013, with OLI (Operational Land Imager) and TIRS (Thermal Infrared Sensor), eleven channels, including eight channels with a resolution of 30 m/pixel in the visible near IR (1–5 and 9) and medium IR (6, 7) bandwidths, two channels (10, 11) with a resolution of 100 m/pixel (thermal band), and one panchromatic channel (8) of 15 m/pixel.

In this study, we used Landsat satellite imagery rather than other techniques, e.g., synthetic aperture radar (SAR) [43] applicable to monitor changes in geometry and the size of tundra lakes. SAR imaging is successfully used for mapping forests, flooded meadows and other vegetation, oil spills, and floods, as well as for monitoring soil water contents and sea ice [44]. It has high spatial (lateral) precision and ensures high-quality data on local features. However, Landsat appears to be better suited for our purposes of (i) tectonic interpretation and lineament analysis and (ii) mapping local geomorphology and periglacial features, including lineaments and pingo-like structures. The precision of 15 m/pixel is sufficient for the detection of tectonic features. The detected linear tectonic elements can be easily tied to drone photographs which, in turn, better resolve frost-heaving features than the SAR data. The scope of work included a search for deformed zones as possible fluid conduits. Such zones look in satellite images as lineaments and represent faults or permeable fractures used by migrating gaseous and liquid fluids, which are commonly warmer than the ground surface, especially in permafrost areas. The lineaments along the fractures are topographically marked by chains of thermokarst lakes, valleys of springs, and zones of ice shattering and secondary heaving that trace the migration paths of warm fluids. The properties of lineaments and their permeability to fluids can be inferred from data on regional- and local-scale tectonics and reconstructed directions of major tectonic forces.

Thus, remote sensing data were interpreted in terms of deformation patterns to identify faults proceeding from the following:

• aligned straight valley segments of large and small rivers or ephemeral streams and clusters of quasi-parallel straight valley segments;
• chains of small and large lakes;
• geomorphically expressed scarps.

Satellite images were deciphered first for a relatively large territory to outline a regional-scale deformation pattern (Landsat-8) and then for the study area (higher-resolution Jilin-1). Lineament analyses yielded rose diagrams of faults detectable in the local and regional patterns.

Spectral analysis of Landsat-8 images to a resolution of 30 m/pixel was applied to identify lake types within the study area, automatically in the geographic information system (GIS) with the QGIS open code, using the SCP plugin [45]. Periglacial landforms were deciphered in satellite images with the QGIS v.3.26 and MapINFO v.16 software. The indicators of older or younger thermokarst lakes, pingo-like structures, and frost cracks were revealed by visual examination of satellite images, on the basis of morphology, color, and shade. The spectral analysis of satellite images also provided evidence on seasonal ice cover on lakes.

Another essential part of the work consisted of an inventory of archived and published data, a review of data on pingo ice and surface waters obtained earlier by researchers from the Arctic Research Center of the Yamal-Nenets Autonomous District, as well as data on climate change (air and ground temperature, permafrost degradation), groundwaters (major-ion chemistry and stable isotopes in meteoric water and pore water in Quaternary aquifers), and ground ice, including pingo ice.

The origin of pingo ice was studied using data on its oxygen isotope composition known as a paleotemperature proxy [46,47,48]. For our purposes, it is essential that the similarity of δ¹⁸O values in pingo core ice and ice wedges with water in the surrounding lakes and rivers indicates a meteoric origin of the ice [49]. We, however, expect the δ¹⁸O patterns of pingo ice to differ from those of surface waters in the area because the pingo-like structures are connected with deeper strata via conduits for the migration of both gas and groundwater.

We have been planning high-resolution monitoring surveys in methane emission zones [50] on lake surfaces within the chosen site since the following field season.

The subsurface can be effectively imaged by several combined geophysical methods, each with its advantages and drawbacks, from few tens of cm to 400–500 m depths [51,52], with a focus on nearly vertical conduits (zones of weakness) as paths for the migration of fluids. These combined geophysical methods include the following: ground penetrating radar (GPR), contactless electrical resistivity tomography (ERT), passive seismic survey, and transient electromagnetic (TEM) soundings.

