High-Resolution Reflection Seismic Investigations of Quick

Journal of Applied Geophysics 92 (2013) 84–102

Contents lists available at SciVerse ScienceDirect

Journal of Applied Geophysics

journal homepage: www.elsevier.com/locate/jappgeo

High-resolution reﬂection seismic investigations of quick-clay and associated formations at a landslide scar in southwest Sweden

Alireza Malehmir a,⁎, Muhammad Umar Saleem a, Mehrdad Bastani b a Uppsala University, Department of Earth Sciences, Villavägen 16, SE75236, Uppsala, Sweden b Geological Survey of Sweden, Villavägen 18, SE75128, Uppsala, Sweden article info abstract

Article history: We present high-resolution reflection seismic data from four lines (total 1.9 km) that cross a quick-clay land- Received 12 October 2012 slide scar located close to the shore of the Göta River in southwest Sweden, and compare the results with geo- Accepted 11 February 2013 technical data from boreholes. The seismic data allow the imaging of bedrock topography and normally to Available online 1 March 2013 weakly consolidated sediments to a subsurface depth of about 100 m. Different types of seismic sources, including sledgehammer, accelerated weight-drop and dynamite were utilized and compared with each other. Keywords: Analysis of their power spectra suggests that weight-drop and dynamite have higher frequency content and Reflection seismic Landslide energy than the sledgehammer, which makes these two sources suitable also for waveform tomography and Quick-clay surface-wave data analysis. The shallowest non-bedrock reflector is observed at about 10–20 m below the Coarse-grained materials surface, it overlays the bedrock, and is interpreted to originate from the contact between clay formations Göta River above and a coarse-grained layer below. The coarse-grained layer appears to be spatially linked to the presence of quick-clays. It is a regional scale formation, laterally heterogeneous, which deepens to the west of the study area and correlates well with the available geotechnical data. Continuity of the coarse-grained layer becomes obscured by the landslide scar. There may be a link between the coarse-grained layer and landslides in the study area, although this possibility requires further hydrogeological and geotechnical investigations. Re- flectors from the top of the bedrock suggest a depression zone with its deepest point below the landslide scar and a bowl-shaped structure in the northern portion of one of the seismic lines. © 2013 Elsevier B.V. All rights reserved.

1. Introduction larger deposits are sensitive to greater stress changes, such as increased saturation by excess rainwater (e.g., Vanneste et al., 2012). Landslides are one of the most commonly occurring natural Considering the nature of quick-clay formations, application of disasters; the economic cost of the damage caused by them is estimated geoelectrical methods alone to delineate them is a challenge, al- to be in billions of dollars, and they claim hundreds of human lives every though they are often used. The presence of thick and conductive ma- year (Petley, 2011). Sweden is also affected by this natural hazard rine clay does not allow deep current penetration (Bogoslovsky and (Nadim et al., 2008; Fig. 1). This study is about one particular kind of Ogilvy, 1977). These thick layers of conductive clay, which can also rapid earth flow that is caused by quick-clays, which mainly exist in be fully water-saturated at depth, sometimes already as shallow as Sweden, Norway and Canada (Rankka et al., 2004; Rosenquist, 1953; 0.5 m, make ground penetrating radar (GPR) techniques inefficient Solberg, 2007). Undisturbed quick-clay resembles a water-saturated (Annan, 2005). However, each geophysical method has its own ad- gel that accumulated as flocculated silty-clay to clayey-silt sediment vantage and disadvantage for a given site location or problem and in a marine to brackish environment; uplifted above sea-level, and has for specific geologic target at different depths (Bichler et al., 2004; been leached to low salinity by fresh water flow. The quick-clay liq- Bogoslovsky and Ogilvy, 1977; Carpentier et al., 2012; Jongmans uefies at its natural water content; the flocculated structure collapses and Garambois, 2007). Success is not always guaranteed for a single if its strength is exceeded, which results in landslides of variable magni- geophysical method. Therefore, a combination of various geophysical tude (Bryn et al., 2005; Kvalstad et al., 2005; L'Heureux et al., 2010, methods and implementing those that can also provide deeper infor- 2012; Lundström et al., 2009; Rankka et al., 2004; Rosenquist, 1953; mation is needed (Bogoslovsky and Ogilvy, 1977; Jongmans and Solberg et al., 2012; Torrance, 1983). A small block of quick-clay can liq- Garambois, 2007; Solberg et al., 2012). This is particularly important uefy due to a stress change as little as the touch of a human hand, while if quick-clays extend to greater depths (e.g., >20 m), such as in this case study. According to our knowledge, there have been no previous applica- ⁎ Corresponding author. Tel.: +46 184712335; fax: +46 18501110. fl E-mail addresses: [email protected] (A. Malehmir), tions of re ection seismic methods for delineation of quick-clays and [email protected] (M.U. Saleem), [email protected] (M. Bastani). associated formations in Sweden. Refraction seismic methods have

High Intermediate Lake Low Vänen Pre-historic Småröd landslide scars (2006) in till Luleå Göta River Study Lilla Edet area (1957)

Härnösand

Surte (1950) Tuve 50 km (1977)

Study area Landslide scars Historical landslides Stockholm

Flow direction in Göta River Göteborg

Fig. 1. (right) Landslide risk map of Sweden (from the Geological Survey of Sweden), and (left) showing location of the study area and recent quick-clay landslides in southwest Sweden (e.g., Surte, 1950, Göta, 1957, Tuve, 1977, and Småröd, 2006). Our study area is about 7 km north of the city of Lilla Edet, where the Göta landslide occurred in 1957. been extensively used for landslide studies (Bekler et al., 2011; Carroll metres (smaller than one wavelength) of the near surface (b5 m), but et al., 1972; Jomard et al., 2007; Samyn et al., 2012), however, the use this gap can often be filled by geoelectrical and GPR methods (Bichler of reflection seismic methods has been limited to countries outside et al., 2004; Bogoslovsky and Ogilvy, 1977). Sweden (e.g., Bachrach and Nur, 1998; Baker et al., 1999a,b; Bichler et Our geophysical investigations of areas prone to quick-clay land- al., 2004; Büker et al., 1998; Eichkitz et al., 2009; Kaiser et al., 2009; slides began in September 2011 over a known landslide scar near the L'Heureux et al., 2010, 2012; Medioli et al., 2012; Polom et al., 2010; Göta River (Figs. 1 and 2a) in southwest Sweden. Göta River (Fig. 1)is Roberts et al., 1992; Schmelzbach et al., 2005). Reflection seismic the source of drinking water for about 700,000 people and is used ex- methods are more expensive than the traditional geophysical methods tensively for industrial transportation. Therefore, areas near the river for landslide studies, but if properly designed and implemented they are highly industrialized and populated. The geophysical investigations can provide high-resolution images of layer boundaries, provided that involved 2D and 3D P- and S-wave source and receiver seismic surveys, there is sufficient acoustic impedance contrast (product of seismic ve- geoelectrics, controlled-source and radio-magnetotellurics, Slingram, locity and density) between the layers. For example, seismic methods ground gravity and magnetic surveys as well as passive seismic moni- can be used to map bedrock geometry and overlying clays that can toring (Fig. 2a and b). Prior to our investigations, the Swedish Geotech- sometimes be critical for landslide risk assessments. Integration with nical Institute (SGI) studied the site using various geotechnical and other geophysical and geotechnical methods can also be important in hydrogeological methods (Löfroth et al., 2011). Therefore, a wealth of modelling and interpreting the data (Eichkitz et al., 2009). Reflection geotechnical borehole data, mainly CPT (cone penetration test with seismic methods experience problems when imaging the top few tip friction generated by the rod string; Robertson, 1990), CPTU (cone

Fig. 2. (a) Surface geological map and (b) Lidar (light detection and ranging) elevation map overlaid on an aerial photograph of the study area. Locations of the seismic lines (lines 1 to 5), available geotechnical boreholes (blue circles), and the landslide scar are also shown (see the labelled blue region in (a)). Lines S1a, S1b and S2 were surveyed using hori- zontal source and receivers. Line 6 was only surveyed using electromagnetic methods. Seismic data (vertical geophone) obtained along lines 2, 3, 4 and 5 are the focus of this paper. Note the retrogressive (fan shape) nature of the quick-clay landslide in the northern portion of line 5. A few small landslide scars (or likely erosion surfaces) are also evident in the southern margin of the Göta River and in the western part of the study area. Boreholes 7063 and 7065 were used for pore-pressure monitoring (Swedish Geotechnical In- stitute, 2011). White arrows show approximate locations of two cross sections shown later in the paper. CSTMT is location of an electromagnetic source used during the project. Geological map is kindly provided by the Geological Survey of Sweden. Lidar data and aerial photo are provided by Lantmäteriet. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web of this article.) 86

Northing (m) b Northing (m) a 6454000 6454400 6454800 6454000 6454400 6454800 380310 360320 332400 332000 331600 331200 330800 332400 332000 331600 331200 330800 380310 360320 332400 332000 331600 331200 330800 332400 332000 331600 331200 330800 Bedrock (granitetogranodiorite) Sandy-silty till Glacial clay

1 01 025 20 15 10 0 -10 Göta River Göta River Sea-level Göta River Göta River

.Mlhi ta./Junlo ple epyis9 21)84 (2013) 92 Geophysics Applied of Journal / al. et Malehmir A. Line 6 Line

7069 Göta River Göta River 7070

7071 Line 4 Line 7072 Line 1 (S1a,S1b) Post-glacial clay/gyttja Post-glacial sand Post-glacial silt 7064 7208 Line 3 7065 7063 7066 Easting (m) Easting (m) Easting (m) Easting (m) Line 1 7067 Lidar (m) Line 3

Göta River Göta River

7206 Line 4 Line Line 2 Line 2 7204 7062 Line 2S 7059 CSTMT 7203 7202 River sediment/silt Line 5 Stream/flow direction Scar/erosion surface Line 5 scar Landslide scar Landslide – 102 survey 3D

Sweden

6454000 6454400 6454800 6454000 6454400 6454800 Northing (m) Northing (m) A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 87

