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Refraction seismics to investigate a creeping hillslope in the Austrian Alps M. Rumpf , U. Böniger, J. Tronicke Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24, 14476 Potsdam, Germany abstract article info Article history: Received 7 February 2012 Received in revised form 9 August 2012 Accepted 13 September 2012 Available online 21 September 2012 Keywords: Hillslope Refraction seismics Austrian Alps Layer-based inversion Assessing the human and economic threat introduced by sliding or creeping masses is of major importance in landslide hazard assessment and mitigation. Especially, in the densely populated alpine region unstable hillslopes represent a major hazard to men and infrastructure. Detailed knowledge, especially, of the dominant site-specic controlling factors such as subsurface architecture and geology is thereby key in assessing slope vulnerability. In order to quantify the geological variations at a creeping hillslope in the Austrian Alps, we have collected six 2D refraction seismic proles. We propose using a layer-based inversion strategy to reconstruct P-wave velocity models from rst arrival times. Considering the geological complexity at such sites, the selected inversion approach eases the interpretability of geological structures given intrinsic optimization for only a discrete, user-dened, number of layers. As the applied layer-based inversion approach ts our travel time data equally well as traditional smooth inversion approaches, it represents a feasible mean to summarize the structural complexity often present at such sites. Analysis of the inversion results illustrates that bedrock topog- raphy clearly deviates from a previously assumed planar surface and exhibits distinct variations across the slope extension. Bedrock topography additionally impacts the intermediate geological units and, thus, this information is critical for further analyses such as geomechanical modeling. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In many areas worldwide, unstable mountain and hill slopes threaten human safety and infrastructure. In Europe, the densely populated alpine region is one of the areas where such slopes represent a serious hazard. The stability of these systems is usually controlled by multiple factors such as surface topography, geological settings, and hydrological condi- tions. Thus, it has been recognized that an interdisciplinary research approach is required to understand such complex and heterogeneous systems. McCann and Forster (1990) dened three areas which need to be examined, rst, the denition of the landslide shape with particular reference to shear surfaces and failure planes; secondly, the denition of the hydrogeological regime with regard to water input to the landslide and its distribution within the slip mass; and, nally, the detection of movement by or within the slip mass and the characterization of such movements. Furthermore, a geological model including bedrock topog- raphy is important for understanding and modeling such systems, as bedrock topography may a have critical inuence on the kinematics and behavior of landslides (e.g., Flageollet et al., 2000; Coe et al., 2009; Bièvre et al., 2011). Within the past decade, there has also been a growing interest to use modern geophysical approaches, which may provide useful information regarding subsurface structures and material properties (Hack, 2000; Jongmans and Garambois, 2007). Refraction seismics is a robust and one of the most popular geophysical methods to explore the shallow subsurface, especially, to map the geometry of geological interfaces (Butler, 2005). It has also been widely used to investigate unstable nat- ural slopes and landslides (e.g. McCann and Forster, 1990; Caris and Van Asch, 1991; Mauritsch et al., 2000; Jongmans and Garambois, 2007). In refraction seismics, rst arrivals of seismic events are used to recon- struct subsurface velocity models. To generate such a refraction seismic velocity model, different techniques for analyzing and interpreting refraction traveltimes have been proposed (Butler, 2005); including the plus-minus method (Hagedoorn, 1959), the general reciprocal method method (GRM; Palmer, 1981), the generalized linear inversion method (GLI; e.g. Hampson and Russell, 1984), and tomographic methods (e.g. Iyer and Hirahara, 1993). Regardless of the applied analysis method, the resolution of refraction seismic velocity models decreases with increasing depth, while depth of penetration is roughly 1/5 to 1/3 of the geophone spread (Jongmans and Garambois, 2007). The success of the method depends on the site specic settings and conditions inuencing, for example, data quality and frequency content. Typically the nal velocity models are interpreted in terms of subsur- faces structures. For a detailed geologic interpretation, basic background information as provided by nearby boreholes are important to calibrate the model at selected points. In this study, we present the results of refraction seismic surveying across the Heumöser, a creeping slope in the Austrian Alps, which was carried out to derive high resolution 2D and pseudo 3D seismic velocity subsurface models in order to better understand and mechanically model the behavior of this slope. In the following, we introduce our Engineering Geology 151 (2012) 3746 Corresponding author. Tel.: +49 331 977 5795; fax: +49 331 977 5700. E-mail address: [email protected] (M. Rumpf). 0013-7952/$ see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2012.09.008 Contents lists available at SciVerse ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo

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Page 1: Refraction seismics to investigate a creeping hillslope in the Austrian Alps

Engineering Geology 151 (2012) 37–46

Contents lists available at SciVerse ScienceDirect

Engineering Geology

j ourna l homepage: www.e lsev ie r .com/ locate /enggeo

Refraction seismics to investigate a creeping hillslope in the Austrian Alps

M. Rumpf ⁎, U. Böniger, J. TronickeInstitut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24, 14476 Potsdam, Germany

⁎ Corresponding author. Tel.: +49 331 977 5795; faxE-mail address: [email protected] (M. Rum

0013-7952/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.enggeo.2012.09.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 February 2012Received in revised form 9 August 2012Accepted 13 September 2012Available online 21 September 2012

