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Electrical and electromagnetic investigation for landslide characterisation
Phys. Chem. Earth (C). Vol. 26, No. 9, pp. 705-710,200l 0 2001 Published by Elsevier Science Lid.
Pergamon
All rights reserved 1464-1917/01/$ - see front matter PII: S1464-1917(01)00070-8
Electrical and Electromagnetic
Investigation for Landslide Characterisation
A. Godio and G. Bottlno Dip. Georisorse Received
e Territorio - Politecnico
di Torino - C,so Duca degli Abruzzi, 24 - 10129 Torino, Italy
19 July 2000; accepted I February 2001
Abstract.
cases the information on the depth and the lateral continuity of the sliding surfaces can not be obtained through boreholes or geological investigations. Geophysical investigation can however complete the data set of technical information on the parameters of the subsoil for a complete understanding of the physical behaviour of a slope. Seismic investigations have been widely used to characterise the mechanical properties of rock-slopes; seismic refraction and high resolution seismic reflection are particularly useful to detect discontinuities in the subsoil where large contrast of seismic velocity or acoustic impedance have occurred between near surface materials and bedrock. However these methods can be very expensive (seismic reflection) or unreliable when sub-surface materials are particularly heterogeneous. On the other hand, electrical and electromagnetic methods allow one to obtain high resolution in 2D imaging of the slope; the method can usually image the distribution of the electrical conductivity, but there is not any reliable correlation between the electromagnetic properties and the mechanical behaviour of the rock mass. However, in such a context, the experimentation of high resolution methods, such as electrical tomography or ultra fast time domain electromagnetic methods (TDEM) should be verified in different geological and hydrogeological situations. The aim of the work is to present the results of applications of electrical resistivity tomography and time domain electromagnetic methods as powerful tools for slope site investigation. Particularly, the reliability and the performances of ultra-fast time domain methods for the study of the landslide behaviour will be enhanced and the advantages and shortcomings of these rather new methods will be discussed. In the selected example (Paroldo- Langhe Italy), the geophysical investigation was supported by
A geophysical investigation was applied to an area, subjected to a large slow moving landslide in the region oi Langhe - Piemonte, Italy. The joint application of vertical electrical soundings (VES), resistivity tomography (ERT) and electrical electromagnetic soundings (TDEM) has led to the detection of the geo-morphological conditions of the slope up to a depth of 30-40 m. The geophysical survey has detected the existence and continuity of a potential sliding surface, which had previously pointed out by inclinometer measurements. The test has permitted the verification of the reliability and the resolution of electrical and electromagnetic data analysis for landslides, character&d by marls and clay with low resistivity values (below 100 Ohm-m). o 2001 Published by Elsevier Science Ltd. All rights reserved
1 Introduction
In recent years, the application of geophysics to the study of slopes and landslides has widely increased; in such a context, great attention has been dedicated to seismic, electrical and electromagnetic methods (Chiara et al., 1996, Jongmans, D. et al., 2000). During a wide experimentation that has involved different phenomenon in the Langhe region (Piemonte - Italy), the reliability of the geophysical investigations has been analysed with the aim of improving the geological knowledge of the area and to locate joints and sliding surfaces. The main problems in landslide characterisation are related to the necessity of knowledge on the subsurface geology and hydrogeology site conditions; in many
705
706
A. Godio and G. Bottino: Electrical and Electromagnetic
an accurate geological and hydrogeological characterisation of the site. The final calibration of the geophysical data has been carried out through the correlation of the geophysical results with geological data and borehole results.