Göta River

Seismic line 4 Seismic crew

Seismic line 5

Fig. 3. A series of ﬁeld photos taken (September 2011) from the study area, showing (a) the landslide scar (also see Fig. 2) and location of the seismic lines 4 and 5, its retrogressive nature and (b) tilting trees suggesting a slow mass movement (soil creep) toward the river.

penetration test with friction sleeve measuring pore pressure data) and An overall assessment of different geophysical methods used in the laboratory measurements are available from the site. Among geophysical study area was recently reviewed by Malehmir et al. (in press).Here,we methods, only surface electrical resistivity tomography (ERT) and in- present results and details of high-resolution shallow reﬂection seismic duced polarization (IP) methods had been carried out by SGI (Löfroth et data acquisition, processing and interpretation along four seismic pro- al., 2011). ﬁles (lines 2, 3, 4 and 5), some of which cross the landslide scar (see

Fig. 4. Two cross sections from (a) central and (b) western parts of the study area showing CPT and CPTU measurements including cone resistance (shown in MPa) and sleeve friction (shown in kN) in the boreholes (Fig. 2b). Section (b) partly coincides with the location of seismic line 2. CPTU data (sleeve friction) clearly suggest presence of a coarse-grained layer in the central part of the study area. This layer is not clearly observed in the western parts of the study area but is believed to be intersected where the boreholes ended (dif- ﬁcult to drill into it). A thick layer of quick-clay above the coarse-grained layer is estimated to be present. The coarse-grained layer is one of the main targets of our seismic investigation. Boreholes U07063 and U07065 were used for monitoring pore-pressure at four depth levels (P1–P4). These sections are modiﬁed from Löfroth et al. (2011). 88 a +25 S N U07062 U07061 +20 +20 U07060

+15 +15

Cone resistance +10 +10 U07059 + 7.4 m Quick-clay Friction sleeve 2010-05-06 +5 +5 Quick-clay Quick-clay 0 Göta River 0 4 MPa -5 -5 4 MPa

Elevation (m) 4 kN 4 MPa 4 MPa -10

-10 Elevation (m)

Coarse-grained layer 4 kN 84 (2013) 92 Geophysics Applied of Journal / al. et Malehmir A. -15 -15 Coarse-grained layer Coarse-grained layer -20 -20

Coarse-grained layer -25 -25 4 kN

-30 -30 b SE NW+25 +25 U07067 7208 (CPTU-R) U07065-P2 U07066 U07065-P1 U07065-P4 U07065-P3 +20 U07065 +20 W-18.4 U07064 W-17.7 W-18.7

+15 Level: 3 m +15 U07063-P2 U07063-P4 W-11.2 U07063-P1 U07063-P3 +10 Level: 5 m +10 4 MPa W-8.2 U07063 W-7.7 W-7.7 + 7.2 m W-6.9 W-7.9 W-7.3 2010-05-11

Bedrock? – 2010-05-06 +5 +5 102 Level: 15 m Level: 3 m 4 MPa 0 Bedrock? Göta River 0 Level: 5 m

-5 -5 Level: 15 m Cone resistance Elevation (m) Elevation (m) -10 -10 Pore pressure monitoring hole -15 (Level: 30 m, measured 2010-05-18) -15 Friction sleeve 4 kN -20 4 MPa -20 4 MPa 4 kN 4 MPa -25 Bedrock? No bedrock -25 Pore pressure monitoring hole 4 kN -30 (Level: 30 m, measured 2010-05-18) No bedrock -30

-35 -35 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 89 lines 4 and 5 in Fig. 2). Only data along line 5 were presented by 3. Study area Malehmir et al. (in press). The main objectives of this study are (i) to evaluate the performance of high-resolution reflection seismic methods The study area has been recently studied by SGI in a nation-wide for quick-clay studies in Sweden, (ii) to image a coarse-grained layer project, which included investigations of areas prone to sliding that is overlaid by beds of quick-clay formations in the site, and (iii) to along the Göta River (Löfroth et al., 2011). The focus of our study is image the bedrock topography, and to obtain a better understanding a landslide scar about 300 m by 300 m in size located in the northeast of its relationships with the formations above it that might be important of the selected area (Fig. 2). According to the Quaternary geological to pre-condition landslides in the site. map of the study area (Fig. 2a), the southern margin of the river is dominantly composed of post-glacial tills and occasionally contains 2. Quick-clays pockets of organic silt and clay. In locations where the river mean- ders, small pockets of river sediments (mainly silt) are also observed. Quick-clays in Sweden and Norway were (and still forming) formed Highland areas (Fig. 2b) consist primarily of glacial clays. Bedrock, of as a result of processes that occurred since the last deglaciation crystalline nature (granite to granodiorite), is occasionally exposed (Bjerrum, 1954), which caused the study area to become ice-free c. within the glacial clays; it is often surrounded by small unit of 13 ka BP. As a result of isostatic depression, the deglaciation was accom- sandy to silty materials (Fig. 2a). This sudden exposure of the bedrock panied by eustatic sea-level rise and a regional marine transgression. within the clay materials is an indication of its undulating nature. Melting water from the glaciers brought silt and clay materials from Fig. 3a shows a series of field photos taken from the scar, which land into marine environment (Magnusson et al., 1963). Wasting of depict its retrogressive nature (fan shape). The landslide occurred at the Fennoscandian ice-sheet caused isostatic rebound and regional sea the southern bend of the Göta River where a small stream and river regression (e.g., Berglund, 1992 and references therein). The highest enter the main channel at about 100 m and 200 m east of the scar post-glacial coastline in the study area is approximately 150 m. Deeper (Fig. 2), respectively. Tilting trees close to the riverbank suggest a clay formations that were deposited before the marine regression took slow mass movement towards the river (Fig. 3b). The scar is located place have had their salts leached out by artesian groundwater flow at the southern bend of the river with possible large fluvial deposits, through fissures in the bedrock and permeable coarse-grained mate- which may be leading to less erosion. However, the small stream rials such as sandy to silty layers (Solberg, 2007; Solberg et al., 2012). and river may contribute to increased erosion at the southern bank When salt is leached from the clay formation, it becomes sensitive to ex- of the river, and also locally disturbing the direction of the river cess stress (such as load from constructions or a significant increase in flow. In following section, we describe available geotechnical, geo- the pore water pressure due to excess water from rain or melting physical and hydrogeological data from the study area. Malehmir et snow) and can result in a quick-clay landslide (Lundström et al., 2009; al. (in press) summarizes these investigations. We present them Vanneste et al., 2012). Quick-clay landslides are usually retrogressive again here for completeness. (and often fan shape) or flake-type (Gregersen, 1981), starting near a river or lake, and progress gradually upslope. They have been known to penetrate several kilometres inland, and consume everything in 4. Previous geotechnical and geophysical investigations their path. A well-known example of this retrogressive nature is the Rissa landslide in Norway, which lasted more than 45 min and 4.1. Site characteristics and geotechnical boreholes progressed at a rate of 10–20kmperhour(Gregersen, 1981; L'Heureux et al., 2010, 2012;seealsohttp://en.wikipedia.org/wiki/Rissa,_Norway). For quick-clay identifications, CPT, CPTU, CPTU-R (cone penetra- Quick-clay landslides are not particularly constrained to steep topo- tion test with resistivity measurements) and laboratory measure- graphical slopes and have been known to slide even in low-to-moderate ments had been done. Unfortunately, most of these measurements angle topographical slopes (e.g., 1:10 for the Rissa landslide). Some provide information at points only and thus no geological profile known examples of quick-clay landslides from Norway are Trögstad (cross section) is available from the site. These studies, especially (year 1967, 1 million-m3, 4 human casualties), Rissa (year 1978, 33 ha, CPTU and some laboratory measurements (at some selected depths) one casualty), and Verdal, (year 1893, 55 million-m3, 116 casualties), suggested the presence of coarse-grained materials at varying depth and from Sweden are the Surte (year 1950, 22 ha, 1 casualty; Caldenius (from the surface) but in average about 20–30 m below the current and Lundström, 1956), Göta (year 1957, 15 ha, 3 casualties), Tuve (year surface topography. Interestingly, in most places quick-clays were 1977, 27 ha, 9 casualties), and Småröd (year 2006, 10 ha, no casualty) found to overly coarse-grained materials. Due to the limitations of in Sweden (Fig. 1). The Göta landslide took place closest to our study CPT methods, only a very few boreholes could penetrate through area (Fig. 1), caused large infrastructure damage as well as a significant the coarse-grained layer. Geotechnical boreholes (CPT and CPTU) in- environmental problem including the flow damming of the river for directly suggest that the coarse-grained layer varies in thickness sometimes (Göransson et al., 2009). from 0.5 m to more than 10 m. Fig. 4 shows two cross sections (no Despite the frequency and impact of quick-clay landslides, our geo- geological observation is available from most of these boreholes) physical understanding of quick-clay behaviour in terms of changes in from previous geotechnical investigations (Fig. 2) carried out by SGI the geometrical shape and the physical (as well as geochemical) prop- in the study area. The results of CPT and CPTU measurements clearly erties is limited and requires a better understanding (Khaldoun et al., show the presence of coarse-grained materials intersected by the 2009; Solberg et al., 2012). Traditionally, quick-clay landslides have boreholes. Detailed description of the cross sections can be found in been studied using geomorphic observations with, when possible, the Löfroth et al. (2011) and we present only a short summary relevant aid of limited subsurface data obtained by boreholes or excavations to our study. (e.g., Bichler et al., 2004). Borehole data are expensive and labour inten- In the section just west of the seismic line 5 (Fig. 4a), the upper- sive. Additionally, a lateral interpolation between the boreholes is not most layer consists of 2 m dry surficial crust. The results from CPT accurate enough and may provide misleading results. The motivation and CPTU measurements in point U07060 (for simplicity shown as behind our study is this: Geophysical measurements can fill the large in- 7060 in Fig. 2a) show that a 24 m thick sequence of clay may exist un- formation gap between the boreholes; moreover, they can be used to derneath the dry crust. A thick layer of coarse-grained material re- access information at greater depth where expensive, and in some places a sandy layer found in the boreholes east of the scar. Results cases practically impossible, boreholes cannot be drilled (Benjumea et from static pressure soundings indicate that the thickness of this al., 2001; Bichler et al., 2004; Jongmans and Garambois, 2007; Medioli coarse-grained layer exceeds 15 m immediately west of the landslide et al., 2012; Roberts et al., 1992). scar. It could not be verified if there is clay underneath this layer. The 90 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102