Keywords:HillslopeRefraction seismicsAustrian AlpsLayer-based inversion

Assessing the human and economic threat introduced by sliding or creeping masses is of major importancein landslide hazard assessment and mitigation. Especially, in the densely populated alpine region unstablehillslopes represent a major hazard to men and infrastructure. Detailed knowledge, especially, of the dominantsite-specific controlling factors such as subsurface architecture and geology is thereby key in assessing slopevulnerability. In order to quantify the geological variations at a creeping hillslope in the Austrian Alps, we havecollected six 2D refraction seismic profiles. We propose using a layer-based inversion strategy to reconstructP-wave velocity models from first arrival times. Considering the geological complexity at such sites, the selectedinversion approach eases the interpretability of geological structures given intrinsic optimization for only adiscrete, user-defined, number of layers. As the applied layer-based inversion approach fits our travel timedata equally well as traditional smooth inversion approaches, it represents a feasible mean to summarize thestructural complexity often present at such sites. Analysis of the inversion results illustrates that bedrock topog-raphy clearly deviates from a previously assumed planar surface and exhibits distinct variations across the slopeextension. Bedrock topography additionally impacts the intermediate geological units and, thus, this informationis critical for further analyses such as geomechanical modeling.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Inmanyareasworldwide, unstablemountain andhill slopes threatenhuman safety and infrastructure. In Europe, the densely populated alpineregion is one of the areas where such slopes represent a serious hazard.The stability of these systems is usually controlled by multiple factorssuch as surface topography, geological settings, and hydrological condi-tions. Thus, it has been recognized that an interdisciplinary researchapproach is required to understand such complex and heterogeneoussystems. McCann and Forster (1990) defined three areas which needto be examined,first, the definition of the landslide shapewith particularreference to shear surfaces and failure planes; secondly, the definition ofthe hydrogeological regime with regard to water input to the landslideand its distribution within the slip mass; and, finally, the detection ofmovement by or within the slip mass and the characterization of suchmovements. Furthermore, a geological model including bedrock topog-raphy is important for understanding and modeling such systems, asbedrock topography may a have critical influence on the kinematicsand behavior of landslides (e.g., Flageollet et al., 2000; Coe et al., 2009;Bièvre et al., 2011).

Within the past decade, there has also been a growing interest to usemodern geophysical approaches,whichmay provide useful informationregarding subsurface structures and material properties (Hack, 2000;Jongmans and Garambois, 2007). Refraction seismics is a robust and

: +49 331 977 5700.pf).

rights reserved.

one of the most popular geophysical methods to explore the shallowsubsurface, especially, to map the geometry of geological interfaces(Butler, 2005). It has also been widely used to investigate unstable nat-ural slopes and landslides (e.g.McCann and Forster, 1990; Caris andVanAsch, 1991; Mauritsch et al., 2000; Jongmans and Garambois, 2007). Inrefraction seismics, first arrivals of seismic events are used to recon-struct subsurface velocity models. To generate such a refraction seismicvelocity model, different techniques for analyzing and interpretingrefraction traveltimes have been proposed (Butler, 2005); includingthe plus-minus method (Hagedoorn, 1959), the general reciprocalmethod method (GRM; Palmer, 1981), the generalized linear inversionmethod (GLI; e.g. Hampson and Russell, 1984), and tomographicmethods (e.g. Iyer and Hirahara, 1993). Regardless of the appliedanalysis method, the resolution of refraction seismic velocity modelsdecreases with increasing depth, while depth of penetration is roughly1/5 to 1/3 of the geophone spread (Jongmans and Garambois, 2007).The success of the method depends on the site specific settings andconditions influencing, for example, data quality and frequency content.Typically the final velocity models are interpreted in terms of subsur-faces structures. For a detailed geologic interpretation, basic backgroundinformation as provided by nearby boreholes are important to calibratethe model at selected points.

In this study, we present the results of refraction seismic surveyingacross the Heumöser, a creeping slope in the Austrian Alps, which wascarried out to derive high resolution 2D and pseudo 3D seismic velocitysubsurface models in order to better understand and mechanicallymodel the behavior of this slope. In the following, we introduce our

Page 2: Refraction seismics to investigate a creeping hillslope in the Austrian Alps

38 M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

study site in more detail. After presenting the methodological details ofdata acquisition, analysis and inversion, we present and discuss theobtained 2D P-wave velocity models which are finally calibrated andinterpreted in terms of major geological formations using data fromtwo available boreholes.

2. The study site

Our study site is located in the AustrianAlps near the village of Ebnit,approximately 10 km southeast of the city of Dornbirn (Figure 1). TheHeumöser has an extent of 1800 m in east–west and 500 m in north–south direction. As illustrated in Figure 1 elevation ranges from 900 to1360 m above sea level across this site. The slope is partly forested,used as meadow in summer and for skiing in winter. In the 1970s, aholiday village was built in the central northern part of the slope.Today, most of the buildings show creeping related damage, e.g., cracksof up to several centimetres in diameter are observed.

The regional geology is dominated by the Vorarlberg Helveticum,which is represented mainly by the Säntis nappe around our studysite. The Säntis nappe, which extends from the cretaceous Valanginianformation to the cretaceous Wang formation, originated in the shelfenvironment of the Cretaceous Pennin Ocean (Oberhauser, 1980) andcomprises theAmden andWang formation in the study area. Transgres-sion and regression cycles led to distinct interbedded strata withinthese formations (Lindenmaier, 2007). The Amden formation consistsof lithologically hardly distinguishable members (Fessler et al., 1992),which are mainly clayey, silty, and sandy detritic marls and carbonates(Schneider, 1999). The Wang formation is made up of sandy andschistose marls as well as massif limestones and is more consolidatedthan the underlying Amden formation (Schneider, 1999). Additionally,small remnants of the cretaceous to paleogene Leimern formationbelonging to the Liebenstein nappe are present in the area. The Leimernformation consists of marlstones (Lindenmaier, 2007).