A reconnaissance ground penetrating (GPR) survey was performed using an OKO-2 system with 150 MHz antennas. The GPR data were processed in GeoScan32 [53]. The distance of the profile line was 260 m, and the penetration depth was 6 m.

Resistivity was measured in a non-contact mode using a VEGA instrument in a dipole axial-sounding configuration, and the data were processed in ZondRes2d [54]. Ungrounded lines AB and MN were 10 m long, at 10 mA current in the source dipole. To achieve deeper penetration, the spacing of dipole centers was increased progressively from 10 m to 60 m at a step size of 5 m. The sampling was at every 10 m. Altogether, 22 soundings were acquired.

4. Results

4.1. Periglacial Landforms

The identified periglacial landforms were of several types (Figure 2A–F): forested gently sloping flat watersheds (Figure 2A); bottoms of water-logged depressions, with patterned ground of small polygons (Figure 2B); large drained thermokarst lakes (Figure 2C); low-angle slopes and watersheds with patterned ground of large polygons and runoff hollows (Figure 2D); thermokarst lakes (Figure 2E); and mounds (Figure 2F).

Low flat forested watersheds appear as large (200–500 m) round isometric or rarely elongated blocks of plain light shades (Figure 2A) in the southern part of the study area. The forests consist of sparse conifers. The depressions are most often swampy, have slightly wavy surfaces, are and free from microtopographic signatures of periglacial processes.

Bottoms of water-logged depressions are covered with patterned ground of small (20–40 m) polygons (Figure 2B), some filled with water, that enclose pingo-like mounds, frost cracks, and thermokarst features.

Large drained thermokarst lakes, locally called khasyreys, are detectable as dark round depressions (Figure 2C), often with terraced bottom surfaces that record shrinking and water level fall. They often accommodate mounds and have patterned ground on the surface produced by frost cracking. It is a well-pronounced pattern of square, rectangular, trapezium-like, and other polygons separated by swampy depressions. Their color varies from pale yellow to dark green depending on the phenological phase on the shooting date, the evolutionary phase of the polygonal landscape [55], and water contents.

Low-angle slopes and watersheds with large polygons and runoff hollows (Figure 2D) show up in satellite images as clusters of ≤100 m circles and darker linear features of hollows. The zones bear signatures of frost heaving and frost cracking.

Thermokarst lakes in the study area (Figure 2E) are at different evolution stages. Thermokarst is associated with thawing and subsidence of ice-rich permafrost, which leads to the formation of round or elongated water-filled closed depressions of different sizes easily detectable in satellite images [56]. The observed lakes are 500 m long on average, and the largest is 1300 m. Incipient thermokarst lakes look like small dark spots, while the mature ones are surrounded by zones of disturbed vegetation, thermal erosion, and solifluction. Some mature lakes coalesce into large bodies, often connected with channels. Degrading lakes shoal down, and their bottoms become grown with sedge, grass, or moss, with sporadic small windows of water. The formation of thermokarst accelerates under industrial loads. For instance, traffic on the non-paved road within the study area caused surface thermal erosion.

Pingo-like mounds (Figure 2F) appear as round or oval hills looking lighter than the background [57]. The mounds may look similar to other landforms, such as remnants of lake terraces or other features related to thermokarst, and thus require field checking.

4.2. Deformation Pattern

The local tectonic features are detectable in the 1:1,000,000 geological map [58] of the area within the Salekhard uplift of the Lower Ob’ bench in the northwestern West Siberian Basin (Figure 3). The neotectonic uplift in this part of the basin slightly exceeds 300 m, while the surface of the highest elevation dips in the NW–SE direction. The area is located in the Ob’-Polui interfluve in an NW-oriented fault-bounded block. Two lineaments along the block boundaries have been confirmed geophysically and can be reliably identified as geomorphically expressed faults. The northeastern and southwestern boundaries of the block correspond to the Ob’ and Polui valleys, respectively. Many other lineaments strike in the northeastern direction and control valleys of large rivers or their segments.