Fig. 5. CPTU-R and laboratory measurements carried out in boreholes (a) 7202, (b) 7203, (c) 7206 and (d) 7208 (Fig. 2b), showing a sudden change in the resistivity in boreholes 7202, 7203 and 7206 due to the presence of a coarse-grained layer. Borehole 7208 does not intersect this layer at least down to 35 m below the surface. Note that quick-clays are estimated to be present above the coarse-grained layer in both boreholes 7202 and 7203. Borehole 7202 is located in the landslide scar (Fig. 2a). A large thickness of quick-clay is estimated (based on CPT measurements; not shown here but reported in Löfroth et al. (2011)) to be present in borehole 7206 (see also Fig. 4a) but it is not clear if this occurs immediately above the coarse-grained layer. These data are kindly provided by SGI (from Löfroth et al., 2011). A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 91 quick-clay in this section also is about 15 m thick and overlies the gentle dip towards the landslide scar or to the east. In their report, friction materials (i.e., coarse-grained materials). Löfroth et al. (2011) concluded that the electrical resistivity data could In the section shown in Fig. 4b, the thickness of the fluvial deposits not be used to identify both the coarse-grained layer and quick-clays. is about 3 m, which increases to more than 6 m towards the west. However, they mentioned the importance of using electrical resistiv- The fluvial deposits consist of sand and silt with small amounts of or- ity data, which enabled them to distinguish a top resistive crust and a ganic materials and gyttja. Underneath the fluvial deposits, there is transition to an underlying more conductive clay that may be quick clay that continues to a depth greater than 45 m. The friction mate- at the transition zone. Sudden variations of the electrical resistivity rials observed in the eastern section (Fig. 4a) are not clearly seen in at depth detected by CPTU-R and CPT data, especially those from any of the boreholes, which extend to a maximum depth of 45 m. the coarse-grained layer (e.g., variations observed in boreholes However, we believe that most of the boreholes (e.g., 7063 to 7067) 7202 and 7203; Fig. 2) could not be resolved by the ERT models ended in hard materials (or presumably coarse-grained materials) (Löfroth et al., 2011). and this is the reason why some of them are much shallower than the others (Fig. 4b). This is just a speculation and further investiga- 4.3. Hydrogeological investigations tions are required to prove this interpretation. The northwestern part of seismic line 2 overlaps with the section shown in Fig. 4b. Inter- Pore-pressure monitoring carried out by SGI during 2010 in two bore- estingly, no quick-clay is found neither in this section, nor in the boreholes (e.g., 7063 in Fig. 4)neartheriverforaperiodofabouttwoyearsat holes further towards the west (Fig. 2b). Our seismic lines, which are 4 depth levels (3 m, 8 m, 15 m, and 30 m) did not show any strong tem- shown later, suggest the presence of coarse-grained layer or a contin- poral correlations between precipitation and pore-pressure changes uation of the coarse-grained materials that deepen westwards, imply- (Swedish Geotechnical Institute, 2011). Therefore, pore-pressure effect ing that the boreholes in the western part of the study area ended in for triggering landslides was suggested to be minimal, at least close to these materials. The coarse-grain materials come again closer to the the river or where these investigations were carried out. Pore water pres- surface and are intersected in line 3, which may also be an indication sure measurements in the area indicate that they are generally lower than of its lateral heterogeneity and discontinuity (e.g., size and shape). the hydrostatic pressure implying a downward groundwater flow. At the Geotechnical data along line 3 (Fig. 2) are reported in Löfroth et al. time of measurements, pore water pressures were low, especially above (2011), thus, not shown here as their original format but discussed the coarser-grained layer. Nevertheless, hydrogeological investigations later in the paper. suggest possible artesian groundwater flow especially near the river Fig. 5 shows a selection of CPTU-R and laboratory measurements (Swedish Geotechnical Institute, 2011). (e.g., fall-cone tests) on samples taken at a few locations from four boreholes adjacent to the seismic lines (Fig. 2b). We present the 5. Seismic data acquisition CPTU-R data here since they show a clear increase in the frictional resistance and electrical resistivity (Fig. 4) where the coarse-grained Seismic data acquisition (2D and 3D) was carried out in September layer is intersected (Löfroth et al., 2011). As evident from the bore- 2011 for about two weeks. Four different types of seismic sources, holes and the cross sections (Figs. 4 and 5) from the east to the namely weight-drop, sledgehammer, shear-wave source and explosive, west of the study area, the coarse-grained materials deepen with no were used. While majority of the seismic lines (including the 3D area) indication of the coarse-grained layer in borehole 7208. The resistivi- were acquired using a weight-drop source, data along lines 4 and 5 ty measured in borehole 7203 shows a clear change at about 20 m were acquired using 10–20 g of dynamite fired in holes about 0.5 m below the surface, close to the landslide scar and near the seismic deep. The main reason to use explosives was to test their potential espe- line 5 (Fig. 2). Borehole 7202 shows a similar feature at about 10 m cially for imaging the bedrock at depth, reflections from and within the below the surface in the landslide scar, which is likely the same bedrock that may be important to understand the general structures of layer as observed in borehole 7203. The sensitivity of clays was also ex- the subsurface, wide-angle reflections coming at large offsets and also amined in both laboratory (e.g., fall-cone tests) and using CPT methods suggesting the presence of quick-clays above the coarse-grained Table 1 materials (e.g., boreholes 7202 and 7203). However, a thick layer of Main reflection seismic data acquisition parameters, September 2011. quick-clay was estimated (based on CPT rod friction results) to be present in borehole 7206, but it is not clear if it is situated immediately above Line 2 Line 3 Line 4 Line 5 the coarse-grained layer. Survey parameters Recording system SERCEL 428 SERCEL 428 SERCEL 428 SERCEL 428 No. of receivers 120 96 144 120 4.2. Previous geophysical investigations No. of shots 108 78 47 55 Receiver interval 4 m 4 m 4 m 4 m Previous geophysical investigations only consisted of ERT and IP Shot interval 4–8m 4–8m 4–12 m 4-–12 m methods and were carried out in an irregular grid consisting of eight Maximum source– 480 m 384 m 576 m 480 m 2D profiles (Löfroth et al., 2011). The 2D resistivity model along a line receiver offset Source Weight-drop/ Weight-drop/ Dynamite Dynamite close and nearly parallel to the seismic line 5 showed a sharp vertical hammer hammer (10–20 g) (10–20 g) change in the resistivity from more resistive (30–200 Ω m) top layer CDP size 2 m 2 m 2 m 2 m (20–25 m thick) and a considerably more conductive (1–16 Ω m) underlying layer. The conductive layer dips gently towards the Spread parameters Record length 6 s 6 s 6 s 6 s river. Note that these observations contrast with the CPTU-R results Sampling 0.5 ms 0.5 ms 0.5 ms 0.5 ms (e.g., from borehole 7202) and suggest a slightly resistive layer at the rate interface between the two layers (Fig. 5a). Nevertheless, they do also suggest material properties change at these depths. A highly resistive Receiver and source parameters zone, indicating the crystalline bedrock, was well imaged, but only at Geophone frequency 28 Hz 28 Hz 28 Hz 28 Hz fi Type of base 7.5 cm spike 7.5 cm spike 7.5 cm spike 7.5 cm spike the rst 100 m distance of the southern portion of the line and it has No. of geophone Single Single Single Single no expression in the central and northern portions of the line. Our per set own ERT measurements along line 5 later confirmed this interpretation Source pattern 5 impacts/ 5 impacts/ Single point/ Single point/ (Malehmir et al., in press). An ERT line almost parallel to the seismic line point point hole hole Shot depth 0 m 0 m ~0.5 m ~0.5 m 4 also showed a conductive layer at 10 m depth and below this with a 92 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102

a SE Receiver location NW 10 20 30 40 50 60 70 80 90100 110 120 0.00 Raw shot

0.04

0.08

Time (s) Time 0.12

−20 0.16

Amplitude (dB) Frequency (Hz) −40 0.20 0 200 400 296 216 136 56 0 -64 -144 Offset (m)

b SE Receiver location NW 10 20 30 40 50 60 70 80 90100 110 120 0.00 Vertical stack

0.04

0.08 400

Time (s) Time 0.12

−20 0.16

Amplitude (dB) Frequency (Hz) −40 0.20 0 200 400 296 216 136 56 0 -64 -144 Offset (m) c SE Receiver location NW 10 20 30 40 50 60 70 80 90100 110 120 0.00 Processed shot

0.04

0.08 B1 B1

Time (s) Time 0.12

−20 0.16

Amplitude (dB) Frequency (Hz) −40 200 400 0.20 0 296 216 136 56 0 -64 -144 Offset (m)

Fig. 6. (a) An example of a raw shot gather from line 2 (weight-drop source), (b) after vertical stacking of five repeated shots and (c) processed shot (including field statics, bandpass filtering and top mute). Bedrock reflection, B1, is clearly visible after some processing steps. Note the power spectra (0–200 ms window) in the lower right corner after each processing step. to obtain wide frequency band seismic data that might be potentially 15 kg iron sledgehammer (only in a few places where mobilizing the useful for other studies, such as surface-wave analysis and waveform weight-drop was not possible) were used to generate the seismic signal. tomography. A SERCEL 428 acquisition system was used for the data re- At every source location, five shots were generated and vertically cording and a differential global positioning system (DGPS) registered stacked to increase the signal-to-noise ratio. We used 120 and 96 accurately the locations of shot and receiver positions. For lines 2 and active/live geophones for lines 2 and 3, respectively. For lines 4 and 5 3(Fig. 2) a 110-kg accelerated weight-drop (in most places) and a (Fig. 2), respectively 144 and 120 active/live geophones were used. A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 93 a Receiver location S 25 50 75 100 N 0.00 Raw shot Direct arrivals (1350 m/s) 0.02 0.04 Refracted arrivals (1500 m/s)