During the north–south compression caused by the alpine orogeny,the rocks were deformed resulting in various anticline and synclinestructures with east–west orientated fold axes (Oberhauser, 1980).The limestones show larger and stronger folding compared to themarlstones (Schneider, 1999; Lindenmaier, 2007). The area was finally

I

II

IV

III

HH 4 KB 3

200 400 600Meters

9° 44’ 30’’ N

Fig. 1. Map of the study area including locations of boreholes KB3 and HH4, the location of sixwhich was used for measuring movements of the surface by Depenthal and Schmitt (2003). Pomediate (black, displacements of ca. 5.5–11 cm/year), slow (blue, displacements of ca. 1.5 cm/portional to the displacement. The inlet shows the location of our study site within Austria.

shaped by glaciation events during the quarternary (Würm glaciation;Smit Sibinga-Lokker, 1965). Subglacial till was accumulated and erodingglaciers caused steep slopes (Schneider, 1999). The subglacial till consistsof silty-clayey sometimes sandy material as well as rock fragments(Lindenmaier and Zehe, 2005). After the retreat of the glaciers, theflanks became instable, collapsed, and delivered debris to the Heumöser,which was deposited on top of the subglacial till (Schneider, 1999;Lindenmaier, 2007). These debris flows and rock falls varied accordingto changing climatic conditions andweathering processes, hence leadingtoheterogenous to heterogenous scree (Lindenmaier and Zehe, 2005),which consists mainly of the mentioned cretaceous marls.

Within the past years, various investigations were carried out to un-derstand the structure and processes of the Heumöser (e.g. Schneider,1999; van den Ham and Czurda, 2002; Depenthal and Schmitt, 2003;Lindenmaier, 2007; Wienhöfer et al., 2009; Walter et al., 2011). Thesestudies have shown that theHeumöser exhibits the basic characteristicsof a slow creeping hill slope. GPS based geodetic surveying suggests asubdivision of the Heumöser into three parts: an upper western, acentral, and a lower eastern part (Depenthal and Schmitt, 2003). Thewestern part (Figure 1, area with black GPS points and displacementvectors) of the slope exhibits movement rates of ~5.5–11 cm per yearand is characterized by topographic slopes of up to 19% (Schneider,1999) whereas the central part (Figure 1, area with blueish GPS pointsand displacement vectors) shows movement rates of only ~1.5 cmper year (Depenthal and Schmitt, 2003) and a mean topographicslope around5%. The eastern part (Figure 1, areawith orangeGPSpointsand displacement vectors) shows the largest movement rates withvalues of up to 23 cm per year for individual measurement points(Depenthal and Schmitt, 2003) and is characterized by a mean topo-graphic slope around 12%. In addition to the geodetic surveying, twoboreholes (KB3 and HH4) have been drilled and equipped with incli-nometer devices (Schneider, 1999;Wienhöfer et al., 2009). Piezometerswere installed in order to measure the variations of the groundwatertable in the boreholes. These measurements indicate a varying ground-water table with a mean depth of about 3 m (Lindenmaier, 2007).Furthermore the measurements show that the corresponding rise ofthe groundwater table due to rainfall events is fast and takes in averageabout 14.5 hours (Lindenmaier, 2007). Fig. 2 compares results from

VVI

GPS points with displacementHoliday village

Forest

Creek

RoadBoreholesSeismic profiles

Heumöser boundary

47° 20’ 30’’ E

9° 45’ 00’’ N

47° 20’ 45’’ E

refraction seismic profiles (I–VI) analyzed in this study and the geodetic survey networkints with arrows indicate the initial survey position and their displacement vectors, inter-year) and fast (orange, displacements of up to 23 cm/year). Lengths of the arrows are pro-

Page 3: Refraction seismics to investigate a creeping hillslope in the Austrian Alps

39M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

drilling (lithological log) with inclinometer measurements shown ascumulative deformation in time (22 months for KB3 and 14 monthsfor HH4). In both boreholes, the bedrock (Amden Marlstone) is foundat a depth of ~20 m. It is overlain by glacial and postglacial sedimentsin KB3whereas in HH4 only postglacial sediments have been identified.The inclinometer measurements show a distinct deformation zone,which suggests that the main movement is related to a slip surface at~7 m (borehole KB3) to ~12 m (borehole HH4) depth (Schneider,1999; Wienhöfer et al., 2011). Regarding borehole KB3 the slip surfacecould be related to a lithological change between the clayey gravellytop layer and the gravelly and clayey second layer, which shows a softerconsistency than the first layer (Figure 2). For HH4 we do not observean obvious lithological change at the depth of the slip surface, onlyminor consistency changes (Figure 2). Thus, considering the availabledata, the slip surface seems to be not related to a significant lithologicalboundary. The deformation rates measured by the inclinometers are ingood accordance with the displacement rates determined through geo-detic surveying (Wienhöfer et al., 2009). Furthermore, the inclinometermeasurements indicate a seasonal variability of the mass movementwith higher rates in spring and summer (Schneider, 1999; Wienhöferet al., 2009). However, inclinometer measurements are only availablefor two points in the upper western part of the Heumöser and, thus,the geometry of the slip surface across the site (especially, in the centraland lower eastern part) is largely unknown. Furthermore, the twoboreholes are insufficient to establish a model of subsurface structuresand geology as needed, for example, for 2D and 3D geomechanicalmodeling.

