GEOPHYSICAL SURVEYS AT MACHU PICCHU, PERU:

RESULTS FOR LANDSLIDE HAZARD INVESTIGATIONS

Melvyn Best, Bemex Consulting International, Victoria, B.C., Canada Peter Bobrowsky, Geological Survey of Canada, Ottawa, Ontario, Canada Marten Douma, Geological Survey of Canada, Ottawa, Ontario, Canada

Victor Carlotto, Instituto Geologico Minero y Metalurgico de Peru, Lima, Peru Walter Pari, Instituto Geologico Minero y Metalurgico de Peru, Lima, Peru

Abstract

Geophysical methods are being used more frequently to assess slopes for landslide hazard potential, especially in areas where traditional methods such as trenching and drilling are either difficult to employ or not allowed. This paper presents the results of joint DC resistivity and EM surveys to map fractures and zones of weakness in crystalline bedrock at Machu Picchu, Peru. DC resistivity surveys were carried out along the upper 8 switchbacks leading to the sanctuary as well as across the sanctuary. EM-34 surveys were carried out along the upper 3 switchbacks and across the sanctuary. Inversion of the resistivity data located several lower resistivity zones along the switchbacks. These zones were associated with water seeping out of the rock in ditches. The water is confined to the upper switchbacks which is consistent with the disappearance of lower resistivity zones in the lower switchbacks. EM-34 results along the switchbacks, although more subtle to recognize, located several coincident zones of lower resistivity. The DC resistivity data across the sanctuary located a lower resistivity zone on the east side of the main plaza. There is presently no information on whether any of these fractures have been active in the recent past. Consequently the results from this study are still under investigation.

Introduction

The UNESCO World Heritage Site of Machu Picchu, Peru, the royal estate of the Inca ruler Pachacuti in the 1400’s, remained covered by vegetation and abandoned in the jungle for hundred of years following the Spanish occupation of Peru (Wright and Zegarra, 2000). Discovered early in the last century, the site is now host to some 1 million tourists per year. Recent shallow translational landslides, rock falls and debris torrents in the area have drawn international attention to the region surrounding the site and the adjacent town of Aguas Calientes (Klimes et al. 2007). The impact of the failures has ranged from significant including the loss of life (11 individuals in 2004) to economic concerns involving closure of the only road access between the site and the outside world (Hiram Bingham Road in 1996). Figure 1 Map showing location of Machu Picchu, Peru.

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The additional potential threat for large-scale landslide events affecting the archaeological site itself prompted the Instituto Geologico Minero y Metalurgico de Peru (INGEMMET) to initiate a series of multi-national and multi-disciplinary studies to assess the slope instability conditions of the site. This included geomorphological studies (Vilimek et al. 2005), surface monitoring (Canuti et al. 2005), geotechnical evaluation (Sassa et al. 2005) as well as an evaluation of the sub-surface conditions in the area. This paper discusses the efforts and results of this latter geophysical work. INGEMMET in cooperation with the Geological Survey of Canada (GSC) and under the auspices of the Canadian International Development Agency (CIDA) began a multi-year multi-parameter shallow geophysical assessment program at Machu Picchu in 2003. Geophysical and other remote sensing techniques are an essential component of this integrated study because standard trenching and drilling techniques are not permitted at the site. The data collected will be used to enhance other data collected by complementary studies of the area as conducted by collaborative international scientific and engineering teams.

Figure 2 Road from Aguas Calientes leading to the sanctuary of Machu Picchu. The objectives of this study are to determine geological parameters such as bedrock faults and fractures, overburden lithology and thickness, etc. that are important for understanding potential landslide hazards in the area. The aim of this paper is, therefore, to present the results of the geophysical surveys in terms of these geological parameters. We focus on 3 of the surveys carried out where there were sufficient data to address the objectives - the switchbacks, the sanctuary line and the main plaza.