0.06 Refracted arrivals (> 8000 m/s) 0.08 0.10

Time (s) 0.12 0.14 0.16 0.18 0.20 −200 −150 −100 −50 0 50 100 150 200 Offset (m)

b Receiver location S 25 50 75 100 N 0.00 0.02 + Bandpass filter 0.04 0.06 0.08 S1 S1 0.10

Time (s) 0.12 B1 0.14 0.16 B1 0.18 0.20 −200 −150 −100 −50 0 50 100 150 200 Offset (m)

c Receiver location S 25 50 75 100 N 0.00 0.02 + Field statics 0.04 0.06 0.08 S1 S2S S1 0.10

Time (s) 0.12 S2 First arrivals + 5 ms B1 0.14 B1 0.16 0.18 0.20 −200 −150 −100 −50 0 50 100 150 200 Offset (m)

Fig. 7. (a) A raw shot gather acquired in the middle of line 5 (dynamite source), (b) after bandpass filtering and (c) field (refraction and elevation) static corrections. Note that at least three distinct reflectors (S1, S2 and B1) are already visible (see the red arrows) in the shot gather. Clear direct arrivals (1350 m/s) and refracted arrivals (apparent velocities are shown) are also observed on the raw shot gather. Also note that the critically refracted arrivals coincide with critically reflected arrivals (e.g., S1 and B1). Green curve shows the top mute function used to cancel the energy above and including the first arrivals.

Receiver spacing was 4 m while the shot spacing varied between 4 m to nor presented in this paper. Table 1 summarizes main acquisition pa- 12 m. These data were collected in about 8 days. We also collected a rel- rameters used to acquire data along lines 2, 3, 4 and 5. atively higher resolution (2 m source and receiver intervals) reﬂection Seismic data quality is generally good with clear direct and line (line 1) and only sledgehammer as the source to provide data for refracted arrivals observed on raw shot gathers but no clear shallow comparison with a shear-wave source and receiver set-up (lines S1a, reﬂections on the raw shot gathers. Data quality for the data along S1b and 2S; see Malehmir et al., in press). These data are not discussed line 4 is partly lower than line 5 due to the presence of the river 94 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102

frequency range than the others. The loss of amplitude at higher frequencies represents the earth ﬁltering effect at higher frequencies −5 and at lower frequencies represents the effect of natural geophone Weight-drop frequency (i.e., 28 Hz). Power spectrum for the weight-drop source shows a different pattern for lines 2 and 3. Power spectrum along −15 line 2 shows higher frequencies than line 3. Similar differences in spectral behaviour are observed along lines 4 and 5 for the dynamite. Dynamite Near-surface variations or topography may explain this behaviour. −25 Pullan and MacAulay (1987) explained a similar effect while compar-

Power (dB) ing weight-drop data (75 kg) with shotgun data. When the surface Sledgehammer fi −35 was wet and contained ne-grained materials the shotgun produced Sledgehammer raw shots - Line 2 a stronger signal than the weight-drop. However, in the case of dry Weight-drop raw shots - Line 2 and coarse-grain materials, the shotgun signal was weaker than the Sledgehammer raw shots - Line 3 −45 Weight-drop raw shots - Line 3 weight-drop. In our case, weather conditions were variable during Dynamite raw shots - Line 4 the seismic data acquisition, including calm sunny days to rainy and Dynamite raw shots - Line 5 stormy days. This variable weather can explain the difference be- −55 0 70 140 210 280 350 tween the power spectra from one line to another. Frequency (Hz) In summary, the explosive source produces the highest frequency band and best signal-to-noise ratio; because it contains higher energy Fig. 8. Comparisons of power spectra (0–200 ms window) from sledgehammer, acceler- related to higher burn/blast velocity and source containment than the ated weight-drop and dynamite source data. The dynamite source produces the highest others (Praeg, 2003; Yordkayhun et al., 2009). If the power spectra frequencies and best signal-to-noise ratio. On the other hand, the weight-drop shows are described in terms of bandwidth, the weight-drop source is stron- higher amplitude energy at intermediate frequencies than the two other sources. Sledge- hammer produces the weakest signal of all the three sources. Note that an average power gest in the intermediate frequency range compared with dynamite spectrum containing more than ten shots is shown for every line or source type. See text and the sledgehammer. Therefore, the weight-drop source might be for a detailed description about the figure. better suited if for example a combination of shallow and deeper targets were considered. Moreover, it is cheaper and involves less per- (Fig. 2). Fig. 6a shows an example of a raw shot (only one impact) mitting issues and is safer than dynamite. gather from line 2 and the effects of the vertical stacking of the repeated five shots (Fig. 6b). A comparison between Fig. 6a and b demonstrates improvement in the shot quality after the vertical staking, 6. Seismic data processing especially at far-offsets. Fig. 7a shows an example of a raw shot gather using a dynamite source in the middle of line 5. For the data processing, we used a standard post-stack migration method but focused on important processing steps required for fi fi 5.1. Comparison of various seismic sources high-resolution imaging, such as eld static corrections, bandpass l- tering and velocity analysis (e.g., Bachrach and Nur, 1998; Baker et al., Selection of a suitable seismic source is important for acquiring good 1999a,b; Büker et al., 1998; Hunter et al., 1984; Kaiser et al., 2009; fl quality data during a shallow high-resolution seismic survey. Selection Malehmir and Belle eur, 2010; Malehmir et al., 2011; Schmelzbach of source is also dependent upon the exploration targets and required et al., 2005; Steeples et al., 1997). Low frequencies below 80 Hz for resolution. The compromise between cost and effectiveness is always the dynamite data and 60 Hz for the weight-drop and sledgehammer hard to balance; environmental restrictions and permission limitations data (Fig. 8) were rejected to reveal shallow hidden high-frequency are also other issues to keep in mind when choosing a seismic source. A sledgehammer is always cheaper and easier to operate and maintain in Table 2 Principal seismic data processing steps applied to all the four lines. a near subsurface study. On the other hand, weight-drop and dynamite sources often consistently produce a higher frequency signal with Step Parameters higher amplitude, but cost more money and involve more safety and 1. Read 0.5 s SEG-Y data permission issues (Rashed, 2009). While there were various reasons 2. Extract and apply geometry (CDP bin size 2 m) (some mentioned above) to try different types of sources in the study 3. Trace editing 4. Pick first breaks: full offset range, automatic neural network algorithm but area, the ultimate goal was to see which source could provide higher- manually inspected and corrected resolution image for future studies in the site and in general for 5. Refraction static corrections: datum 25 m, replacement velocity 1500 m/s, quick-clay landslide studies. Unfortunately, due to the limitation with v0 600 m/s time and budget, we were unable to test all these sources on one single 6. Geometric-spreading compensation: v2t fi – – – – – line, but consider that near-surface conditions within such a small area 7. Band-pass ltering: weight-drop: 40 60 220 240 Hz, dynamite: 70 80 260–280 Hz is likely similar to allow such a comparison possible using different 8. Spectral whitening: weight-drop: 30–50–200–220 Hz, dynamite: 60–80– lines. 250–270 Hz Fig. 8 shows the power spectra of reflection seismic data using 9. Direct shear-wave muting (near-offset) or attenuation (far-offset) sledgehammer, weight-drop and dynamite sources. We used every 10. Air-blast attenuation 11. Trace balance using data window fifth shot and then averaged all the power spectra for that specific 12. Velocity analysis (iterative) source type. Our analysis of the power spectra suggests that the 13. Residual static corrections (iterative) weight-drop source (e.g., solid black line in Fig. 8) shows the highest 14. Normal moveout corrections (NMO): 60% stretch mute frequency content with broader bandwidth spectra as compared with 15. Stack the sledgehammer (e.g., solid grey line in Fig. 8). The notably broader 16. fx-deconvolution 17. FK filtering frequency band of the weight-drop indicates better resolution than 18. fx-deconvolution the sledgehammer. All sources contain dominant signal frequencies 19. Trace balance: 0–200 ms around 30–150 Hz (>2 octave). The high frequency portions of the 20. Migration: finite-difference power spectra are different for weight-drop and dynamite. Overall, 21. Time-to-depth conversion: using smoothed stacking velocities (1200 m/s to1500 m/s) dynamite (e.g., solid grey and black arrowed lines) shows broader a SECDP NW c SECDP NW 100 125 150 175 200 225 250 275 100 125 150 175 200 225 250 275 0.00 0.00 S1 0.02 B1 0.02 B1 0.04 0.04 S1 0.06 0.06 S2 0.08 0.08 S2 0.10 0.10 .Mlhi ta./Junlo ple epyis9 21)84 (2013) 92 Geophysics Applied of Journal / al. et Malehmir A. Time (s) 0.12 Time (s) 0.12 B1 0.14 0.14 B1 0.16 0.16 0.18 1350 m/s 0.18 Merged 0.20 0.20 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Distance along profile (m) Distance along profile (m)

b SECDP NW d SECDP NW 100 125 150 175 200 225 250 275 100 125 150 175 200 225 250 275 0.00 0.00 S1 0.02 B1 S1 0.02 B1 0.04 S1 0.04 S1 0.06 S2 0.06 S2 0.08 S2 0.08 S2 0.10 0.10 B1 – 102 Time (s) 0.12 Time (s) 0.12 0.14 0.14 B1 0.16 0.16 0.18 0.18 800 m/s FK-filtered 0.20 0.20 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Distance along profile (m) Distance along profile (m)

Fig. 9. A series of unmigrated stacked sections along line 3 obtained using (a) 1350 m/s stacking velocity and (b) 800 m/s stacking velocity. (c) Merged data from (a) and (b). (d) FK-filtered and coherency enhanced version of (c). Reflection B1 likely represents bedrock topography (see Fig. 2); S1 and S2 are interpreted to be two sets of normally consolidated sedimentary layers above the bedrock. See text for further explanation about the figure and events marked in the sections. 95 96 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102