3. Methodology

3.1. Data acquisition

Refraction seismic data were recorded along six profiles across theHeumöser (Figure 1). Themain goal of this studywas to image thedom-inant subsurface structures and the shallow geology of the Heumöser,which are believed to be critical to understand the processes related

soft firm stiff hard

Consistency

bedrock

0 2 4 6 8 10

deformation [mm]

0

clay, gravellyvery soft - stiff

core loss

depth [m]

KB3

glacial sedimentsstiff - firm

glacial sediments/Amden marlstone

gravel, clayeywet, soft - stiff

10

12

6

14

16

18

20

22

24

2

4

8

very soft

Fig. 2. Borehole and inclinometer observations for boreholes KB3 (left) andHH4 (right) (modifias Amden marlstone is found in a depth of ~20 m. It is overlain by subglacial sediments and ppostglacial sediments show different compositions, dominated by clay and gravel. The consisteindicate that the slip surface is located at a depth of about 7.5 m for borehole KB3. For borehodetermined for a period of 22 month (KB3; Schneider, 1999) and 14 month (HH4; Wienhöfer

to the observed mass movement. Furthermore, improved structuralgeological models are of major importance toward realistic mechanicalmodeling used to quantify shear zone generation (Ehlers et al., 2010).Preliminary seismic data acquired by Walter et al. (2011) indicatedthe general applicability of the refraction seismic technique at theHeumöser but also indicated that a high-resolution data acquisitionstrategy might be needed to reliably image subsurface architecture. Touse small geophone spacing on the entire profiles, the data needed tobe acquired in a roll-along strategy. In 2009, three profiles (I, IV, VI)were recorded using a Geode system (Geometrics Inc.) with 168 activechannels, 14 Hz vertical geophones, and a 5 kg sledgehammer source.Receiver spacing was 1 m, source spacing 1 m, and 2 m, respectively.In 2010, three additional profiles (II, III, V) were measured usingthe same Geode system with 240 active channels, 10 Hz vertical geo-phones, the same sledgehammer source, and a receiver spacing of 1 mand source spacing of 2 m. The coordinates of all source and receiverlocations were measured using a total station; i.e., the accuracy of thepositioning data is in the cm-range. For all profiles, the samplingintervalwas 0.5 ms and the record lengthwas>1 s becausewewantedto properly record the entire seismic wavefield, including reflectionsand Rayleigh waves (Table 1). In this study, we focus on the first arrivaltimes, which are inverted to generate P-wave velocity models of theshallow subsurface.

At least two shots have been recorded at each source position fromwhich the one with the highest signal-to-noise ratio has been selectedfor travel time picking. A total of 2217 shots were conducted resultingin 421128 recorded traces. For the entire data set, data quality showssignificant variations as demonstrated in Fig. 3 by two exemplaryraw shot gathers from profile I. This observation can be explained byvariable source-coupling conditions along all profiles (top layer rangesfrom hard rock to loose and/or water saturated soils). Therefore, onlyon ~56% of all recorded traces reliable picking of first arrival timeswas possible; with maximum pick coverage for profile I (~64%) andminimumcoverage for profile IV (~45%) (see Table 1). Travel time pick-ingwas performedmanually on the raw data using the picking routinesin GeoTomo's TomoPlus Software package.

10

12

14

16

18

20

22

24

26

28

0

2

4

6

8

clay,slightly gravellyvery soft - soft

clay, gravellyfirm - stiff

clay, gravel stiff

Amden marlstone

HH4

0 40 80

deformation [mm]

depth [m]

ed after Schneider, 1999;Wienhöfer et al., 2011). In both boreholes, the bedrock identifiedostglacial sediments in KB3 whereas only postglacial sediments are detected in HH4. Thency shows that there are minor differences within the layers. Inclinometer measurementsle HH4 the slip surface was detected at a depth of about 11 m. The deformation rates areet al., 2011).

Page 4: Refraction seismics to investigate a creeping hillslope in the Austrian Alps

Table 1Acquisition parameters (t — recording time, Δt — sampling rate), number of picks,number of traces, inversion parameters used within the GLI2D inversion algorithm,root mean square (RMS) errors of the initial models and the resulting RMS errors forprofiles (P) I–VI. Lay: number of layers, DS: depth smoother, VS: velocity smoother.

P t [s] Δt[ms]

Picks Traces Lay DS[m]

VS[m]

RMSinitial[ms]

RMSfinal[ms]

I 1.3 0.5 49,878 97,680 4 25 200 6.32 2.52II 1.3 0.5 118,643 186,480 4 22 200 4.19 2.69III 1.3 0.5 18,016 31,680 4 22 200 4.18 2.91IV 1.05 0.5 24,472 53,928 4 25 200 4.58 3.54V 1.3 0.5 18,214 32,880 4 22 200 4.37 3.71VI 1.3 0.5 8987 18,480 4 22 200 2.74 2.28

40 M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

3.2. Traveltime inversion

Various techniques are commonly used for the interpretation andinversion of traveltime datasets from refraction seismic experiments.

Considering the complexity, strong topographic variations andsteady subsurface deformation, we were expecting difficult to interpretinversion results, when using a tomographic inversion approach. Giventhe fact that distinct refractors can be observed (see Figure 4, no purevelocity gradientmedia), we decided to ease interpretability bymakinguse of a layer-based inversion strategy, provided its misfit equivalencycompared to the tomographic approach. In Fig. 5 we compare the re-sults of a layer-based inversion with a standard tomographic inversionfor Profile IV. For the layer-based inversion a RMS error of 3.54 mswas achieved and 3.34 ms for the tomographic inversion approach.The layer-based inversion approachwe use is known as generalized lin-ear inversion (GLI) method and was originally proposed by Hampsonand Russell (1984).