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Geological and geographical setting

The Machu Picchu world heritage site is located approximately 100 kilometers north of Cusco, Peru (Figure 1). The site is perched high in the Andes at an elevation approximately 2500 m above sea level. The archaeological site straddles a saddle ridge stretching between the peaks of Machu Picchu to the south and Huayna Picchu to the north. Access to the site from the town of Aguas Calientes located in the valley some 500 m below Machu Picchu is along a steep road consisting of multiple switchbacks (Figure 2). The slopes are covered with thin (1-2 m thick) overburden consisting mainly of rubble. The area has poorly developed soils, moist tropical conditions, and localized high annual precipitation. The high relief topography (Figures 3 and 4) in the area is underlain by part of the Vilcabamba Batholith, a white grey colored granitic complex dated by Rb/Sr to be about ±246 Ma (Carlotto et al. 1999). The granitic complex is cut by several large faults and is characterized by an extensive jointing pattern. Landslides (Figure 3) and debris flows (Figure 5) are common occurrences throughout this region. Both faults and joints are thought to be primary contributors to rock slope instability in the area.

Figure 3 Rock slides are common in the area. Figure 4 Debris slides occur on the steep This picture was taken from the west side of slopes near the village of Aguas Calientes the sanctuary of Machu Picchu looking south. in the valley below Machu Picchu.

Geophysical surveys The specific objectives of the geophysical study were to determine thickness and type of overburden as well as to map fractures, faults, structure, slip planes and lithology within the bedrock. Additional archaeological objectives were to locate buried construction items, particularly large foundation blocks, voids and old walls within the sanctuary. In order to achieve these objectives several geophysical methods were employed at the site. Geophysical methods have been used to map landslide-prone areas for many years. Recent examples include papers by Wang and Lu (2002), Lapenna et al. (2005), Pant et al. (1999), Sendlhofer et al. (1990), Hyde et al. (1997), and Godio and Bottino (2000). Most of these studies however use a single geophysical method to map the geological features. One of the best examples employing multi-parameter geophysical methods to study a single landslide is a paper by Bichler et al. (2004). These authors showed that an integrated approach using ground penetrating radar (GPR), DC resistivity, seismic reflection and seismic refraction surveys to map subsurface geology of the Quesnel Forks

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landslide in central British Columbia, Canada provided significantly more information than any one of the systems used on their own.. The present study employed electromagnetic (EM), DC resistivity and ground penetrating radar (GPR) geophysical methods, although not all 3 methods were used at each location on the site. Additional geological results, slope stability implications and archaeological applications of this project appear elsewhere (Mamani et al. 2005, Carlotto et al., 2007)

Acquisition Geonics EM-31 and EM-34 systems (Geonics, 1991, McNeill 1980) operating in the horizontal coplanar (vertical dipole) mode were used to collect the EM data. EM-31 in-phase (ppt) and conductivity (mS/m) data were collected at a spacing of 3 to 5 m. EM-34 conductivity data (mS/m) were collected using 10 m and 20 m transmitter-receiver separations. The EM-34 station spacing was 10 m for the 10 m separation and 20 m for the 20 m separation. The plotting point for the EM-31 and EM-34 data was the mid-point between the transmitter and receiver.

Figure 5 Debris from a slide that occurred during heavy rains in 2004. The slide occurred on the left slope just past the houses in Figure 4. A 48 electrode Iris Syscal DC resistivity system with a 5 m electrode separation was used to collect the DC resistivity data (Iris, 2006). Apparent resistivity data were collected using a Wenner array with “a” values between 5 m and 75 m. Lines longer than the array length of 235 m (5 m x 47 electrode spacings) were acquired by continuously moving the back 24 electrodes to the front until the entire line was surveyed. The data were then concatenated to produce an apparent resistivity profile along the entire line. A Sensors and Software PulseEkko 100 system using a 100 MHz antenna and a pulser voltage of 400 V was used to collect the GPR data (Sensors and Software, 1999). The GPR system was operated with the antennae perpendicular to the line direction. No common midpoint (CMP) velocity analysis was carried out so a radar velocity was set at 0.1 metre/nanosecond. The transmitter-receiver separation was fixed at 1 m and the two antennae were moved together in steps of 25 cm. The measurement point is located at the midpoint between the transmitter and receiver antennae. Figure 6 is an air photo showing the location of the geophysical surveys and Table 1 lists the surveys carried out at these locations.