SE NW 7067 a CDP 7066 7065 7063 100 125 150 175 200 225 250 275 300 325 25 B1 10 -5 -20 B1 -35 -50 -65 B1? -80 Approx. elevation (m) -95 -110

Line 2 Line 4 -125 0 50 100 150 200 250 300 350 400 450 Distance along profile (m)

SE NW b CDP 7072 7071 7070 7069 100 125 150 175 200 225 250 275 Coarse-grained layer 25 S1? or hard materials 10 S2 -5 B1 S1?S -20 -35 -50 S2 -65 -80 B1 Approx. elevation (m) -95 -110 Line 3 -125 0 50 100 150 200 250 300 350 Distance along profile (m)

Fig. 10. Migrated and time-to-depth converted stacked section along (a) line 2 and (b) line 3 showing how variable the bedrock topography (B1) is along these two lines. Bedrock reaches near the surface in the middle of line 2 and dips with higher angle than the southeastern part towards the river. Bedrock in line 3 shows a similar character to what is observed in line 5 (Fig. 11). Geotechnical boreholes along line 2 (Fig. 4b) most likely end in hard materials present at shallow depths and correlate with the reflection marked as dashed green line. Geotechnical boreholes along line 3 also suggest presence of hard materials where reflectors S1and S2 are labelled further supporting our processing approach (Fig. 9) used for this line that it did not result in shallow artefacts. Reflector S2 shows similar character to what is observed along line 5 (Fig. 11) and is likely from similar coarse-grained materials onlapping the bedrock. reflections (Steeples et al., 1997). This rejection proved to be critical remained which were attenuated at near-offsets (using a median filter) in revealing reflectionsasshallowas20–30 ms (Figs. 6cand7b). We and muted at far-offsets (Table 2). After various tests, we eventually de- used a time-variant bandpass filter to enhance deeper reflections cided to use 60% for the stretch mute. Higher percentage of stretch mute from within the bedrock. Refraction static corrections were estimat- was still possible but generally resulted in lower frequency and partly ed using a single-layer model (e.g., Malehmir and Juhlin, 2010). distorted seismic images and, therefore, it was not applied. Bedrock re- Combined with elevation statics helped improving the continuity flections (e.g., B1 in Fig. 7c) are often observed at wide-angle (and large of the reflections in raw shot gathers (Figs. 6cand7c). As demon- offsets). Significantly lower stretch mute than 60% (e.g., 30%) resulted in strated by several authors (e.g., Schmelzbach et al., 2005), top mute poor image of the bedrock and lower fold coverage at shallower travel (cancelling amplitudes above and including the first arrivals) and the times. Such low stretch mutes were not applied. amount of stretch mute (wavelet stretching) allowed after NMO correc- Stacked sections were extremely sensitive to the velocities used, es- tions are important to cancel first arrivals and avoid misinterpreting pecially for the data collected along lines 2 and 3. To obtain best stacking them as reflections. We added 5 ms to the picked first arrivals and velocities, we ran a series of constant velocity stacks using velocities used that as a top mute function (Figs. 6cand7c). As evident from ranging from 500 m/s to 2500 m/s. These allowed us to successfully de- Fig. 7 (also Fig. 6), at far-offsets especially greater than 150 m, direct ar- fine velocities required to image individual reflections. Final stacking rivals are received later than the refracted arrivals. Obviously, our top velocities used to image the reflections above the interpreted bedrock mute function is not able to remove these arrivals. Although a manual range from 800 m/s to 1500 m/s. Higher-velocities up to 5000 m/s, mute was possible, we left these arrivals in the data and hoped they were used below the bedrock. This method of finding stacking velocities would cancel each other during the stacking process, which did prove worked well for lines 2, 4 and 5 but failed for the data along line 3. For to be the case. Although most shear-wave and surface-wave energies the data along line 3, we could not use a common stacking velocity func- were attenuated by the bandpass filtering applied to the data, some tion, which can be used to image both bedrock reflection and reflections A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 97

Landslide scar a W 7208 7206 CDP 7204 E 100 125 150 175 200 225 250 275 300 3257202 350 375 25 10 S1 -5 -20 ? S2 S1-35 -50 S2 -65 -80 -95 Approx. elevation (m) -110 Line 2

Line 4 Line 5 -125 0 50 100 150 200 250 300 350 400 450 500 550 Distance along profile (m)

Landslide scar Coarse-grained layer b S CDP 7203 N 100 125 150 175 200 225 250 2757202 300 325 Quick-clay 25 B1 10 S1 -5 S1 -20 S2 -35 S2 -50 -65 B1 -80 -95 Approx. elevation (m) -110

Line 5 Line 4 -125 0 50 100 150 200 250 300 350 400 450 Distance along profile (m)

Fig. 11. Migrated and time-to-depth converted stacked section along (a) line 4 and (b) line 5 showing that reflectors along line 5 may constitute a depression zone with its deepest point at the location of the landslide scar (Fig. 2). Reflectors S1 and S2 marked by the red arrows are interpreted to originate from the coarse-grained layers that onlap reflector B1 from the bedrock. Note diffuse character of S1 at the location of the landslide scar. Blue and orange boxes represent intersection of the geotechnical boreholes (Löfroth et al., 2011; Fig. 2) with the interpreted coarse-grained layer and quick-clay formations, respectively. No coarse-grained material is intersected by borehole 7208, most likely because the borehole is shallower than the coarse-grained layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web of this article.)

above it. They (i.e., bedrock and reflections above it) required signifi- helped to improve the quality of the unmigrated stacked sections. cantly different velocities and choosing one velocity for one target Seismic data were migrated and depth converted using smoothed resulted in loosing the other ones. The time section along line 2 was version of the stacking velocities. Table 2 summarizes main data pro- stacked with stacking velocities ranging from 800 to 1350 m/s. A cessing steps used in this study. DMO (dip-moveout) approach was also tested but it did not improve the results, probably because these reflections are very close to each 7. Results and interpretations other (about one wavelength) our processing approach would not allow simultaneous imaging of all these reflections at the same time. 7.1. Seismic lines 2 and 3 Neither a smooth nor a detailed and significantly varying velocity function was helpful. Fig. 9a and b shows two time sections for the stack Fig. 10a and b shows migrated and time-to-depth converted stacked with different stacking velocities around 1350 m/s and 800 m/s, sections along lines 2 and 3, respectively. The shallowest imaged reflector respectively. To be able to provide an image containing all these (B1),afewmetresbelowthesurface,isobservedinline2andis reflections, we decided to merge the two individual stacked sections interpreted to be from the top of the bedrock. Combined with the bedrock (i.e., Fig. 9a and b) by selecting odd traces from one section and even outcrops observed in both western and eastern sides of the line (Fig. 2), traces from the other one (Fig. 9c) and by applying an FK-filter to the this interpretation suggests that the bedrock has a bowl-shaped geometry merged sections (Fig. 9d). Juhlin et al. (2010) recently reported a similar in the southeastern portion of the line. This geometry implies that clay approach for data acquired to image the upper crust. materials in the most southeastern portion of the line, even if are A coherency enhancement filter was used after the stacking. This quick-clays, are unlikely prone to slide because movement of these mate- filter helped to improve the quality of the reflections, but it also arti- rialsisrestrictedbythesidesofthebedrockdepression.Twosetsofreflec- ficially enhanced steeply dipping structures, which we believe were tions are observed in the northwestern portion of the line, but it is not the artefacts of the surface-wave and direct arrival data. We used a clear which one is from the bedrock. It is possible that the shallower re- dip filter to remove these steeply dipping artefacts in the stacked sec- flector (i.e., dashed green line in Fig. 10a) represents coarse-grained mate- tions. A final pass of coherency enhancement filter successfully rials onlapping the bedrock (i.e., dashed orange line in Fig. 10a)butitis 98 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 also possible that the shallowest reflector is from the top of the bedrock Geotechnical boreholes did not suggest presence of any quick-clay in too. In either case, our interpretation of the geotechnical boreholes this part of the study area (Löfroth et al., 2011). (Fig. 4b), if correct, suggests hard materials as shallow as 15 m below the surface (e.g., 7066 and 7067), which is in agreement with the 7.2. Seismic lines 4 and 5 shallowest reflector observed in this portion of the line. A good agreement between the depth of this reflector and data from geotechnical boreholes Fig. 11a and b shows migrated and time-to-depth converted stacked is also observed in the remaining portion of the line. sections along lines 4 and 5, respectively. Reflectors as shallow as However, reflectors along line 3 show different characters than 10–20 m (e.g., S1 below the landslide scar) are observed along line 4. those observed in line 2 (Fig. 10b). A bedrock reflector (B1) clearly ap- They generally dip gently towards the west and are discontinuous in proaches the surface and correlates well with the exposed bedrock in the middle of the profile. In contrast, reflectors along line 5 show both the southeastern portion of the line. Two sets of reflectors, S1 and S2, flat-lying and moderately- north dipping characteristics, especially in are imaged above the bedrock gently dipping towards the river but the southern portion of the line. In general, three distinct reflectors are ob- onlapping the bedrock. We were not surprised to see some disagree- served along seismic line 5: one at about 10–20 m depth (or about ment between the reflectors and the geotechnical data because the sea-level; S1 in Fig. 11b), one at about 40–45 m depth (i.e., about 20 m data along this line were processed somewhat differently (Fig. 9), how- below sea-level; S2 in Fig. 11b) and another one that dips to the north ever, a careful comparison between the reflectors and geotechnical data (the bedrock reflection). The latter starts at about 10 m depth in the (Fig. 10b) suggests an excellent agreement between the two data sets, southern portion of the line and extends to a depth of about 90 m in which further supports our processing approach for this line. In particular, the middle (B1 in Fig. 11b). We interpret the deepest reflector (B1) to reflection S1 matches well (expect at 7070) with a thin (b2m)layerof represent the bedrock topography, with its deepest point located at a hard materials (possibly coarse-grained) imaged as shallow as 10 m depth of about 100–120 m and potentially located below the landslide (also see Löfroth et al., 2011). Most geotechnical boreholes ended at re- scar (Fig. 11b). No borehole intersects the bedrock along the seismic flection S2 suggesting a second coarse-grained layer at deeper levels. lines, thus, we can only speculate about the nature of the reflector and

a V=H 7202 Landslide scar

7204 7203 N

7206

7208

? 150 m

Line 5

Line 4

b V=H 7202 Landslide scar

7204 7203 N

7206

S1 7208 S2 B1 Bedrock?