Here, we use the implementation GLI2D as available in GeoTomo'ssoftware package TomoPlus. This approach uses a layer-based modelparametrization. Starting with a user-defined initial velocity model,

050

100

150

200

20 40 60 80 100 120 140 1600Trace

Tim

e [m

s]

(a) (

0 100 200 300 400900

950

1000

Ele

vatio

n [m

]

Profile m

(c)(a)

(b)

Fig. 3. Data quality. (a) Shot gather with high signal-to-noise ratio. (b) Shot gather with lowexemplary raw shot gathers. For better visualization of the seismic raw data an AGC with a

theoretical traveltimes are calculated for each seismic source-receiverpair and the algorithm inverts for layer velocities and thicknesses. Thenumber of layers is predefined while the velocity within an individuallayer can vary laterally. The interfaces between individual layersare parameterized using spline functions allowing smoothly varyinginterfaces. A layer may have zero thickness, which accommodates thepossibility that layers pinch out or vary in number along the profile. Ini-tial models can either consist of flat, constant-velocity layers or modelswith varying refractor topography and laterally variable velocities,when such a priori information is available. To create initial modelswith varying layer velocities and surface-topography-independent undu-lating refractors, we have used an approach originally described byHampson-Russell Software Services Ltd (2004). In this approach, veloci-ties and intercept times are obtained from traveltime curves using apredefined number of neighboring shots at arbitrary chosen shotlocations (in our case 10 shots) along the profile (Figure 4) and usedto calculate 1D velocity-depth models using the following equation(Hampson-Russell Software Services Ltd, 2004):

dj ¼T0j

2vjvjþ1

v2jþ1−v2j� �1=2

264

375−

Xj−1

i¼1

divj v2jþ1−v2i� �1=2

vi v2jþ1−v2j� �1=2; ð1Þ

where Tj0 denotes the intercept time of layer j and v(j) same style as in

equation the velocity of the corresponding layer. From these 1D modelsa pseudo-2D model is calculated using cubic spline interpolation, which,then, can be used as initialmodel for the inversion.

The inverse problem of minimizing the residuals between observedand calculated first arrival times is formulated by the following objectfunction J (Hampson-Russell Software Services Ltd, 2004):

J ¼ ∑k

�Pk−Mk E; zi; við Þ

�2; ð2Þ

050

100

150

200

20 40 60 80 100 120 140 1600Trace

Tim

e [m

s]

b)

500 600 700 800 900

eter [m]

signal-to-noise ratio. (c) Topography along profile I illustrating the locations of the twowindow length of 100 ms was used. Spacing between individual traces is 1 m.

Page 5: Refraction seismics to investigate a creeping hillslope in the Austrian Alps

0 20 40 60 80 100 1200

0.01

0.02

0.03

0.04

0.05

0.06

0.07

v = 276 m/s

v = 1102 m/s

v = 2151 m/s

v = 3587 m/s

Offset [m]0 20 40 60 80 100

0

0.01

0.02

0.03

0.04

0.05

0.06

v = 316 m/s

v = 1399 m/s

v = 3612 m/s

Offset [m]

Tim

e [s

]

Tim

e [s

]

0 100 200 300 400 500 600 700 800 900900

950

1000

Profile meter [m]

Ele

vatio

n [z

]

(a)

(c)

(b)

(a)

(b)

Number of counts per bin

0 2010

Fig. 4. Data interpretation for two shot locations along profile I. (a) Count matrix over 10 neighboring shot locations and observed apparent velocities and intercept times derivedfrom the upper part of the slope. (b) Count matrix over 10 neighboring shot locations and observed apparent velocities and intercept times derived from the central part of theslope. (c) Topography along profile I illustrating the locations of the two examples. This example illustrates a four layer case at location (a) and a three layer case at (b). Thewhite lines show the mean traveltime calculated for the 10 neighboring shots. Red lines indicate our interpretation of the traveltimes. Apparent velocities and intercept timeswere determined using these plots at arbitrary shot locations and 1D velocity-depth models were created fromwhich we have generated the initial models for inversion. For furtherinformation see the text.

41M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

where Pk are the picked traveltimes for the kth trace, Mk the modelingfunction for calculating first break times for trace k, E the surface eleva-tion, zi the depth of the ith interface, and vi the velocity of the ith layer.As Eq. (2) is non-linear regarding the unknown parameters zi and vi, it issolved by linearization in the vicinity of the initial guess. Therefore,updating zi and vi is performed stepwise using Gauss–Seidel andconjugate-gradient algorithms.

In addition to the number of layers, the user also has to specifyvelocity and depth smoothers. Velocity and depth smoothers definethe size of a cubic spline patch system used to set up the depth andvelocity models (Hampson-Russell Software Services Ltd, 2004) andare used to stabilize the inversion by constraining velocity variabilitywithin the layers and interface roughness, respectively. Thus, carefulparameter tests are needed to obtain optimum results. Typically, thevelocity smoother is chosen to be longer than the depth smootherenforcing models with moderate lateral velocity variations. We testedthe influence of different depth and velocity smoother values onproblem-oriented synthetic data (i.e., same dimensions, same sourceand receiver spacings, same subsurface variations) as well as on thefield data to find a good compromise. Although the layer-based modelparametrization reduces the number of model parameters comparedto cell-based approaches typically used in refraction tomography(Ivanov et al., 2005), non-uniqueness issues have to be considered.Many inverse geophysical problems are non-unique; i.e., a number ofdifferent models explain the data equally well. In addition, as we use alinearized inversion scheme, we know that the final model maycritically depend on the starting model; i.e., the inversion can betrapped in a local minimum. Careful setup of the initial model (alsoconsidering available background information) and repeating theinversion using different reasonable starting models is a common wayto address such problems (Jongmans and Garambois, 2007).

Using the above outlined inversion procedure, we have inverted ourtraveltime data to generate 2D layered P-wave velocity models alongprofiles I-VI (Figure 1). Various systematic parameter tests using fieldand synthetic data examples have been performed to select an appro-priate number of layers and reasonable velocity and depth smoothers.Furthermore, the consistency of the models at the crossing points ofthe profile has been considered to select these parameters.