2 Geology of the landslide The morphology of the hilly region shows that the whole area is, at present, subjected to a great morphological rejuvenation that has partially modified the old forms; this rapid evolution is due both to the effects produced by the deepening of the basic level of the plain hydrography and to the effects produced by the raising of the hilly areas. From a geomorphological point of view, a close link between this rapid and recent morphological evolution and the existence of failures of different types resulted. The lithology involved in the instability phenomena is made up, in the upper part, of terrigenous deposits character&d by rhythmic altemances of marls, silts and arenaceous-sandy layers of variable thickness and, in the lower part of the hillside, of stiff marl sequences (Fig. 1). The general dipping of the strata is about ten-twelve degrees up dip. These sediments are intensely fractured on the top with different families of discontinuities and faults with modest throws; a low circulation of water is associated to these fractures and this can also be seen by frequent oxidation coatings. The landslide cinematic model refers to a planar slide with an approximately 20 m deep sliding surface. The movements are slow but continuous with periodic increasing due to heavy rains.
3 Materials and methods Electrical and electromagnetic methods are based on the observation of the spatial change of the electromagnetic constitutive parameters (electrical resistivity or electrical permittivity) of the subsoil; AC low frequency methods or DC measurements are mainly affected by changes in the electrical conductivity. The conductivity takes place through the moisture-filled pore of the subsoil; therefore, the conductivity value of the subsoil depends on the porosity in terms of the shape size of the pores and the characteristic of the interconnecting pathways, the total amount of pores filled with fluids, the concentration and the mobility of the electrolytes in the moisture and the temperature. A review of the role that each parameter play on the behaviour of the electrical conductivity of soils and rocks is described by McNeil1 (1980). In measuring the electrical properties of
Investigation
for Landslide Characterisation
heterogeneous landslides, mainly character&d by unconsolidated sediments, the importance of the colloidal conductivity must not be neglected; the addition of clay materials affects the electrical conductivity as the ion exchange capacity of clays is very high. The presence of a small amount of clay particle in the soil moisture can therefore substantially increase the electrical conductivity. Starting fkom these consideration, a strong contrast of electrical properties can be expected in a slope and landslide characterised by near surface filled-moisture fractures; in rock and soil where clay or silty materials are prevalent, the presence of fine silicates dispersed in the fluid phase could enhance the contrast of the electrical properties, between the rock matrix and the moisture embedded in the fractures. Electrical and electromagnetic methods are therefore suitable to map hydrogeological site conditions with reference to pathways and the circulation of groundwater. The main goal of the electrical and electromagnetic geophysical survey in the selected site was to delineate the presence of near surface heterogeneous materials that can be distinguished from the bedrock; the bedrock is not involved by sliding movements and, therefore, more consolidated and less affected by fractures and groundwater circulation.
Fig. 1. Schematic cross-section of the slope and location of the electrical resistivity tomography (ERT) and TDEM measurements.
In order to achieve these results, several methodologies that are capable of offering high resolution in the investigation range up to SO meters from the surface were applied. The geophysical survey was performed using the following methodologies: l preliminary electro-stratigraphical characterisation of the subsoil by means of DC vertical electrical soundings (VES); . electrical resistivity tomography (ERT) for a detailed characterisation of the electrical behaviour of the slope to 20-30 meters in depth; l execution of electromagnetic soundings (TDEM) to obtain information on the hydrogeological and geological setting of the slope to 50 meters in depth;
A. Godio and G. Bottino: Electrical and Electromagnetic l
detailed electromagnetic survey by means of ground conductivity meter equipment for a mapping of the near-surface hydrogeological condition on the top of the slope. This last survey was confined to a small area and is not discussed in this paper.
Vertical Electrical Soundings were performed along the slope using the Schlumberger array with a maximum spread of 150 m&.-s and 10 measurements for each decade; the data processing involved the 1D inversion of each sounding and the 2D correlation of the results by interpolation of 1D inversion. The methodology is straightforward and literature provides both theory and application examples for site character&&ion (e.g. Telford et. al., 1990, Reynolds, 1997). This procedure permits one to obtain a preliminary evaluation of the electro-stratigraphic condition of the slope (figure 2). PAROLCD'_
Investigation
for Landslide Characterisation
707
medium and TDEM sounding methods can be found in Kaut?na.n and Keller ( 1983) and in Nabighian ( 199 1). The Time Domain Electromagnetic acquisition involved the execution of 15 vertical electrical sounding& acquired using an ultra fast time domain equipment (TEM-Fast Pro-System, NL) in the coincident loop mode (12.5 x 12.5 m and 25 x 25 m loops). The investigation depth and the spatial resolution are affected by the conductivity of the subsoil and by the dimension of the loop: as a preliminary evaluation, due to the condition of the site, the adopted loop configurations allowed the reconstruction of the electrical resistivity of the subsoil in ranges of between 5-10 meters and 50-60 meters. The results of a preliminary test, performed at the bottom of the landslide, are shown in figure 3.