Bedrock? Bedrock? S1 150 m S2 Bedrock?

B1 Line 5

Line 4

Fig. 12. (a) 3D visualization of the seismic lines 4 and 5, showing the correlations of the reflectors in different lines. (b) Correlation between the reflectors and a coarse-grained layer (blue line) identified from the geotechnical data (Löfroth et al., 2011). Solid blue, yellow and red lines represent our interpretation of different layers along the seismic lines. A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 99

V=H

S1 7208 S2 7206 ? S1 N 7204 Bedrock Bedrock? 7202 Line 4 7203 S1

Bedrock S1 Line 3

150 m Bedrock? Line 2 Line 5

Fig. 13. 3D visualization of the seismic lines 2, 3, 4 and 5, showing the correlations of the reflectors in different lines. Bedrock (red lines) is clearly imaged in lines 2, 3 and 5 but not in line 4. Line 2 shows rather a different image than all the other lines and no indication of S1 and S2 reflections as observed in the other lines. what it may imply concerning the landslide evolution. Bedrock is ex- the dip direction) and this results in imaging out-of-plane reflections posed at about hundred metres west of the southern portion of the from undulated surfaces of the bedrock (Fig. 11a). line 5 and this maybe an indication that the reflection B1 is also from the shallow bedrock in the southern part of the line (Malehmir et al., 8. Correlation and 3D visualization with geotechnical boreholes in press). In contrast with the other three lines, the bedrock reflector is not evident at the landslide scar nor along line 4 and is as deep as Fig. 12 shows 3D visualization of the seismic lines 4 and 5 with the 80–100 m in the northern part of the study area. geotechnical boreholes and it demonstrates a good correspondence be- Another reflector located nearly at sea-level is observed along tween the reflectors and available borehole data. The borehole data are line 4 (S1 in Fig. 11a) and is similar to the one observed in lines 3 the CPT and CPTU-R measurements carried out by SGI (Löfroth et al., and 5. The reflector is nearly flat lying along line 5 but shows clear 2011), which attributes the sudden increases in the penetration resis- westwards dip component along line 4 (Fig. 11a). This reflector tance and resistivity to the presence of a coarse-grained layer. Seismic onlaps the bedrock in line 5 and shows a good correspondence in data clearly indicate the presence of a reflector at the location where both lines 4 and 5. However, its character becomes unclear at the lo- this layer was detected and suggest that the coarse-grained layer can cation of the landslide scar (Fig. 11b). Borehole data suggest that the be a regional scale formation in the study area. Seismic data have reflector (S1) is from the contact between the clay formations above been able to clearly image this reflector along line 4, showing a west- and the coarser-grained materials that were identified by CPT and ward dipping component of about 10 degrees. This depth also correlates CPTU-R measurements, potentially sandy-silty or coarse-grained well with the borehole information further demonstrating why the materials (Figs. 4 and 5). This reflector occurs at about 10–15 m layer was not intersected by for example borehole 7208 (Fig. 5)or below the surface at the landslide scar and reaches a depth of more any of those located in the western part of the study area (Fig. 4b). Pen- than 50 m (below the surface) in the westernmost portion of line etrations of these boreholes were simply too shallow (about 10 m) to 4. This depth may correlate with the reflector (green dashed line in intersect the layer or there is a sudden break in the continuity of the Fig. 10a) observed in line 2, although not clearly visible because it oc- coarse-grained layer from the east to the west at the location of seismic curs, in both lines, at the edge of the seismic lines where the line 2 (Fig. 10a). Geotechnical data from CPTU, CPT and laboratory mea- signal-to-noise ratio is usually low. It is also possible that there is surements (Figs. 4 and 5) suggest the presence of a thick quick-clay for- no S1 reflector in line 2 which would then indicate geological mation above the coarse-grained layer especially in the eastern side of changesinthislocation. the study area except in the old landslide area. It should be noticed We also observe a short reflector (S2 in Fig. 11)belowthecoarse- that borehole 7206 and those shown in Fig. 4a intersect a large thick- grained layer and above the bedrock, which similar to our interpreta- ness of quick-clay formation near where S1 reflector suggests a depres- tion for line 3 (Fig. 10b) may represent another contact between a sion in the western part of line 4 (Figs. 11 and 12). coarse-grained layer and clay formations. This layer is located below Fig. 13 shows 3D visualization of all the four seismic lines and our present day sea-level and may not directly be relevant for triggering interpretation of the bedrock geometry and the S1 and S2 reflectors. landslides in the study area. However, it may be important for the pres- Bedrock is well imaged in all the profiles except line 4 and this ence of another artesian aquifer below the landslide scar. No borehole maybe a confirmation that recognizing the bedrock, especially if its reaches the S2 reflector along lines 4 and 5. However, the presence of structural geometry is parallel to the line is difficult. Seismic data marine clays below the coarse-grained layer has been confirmed by a along lines 2 and 4 at where they cross each other show a correlation few boreholes (e.g., 7202). A noticeable difference in the reflection in terms of reflectivity pattern but do not support the number of re- characteristics of the data between lines 4 and 5 maybe due to the flectors observed in each of them. The fact that this occurs at a place lower signal-to-noise ratio along line 4 since it is close and parallel to where the seismic fold is low and this inconsistency maybe resulted the river. A more plausible scenario to explain this could also be that from that does not allow further speculation about the results in line 4 is parallel to the structural geometry of the bedrock (opposite to this part of the study area. It is clear that line 3 shows both S1 and 100 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102

a b

1 2 Göta River Göta River

3 4 Fault Fault

Göta River

Fault

1 Leached/quick-clay 2 Coarse-grained layer 3 Marine clay (normal)

4 Bedrock Underground water flow

Fig. 14. Schematics of three hypothetical scenarios explaining the formation and potential triggering mechanism of quick-clay landslides in the study area. (a) The coarse-grained layer acts as a conduit allowing excess surface water inﬁltrated across the bedrock topography to leach the salt from the clay materials. Increase in pore-pressure (from excess rain/ snow meltwater) in this scenario is the main landslide triggering mechanism. (b) The erosion and increased pore-pressure from fresh river water are the main triggering mechanisms. (c) A combination of erosion and increased pore-pressure from excess water from the river and surface water inﬁltration into the coarse-grained layer is the main cause of quick-clay formation and landslide. These are all hypothetical scenarios and need to be validated by additional measurements.