We found that generallyfive iterations are sufficient to reach a stableinversion result. The inversion parameters, root mean square (RMS)errors of the initial model and the resulting RMS errors are shown inTable 1 for each profile. All profiles were inverted using pseudo 2Dinitial models consisting of four layers generated using the aboveoutlined procedure. Fig. 4 shows data examples for two shot locationsusing the procedure presented above to demonstrate how we derivedthe number of layers in our datasets. For the upper part of the slopefour layers can be identified from the data (Figure 4a). In the centralpart of the slope only three layers are visible as can be seen in Fig. 4b.Furthermore, we used a depth smoother between 22 m and 25 m,and a velocity smoother of 200 m for the inversion. The final RMS errorslie between ~2 and ~3 ms formost of the profiles (I, II, III, VI). Profiles IVand V show increased RMS errors, which can be related to poorer dataquality caused by worse acquisition conditions (bad coupling) com-pared to the other profiles. To better assess the quality of the RMS weincluded some statistical measures in Table 2.

4. Results

In the following, we present and discuss the results for all profilesand provide a geological interpretation of the detected layers, also con-sidering the available background information from boreholes KB3 andHH4. These boreholes are located close to profile I and profiles I and III,

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Table 2Statistics of inversion results to asses the quality of the final RMS errors. We calculatedthe mean and the standard deviation (Std) of the differences between observed andmodeled travel times. These values show that the models fit our travel time data well.

Profile I II III IV V VI

Mean [ms] 1.02 1.73 1.44 1.92 2.28 0.57Std [ms] 1.02 1.64 1.25 2.03 4.01 0.55

S N

Ele

vatio

n [m

]

Profile meter [m]

250200150100500

880

900

920

940

(a)

S N

Ele

vatio

n [m

]

Profile meter [m]

250200150100500

880

900

920

940

(b)

0

1000

2000

3000

V [m/s]

Fig. 5. Comparison of layer-based (a) and tomographic (b) inversion approach on the example of profile IV. As root mean square errors are in the same range (3.54 ms of the layer-basedand 3.34 ms for the tomographic inversion approach), we decided to use a layer-based inversion approach to invert our traveltime data due to the ease of interpretation.

42 M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

respectively (Figure 1). Thefinal P-wave velocitymodels obtained usingthe discussed layer-based inversion strategy are presented in Figs. 6 and7.

4.1. Slope parallel profiles

Figure 6 shows the three slope-parallel profiles I, II, and VI. Invertedvelocities range from a around 300 m/s in the uppermost layer up tomaximum values of ~3500 m/s for the deepest layer within the threeprofiles. In the uphillparts of the slope, namely in the upper part ofprofile I (profile meter 0–600) and in the entire profile II, two layerswith intermediate velocities (~1200 m/s–1500 m/s and ~2000 m/s)are needed to fit the traveltime data, whereas in the downhill partsof the slope, namely in the lower part of profile I (profile meter600–940) and in profile VI a single intercalated layer with velocitiesbetween ~1500 m/s and ~1700 m/s is sufficient. Structurally, the fourlayer (western slope) and the three layer (central slope) part showdistinct differences. Profile I shows a bedrock bulge between 550and 600 m dividing these parts. This bulge is also visible on profile IIbetween profile meter 375 and 410, but with a minor extent comparedto profile I. The depth to bedrock (velocities>~3000 m/s) in the west-ern part of the slope is shallower than in the central part of the slope(~20 mcompared to ~50 m). The identified bulgemay act as amechan-ical barrier, attenuating the sliding material, and hence reducing themovement rates in the eastern part of the Heumöser as observed byDepenthal and Schmitt (2003) in their GPS studies.

Considering the borehole core interpretation of Schneider (1999)and Lindenmaier (2007), we performed a geological interpretationof the imaged layers. In all three profiles the uppermost layer withthicknesses of 1 m to 2 m is interpreted as weathered topsoil. Thesecond layer represents postglacial sediments, e.g., debris and scree.The third layer of the uphill part of the slope can be subdivided intotwo different geological sediment types. For the upper part (profile

meter 0 to 350 in profile I, and profile meter 0 to 300 in profile II)the layer is interpreted as postglacial sediments, e.g., debris andscree. These postglacial sediments are stiffer and show a higherdegree of compaction than the postglacial sediments of the secondlayer. Therefore, the seismic velocities are higher. Between profilemeter 350 to 550 in profile I, and profile meter >350 in profile II,the third layer is regarded as subglacial sediments accumulatedduring the Würm glaciation event. In layer three the location of thecontact between the postglacial and the subglacial sediments isunclear and queried, but must be located somewhere between bore-holes KB3, where the subglacial sediments have been identified, andHH4, where they are missing. This inferred change in sediment typeat profile meter 330 to 400 for profile I, and profile meter 220 and275 for profile II, is also supported by a slight change in velocitieshere (about 100 m/s). The intermediate layer in the central part(profile meter 600 and longer in profile I) and eastern part of theslope (profile VI) is interpreted as subglacial sediments accordingto Lindenmaier (2007). The velocity differences between subglacialsediments of the upper and central parts of the Heumöser mightoriginate from the blocking effect the bulge exerts on the sedimentsuphill resulting in possibly more compacted sediments in the upperpart. The bedrock layer is interpreted as Amden marlstone.

Comparison of the obtained slope parallel models with boreholesis only possible for profile I. This profile is in good accordance with

Page 7: Refraction seismics to investigate a creeping hillslope in the Austrian Alps

20 40 100 120 140 160 200

890

870

850

Ele

vatio

n [m

]

Profile meter [m]

NW SE(c)

? ? ? ?

