基于凍土視電磁電阻率劃定融冰/富冰區(qū)

Vladimir N.Efremov

(俄羅斯科學院西伯利亞分院麥爾尼科夫凍土研究所，雅庫茨克677010，俄羅斯)

0 Introduction

Thawed and ice-rich zones in frozen soils cause many problems during construction of buildings，roads，railways and laying of pipelines.Geophysical methods of electrical prospecting based on constant current，high or low frequencies are traditionally used to identify such zones.However，delineating the thawed zones in clayey silts and ice-rich zones in sand deposits is complicated by low contrast of electrical resistivity and permittivity of the objects relative to enclosing rocks.A new parameter for estimation of permafrost electrical properties-apparent electromagnetic resistivity in the frequency range of 10～1 000 kHz，which can improve the identification of such objects in frozen soils is suggested.This parameter is a ratio of the effective values of electrical resistivity to permittivity.It can be obtained by measuring the module and the argument (phase)of surface impedance.For this purpose it is useful to apply a radio-interferometry method［1］，MT sounding［2］or radioimpedance sounding［3］in the medium-frequency band，for which conduction currents，depending on ground electrical resistivity，and displacement currents，depending on their permittivity，are significant.

1 Methods

Electromagnetic field，penetrating into the ground depending on the field frequency and electrical properties of soils，causes conduction currents in it，determined by ground conductivity and displacement currents，determined by their permittivity.From the first Maxwell's equation a known ratio of displacement current density to conductivity follows:

Where σ is ground conductivity，S;ε is ground relative permittivity，relative unit;jcmis displacement current density，A/m;jccdis conduction current density，A/m2;ω is angular frequency，Hz;εais absolute permittivity，F(xiàn)/m;E is electric field component，V/m.

The ratio of displacement currents density to conduction currents density is a parameter that determines the electrical properties of a medium for alternating field of a certain frequency.If σ/ω? ε，conduction currents are dominant in the medium and it is in properties close to a conductor.If σ/ω?ε，displacement currents are dominant and the medium in properties is close to a dielectric.

The ratio between conduction currents and displacement currents is contained in one form or another in the known expression for module and argument of homogeneous half-space surface profiling［4］:

When displacement currents become comparable in magnitude with conduction currents，the observations are presented as effective values of specific electrical resistivity ρ and permittivity ε，determined by the formulas［4］:

A new estimated parameter is apparent electromagnetic resistivity rem，which is a product of reciprocals of conductivity and permittivity at the frequency，for which the displacement currents effect is significant (in longwave and mediumwave bands).In other words，it is the ratio of apparent electrical resistivity ρefto effective permittivity［5］:

Where Z is surface impedance;φ is its phase angle or phase;f is frequency，kHz.

During discrete radioimpedance profiling the magnetic(framework)antenna and the detector line of electric field component are oriented to the radio station strictly by bearing.This helps to avoid amplitude and phase errors during impedance measurements.

Radioimpedance profiling of taliks or ice-rich formations is carried out using the symmetric ungrounded detector line of electric field component with arm length from 4 to 10 m，depending on required degree of details.

During measurements in motion，the detector line stretches after a going on the profile operator.This allows to find out even such local objects，for example，vertical ground ice wedges with about 1 m cross section.A detector line is an electrical cable 8 m and more long or a steel strip with insulated surface.An operator observing impedance phase indications stops to take the readings where the phase values increase，decrease or become extreme on the general background.

To test the method the measurements were carried out in specific areas with known geological and geocryological structure and the interpretation the results were compared with the drilling data.

2 Experimental results and discussion

Apparent electromagnetic resistivity，which is the ratio of apparent electromagnetic resistivity values to permittivity，reduces for thawed soils by a decrease in electrical resistivity of thawed soils and increase in their permittivity caused by water content increase in liquid state.For ice-rich soils this ratio increases as a results of increase in ice-rich soils electrical resistivity in the denominator and decrease in their permittivity in the numerator，caused by iciness increase.Changes accounting of not only electrical resistivity but also of permittivity based on this parameter allows to increases the difference and to identify more efficiently such objects in permafrost as thawed，water-saturated or icerich zones，which differ in electrical conductivity and permittivity from enclosing rocks.