S2. Seismic data along line 1 (Fig. 2; see Malehmir et al., in press) also might be an indication that the layer was disturbed during the landslide show two clear sets of reflectors down to the bedrock which further or is an indication of large lateral heterogeneity across the layer. A de- confirm the presence of two distinguishable units, most likely pression zone of similar character, but of lesser extent, is also observed coarse-grained materials, above the bedrock in the study area. in the middle of line 4 and this zone can be an important factor for future landslides in the study area (Fig. 11a). The geotechnical data sug- 9. Discussion and implications gest the presence of a thick quick-clay formation at this location (Figs. 4 and 5) and we recommend monitoring and detailed studies of The most significant feature in the seismic data is the reflection that this part of the line. originates from the coarse-grained layer (S1). If in fact this reflector is Bedrock topography is particularly well defined along lines 2, 3 and 5 originated from a sandy-silty layer with higher permeability than the (Fig. 13). The surface elevation map (Fig. 2b) shows a gentle dip (about clays above and below, it could act as conduit for fresh groundwater 5°) toward the river; however, bedrock topography imaged by the seis- water from the highland areas in the south into the landslide scar and mic data demonstrates a dip of about 25° towards the river (Figs. 10 eventually into the river. The bedrock is closer to the surface in the and 11). This is certainly an important outcome of the seismic survey southern part of the study area (Figs. 10 and 11), and the reflector suggesting that, locally, surface topography is not exactly controlled by (the coarse-grained layer) onlaps the bedrock, suggesting that there the bedrock topography and that the bedrock dips steeper than the sur- might be stronger water infiltration into the clay from the southern face topography. This is not surprise since this was an aquatic sedimen- part of the study area than the northern part. In this scenario, ground- tary nature prior to regression. Although geological features located water recharge (infiltration) can take place where there are outcrops below present day sea-level maybe irrelevant for quick-clay formation of bedrock. It is not clear at the moment if a change in the water flow and generation of landslides, they are important structures that define (pore-pressure) in the coarse-grained layer could alone or in combina- the subsurface geometry of the normally consolidated sediments. Their tion with the water flowing from the river into the formation be the shape and geometry may control the extent of the area that a landslide main pre-condition of the landslide in the site. Therefore, it is neces- can affect. The fact that the bedrock is imaged as shallow as only a few sary to investigate this scenario using down-hole geophysical and metres (Figs. 10 and 11) again proves the potential of high-resolution re- hydrogeological methods in the southern part of the line 5. The fact flection seismic data, provided that they are properly acquired and that we are able to image the coarse-grained layer is encouraging and processed. Good ground conditions with highly saturated clays and a sig- demonstrates the value of high-resolution reflection seismic methods nificant seismic contrast between the clay and coarse-grained layers in delineating such detailed (thin) structures that may contribute to have contributed to such an excellent data quality. the sliding potential of quick-clays and help to improve landslide risk Although our ultimate goal in this study was to assess the potential assessment in Sweden. Shear-wave reflection seismic data (e.g., S1a) of seismic methods for imaging structures associated with quick-clays, collected from the southwestern part of the study area also suggest on the basis of the results, we are able to suggest a few different scenar- two sets of reflections above the bedrock, consistent with the P-wave ios that may explain the formation of quick-clays and their landslide reflection seismic data (Malehmir et al., in press). Previous (Löfroth et generation in the study area. Various people (e.g., Hjördis Löfroth, per- al., 2011) and preliminary ERT models (Malehmir et al., in press) sonal communication, 2012) have discussed these ideas but now we could not image the coarse-grain layer. The loose character of the provide seismic evidence about the subsurface geometry, which can coarse-grained layer at the location of the landslide scar (Fig. 11b) support some of these ideas. The first scenario (Fig. 14a) suggests that A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102 101 the coarse-grained layer, a regionally important formation, but laterally the geotechnical boreholes, which are often far apart. They should be heterogeneous, acts as a conduit for groundwater flowing from highland attempted regularly in combination with other geophysical and geo- areas. As it plummets below the clay formation, an artesian aquifer is technical methods especially in areas like the one shown in this study formed and salts can be leached from the nearby clays and transported where thick clay formations would limit depth penetration of tradition- to the river. Periods of high rainfall or snowmelt, and hence recharge of al geoelectrical methods to only a few tens of metres. the aquifer would increase pore-pressure. A second scenario (Fig. 14b) suggests that during periods of higher river level (perhaps due to precipitation events or snowmelt further upstream) water is directed into the 10. Conclusions coarse-grained layer and a combination of erosion at the riverbank and increased pore-pressure in the layer close to the river is responsible for Shallow high-resolution reflection seismic data, for the first time in the generation of landslides in the study area. A high rate of erosion, Sweden, were acquired and processed along four lines to image subsur- due to the stream and the small river close to the landslide scar, perhaps face formations at a quick-clay landslide site. All the seismic lines con- expedited this process at this site. The third scenario (Fig. 14c), our pre- tain numerous reflectors with different characters. Reflectors that are ferred one, is a combination of the first and the second scenarios with probably of regional importance in the study area are (1) a flat-lying re- some exceptions, suggesting that the quick-clays are formed mainly flector at about 20–30 m below sea-level that originates from the con- due to the infiltration of surface water into the clay formations and partly tact between the clay formations and a coarse-grained or sandy-silty due to the circulation of fresh water into the clay formation from an layer. The estimated depth of this reflector at the eastern parts of the artesian aquifer (i.e., the coarse-grained layer). It is possible that the study area correlates well with the results from the geotechnical inves- coarse-grained layer, locally highly permeable, has acted as a conduit tigations carried out in a few boreholes located close to the seismic for water from either highland areas or the river and has contributed to lines. Borehole information suggests that the quick-clay usually occurs the formation of quick-clays. It is also equally, if not highly, possible above this layer. However, exact relationships between the reflector that the coarse-grain layer has acted as a drainage conduit directing and the occurrences of the landslides in the site require further investi- the salts leached downwards from the overlying sediment toward the gations. The continuity of the reflector becomes loosely defined where river. In their review article, Malehmir et al. (in press) suggested that a the landslide scar exists, which maybe an indication that there is a decreasing value of the resistivity observed on CPTU-R data (Fig. 5a) link between the reflector and the occurrence of landslides in the site. from the top of the quick-clay zone to its base might imply an increase The reflector dips about 15° towards the west also partly confirmed in the pore water conductivity with depth and hence, an increasing salin- by geotechnical boreholes which show no evidence of its presence in ity value (even if it is low). They concluded a possibility that the original boreholes drilled down to about 45 m depth in the western part of marine pore water was displaced by water percolating downward from the study area, and (2) reflectors from the bedrock. the surface. The coarse-grained layer would have then facilitated move- Various types of seismic sources were utilized in this study. Com- ment of water and its dissolved chemicals to the Göta River, as they parisons between the power spectra of the raw shot gathers suggest exited from the base of the overlying clay layer (Malehmir et al., in that the weight-drop and dynamite are better suited for seismic im- press). What this implies is that the coarse-grained layer has a little sig- aging in the study area than the sledgehammer. Dynamite source nificance in the development of the quick-clay zone, a point that needs to shows broader frequency bandwidth, which is useful if additional be confirmed by additional investigations (Malehmir et al., in press). A studies such as surface-wave and waveform tomography are also landslide triggering mechanism due to increased pore-pressure in the intended. coarse-grained layer cannot be entirely ruled out, but this hypothesis is The level of the details revealed by the high-resolution reflection not supported by the results of pore-pressure monitoring carried out seismic data demonstrates the ability of the method to image fine close to the river. Nevertheless, it should be noted that if the layer drains structures associated with quick-clay landslides, which are important readily to the Göta River, it probably does not build up substantial excess and valuable for any site risk assessment especially in areas where pore pressures or artesian pressures. Erosion by the Göta River that thick clay formations and deep bedrock topography are expected. steepens its bank, and saturation of the clay slopes associated with flooding stages of the Göta River, or periods of heavy or prolonged rainfall that saturates the slope, are more probable triggers; possibly after a Acknowledgements flushing event and fall in river level after a high stand. The very small slides along the Göta River (Fig. 2) are most likely caused by erosion The Society of Exploration Geophysicists (SEG) through its Geosci- and slope saturation. An initial landslide in the slope materials, which al- entists Without Borders (GWB) program sponsored this study. Addi- most always precedes a larger landslide that involves quick-clay, was tional funding from the Swedish Research Council FORMAS, for urban probably also caused by the erosion/rainfall combination. infrastructure construction and planning, supported a continuation of To check the validity of all these scenarios, further investigations the project for various geophysical tests. This is also part of a larger and studies using ground-based and down-hole geophysical as well joint research project involving collaboration among Uppsala Univer- as hydrogeological site investigations should be carried out. A bore- sity, Geological Survey of Sweden (SGU), Leibniz Institute for Applied hole located in the southern portion of the line 5 is best suited for Geophysics (LIAG), University of Cologne, Syiah Kuala University, Pol- this purpose. These scenarios are all important when climate change ish Academy of Sciences (PAN), NORSAR, and International Centre for and water-soil interactions are considered. Long term geophysical Geohazards (ICG) in Norway. We appreciate discussions with Mats (4D) and hydrogeological investigations using active and passive Engdahl from the Geological Survey of Sweden (SGU), Hjördis Löfroth monitoring techniques (e.g., Pennington et al., 2009)wouldcertain- and several other personnel from the Swedish Geotechnical Institute ly be helpful to provide useful insights to explain mechanisms (SGI) who helped us to design our survey and interpret the seismic behind quick-clay formations and landslides. Future site investiga- data. Graduate and undergraduate students from Uppsala University tions should also aim at drilling deeper boreholes at a few locations and SEG Student Chapter are acknowledged for their fieldwork con- along lines 2, 4 and 5 to constrain our interpretations (e.g., Medioli tribution. M.U. Saleem conducted part of this study during his MSc et al., 2012) and if possible to include subaquatic geophysical mea- degree project at Uppsala University. An early version of this manu- surements in the river. script was critically reviewed and improved by several people to Our results demonstrate the potential of high-resolution reflection whom we are grateful. We are grateful to I. Snowball and two anon- seismic methods for not only delineating shallow structures that are as- ymous reviewers for their comments and suggestions, which helped sociated with quick-clays but also filling the information gap between improve the quality of our paper. 102 A. Malehmir et al. / Journal of Applied Geophysics 92 (2013) 84–102