The basin sediments have an intricate fault pattern, with more rigid blocks separated by heavily fractured zones and relatively large faults mainly originated from the basement, according to physical modeling and data from some basin areas [59]. Like the setting of the northern Yamal Peninsula [59], many large faults in the area within the northwestern West Siberian basin (Figure 3B) evolved in several stages, as the intensity and duration of unidirectional motion was insufficient to produce a major fault plane. Most of the faults are of an old generation while some formed during more recent tectonic events. Pre-Quaternary fracturing was mainly of strike-slip geometry, controlled by N–W compression and W–E extension, whereas the modern extension direction is rather from north to south. Therefore, the deformation pattern consists of multiple fracture zones rather than a single fault prominent in geophysical fields and in the surface topography, and not all fractures can act as conduits for fluids, including hydrocarbons and groundwater.

The structure of the study area (Figure 3B) within the regional tectonic framework comprises lineaments and circular features. Local-scale fractures are often poorly expressed geomorphically because of viscoplastic deformation, partial thawing of soft sediments under the warming effect of fluids, and the presence of periglacial landforms.

The lineaments within the study area coexist with circular features (Figure 3 and Figure 4), which are likewise of interest in terms of active fluid dynamics. Circular patterns can be produced by surface or subsurface processes: (i) degradation (thawing) of permafrost that may cause subsidence and (ii) compaction of sediments due to redistribution of fluids within reservoirs bearing hydrocarbons and formation waters. Furthermore, the circular features may correspond to small dome-shaped uplifts, intrusions, etc.

Corners 1, 3, and 4 of the site (Figure 4) fall within three different regional-scale circular features corresponding to domes northwest of corners 1 and 2 and large depressions in the southern part. The Landsat images reveal zones of deformation along lineaments of different lengths which fit linear depressions striking in the NEN direction in the site center (chains and clusters of lakes), as well as W–E ridges near corners 1 and 4 along the faults that bound the Salekhard block (Figure 3 and Figure 4).

All lineament directions traceable on the regional scale appear at the local level within the site (Figure 5 and Figure 6).

The rose diagrams of lineaments for the site (Figure 6) match the regional pattern. The W–E lineaments obviously predominate at both scales among 141 and 402 lineaments in the site and in the region as a whole, respectively (Figure 4), but they are most often subsidiary in origin with respect to the mostly NW major regional zones. Another well pronounced peak highlights NE directions corresponding to a regional lineament expressed in the surface topography as an elongate depression that accommodates thermokarst lakes. Especially large topographic lows occupied by swamps, rills, and lakes, occur at junctions of fractures striking in different directions.

Zones of frost heaving within a few tens of m in diameter follow the NW lineaments and broad NE zones and their intersections with NE zones, or partly zones of W–E orientation.

NE and W–E faults developing under N–S or rather NEN extension appear to be most widely open conduits for gas and water migrating into shallow sediments. Therefore, these zones, as well as their junctions with the NW structures, are the best target locations to conduct geological and geophysical studies of fluid dynamic processes.

4.3. Relation of Periglacial Landforms to Deformation Pattern

Lineaments within the site (Figure 7) were detected as linear features associated with rivers and zones of permafrost degradation. A chain of pingo-like mounds in the central part of the area is aligned with an NW fault crossed by an NE lineament. The mounds are especially numerous within a zone of major lineaments consisting of smaller lineaments along the axis of a broad fault zone.

Judging by the relative spatial position of lineaments and periglacial landforms at the site, permafrost within major lineaments is subject to degradation, probably, under a warming effect of ascending fluids. The thermokarst lakes and depressions with patterned ground of small polygons on their bottom predominate within the detected lineaments (

Table 1. Spatial relation of periglacial landforms and lineaments.

Site as a Whole			Within Lineaments			Outside Lineaments
Surface Area, km2	Perimeter, km	Number	Surface Area, km2	Perimeter, km	Number	Perimeter, km	Number	Surface Area, km2
Thermokarst Lakes
1.7	27.12	39	0.93	20.4	29	0.76	6.7	10
0.04 *	0.69		0.03	0.7		0.07	0.6
Depressions
34.77	7.01	20.16	10.67	4.04	37.9	24.1	2.97	12.3
100%	100%		30.7%	57.6%		69.3%	42.4%
Large Polygons
8.48	115.6	55	1.69	26.81	18	6.79	88.8	37
0.15 *	2.03		0.09	1.75		0.18	2.33
Pingo-Like Structures
34.77	-	317	10.67	-	143	23.1	-	174
100%	-	100%	33.6%	-	45.1%	66.4%	-	54.9%

*—the bottom line shows average values.