Measurements by radioimpedance profiling in areas with known geological and geocryological structure were carried out for comparative estimation of anomalies magnitude of apparent electromagnetic resistivity in comparison with anomalies of electrical resistivity.

Section 1."Suprapermafrost talik".The section is located near Yakutsk.Previously in this area during drilling the borehole№21 in sandy silts and underlying sands a suprapermafrost talik with the thickness of 4.2 m was found out.The boundaries of this talik were identified with the help of radioimpedance profiling in April at frequency of 549 kHz by anomaly of electromagnetic resistivity.The negative anomaly of electromagnetic resistivity at frequency of 549 kHz was－97%and the anomaly of apparent resistivity was－59%(Fig.1).

Fig.1 The anomalies of apparent electrical and apparent electromagnetic resistivity at frequency of 549 kHz on profile with a step of 10 m across the suprapermafrost talik in a suburb of Yakutsk

Section 2."Pingo".The section is located also in a suburb of Yakutsk.Here by drilling the borehole№22 on the pingo top its ice core with the thickness of 0.45 m at the depth of 7 m was opened.The overlapping soils are presented by clayey silts and peats.The ice-rich clayey silt，which underlies the ice core of the pingo，has a thickness of 1.4 m.The positive anomaly of electromagnetic resistivity above the pingo's ice core，also obtained in April，was 74%and the anomaly of apparent resistivity was 32%(Fig.2).

Fig.2 Anomalies of apparent electrical and apparent electromagnetic resistivity at frequency of 864 kHz on profile with a step of 5 m across the pingo in a suburb of Yakutsk

Section 3."Subsidence 1"along"Amur"，a federal highway.The presented method of radioimpedance profiling showed its high efficiency during highway works.In the autumn of 2012 with the author's participation some works were carried out in key areas of the"Amur"federal highway in Amur region and in Zabaykalsky Krai.Due to lack of received frequencies in some areas，profiling was the only possibility to apply the radio-interferometry method.On each measurements profile one borehole was drilled.

The analysis of the module and the phase values of surface impedance in certain points of the profile，which change to a considerable degree，allowed to identify anomalous zones，characterized by a certain combination of qualitatively estimated parameter changes.In areas with thawed and ice-rich zones，taking into account the nature of these combinations and drilling data，the following types of anomalous zones are identified:

1)with a negative anomaly of the module combined with its phase decrease，caused by presence of a thawed water-saturated layer formed as result of frozen base soils thawing;

2)with a positive anomaly of the module combined with its phase decrease，caused by ice-rich soils heaving of the frozen road base.

As a result of the visual examination it was noted，that anomalies of the first type correspond to subsidences and anomalies of the second type correspond to considerable heavingsofthe roadbed on ice-rich ground.

As stated above，roadbed heaving can be associated with anomalies of the second type.Frost-susceptible soils can be identified by anomalies of apparent electromagnetic resistivity，as shown in Fig.3.According to the results of radioimpedance profiling in interval 28 an anomalous zone was identified，presumably caused by high ground ice content.The positive anomaly of electromagnetic resistivity over an area with high ice content was 94%and the anomaly of apparent resistivity was 47%.Then between the points 25 and 28 some warm ice-rich sandy silts at the depth of 5 m with water-saturated soil at their surface were opened by drilling.Water-saturated ground freezing caused heaving of the roadbed above the ground with the highest ice content(Fig.3).

3 Conclusions

Fig.3 Changes of apparent electrical and electromagnetic resistivity at frequency of 22.3 kHz on profile with a step of 10 min the“Subsidence 1”area of the“Amur”federal highway

Application of new parameter values of electromagnetic resistivity as a parameter，estimated while profiling，causes a considerable increase in anomalies from taliks，ground ices and ice-rich objects in permafrost.Anomalies，obtained by this parameter，are approximately twice the existing anomalies，obtained by the parameter of apparent electrical resistivity.

Using the potential of radioimpedance profiling based on apparent electromagnetic resistivity allows to identify more effectively most dangerous areas for drilling during surveys of sites and line structure traces.

During surveys of operating highways，radioimpedance profiling based on apparent electromagnetic resistivity allows to identify and delimit zones of thawing and frozen base ground heaving，ground water filtering methods in problem areas.

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