References Lundström, K., Larsson, R., Dahlin, T., 2009. Mapping of quick clay formations using geotechnical and geophysical methods. Landslides 6, 1–15. Annan, A.P., 2005. GPR methods for hydrogeological studies. In: Rubin, Y., Hubbard, S.S. Magnusson, N., Lundqvist, G., Regnéll, G., 1963. Sveriges geologi. Nordstedts, Stockholm (Eds.), Hydrogeophysics. Springer. (In Swedish). fl fl Bachrach, R., Nur, A.M., 1998. High-resolution shallow-seismic experiments in sand, Malehmir, A., Belle eur, G., 2010. Re ection seismic imaging and physical properties of Part 1: water table, fluid flow, and saturation. Geophysics 63, 1225–1233. base-metal and associated iron deposits in the Bathurst Mining Camp, New Brunswick, – Baker, G.S., Schmeissner, C., Steeples, D.W., Plumb, R.G., 1999a. Seismic reflections from Canada. Ore Geology Reviews 38, 319 333. depths of less than two meters. Geophysical Research Letters 26, 279–282. Malehmir, A., Juhlin, C., 2010. An investigation of the effects of the choice of stacking fl Baker, G.S., Steeples, D.W., Schmeissner, C., 1999b. In-situ, high-frequency P-wave ve- velocities on residual statics for hardrock re ection seismic processing. Journal of – locity measurements within 1 m of the earth's surface. Geophysics 64, 323–325. Applied Geophysics 72, 28 38. fl Bekler, T., Ekinci, Y.L., Demirci, A., Erginal, A.E., Ertekin, C., 2011. Characterization of a Malehmir, A., Dahlin, P., Lundberg, E., Juhlin, C., Sjöström, H., Högdahl, K., 2011. Re ec- landslide using seismic refraction, electrical resistivity and hydrometer methods, tion seismic investigations in the Dannemora area, central Sweden: insights into Adatepe–Çanakkale, NW Turkey. Journal of Environmental and Engineering Geo- the geometry of poly-phase deformation zones and magnetite-skarn deposits. physics 16, 115–126. Journal of Geophysical Research 116, B11307. Benjumea, B., Hunter, J.A., Good, R.L., Burns, R.A., Ross, M., 2001. Application of high- Malehmir,A.,Bastani,M.,Krawczyk,C.,Gurk,M.,Ismail,N.,Polom,U.,Persson,L.,inpress.Geo- — resolution seismic-reflection techniques in Champlain sea sediments near Lachute- physical assessment and geotechnical investigation of quick-clay landslides aSwedish Saint-Benoit, Quebec. Geological Survey of Canada, Current Research 2001-D6, p. 7. case study. In Near Surface Geophysics. http://dx.doi.org/10.3997/1873-0604.2013010 Berglund, M., 1992. Shore level changes during the Late Weichselian deglaciation in Medioli, B.E., Alpay, S., Crow, H.L., Cummings, D.I., Hinton, M.J., Knight, R.D., Logan, C., Halland, southwestern Sweden. GFF 114, 395–415. Pugin, A.J.-M., Russell, H.A.J., Sharpe, D.R., 2012. Integrated data sets from a buried Bichler, A., Bobrowsky, P., Best, M., Douma, M., Hunter, J., Calvert, T., Burns, R., 2004. valley borehole, Champlain sea basin, Kinburn, Ontario. Geological Survey of Three-dimensional mapping of a landslide using a multi-geophysical approach: Canada. Current Research, 3. http://dx.doi.org/10.4095/289597 (16 pp.). the Quesnel Forks landslide. Landslides 1, 29–40. Nadim, F., Pedersen, S.A.S., Schmidt-Thomé, P., Sigmundsson, F., Engdahl, M., 2008. – Bjerrum, L., 1954. Geotechnical properties of Norwegian clays. Geotechnique 4, 49–69. Natural hazards in Nordic Countries. Episodes 31, 176 184. Bogoslovsky, V., Ogilvy, A., 1977. Geophysical methods for the investigation of land- Pennington, C.V.L., Foster, C., Chambers, J.E., Jenkins, G.O., 2009. Landslide research at slides. Geophysics 42, 562–571. the British Geological Survey: capture, storage and interpretation on a national fi – Bryn, P., Berg, K., Solheim, K., Kvalstad, T.J., Forsberg, C.F., 2005. Explaining the Storegga and site-speci c scale. Acta Geologica Sinica 83, 991 999. Slide. Marine and Petroleum Geology 22, 11–19. Petley, D., 2011. Global deaths from landslides in 2010 (updated to include a compar- Büker, F., Green, A.G., Horstmeyer, H., 1998. Shallow seismic reflection study of a glaci- ison with previous years). Available http://blogs.agu.org/landslideblog/2011/02/ ated valley. Geophysics 63, 1395–1407. 05/global-deaths-from-landslides-in-2010. Caldenius, C., Lundström, R., 1956. The landslide at Surte on the river Göta Älv. A Polom, U., Hansen, L., Sauvin, G., LeHeureux, J.-S., Lecomte, I., Krawczyk, C.M., Vanneste, M., fl Geologico-Geotechnical Study. Swedish Geological Survey, Ca, 27 88. Longva, O., 2010. High-resolution SH-wave seismic re ection for characterization of Carpentier, S., Konz, M., Fischer, R., Anagnostopoulos, G., Meusburger, K., Schoeck, K., onshore ground conditions in the Trondheim harbor, central Norway. Advances in – 2012. Geophysical imaging of shallow subsurface topography and its implication Near-Surface Seismology and Ground-Penetrating Radar. SEG, pp. 297 312. for shallow landslide susceptibility in the Urseren Valley, Switzerland. Journal of Praeg, D., 2003. Seismic imaging of mid-Pleistocene tunnel-valleys in the North Sea — Applied Geophysics 83, 46–56. Basin high resolution from low frequencies. Journal of Applied Geophysics 53, – Carroll, R.D., Scott, J.H., Lee, F.T., 1972. Seismic refraction studies. Geological, Geophys- 273 298. ical and Engineering Investigations of the Loveland Basin Landslide, Clear Creek Pullan, S.E., MacAulay, H.A., 1987. An in-hole shotgun source for engineering seismic – County, Colorado 1963–65: U.S. Geological Survey Professional Paper, 673 C, surveys. Geophysics 52, 985 996. pp. 17–19. Rankka, K., Andersson-Sköld, Y., Hultén, C., Larsson, R., Leroux, V., Dahlin, T., 2004. Eichkitz, C.G., Schreilechner, M.G., Amtmann, J., Schmid, C., 2009. Shallow seismic re- Quick Clay in Sweden. Report 65. Swedish Geotechnical Institute, Linköping. flection study of the Gschliefgraben landslide deposition area — interpretation Rashed, M.A., 2009. GDP: a new source for shallow high-resolution seismic exploration. – and three dimensional modeling. Austrian Journal of Earth Sciences 102, 52–60. Journal of Applied Geophysics 68, 243 248. Göransson, G.I., Bendz, D., Larson, P.M., 2009. Combining landslide and contaminant Roberts, M.C., Pullan, S.E., Huntert, J.A., 1992. Applications of land-based high resolu- fl risk: a preliminary assessment. Journal of Soils and Sediments 9, 33–34. tion re ection analysis to quaternary and geomorphic research. Quaternary Sci- – Gregersen, O., 1981. The quick clay landslide in Rissa, Norway. The Sliding Process ence Reviews 11, 557 568. fi and Discussion of Failure Modes.Norwegian Geotechnical Institute (Publication Robertson, P.K., 1990. Soil classi cation using the cone penetration test. Canadian Geo- – No. 135). technical Journal 27, 151 158. Hunter, J.A., Pullan, S.E., Burns, R.A., Gagne, R., Good, R., 1984. Shallow reflection map- Rosenquist, I.T., 1953. Considerations on the sensitivity of Norwegian quick-clays. – ping of the overburden bedrock interface with engineering seismograph — some Geotechnique 3, 195 200. simple techniques. Geophysics 49, 1381–1385. Samyn, K., Travelletti, J., Bitri, A., Grandjean, G., Malet, J.-P., 2012. Characterization of a Jomard, H., Lebourg, T., Tric, E., 2007. Identification of the gravitational boundary in landslide geometry using 3D seismic refraction traveltime tomography: the La Valette – weathered gneiss by geophysical survey: La Clapière landslide (France). Journal landslide case history. Journal of Applied Geophysics 86, 120 132. fl of Applied Geophysics 62, 47–57. Schmelzbach, C., Green, A.G., Horstmeyer, H., 2005. Ultra-shallow seismic re ection Jongmans, D., Garambois, S., 2007. Surface geophysical investigation and monitoring of imaging in a region characterized by high source-generated noise. Near Surface – landslides: a review. Bulletin de la Societe Geologique de France 178 (2), 101–112. Geophysics 3, 33 46. Juhlin, C., Dehghannejad, M., Lund, B., Malehmir, A., Pratt, G., 2010. Reflection seismic Solberg, I., 2007. Geological, geomorphological and geophysical investigations of areas imaging of the end-glacial Pärvie Fault system, northern Sweden. Journal of Ap- prone to clay slides: examples from Buvika, Mid Norway. Doctoral thesis. Norwegian plied Geophysics 70, 307–316. http://dx.doi.org/10.1016/j.jappgeo.2009.06.004. University of Science and Technology, Faculty of Engineering Science and Technology, Kaiser, A.E., Green, A.G., Campbell, F.M., Horstmeyer, H., Manukyan, E., Langridge, R.M., Department of Geology and Mineral Resources Engineering. Trondheim. McClymont, A.F., Mancktelow, N., Finnemore, M., Nobes, D.C., 2009. Ultra high- Solberg, I.L., Hansen, L., Ronning, J.S., Haugen, E.D., Dalsegg, E., Tonnesen, J.F., 2012. resolution seismic reflection imaging of the Alpine Fault, New Zealand. Journal of Combined geophysical and geotechnical approach to ground investigations and Geophysical Research 114, B11306 (doi: 11310.11029/12009JB006338). hazard zonation of a quick clay area, mid Norway. Bulletin of Engineering Geology – Khaldoun, A., Moller, P., Fall, A., Wegdam, G., De Leeuw, B., Me´heust, Y., Fossum, J.O., and the Environment 71, 119 133. Bonn, D., 2009. Quick clay and landslides of clayey soils. Physical Review Letters Steeples, D.W., Green, A.G., McEvilly, T.V., Miller, R.D., Doll, W.E., Rector, J.W., 1997. A fl 103 (4), 188301. workshop examination of shallow seismic re ection surveying. The Leading Edge – Kvalstad, T.J., Andresen, L., Forsberg, C.F., Berg, K., Bryn, P., Wangen, M., 2005. The 16, 1641 1647. Storegga slide: evaluation of triggering sources and slide mechanics. Marine and Swedish Geotechnical Institute, 2011. Report in Swedish. Document Number 07PM001. Petroleum Geology 22, 245–256. Torrance, J.K., 1983. Towards a general model of quick clay development. Sedimentol- – L’Heureux, J.-S., Hansen, L., Longva, O., Emdal, A., Grande, L., 2010. A multidisciplinary study of ogy 30, 547 555. submarine landslides at the Nidelva fjord delta, Central Norway — implications for Vanneste, M., L'Heureux, J.S., Baeten, N., Brendryen, J., Vardy, M.E., Steiner, A., Forsberg, fl geohazards assessments. Norwegian Journal of Geology 90, 1–20. C.F., Kvalstad, T.J., Laberg, J.S., Chand, S., Longva, O., Rise, L., Ha idason, H., L'Heureux, J.-S., Steiner, A., Longva, O., Chand, S., Vanneste, M., Kopf, A., Haflidason, H., Hjelstuen, B.O., Forwick, M., Morgan, E., Lecomte, I., Kopf, A., Vorren, T.O., Reichel, Kvalstad, T.J., Forsberg, C.F., Vrdy, M.E., 2012. Identification of weak layers and their T., 2012. Shallow landslides and their dynamics in coastal and deepwater environ- role for the stability of slopes at Finneidfjord, northern Norway. In: Yamada, Y. ments, Norway. In: Yamada, Y., et al. (Ed.), Submarine Mass Movements and Their (Ed.), Submarine Mass Movements and Their Consequences. Advances in Natural Consequences. Advances in Natural and Technological Hazards Research, vol. 31. – and Technological Hazards Research, vol. 31. Springer, Dordrecht, pp. 321–330. Springer, Dordrecht, pp. 29 41. Löfroth, H., Suer, P., Dahlin, T., Leroux, V., Schälin, D., 2011. Quick clay mapping by re- Yordkayhun, S., Ivanova, A., Giese, R., Juhlin, C., Cosma, C., 2009. Comparison of surface sistivity — surface resistivity, CPTU-R and chemistry to complement other geotech- seismic sources at the SINK site, Ketzin, Germany. Geophysical Prospecting 51, – nical sounding and sampling. Swedish Geotechnical Institute, report GÄU 30. 125 139. http://dx.doi.org/10.1111/j.1365-2478.2008.00737.x.