; Figure 8). Namely, the lineaments accommodate two-thirds of all thermokarst lakes, while the surface area of depression bottoms and lineaments occupies 57.6% and 30.7% of the total site area, respectively. Note that most pingo-like mounds occur within small polygons, while the large polygons corresponding to less degraded permafrost are mostly located beyond the lineaments. Many pingos likewise are associated with lineaments, and their density within the lineaments is 12.3 pingos per square km against 7.5 per square km outside the lineaments

4.4. Reconnaissance Surveys

The relation of periglacial landforms to faults and fractures was checked additionally by several combined geophysical methods.

The GPR image across a pingo-like mound (Figure 9) reveals a 1.5–2.0 m thick active layer (average thickness for the area) and the permafrost top (better pronounced in the right-hand section).

The electrical resistivity tomography (ERT) yielded a section to a depth of 20 m and revealed resistive rocks no less than 2000 Ohm∙m almost over the whole penetration depth of 20 m (Figure 10). However, the resistivity is lower (400–500 Ohm∙m) below 17–18 m, possibly because of a lithological change from silt to clay silt. The image (Figure 10) resolves a vertical low-resistivity feature at the 40 m point of the profile, which extends downward from a depth of 6 m and may correspond to a fluid conduit feeding a growing pingo.

Thus, the reconnaissance survey at the selected site in the Yamal Peninsula allowed for completing the following objectives:

• confirming the feasibility of reaching the pingo for geophysical studies and drilling;
• planning further field work in the spring season (logistic routes and transport: an off-road wheeled vehicle);
• proving the efficiency of resistivity surveys for sounding periglacial features (pingo case);
• obtaining resistivity and GPR patterns confirming that the pingo can be studied further by geophysical methods.

5. Discussion

The spatial distribution of periglacial landforms in the study area matches the pattern of fault-bounded blocks and is thus related to subsurface features. The blocks, including dome-shaped uplifts, most often geometrically correspond to large permafrost polygons which were produced by frost cracking at cold soil surface temperatures and underwent almost no erosion [55].

The lineaments mostly match eroded zones along the block boundaries and accommodate thermokarst lakes (Table 1; Figure 7), especially in the densely fractured central part of the study area, with mainly NEN fractures. Both the eroded zones and thermokarst lakes result from permafrost degradation, apparently under the warming effect of ascending fluid flows. The presence of fluid flow toward the surface is confirmed by geophysical data across a pingo-like mound in the fractured zone (Figure 10).

The formation of patterned ground with small polygons in the bottoms of dry lakes, as well as frost heaving, are associated with secondary periglacial effects: freezing of degraded permafrost induced by changes in fluid flow in fractured zones and climatic fluctuations. The current climatic fluctuations can also lead to the formation of taliks in uplifted blocks and to frost heaving during cold spells of one year or longer. Therefore, pingo-like mounds occur both in blocks and in fractured zones in the study area. This distribution is consistent with evidence from the Pestsovy oil and gas field [15], where conduits of ascending fluids were found beneath only 20% of all pingo structures.

One classical model of frost heaving [60] appears to be workable in all cases: freezing in a closed system with related expansion and surface deformation or in an open system with groundwater inputs into the freezing zone. In fractured zones, fluids can penetrate into the area of frost heaving and maintain rapid pingo growth [11], which may end up with explosive gas emissions. Most of the gas in some pingos is of deep origin, as confirmed in our earlier publications (e.g., [8]).

The existence of an additional subsurface agent responsible for the formation of potentially hazardous precursors to craters produced by explosive gas emissions, as well as for methane bubbling in thermokarst lakes [50], requires more thorough studies of periglacial landforms in heavily fractured zones. Such studies are indispensable for predicting the pingo evolution and estimating related geological hazards.