1000

940

880

Ele

vatio

n [m

]

100 150 200

80

250 300

600 180

0 50 350 400 450

W E

Profile meter [m]

(b)

0 100 200 300 400 500 600 700 800 900850

900

950

1000

Profile meter [m]

Ele

vatio

n [m

]W E

? ?

(a)

KB 3

2525

Dep

th [m

]

Dep

th [m

]

III, HH4

IV

V

III

IV

Weathering layer

Postglacial sediments

Subglacial sediments

Marlstone

Contact - inferred, queried?

0 1000 2000 3000

V [m/s]

15

515

5

Fig. 6. P-wave velocitymodels for profiles (a) I, (b) II and (c) VI overlain by overlain by geological interpretation derived fromborehole data and geologicalmapping by Schneider (1999);Lindenmaier (2007). The bedrock (AmdenMarlstone) is overlain by subglacial sediments as well as postglacial sediments and a thin weathering layer (2–3 m). The locations of crossingprofiles and boreholes are indicated by red labels. In (a) the geological profiles derived from boreholes HH4 and KB3 are projected on the P-wave velocity model and a zoom is shown inorder to compare the extracted 1D velocitymodel at the projected borehole locationswith the geological profiles. The black line in the zoom shows the inclinometer data. For deformationdetails see Fig. 2. The distances between the boreholes and the profiles are ca. 3.5 m for borehole HH4 and ca. 30 m for borehole KB3. The bedrock shows velocities greater than 3000 m/sand is colored in the reddish color. Note, that spatial scales and vertical exaggeration are different for all profiles.

43M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

the borehole observationsmade at borehole HH4. (Figure 6a). BoreholeHH4 indicates a change between very soft clayey, slightly gravellysediments to firm and stiff clayey, gravelly sediments between 6 and7 m depth (Figure 2). At the location of HH4 within the velocitymodel, the transition between layer 2 and 3 is detected in at depth ofabout 6 m, suggesting that the change in material correlates with achange in velocity. The bedrock was detected at a depth of ~22 m inthe borehole, which is in good agreement with the seismic velocitymodel (Figure 6a). Furthermore, the inclinometermeasurements indicate

the slip surface to be located at a depth of around 11 m. Our inversionresults do not show a velocity change in this depth.

Borehole observations from borehole KB3 and profile I are less inaccordance with the borehole logs (Figure 6a). This is due to the factthat borehole KB3 is located about 30 m away from profile I. At KB3the slip surface can be found at around 7 m depth. We do observe avelocity change at that depth, but due to the distance betweenborehole KB3 and profile I, this observation has to be regardedwith care.

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(c) S N

Ele

vatio

n [m

]

0

900

10050 150 250200

840

860

880

I

Profile meter [m]

(b) S N

Ele

vatio

n [m

]

Profile meter [m]

250200150100500

880

900

920

940

(a) N

Profile meter [m]

Ele

vatio

n [m

]

S

0 50 100 150 250

860

880

900

920

200

IIIHH4

Marlstone

Subglacial sediments

Postglacial sediments

Weathering layer

0 1000 2000 3000V [m/s]

5

15

25

Dep

th [m

]

Fig. 7. P-wave velocitymodels for profiles (a) III, (b) IV and (c) V overlain by geological interpretation derived from boreholemeasurements and geologicalmapping by Schneider (1999);Lindenmaier (2007). The bedrock (AmdenMarlstone) is overlain by subglacial sediments as well as postglacial sediments and a thin weathering layer (2–3 m). The locations of crossingprofiles and boreholes are indicated by red labels. In (a) the geological profile derived from boreholes HH4 is projected on the P-wave velocity model and a zoom is shown in order tocompare the extracted 1D velocity model at the projected borehole location with the geological profiles. The black line in the zoom shows the inclinometer data. For deformation detailssee Fig. 2. The distance between the boreholes and the profile is ca 0.5 m. Note, that spatial scales and vertical exaggeration are different for all profiles.

44 M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

4.2. Slope perpendicular profiles

The three slope perpendicular profiles III, IV, and V are shown inFig. 7. Depending on their location three or four layers are sufficient toexplain the traveltime data. Velocities for all profiles range betweenaround 300 m/s for the uppermost layer and up to 4200 m/s for thedeepest layer interpreted as bedrock. Profiles III and IV show two inter-mediate layers whereas profile V shows only one intermediate layer.This observation is consistent with the layer distribution in the slopeparallel profiles. In the western part of the slope (profile III and IV)velocities of the intermediate layers vary between ~1300 m/s and~1400 m/s for layer two and between ~1600 m/s and ~2000 m/s for

the layer three. Velocities of the single intermediate layer in the centralpart of the slope (profile V) range from ~1500 m/s up to ~1800 m/s.Structurally regarded all three profiles show a bedrock depressionwith varying extent, which increases downslope. In profile III thedepression extents from profile meter 50 to 180, reaching a maximumdepth of 23 m. The bedrock depression increases downslope reachingabout 40 m in profile IV (profile meter 90 to 160) and up to 50 m inprofile V (profilemeter 50 to 150). The increasing bedrock depth resultsin a thicker sedimentary cover. The depression can be interpreted as astructural feature canalizing the material on its way downhill.