MODEL v===--
TT
Fii. 2. Example of VES sounding apparent resistivity curve and model of electrostratigraphy (resistivity and thickness) of the subsoil.
r ‘“i----r
10 Time Apparent 10
The Electrical Resistivity Tomography (ERT) was carried out using both a dipole-dipole and Wenner configuration array with a 32 electrode device (Sting AGI); an electrode separation distance of 5 meters was used; the data processing involved the 2D inversion based on a smoothness-constrained least square method (Griffiths and Barker, 1993, Loke and Barker, 1996). In TDEM measurements, the current flowing in a loop situated on the ground is abruptly interrupted, inducing eddy currents in the subsoil which diffuse away from the transmitter loop in depth. The diffusion of the eddy current is controlled by the electrical conductivity of the subsoil. The diffusion of the eddy current is determined by measuring the transient decay of the secondary magnetic field induced by the currents. A receiver loop, in this case coincident with the transmitter loop, is used to detect the transient of the magnetic field. An accurate description of the basic principles of electromagnetic propagation in natural
-vFy-
100
[ nicroseconds ] Resistivity 20
[ Ohmm] 30
Fii. 3. Example of the transient of TDEM sounding (top) and transformation in apparent resistivity versus depth (bottom).
708
A. Godio and G. Bottino: Electrical and Electromagnetic
The application of “extra-small” antennas (less than lo15 meters) to detect layer stratification with high vertical resolution at small depths (less than 20-30 m) is possible only at low resistivity of rocks, of less than 20-30 Ohm-m. The most favourable range of specific resistivity for rocks lays within the limits of 10 Ohmam and 300 0hm.m. The data processing of the TDEM survey involved the smoothing of each voltage decay curve and data conversion into apparent resistivity versus depth. The transformation allows one to obtain apparent resistivity values with respect to the depth, enhancing the presence of low resistivity layers (Meju, 1998). The apparent resistivity pseudosection were calculated by bidimensional interpolation of the results of the TDEM soundings (Fig. 4).
for Landslide Characterisation
4 Discussion The ERT pointed out a great heterogeneity of the near surface material and the presence of clay lenses that can be associated to lateral and vertical limits of permeability that could affect the groundwater pathway and circulation of the slope (Fig.5). The TDEM pseudo-section increases the depth of investigation of the ERT survey and detects the presence of a conductive bedrock (marls). On the other hand, the shortcomings of the electromagnetic
._. __
--A,
0
Investigation
between the fixcturcd -----7
/
20
40
60
80
100
120-)
Distance [ m ] Fii. 4. Pseudosection (values in 0hm.m).
of apparent resistivity
of the TDEM soundings
Resislwdy h ohnm
Fig. 5. Tomographic electrical resistivity section and position borehole S8, close to the lower edge of the tomographic section.
Unil Eledmde
of
Spcug
= 5.0 m.
A. Godio and G. Bottino: Electrical and Electromagnetic
Investigation
for Landslide Characterisation
activated in 1993, involving the slope movements of the near surface materials, and a deeper sliding surface, slowly moving, located at 35 meters in depth.