According to the new results, as well as to our previous experience, areas of periglacial processes can be successfully imaged by several combined geophysical methods, with subsequent correlation of the integrated geophysical data to analytical data on core samples. Thus, the collected evidence can be further used for the modeling of pingo evolution. The geophysical methods aim at imaging the permafrost structure and include GPR, ERT, sTEM, and passive seismic surveys. They include the following:

• ground penetrating radar (GPR) survey with 150 MHz antennas, to a depth from a few cm to 5–10 m depending on conductivity, for detecting faults and mapping water-saturated rocks; continuous sampling;
• electrical resistivity tomography (ERT), to depths from a few tens of cm to 30 m or more, with 48 electrodes spaced at 5 m between pairs, for more detailed resistivity imaging until the designed drilling depth, as well as for mapping faults from resistivity contrasts;
• shallow transient electromagnetic (sTEM) sounding to depths from 10 to 400–450 m (principal method), for mapping permafrost base, fluid conduits, and resistivity anomalies presumably associated with gas accumulations beneath frost-heaving features. It is the most efficient method for mapping the complex permafrost structure in the continental Arctic, including the gas hydrate lenses [15]. It is the high resistivity contrast between frozen and unfrozen rocks that ensures the efficiency of TEM soundings in permafrost imaging. Sounding permafrost, with accumulations of free gas and clathrates at shallow depths within 500 m, is challenging. In international practice, gas pockets and gas hydrates outside the permafrost zone have been successfully revealed by seismic and controlled-source electromagnetic (CSEM) surveys [61,62,63,64,65,66,67,68]. However, these methods fail to discriminate between frozen and hydrate-bearing rocks owing to similar resistivity and acoustic velocity in the sediments. sTEM soundings with space-effective loop systems can see permafrost to a depth of 500 m. The sounding technique based on induction field propagation is advantageous over the seismic, GPR, direct current (DC) resistivity, and frequency- or time-domain electromagnetic surveys because it does not require loop grounding and can run in any season and in any terrain [51];
• passive seismic survey, for mapping the structure to the basement top or deeper, detecting S velocity anomalies possibly produced by gas, and mapping potential fluid conduits. This technique, complemented with spatial autocorrelation for measuring seismic velocities and density of rocks, can detect vertical fluid conduits beneath mounds. Data from remote earthquakes along profiles traversing the area of interest may be useful to evaluate the fluid contents at local sites. The combination of seismic methods can yield a velocity depth profile to the upper mantle. It allows for estimating the total attenuation and contributions of its different components, with implications for the origin of the fluid conduits.

It is also important to know the composition and origin of the fluids that formed the ice core and pore gas in periglacial fractured zones. For this purpose, laboratory analyses are applied to separate fluid (liquid or gas) and solid (mineral) fractions of permafrost samples.

The data on stable isotopes in ice cores (mostly δ¹⁸O, δD, d-excess, and regression lines between them) have implications for the ice origin and the paths of water in pingos, system type (open vs. close), and freezing conditions (equilibrium vs. non-equilibrium) [7,69,70]. Additionally, d-excess, expressed as δD shift relative to δ¹⁸O, refers to isotope fractionation and is a marker of injection and condensate water types.

Another essential line of studies should focus on the detection and analysis of gases in pingo ice, including hydrocarbon gases which may be of biogenic (surface) or abiogenic (deep) origin. Chromatography of the permafrost gas phase allows for separating hydrocarbon gases from the ice of drilled mounds (alkanes, alkenes, isomers, and C₁-C₆ compounds) and other types of gases: N₂, O₂, H₂, He, CO₂, Ar and aromatic gases (benzene, toluene, ethylbenzene, p-xylene, o-xylene, m-xylene).

After the geophysical survey, drilling, and laboratory work, the data collected by different methods are to be interpreted jointly (Figure 11).

At present, there is no unanimity about the causes of explosive pingo failure with the formation of craters. Explaining and predicting a potential pingo collapse hazard similar to the event of 2014 that produced the Yamal crater [71] requires special approaches to study permafrost and related surface features, as well as their possible linkage with fluid dynamic processes.