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Figure 6 Air photo showing location of the geophysical surveys. Processing The EM-31 and EM-34 data required little processing. The EM-31 data were plotted as conductivity (mS./m) and in-phase (ppt) profiles and the EM-34 data were plotted as conductivity (mS/m) profiles. The EM-31 conductivity data for the temple grid and main plaza grid were plotted as images. The Syscal DC resistivity data were downloaded into Res2Dinv software for plotting and inversion (Loke and Barker, 1996). The Res2Dinv program generates a ‘best fit’ resistivity model of the earth from the apparent resistivity profiles. Res2Dinv plots have the apparent resistivity profile of the field data at the top, the computed apparent resistivity profile from the best fit model in the middle and the best fit resistivity model at the bottom. When topography is included in the model only the best fit resistivity model is plotted. The GPR data are stored on a PC during acquisition. These data files can be plotted as GPR sections which are plots of two way travel time (vertical axis) versus position of transmitter-receiver midpoint (horizontal axis). Depth (m) can be plotted on the vertical axis as well using the velocity of 0.1 m/ns. Processing of the GPR data was limited to trace averaging and filtering.

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Figure 7 Plot of resistivity data for line 1. Top profile is measured apparent resistivity, middle profile is best fit apparent resistivity, and bottom profile is best fit resistivity earth model.

Figure 8 Plot of resistivity data for line 4. The profiles are in the same order as those in Figure 7.

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Table 1 Location of geophysical surveys

Line EM-31 EM-34 Resistivity GPR 10 m 20 m Sanctuary/citadel x x x x Huayna Picchu x Inca Trail south x Inca Trail north x x Switchback lines (starting with 0 at the top) 0 x x 1 x x 2 x x 3 x x 4 x x 5 x x 6 x x 7 x x Main Plaza grid x x Temple grid x

Lower Plaza grid x

Results and Discussion In this paper we concentrate on the switchback lines, sanctuary line and main plaza area since they were surveyed in greatest detail. The final processed geophysical data discussed above provides the basis for the following discussion. Switchbacks The data collected along the switchbacks consisted of eight DC resistivity lines (lines 0 to 7) and EM-34 data (20 m transmitter-receiver separation) along lines 0, 1 and 2 (Figure 2). Resistivity data Figures 7 and 8 are example plots of the inverted resistivity data for lines 1 and 4 respectively. The upper plot is the measured apparent resistivity, the middle plot is the calculated apparent resistivity using the best-fit inversion model, and the bottom plot is the best-fit resistivity model obtained from the inversion. The resistivity colour bar is the same for both plots. There are only minor differences between the measured and calculated apparent resistivity sections indicating the error of fit is small (Table 2). Figure 9 is a composite plot of all 8 lines showing the best-fit resistivity models with topography effects included. There are no significant differences between these plots and the best-fit model plots (Figures 7 and 8 for example) since the topographical surface along each switchback, although steep and inclined, has no local relief and therefore is a simple plane surface.

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Figure 9 Plot of all 8 topographically corrected best fit resistivity profiles.

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The blue colours on the model plots represent lower resistivity values. The average or background resistivity associated with these cross sections is around 4000 ohm-m which is consistent with granites. The lower resistivity values are only “relatively” low since the values vary between 700 and 1000 ohm-m, or 7 to 4 times lower than the background value. These low resistivity zones outcrop at some locations and occur at depth at other locations along the sections. Figure 10 is a plot showing the location of these low resistivity zones. The solid black lines are areas where the low zones outcrop and the dashed black lines are where the low zones are at depth. The yellow lines show the trends of the low resistivity values. It is interesting to note that all these zones only occur along the upper switchbacks, except for the continuous trend on the north side (right hand side) of the survey area that seems to follow a ridge that starts near the hotel (north end of line) on line 0. The low resistivity zones are likely associated with water in minor fractures and/or shears within the granites. These fracture zones are not highly altered or they would have lower resistivity values. The water that percolates into these fractures at higher elevations apparently does not migrate all the way down the mountain which is one reason why the low resistivity zones are confined to the upper switchbacks. This is consistent with visual observations of where water was seeping out of the bedrock into the ditches along the road. The continuous trend on the north side may be associated with a larger fracture that allows water to further migrate. The ridge may structurally control this fracture.