5 Conclusions
Clay marls aod (grey colau r)
Yrodstooes
Grey marl
Grey
Fig. 6. Sbatigraghy
mad
of borehole S8, located at the base of the slope.
investigation are related to a lack in the lateral resolution, compared to electrical tomography. The borehole, performed at the base of the slope investigated by the ERT (Fig. 6), pointed out the presence of a near surface weathered clay and sandstone layers (up to 10 meters in depth) overlying alternances of clay and marl layers: these sequence justified the low resistivity values encountered in the lower edge of the ERT section (values below 30 Ohmem). The borehole data allow an accurate calibration of the resistivity section, permitting one to extrapolate the typologies of the materials in the slope subsoil. The slope is characterised by lateral alternances of low resistivity materials (weathered clay and sandstone) with higher resistivity zones, which probably correspond to arenaceous materials. The geophysical survey confirmed the data of the inclinometer measurements, that have pointed out a frost sliding surface at 20 meters in depth, which was
Electrical resistvity tomography, when combined with the ultra-fast time domain method, permits a fast mapping of the subsoil electromagnetic properties that can be related to the geology and hydrogeology of the site. This test, in a complex geological situation, such as a landslide in a gently slope, permits one to verify the reliability of these methodologies as an aid to study the geological and hydrogeological conditions of rock mass. The ERT, using the dipole-dipole array, permits one to obtain an high lateral resolution and to point out the presence of sub-vertical water and moisture-filled fractures in the near surface layers; the TDEM section allows one to study the subsoil up to 40-50 in depth with good vertical resolution (l-2 meters). In data processing of TDEM soundings in very complex situations, the best choice is to adopt simple pseudogradient transform of the signal into apparent resistivity versus depth. An improvement in the efficiency of the TDEM data processing will be achieved when more powerhd algorithms for twodimensional interpretation are available. From a geological point of view, the joint analysis of electrical and electromagnetic data confirms the great heterogeneity of rock materials in the first lo-15 meters, the ‘presence of very near surface of sliding phenomena and the existence of two main sliding surf&es in the first 30-40 meters in depth (Fig. 7). On the basis of these observations it can be presumed that the instability model in the region is very complex. The planar landslides are characterised by multiple sliding surfaces, involving overall thickness that is greater of those that is normally suspected, where the deeper instability phenomena gradually trigger movements in the shallow part of the slope.
709
A. Godio and G. Bottino: Electrical and Elect] -omagnetic Investigation
8”
----
sail slip
Fig. 7. Schematic geological model of the landslide, brought to light out by the geophysical and borehole investigations.
References Chiara P., Godio A., Sambuelli L., Combined electrical investigation on a landslide, Proc. 2nd Meeting Environmental and Engineering Geophysics, Nantes, 113-l 16,1996.
for Landslide Characterisation
Griffiths, D.H. and Barker, R.D, Two-dimensional resistivit y imaging and modelling in areas of complex geology, Journal of Applied Geophysics, 29,21 l-226, 1993. Loke, M.H. and Barker, R.D., Rapid least-square inversion of apparent resistivity pseudosections by a quasi-Newton method, Geophysical Prospecting, 44, 131-152, 1996. Jongmans, D., Hemroulle, P., Demanet, D., Renardy, F., Vanbrabant, Y., Application of 2D electrical and seismic tomography techniques for investigating landslides, European Journal of Env. and Eng. Geophysics, 5, n.l,75-89,200O. Kaufman, A. and Keller, Ci., Frequency and transient soundings, Elsevier Science Publishers, Amsterdam, 685 pp., 1983. McNeil1 J.D., Electrical conductivity of soils and rocks, Tecnichal Note W-5, Geonics Ltd., Ontario, Canada, 22 pp., 1980. Meju, M. A., A simple method of transient electromagnetic data analysis, Geophysics, 63,2,405-410, 1998. Nabighian, M.N. and Macnae, J.C., Time domain electromagnetic prospecting methods. In: Nabighian, MN. (ed.), Electromagnetic Methods in Applied Geophysics, Vol 2A. Tulsa: Society of Exploration Geophysicists, 427-520, 1991. Reynolds, J. M, An introduction to applied and environmental geophysics, Wiley, Chicester, England, 796 pp., 1997. Tel&d, W.M., Geldart L.P. and Sheriff, R.E., Applied Geophysics, Cambridge University Press, Cambridge, 770 pp., 1990.