The present study prepares further field work, as to the choice of the optimal combination and sequence of project activities and methods: (i) the choice of the site for detailed studies based on remote sensing data and lineament analysis, (ii) the study of the local deformation pattern; (iii) a geophysical survey as a basis for modeling the permafrost structure and search for fluid conduits; (iv) the choice of a pingo, with reference to the obtained structural and geophysical data; (v) drilling and logging in the pingo zone, with continuous monitoring of released gases and markers such as stable isotopes in ground ice and water-salt budget of pore water. The site we selected (the star in Figure 7) is well-suitable for further full-scope studies, including drilling, as it was confirmed by structural data combined with satellite imagery for the classification of periglacial landforms, as well as by the reconnaissance surveys.

Organizing a project of integrated studies by different methods is extremely challenging, for several reasons: geophysical surveys are very rarely used, especially for the purpose of pingo evolution modeling; the methods for modeling pingos and underlying formations remain unclear; joint interpretation of geophysical and core (stable isotopes, element contents) data lacks any solid methodological background. Meanwhile, it is only a review of all collected data jointly that can provide insights into the linkage between subsurface fluid dynamic processes and formation mechanisms of periglacial landforms in the Arctic.

The main goal of geophysical surveys is to model the structure of pingos and the underlying rocks. The detection of quasi-vertical zones of weakness allows for tracing presumable pathways of fluid migration into the pingo. The mapping of permafrost, including its base, cryopegs, taliks, and faults, can reveal resistivity anomalies caused by accumulations of free and hydrate gas beneath the pingo, posing risks of blowout and pingo failure. The isotope and geochemical analyses of core samples can shed light on the pingo formation mechanisms and the origin of pore gas, with implications for failure risks at specific pingos.

6. Conclusions

Periglacial processes in the Arctic, including frost heaving, can produce pingos and gas-bearing pingo-like mounds, which can collapse with explosive gas emissions, producing large craters on the surface. Understanding the origin and evolution of hazardous frost-heaving features requires comprehensive studies by various remote sensing, geophysical, and laboratory analytical methods, with the joint interpretation of the results. In search of the appropriate strategy of such studies, we performed lineament analysis of satellite images and reconnaissance geophysical surveys at a specially chosen site in the Yamal Peninsula on the right side of the lower Ob’ River, and reviewed the available geological, geocryological, and analytical data from the area.

In order to ensure the well-justified design of surveys, the local deformation pattern has been interpreted from deciphered satellite images which highlight lineaments corresponding to faults and other deformed zones, surface processes, as well as lakes of different types, often located along faults. The data of satellite imagery were used to compile a cryological–geomorphological model of the area, including several typical landscape features: forested low flat watersheds and low-angle slopes; large polygons on watersheds; low-angle slopes and watersheds with signatures of frost cracking and runoff hollows; depression bottoms with patterned ground (small polygons), mainly water-logged; runoff hollows and thermokarst lakes, mainly swampy; thermokarst lakes; partly drained thermokarst lakes; and completely drained thermokarst lakes (khasyreys).

The pingo-like mounds detected within the study area follow NW-striking fault zones and their intersections with NE and partly W–E zones. The faults striking in the NE and W–E directions evolving in the modern NEN extension stress field were inferred to be potentially the most widely open channels for the migration of gas and water into shallow sediments. Thus, these zones, along with junctions with NW structures, are advantageous targets for studying fluid dynamic processes by geological and geophysical methods.

The traced major and subsidiary lineaments correspond to zones of permafrost degradation. This distribution is illustrated by the relative percentages of periglacial landforms associated with thawing permafrost (pingos, thermokarst lakes, and patterned ground of small polygons in depression bottoms) located within and outside the lineaments. The reason may be that fluids penetrating through fractured rocks cause a warming effect on permafrost.

The reconnaissance geophysical surveys confirmed that the selected site is well-suitable for developing and justifying the research approaches to the permafrost structure and landforms in the Arctic and their possible fluid dynamic linkage. The obtained resistivity and GPR patterns demonstrate that the periglacial processes can be studied successfully by geophysical methods.

The integrated interpretation of geophysical data acquired by several combined methods together with data on stable isotopes and element contents in core samples with the suggested workflow can provide constraints on fluid dynamic processes and deep gas sources responsible for hazardous frost heaving leading to explosive gas emission in the Arctic. More detailed investigations of fluid dynamics in the Yamal Peninsula require a sufficient amount of test drilling.