Figure 10 Interpretation of resistivity lows overlain on photo of switchbacks.

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Table 2 Error of fit from the resistivity inversions

Location Error of fit (%) Comments Switchbacks gentle topography Line 0 4.0 Line 1 2.9 Line 2 2.9 Line 3 3.2 Line 4 2.3 Line 5 3.6 Line 6 3.3 Line 7 3.8 Sanctuary 20.0 more rugged topography EM-34 data EM-34 data were collected along lines 0, 1 and 2 using a transmitter-receiver separation of 20 m. The apparent conductivity plots for lines 0 to 2 are given in Figure 11. The two higher conductivity values on line 1 (near positions 125 and 250) seem to correlate with the lows on the corresponding resistivity line. The average background conductivity along these lines is around 0.75 to 1.0 mS/m or approximately 1400 to 1000 ohm-m which is lower than the resistivity computed from the resistivity inversions but still quite resistive. The increase in conductivity near the north end of line 0 is most likely associated with the fill used for the vehicle turn around area near the hotel. The negative value on this line is associated with metal from one of the nearby buses. Although there are subtle variations in conductivity along these lines they are difficult to correlate to the geology. Electromagnetic methods had limited success in mapping fractures or shear zones on the switchbacks. This is due to the high resistivity of the granite and the “relatively” high resistivity values associated with the fractures. Figure 11 Plot of EM-34 profiles for lines 0 and 2.

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Sanctuary line The data collected along the sanctuary line consisted of EM-31 conductivity and in-phase data, EM-34 conductivity data (10 m and 20 m separations) and DC resistivity data. Figure 12 Plot of resistivity data for sanctuary line. The profiles are in the same order as those in Figure 7. Resistivity data Figure 12 shows the results of the resistivity inversion along the sanctuary line. The 3 plots in the figure are in the same order as those discussed in the switchback section. Figure 13 is a plot of the best-fit resistivity inversion model with topographic effects included. The topography along this line is much more rugged than the switchbacks and is at least part of the reason why the error of fit is larger than that for the switchbacks (Table 2). The line starts at zero on the west side and rises up the west terraces, down the terraces on the west side of the main plaza, across the main plaza between 65 and 100 m, again goes up the terraces on the east side of the plaza into the building complex on the east side of the plaza between 120 and 160 m, and finally down the east terraces. There are low resistivity zones associated with the upper parts of the east and west agricultural terraces, most likely associated with thicker soils on the terraces. The resistivity values over the main plaza are high which is consistent with the rock debris found filling the zone above the bedrock (see GPR data later). There is a thin layer of soil over the plaza but the DC resistivity system with 5 m dipole (a) spacing averages the results over this zone and the underlying rock debris. There is a low resistivity zone that parallels the terraces on the east side of the plaza. This could be associated with a fracture zone similar to those inferred on the switchbacks or it may be associated with a different type of fill above bedrock on the east side of the plaza.

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Figure 13 Plot of topographically corrected best fit resistivity profile.