Pergamon
All rights reserved 1464-1917/01/$ - see front matter PII: S1464-1917(01)00070-8
Electrical and Electromagnetic
Investigation for Landslide Characterisation
A. Godio and G. Bottlno Dip. Georisorse Received
e Territorio - Politecnico
di Torino - C,so Duca degli Abruzzi, 24 - 10129 Torino, Italy
19 July 2000; accepted I February 2001
Abstract.
cases the information on the depth and the lateral continuity of the sliding surfaces can not be obtained through boreholes or geological investigations. Geophysical investigation can however complete the data set of technical information on the parameters of the subsoil for a complete understanding of the physical behaviour of a slope. Seismic investigations have been widely used to characterise the mechanical properties of rock-slopes; seismic refraction and high resolution seismic reflection are particularly useful to detect discontinuities in the subsoil where large contrast of seismic velocity or acoustic impedance have occurred between near surface materials and bedrock. However these methods can be very expensive (seismic reflection) or unreliable when sub-surface materials are particularly heterogeneous. On the other hand, electrical and electromagnetic methods allow one to obtain high resolution in 2D imaging of the slope; the method can usually image the distribution of the electrical conductivity, but there is not any reliable correlation between the electromagnetic properties and the mechanical behaviour of the rock mass. However, in such a context, the experimentation of high resolution methods, such as electrical tomography or ultra fast time domain electromagnetic methods (TDEM) should be verified in different geological and hydrogeological situations. The aim of the work is to present the results of applications of electrical resistivity tomography and time domain electromagnetic methods as powerful tools for slope site investigation. Particularly, the reliability and the performances of ultra-fast time domain methods for the study of the landslide behaviour will be enhanced and the advantages and shortcomings of these rather new methods will be discussed. In the selected example (Paroldo- Langhe Italy), the geophysical investigation was supported by
A geophysical investigation was applied to an area, subjected to a large slow moving landslide in the region oi Langhe - Piemonte, Italy. The joint application of vertical electrical soundings (VES), resistivity tomography (ERT) and electrical electromagnetic soundings (TDEM) has led to the detection of the geo-morphological conditions of the slope up to a depth of 30-40 m. The geophysical survey has detected the existence and continuity of a potential sliding surface, which had previously pointed out by inclinometer measurements. The test has permitted the verification of the reliability and the resolution of electrical and electromagnetic data analysis for landslides, character&d by marls and clay with low resistivity values (below 100 Ohm-m). o 2001 Published by Elsevier Science Ltd. All rights reserved
1 Introduction
In recent years, the application of geophysics to the study of slopes and landslides has widely increased; in such a context, great attention has been dedicated to seismic, electrical and electromagnetic methods (Chiara et al., 1996, Jongmans, D. et al., 2000). During a wide experimentation that has involved different phenomenon in the Langhe region (Piemonte - Italy), the reliability of the geophysical investigations has been analysed with the aim of improving the geological knowledge of the area and to locate joints and sliding surfaces. The main problems in landslide characterisation are related to the necessity of knowledge on the subsurface geology and hydrogeology site conditions; in many
705
706
A. Godio and G. Bottino: Electrical and Electromagnetic
an accurate geological and hydrogeological characterisation of the site. The final calibration of the geophysical data has been carried out through the correlation of the geophysical results with geological data and borehole results.
2 Geology of the landslide The morphology of the hilly region shows that the whole area is, at present, subjected to a great morphological rejuvenation that has partially modified the old forms; this rapid evolution is due both to the effects produced by the deepening of the basic level of the plain hydrography and to the effects produced by the raising of the hilly areas. From a geomorphological point of view, a close link between this rapid and recent morphological evolution and the existence of failures of different types resulted. The lithology involved in the instability phenomena is made up, in the upper part, of terrigenous deposits character&d by rhythmic altemances of marls, silts and arenaceous-sandy layers of variable thickness and, in the lower part of the hillside, of stiff marl sequences (Fig. 1). The general dipping of the strata is about ten-twelve degrees up dip. These sediments are intensely fractured on the top with different families of discontinuities and faults with modest throws; a low circulation of water is associated to these fractures and this can also be seen by frequent oxidation coatings. The landslide cinematic model refers to a planar slide with an approximately 20 m deep sliding surface. The movements are slow but continuous with periodic increasing due to heavy rains.