The geological interpretation of the velocitymodels was again basedon the results obtained from Schneider (1999) and Lindenmaier (2007)

Page 9: Refraction seismics to investigate a creeping hillslope in the Austrian Alps

45M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

and basically corresponds to the interpretation of profiles I, II and VI.The uppermost layer represents weathered topsoil. In profiles III andIV the second layer is interpreted as postglacial sediments. Layer threeof profile III and IV is interpreted as postglacial and subglacial sedi-ments, respectively. In the central part of the slope (profile V), the singleintermediate layer represents subglacial sediments. Again there is onlyone profile, which can be compared to borehole geological interpreta-tions. Profile III lies in the vicinity of borehole HH4 and the detectedlayer boundaries are in good accordance with the borehole observa-tions. The bedrock shows a depth of about 20 m in the seismic velocitymodel and about 22 m in the borehole data. The boundary betweenlayer 2 and 3 is in a depth of ~7 m and can be interpreted as the changefrom very soft clayey, slightly gravelly sediments to firm and stiff clayeygravelly sediments as depicted in the borehole logs (see also Figure 2).Furthermore, there is no correlation between the inclinometer mea-surements, which indicate the slip surface at a depth of 11 m, and theobtained seismic velocity model of profile III. No velocity change at adepth of 11 m can be detected, which is in good agreement with theobservations made when comparing borehole HH4 and the slopeparallel profile I (see also Figure 2).

The consistency at the crossing of different profiles is fair. At thecrossing points of different profiles the misfit of refractor depths is inthe order of a meter. The largest difference is found at the crossing ofprofiles II and IV. Here, the discrepancy of the bedrock depth reaches3 m for a depth of around 30 m. As the resolution of refraction seismicmeasurements decreases significantly with depth, the accordance atthe crossing points can be regarded as reasonable. Furthermore, the sixseismic profiles suggest that bedrock topography has an importantinfluence on the mass movement. We believe that bedrock topographyand its spatial variations are a key in understanding themassmovementat our study site. Through convergent interpolation (Haecker, 1992,implemented in Schlumberger's Petrel software) of the 2D P-wavevelocity models a surface-to-bedrock isopach was obtained (Figure 8).The bedrock in the upper part of the Heumöser shows depths ofb20 m and becomes significantly shallower toward the upper centralpart, followed by a basin-like steepening in the lower central part ofthe Heumöser with maximum depth of 60 m. Comparison of our inter-polated surface-to-bedrock isopach with the GPS derived displacement

BedrockDepth below surface [m]

15 45

Horizontal displacement vectorsAugust 1995 - November 2001

100 cm

9° 44’ 10’’ N 9° 44’ 30’’ N

Fig. 8. Shaded relief of the study site overlain by surface-to-bedrock isopach and GPS-pointHeumöser explains the attenuation of mass movement in that part compared to other parsubdivision into intermediate (black, displacements of ca. 5.5–11 cm/year), slow (blue, disp

vectors from (Depenthal and Schmitt, 2003) shows that the detectedbedrock bulge is in good accordance with the change of displacementrates in the central part of the Heumöser, highlighted by a significantdecrease in displacements for GPS points close to the bedrock bulge(Figure 8). This bedrock bulge seems to act as amechanical barrier atten-uating the mass movement, explaining the decrease in displacementrates in the vicinity of this bulge. Furthermore, it can be shown thatareas of considerable mass movement are characterized by steep slopes(upper part of the Heumöser) and smooth bedrock topography (lowerpart of the Heumöser).

5. Conclusions

By using refraction seismic surveys along six crossing profileswe areable to build first structural subsurface models of the Heumöser. Threeto four layers are needed to fit the refraction seismic traveltimes inorder to obtain geologically reasonable models based on the givenbackground information. The obtained velocity models clearly outlinethe major relevant subsurface structures (e.g., depth to bedrock),which are believed to be critical in order to understand themass move-ment at this site. The models in east–west direction (slope-parallel)show a geological reasonable explanation for the observed differencebetween the movement rates of the upper part and the central part ofthe slope. The detected bedrock bulge might attenuate the slidingmasses resulting in the lower movement rates in the central part ofthe Heumöser compared to the upper western part. However, compar-ing the inclinometer data with the seismic velocity models, the slipsurface could not be detected using refraction seismic data, whichimplies no or only small variations of petrophysical parameters acrossthe shear surface and vertical seismic velocity variations being mainlycontrolled by lithological changes. Compared to standard tomographicinversion approaches, the used GLI approach provides well-definedlayered velocity models, which can be more easily interpreted thansmoothly varying tomographic velocity models. In this study, velocitymodels from standard tomographic inversions and the GLI approach re-sult in comparable root-mean-square (rms) errors. This illustrates thatrather simple three-to-four-layer models are sufficient to fit the mea-sured traveltime data in geologically reasonable fashion. As many

N

E

S

W

9° 45’ 00’’ N

47° 20’ 56’’ E47° 20’ 42’’ E

locations and their displacement vectors. The bedrock bulge in the central part of thets. The different colors of the GPS-points and their displacements vectors indicate thelacements of ca. 1.5 cm/year) and fast (orange, displacements of up to 23 cm/year).

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46 M. Rumpf et al. / Engineering Geology 151 (2012) 37–46

near-surface environments are characterized bywell-defined layers,webelieve that layer-based inversion strategies such as the GLI approachare more than an alternative in many typical near-surface seismic re-fraction applications.

Acknowledgment

This work is part of the research unit 581 "Coupling of flow anddeformation processes for modeling the movement of natural slopes"which is supported by the Deutsche Forschungsgemeinschaft (DFG).We thank GeoTomo for supplying the TomoPlus software package andSchlumberger for the access to their E&P software platform Petrel (TMof Schlumberger). We also would like to thank Marcel Delock, MarkoDubnitzki, Jorge Iturralde Jorge E. Iturralde, and Steffen Linder fortheir help during data acquisition. The Helmholtz Research Centre forGeosciences Potsdam (GFZ) and the Helmholtz Centre for Environmen-tal Research Leipzig (UFZ) are thanked for supplying instruments. Wefinally thank Roberto de Franco and an anonymous reviewer for manyuseful comments which helped to improve the quality of this paper.

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