EM-31 data Figure 14(a) is a plot of in-phase and conductivity data for the EM-31 survey along the sanctuary line. The line starts at the west end and goes up the western terraces, over the top (stations 17 to 20), down the west terraces to the plaza, across the plaza (stations 40 to 59) and then up the eastern terraces into the building complex. It does not go down the eastern terraces. The in-phase data are not very useful for geological mapping. EM-31 in-phase data are more often used for locating metallic targets. Consequently these data are not discussed further. The conductivity values from the EM-31 vary between 18 and 19 mS/m (55 and 53 ohm-m). There is very little variation in the conductivity over the entire line. The values of conductivity indicate that the EM-31 system is averaging the upper meter of soil and the lower bedrock and/or rock debris. The subtle variations in conductivity on the terraces are associated with the location of the EM system relative to the outer and inner edges of each terrace. These

Figure 14 (a) Plot of EM-31 data for sanctuary line. (b) Plot of EM-34 data for sanctuary line.

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variations are so minor there is no significance attached to them. The EM-31 did not recognize changes in geology because it only samples the upper few meters of the ground. EM-34 data Figure 14(b) is a plot of the conductivity data for the EM-34 system (with a transmitter-receiver separation of 10 m) along the sanctuary line. The larger separation means the effects of the edges of the terraces are averaged. Moreover this system is capable of penetrating at least twice the depth of the EM-31 system. This is why the average conductivity is much lower (0.5 to 1.5 mS/m or 2000 to 670 ohm-m) since it is sampling more of the bedrock. The variations in conductivity may be related to changes in overburden thickness and/or lithology but they are hard to correlate. The negative value around station 4 is at the top of the western terraces. This is most likely metal associated with the large number of tourists that frequent that area. Main Plaza area We only discuss the GPR data in this section since the EM-31 data did not provide any more information than that discussed previously. The data collected here consisted of 6 GPR lines in the main plaza and 3 GPR lines in the lower plaza. The lower plaza survey was related to archaeological studies and will not be discussed here.

Figure 15 Location of GPR lines across plazas.

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GPR data Figure 15 shows the location of the GPR lines across the main and lower plazas. Figure 16 is a plot showing the GPR sections for the 5 lines across the main plaza. There are 3 GPR facies indicated on these plots. The first (red) corresponds to near surface soil and has a thickness between 0.5 and 1 m. Several weak reflectors can be seen that parallel the ground surface. The second facies lies beneath the soil and continues down to the bedrock. It consists mostly of broken reflectors, although there are several continuous reflectors within this facies. The disorganized nature of this facies is attributed to the rock debris and fill that was placed above bedrock during construction of the site. Trenching within the site confirmed the presence of such debris. The continuous reflectors are assumed to be locations where the workers placed clay and other soil to smooth out the debris-covered areas. The third facies represents the bedrock. There is considerable relief on the bedrock surface. There are broken reflectors within the bedrock and several steeply dipping events. These steep events are most likely associated with reflections from the lower walls of the terraces since they occur at the ends of the lines.

Conclusions. The granite bedrock underlying the archaeological site of Machu Picchu is very resistive with average values between 3000 and 4000 ohm-m and often with much higher resistivity values. Overburden, where it exists, is generally thin. The plazas have been filled with rock debris and covered with a layer of soil. Most likely the agricultural terraces on both east and west slopes of the sanctuary are similar in anture. EM Highly resistive bedrock with a thin overburden cover is a difficult environment for electromagnetic techniques. The EM-31 system provided limited information on overburden thickness and lithology and no information on bedrock fractures. Minor variations in the conductivity readings for the EM-31 do not appear to correlate with any geological features. The variations of the EM-34 data along the sanctuary line may be related to geology but such a relationship is difficult to establish. The subtle variations in conductivity values along the switchback lines correlate with the lows associated with the DC resistivity data. However, these subtle features may have been overlooked if the DC resistivity data were not available for comparison. DC resistivity The DC resistivity system provided useful information on potential locations of fractures/shear zones within the crystalline bedrock. Low resistivity zones associated with these fractures correlates line to line along the switchbacks. These low resistivity values are between 500 and 1000 ohm-m, indicating these zones are not very conductive. Significant clay alteration on the walls of the fractures is not likely since the resistivity values would be lower in that case. The low resistivity zones are most likely associated with relatively fresh water within the fractures. This is consistent with the fact that these zones are only observed on the upper switchbacks where visible water can be observed in the ditches.