3 Materials and methods Electrical and electromagnetic methods are based on the observation of the spatial change of the electromagnetic constitutive parameters (electrical resistivity or electrical permittivity) of the subsoil; AC low frequency methods or DC measurements are mainly affected by changes in the electrical conductivity. The conductivity takes place through the moisture-filled pore of the subsoil; therefore, the conductivity value of the subsoil depends on the porosity in terms of the shape size of the pores and the characteristic of the interconnecting pathways, the total amount of pores filled with fluids, the concentration and the mobility of the electrolytes in the moisture and the temperature. A review of the role that each parameter play on the behaviour of the electrical conductivity of soils and rocks is described by McNeil1 (1980). In measuring the electrical properties of
Investigation
for Landslide Characterisation
heterogeneous landslides, mainly character&d by unconsolidated sediments, the importance of the colloidal conductivity must not be neglected; the addition of clay materials affects the electrical conductivity as the ion exchange capacity of clays is very high. The presence of a small amount of clay particle in the soil moisture can therefore substantially increase the electrical conductivity. Starting fkom these consideration, a strong contrast of electrical properties can be expected in a slope and landslide characterised by near surface filled-moisture fractures; in rock and soil where clay or silty materials are prevalent, the presence of fine silicates dispersed in the fluid phase could enhance the contrast of the electrical properties, between the rock matrix and the moisture embedded in the fractures. Electrical and electromagnetic methods are therefore suitable to map hydrogeological site conditions with reference to pathways and the circulation of groundwater. The main goal of the electrical and electromagnetic geophysical survey in the selected site was to delineate the presence of near surface heterogeneous materials that can be distinguished from the bedrock; the bedrock is not involved by sliding movements and, therefore, more consolidated and less affected by fractures and groundwater circulation.
Fig. 1. Schematic cross-section of the slope and location of the electrical resistivity tomography (ERT) and TDEM measurements.
In order to achieve these results, several methodologies that are capable of offering high resolution in the investigation range up to SO meters from the surface were applied. The geophysical survey was performed using the following methodologies: l preliminary electro-stratigraphical characterisation of the subsoil by means of DC vertical electrical soundings (VES); . electrical resistivity tomography (ERT) for a detailed characterisation of the electrical behaviour of the slope to 20-30 meters in depth; l execution of electromagnetic soundings (TDEM) to obtain information on the hydrogeological and geological setting of the slope to 50 meters in depth;
A. Godio and G. Bottino: Electrical and Electromagnetic l
detailed electromagnetic survey by means of ground conductivity meter equipment for a mapping of the near-surface hydrogeological condition on the top of the slope. This last survey was confined to a small area and is not discussed in this paper.
Vertical Electrical Soundings were performed along the slope using the Schlumberger array with a maximum spread of 150 m&.-s and 10 measurements for each decade; the data processing involved the 1D inversion of each sounding and the 2D correlation of the results by interpolation of 1D inversion. The methodology is straightforward and literature provides both theory and application examples for site character&&ion (e.g. Telford et. al., 1990, Reynolds, 1997). This procedure permits one to obtain a preliminary evaluation of the electro-stratigraphic condition of the slope (figure 2). PAROLCD'_
Investigation
for Landslide Characterisation
707
medium and TDEM sounding methods can be found in Kaut?na.n and Keller ( 1983) and in Nabighian ( 199 1). The Time Domain Electromagnetic acquisition involved the execution of 15 vertical electrical sounding& acquired using an ultra fast time domain equipment (TEM-Fast Pro-System, NL) in the coincident loop mode (12.5 x 12.5 m and 25 x 25 m loops). The investigation depth and the spatial resolution are affected by the conductivity of the subsoil and by the dimension of the loop: as a preliminary evaluation, due to the condition of the site, the adopted loop configurations allowed the reconstruction of the electrical resistivity of the subsoil in ranges of between 5-10 meters and 50-60 meters. The results of a preliminary test, performed at the bottom of the landslide, are shown in figure 3.