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Figure 16 GPR sections for the main plaza. The coloured units are described in the text. The resistivity line over the sanctuary has low resistivity values on the upper terraces on both the east and west sides. The resistivity values over the main plaza are consistent with a thin layer of soil over rock debris situated above the bedrock. The low resistivity zone on the terraces on the east side of the main plaza may be associated with vertical to near-vertical fractures similar to those seen along the switchbacks or to different fill material within these terraces.

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GPR The GPR data collected in the main and south plazas provided detailed information of the material beneath the surface to depths of 5 m or more. There is a thin veneer of soil (0.5 to 1 m thick) at the surface of these two plazas. Rock debris and rubble lie beneath the soil cover. The rubble and debris was used as fill to level out the bedrock topography in these areas. The fill can be as thick as 3.5 m in some sections of the main plaza. The more continuous reflectors seen within this debris facies are thought to be areas where clay/soil was placed to smooth out the debris, making it easier to move material within the plaza area. The reflectors within the bedrock are erratic and not very continuous but provide an indication that GPR may be able to locate fractures within the bedrock. Landslide potential The geophysical evidence collected at this site provides a fairly good indication of the nature of the subsurface materials including both bedrock and overburden. Although isolated vertical fracturing was observed in the bedrock, there is no indication of a deep incipient or existing failure plane cutting through the granite. Similarly, although the site has been subjected to several shallow, surface failures in the past several years, there was no evidence in the overburden to indicate that the site is subject to extensive surficial failure. No parallel or subparallel to slope features were observed in the geophysical data. It is our belief that although the site and region are subject to debris flows and slides, proper drainage and site management provides the best mitigative measures to reduce slope instability at this important site. Evidence for, and the likelihood of deep-seated catastrophic failure in the past or near future is lacking and such events are unlikely to occur. There is presently no information on whether any of the fractures located during this study have been active in the recent past. Future investigations within the sanctuary will have to rely on long term monitoring of these and other known faults and fractures because trenching and drilling are forbidden. Monitoring at the site will require the use of GPS, strain gauges and remote sensing methods.

References Bichler A., Bobrowsky P.T., Best M.E., Douma M., Hunter J., Calvert T., and Burns R., 2004, Three-

dimensional mapping of a landslide using a multi-geophysical approach: the Quesnel Forks landslide. Landslides 1, 29-40.

Brooks, G.R. and Pilon, J.A., 1995, Ground penetrating radar survey of the Katz Slide, southwestern

British Columbia; Cordillera and Pacific margin- Cordillere et marge du pacifique: Geological Survey of Canada, Report: 1995-A, 33-40.

Canuti, P., Margottini, C., Mucho, R., Casagli, N., Delmonaco, G., Ferretti, A., Pollino, G., Puglisi, C.

and Tarchi, D., 2005, Preliminary remarks on monitoring, geomorphological evolution and slope stability of Inca Citadle of Machu Picchu (C101-1): In Landslides: risk analysis and sustainable disaster management. Edited by K. Sassa, H. Fukuoka, F. Wang and G. Wang, Springer Verlag, pp. 39-47.

Carlotto V., Cardenas J., Romero D., Valdivia W., and Tintaya D., 1999, Geologia de los Cuadrangulos

de Quillabamba y Machupicchu: Boletin No. 127, Serie A: Carat Geologica Nacional, INGEMMET, Instituto Geologico Minero y Metalurgico.

363

Carlotto, V, Bobrowsky, P., Best, M., and Douma, M., 2007, An assessment of geophysical methods for landslide studies: Proc. of the international Symposium on Landslide Risk Analysis and Sustainable Disaster Management (IPL 2007), United Nations University, Tokyo, pp 39-44.

Geonics, 1991, EM-31 Operating Manual: Geonics Limited, 1745 Meyerside Drive, Mississauga,

Ontario, Canada. Godio, A. and Bottino, G., 2000, Electrical and electromagnetic investigation for landslide

characterization; monitoring, modelling and mapping of mass movements: Physics and Chemistry of the Earth, Part C: Solar-Terrestial and Planetary Science, 26(9), 705-710.

Hyde, C.S.B., Hunter, J.A., Lawrence, D.E. and Aylsworth, J.M., 1997, Correlating geophysical and

geotechnical parameters of landslide-prone Champlain Sea sediments, Ottawa valley: Program with Abstracts - Geological Association of Canada; Mineralogical Association of Canada; Canadian Geophysical Union, Joint Annual Meeting, 22, 71.

Iris, 2006, Iris systems website: www.iris.instruments.com. Klimes, J., Vilimek, V. and Vlcko, J., 2007, Debris flows in the vicinity of the Machu Picchu village,

Peru: In Progress in Landslide Science, edited by K. Sassa, H. Rukuoka, F. Wang and G. Wang, Springer Verlag, pp. 313-318.

Lapenna, V., Lorenzo, P., Perrone, A., Piscitelli, S., Rizzo, E. and Sdao, F., 2005, 2D electrical

resistivity imaging of some complex landslides in Lucanian Apennine Chain, southern Italy: Geophysics, 70(3), B11-B18.

Loke, M.H. and Barker, R.D., 1996, Rapid least-squares inversion of apparent resistivity pseudosections

by a quasi-Newton method: Geophysical Prospecting, 44, 131-152. Mamani, R. Mucho, Caillaux, V. Carlotto, Pinto, W. Pari, Oviedo, M. Jhonthan, Douma, M., Best, M.

and Bobrowsky, P., 2005, The application of ground penetrating radar (GRP) at Machu Picchu, Peru (C101-1): in Landslides: risk analysis and sustainable disaster management, edited by K. Sassa, H. Fukuoka, F. Wang and G. Wang, Springer Verlag, pp. 55-59.

McNeill, J.D., 1980. Electromagnetic terrain conductivity measurement at low induction numbers:

Technical Note TN-6, Geonics Limited, Ontario, Canada. Pant, S.R., Li, T., Wagner, A., Fu Weiyi and Jiaman, C., 1999, High resolution seismic refraction data

interpretation; an example from Xiakou landslide, Sichuan, China: Journal of Nepal Geological Society, 19, 31-40.

Sassa, K., Fukuoka, H., Wang, G., Wang, F., Benavente, E., Ugarte, D. and Astete, F.V., 2005,

Landslide investigation in Machu Picchu World Heritage, Cusco, Peru (C101-1): in Landslides: risk analysis and sustainable disaster management. Edited by K. Sassa, H. Fukuoka, F. Wang and G. Wang, Springer Verlag, pp. 25-38

364

Sendlhofer, G., Grummt, S. and Roth, T., 1999, Application of near-surface reflection seismic surveys in order to investigate landslides: BHM.Berg- und Huettenmaennische Monatshefte, 144(12), 476-482.

Sensors & Software, 1999, PulseEKKO 100 RUN, User’s Guide version 1.2, Manual 25: Sensors &

Software Inc., Mississauga, Ontario, 66 p. Vilimek, V., Zvelebil, J., Klimes, J., Vlcko, J. and Astete, F.V., 2005, Geomorphological investigations

at Machu Picchu, Peru (C101-1): in Landslides: risk analysis and sustainable disaster management. Edited by K. Sassa, H. Fukuoka, F. Wang and G. Wang, Springer Verlag, pp. 49-54.

Wang, J. and Lu, J., 2002, The use of ground-penetrating radar in environmental geologic hazard

surveys: Geology and Prospecting, 38(3), 70-73. Wright KR, and Valencia Zegarra, A., 2000, Machu Picchu: a civil engineering marvel: ASCE Press.

Acknowledgements This paper is a contribution to the Multinational Andean Project: Geoscience for Andean Communities. We appreciate the financial support of CIDA (Canadian International Development Agency), INGEMMET and the GSC towards the completion of this project.

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