MODEL v===--
TT
Fii. 2. Example of VES sounding apparent resistivity curve and model of electrostratigraphy (resistivity and thickness) of the subsoil.
r ‘“i----r
10 Time Apparent 10
The Electrical Resistivity Tomography (ERT) was carried out using both a dipole-dipole and Wenner configuration array with a 32 electrode device (Sting AGI); an electrode separation distance of 5 meters was used; the data processing involved the 2D inversion based on a smoothness-constrained least square method (Griffiths and Barker, 1993, Loke and Barker, 1996). In TDEM measurements, the current flowing in a loop situated on the ground is abruptly interrupted, inducing eddy currents in the subsoil which diffuse away from the transmitter loop in depth. The diffusion of the eddy current is controlled by the electrical conductivity of the subsoil. The diffusion of the eddy current is determined by measuring the transient decay of the secondary magnetic field induced by the currents. A receiver loop, in this case coincident with the transmitter loop, is used to detect the transient of the magnetic field. An accurate description of the basic principles of electromagnetic propagation in natural
-vFy-
100
[ nicroseconds ] Resistivity 20
[ Ohmm] 30
Fii. 3. Example of the transient of TDEM sounding (top) and transformation in apparent resistivity versus depth (bottom).
708
A. Godio and G. Bottino: Electrical and Electromagnetic
The application of “extra-small” antennas (less than lo15 meters) to detect layer stratification with high vertical resolution at small depths (less than 20-30 m) is possible only at low resistivity of rocks, of less than 20-30 Ohm-m. The most favourable range of specific resistivity for rocks lays within the limits of 10 Ohmam and 300 0hm.m. The data processing of the TDEM survey involved the smoothing of each voltage decay curve and data conversion into apparent resistivity versus depth. The transformation allows one to obtain apparent resistivity values with respect to the depth, enhancing the presence of low resistivity layers (Meju, 1998). The apparent resistivity pseudosection were calculated by bidimensional interpolation of the results of the TDEM soundings (Fig. 4).
for Landslide Characterisation
4 Discussion The ERT pointed out a great heterogeneity of the near surface material and the presence of clay lenses that can be associated to lateral and vertical limits of permeability that could affect the groundwater pathway and circulation of the slope (Fig.5). The TDEM pseudo-section increases the depth of investigation of the ERT survey and detects the presence of a conductive bedrock (marls). On the other hand, the shortcomings of the electromagnetic
._. __
--A,
0
Investigation
between the fixcturcd -----7
/
20
40
60
80
100
120-)
Distance [ m ] Fii. 4. Pseudosection (values in 0hm.m).
of apparent resistivity
of the TDEM soundings
Resislwdy h ohnm
Fig. 5. Tomographic electrical resistivity section and position borehole S8, close to the lower edge of the tomographic section.
Unil Eledmde
of
Spcug
= 5.0 m.
A. Godio and G. Bottino: Electrical and Electromagnetic
Investigation
for Landslide Characterisation
activated in 1993, involving the slope movements of the near surface materials, and a deeper sliding surface, slowly moving, located at 35 meters in depth.
5 Conclusions
Clay marls aod (grey colau r)
Yrodstooes
Grey marl
Grey
Fig. 6. Sbatigraghy
mad
of borehole S8, located at the base of the slope.
investigation are related to a lack in the lateral resolution, compared to electrical tomography. The borehole, performed at the base of the slope investigated by the ERT (Fig. 6), pointed out the presence of a near surface weathered clay and sandstone layers (up to 10 meters in depth) overlying alternances of clay and marl layers: these sequence justified the low resistivity values encountered in the lower edge of the ERT section (values below 30 Ohmem). The borehole data allow an accurate calibration of the resistivity section, permitting one to extrapolate the typologies of the materials in the slope subsoil. The slope is characterised by lateral alternances of low resistivity materials (weathered clay and sandstone) with higher resistivity zones, which probably correspond to arenaceous materials. The geophysical survey confirmed the data of the inclinometer measurements, that have pointed out a frost sliding surface at 20 meters in depth, which was
Electrical resistvity tomography, when combined with the ultra-fast time domain method, permits a fast mapping of the subsoil electromagnetic properties that can be related to the geology and hydrogeology of the site. This test, in a complex geological situation, such as a landslide in a gently slope, permits one to verify the reliability of these methodologies as an aid to study the geological and hydrogeological conditions of rock mass. The ERT, using the dipole-dipole array, permits one to obtain an high lateral resolution and to point out the presence of sub-vertical water and moisture-filled fractures in the near surface layers; the TDEM section allows one to study the subsoil up to 40-50 in depth with good vertical resolution (l-2 meters). In data processing of TDEM soundings in very complex situations, the best choice is to adopt simple pseudogradient transform of the signal into apparent resistivity versus depth. An improvement in the efficiency of the TDEM data processing will be achieved when more powerhd algorithms for twodimensional interpretation are available. From a geological point of view, the joint analysis of electrical and electromagnetic data confirms the great heterogeneity of rock materials in the first lo-15 meters, the ‘presence of very near surface of sliding phenomena and the existence of two main sliding surf&es in the first 30-40 meters in depth (Fig. 7). On the basis of these observations it can be presumed that the instability model in the region is very complex. The planar landslides are characterised by multiple sliding surfaces, involving overall thickness that is greater of those that is normally suspected, where the deeper instability phenomena gradually trigger movements in the shallow part of the slope.
709
A. Godio and G. Bottino: Electrical and Elect] -omagnetic Investigation
8”
----
sail slip
Fig. 7. Schematic geological model of the landslide, brought to light out by the geophysical and borehole investigations.
References Chiara P., Godio A., Sambuelli L., Combined electrical investigation on a landslide, Proc. 2nd Meeting Environmental and Engineering Geophysics, Nantes, 113-l 16,1996.
for Landslide Characterisation
Griffiths, D.H. and Barker, R.D, Two-dimensional resistivit y imaging and modelling in areas of complex geology, Journal of Applied Geophysics, 29,21 l-226, 1993. Loke, M.H. and Barker, R.D., Rapid least-square inversion of apparent resistivity pseudosections by a quasi-Newton method, Geophysical Prospecting, 44, 131-152, 1996. Jongmans, D., Hemroulle, P., Demanet, D., Renardy, F., Vanbrabant, Y., Application of 2D electrical and seismic tomography techniques for investigating landslides, European Journal of Env. and Eng. Geophysics, 5, n.l,75-89,200O. Kaufman, A. and Keller, Ci., Frequency and transient soundings, Elsevier Science Publishers, Amsterdam, 685 pp., 1983. McNeil1 J.D., Electrical conductivity of soils and rocks, Tecnichal Note W-5, Geonics Ltd., Ontario, Canada, 22 pp., 1980. Meju, M. A., A simple method of transient electromagnetic data analysis, Geophysics, 63,2,405-410, 1998. Nabighian, M.N. and Macnae, J.C., Time domain electromagnetic prospecting methods. In: Nabighian, MN. (ed.), Electromagnetic Methods in Applied Geophysics, Vol 2A. Tulsa: Society of Exploration Geophysicists, 427-520, 1991. Reynolds, J. M, An introduction to applied and environmental geophysics, Wiley, Chicester, England, 796 pp., 1997. Tel&d, W.M., Geldart L.P. and Sheriff, R.E., Applied Geophysics, Cambridge University Press, Cambridge, 770 pp., 1990.