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Geomorphology 93 (2008) 55–73 www.elsevier.com/locate/geomorph

Application of field in geomorphology: Advances and limitations exemplified by case studies ⁎ Lothar Schrott a, , Oliver Sass b

a Department of and , University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria b Department of Geography, University of Augsburg, Universitätsstr. 10, D-86135 Augsburg, Germany Received 30 December 2005; accepted 27 December 2006 Available online 10 May 2007

Abstract

During the last decade, the use of geophysical techniques has become popular in many geomorphological studies. However, the correct handling of geophysical instruments and the subsequent processing of the data they yield are difficult tasks. Furthermore, the description and interpretation of geomorphological settings to which they apply can significantly influence the data gathering and subsequent modelling procedure (e.g. achieving a maximum depth of 30 m requires a certain profile length and geophone spacing or a particular frequency of antenna). For more than three decades geophysical techniques have been successfully applied, for example, in studies. However, in many cases complex or more heterogeneous subsurface structures could not be adequately interpreted due to limited computer facilities and time consuming calculations. As a result of recent technical improvements, geophysical techniques have been applied to a wider spectrum of geomorphological and geological settings. This paper aims to present some examples of geomorphological studies that demonstrate the powerful integration of geophysical techniques and highlight some of the limitations of these techniques. A focus has been given to the three most frequently used techniques in geomorphology to date, namely ground-penetrating radar, seismic refraction and DC resistivity. Promising applications are reported for a broad range of and environments, such as talus slopes, block fields, , complex fill deposits, karst and covered landforms. A qualitative assessment highlights suitable landforms and environments. The techniques can help to answer yet unsolved questions in geomorphological research regarding for example thickness and internal structures. However, based on case studies it can be shown that the use of a single geophysical technique or a single interpretation tool is not recommended for many geomorphological surface and subsurface conditions as this may lead to significant errors in interpretation. Because of changing physical properties of the subsurface material (e.g. sediment, content) in many cases only a combination of two or sometimes even three geophysical methods gives sufficient insight to avoid serious misinterpretation. A “good practice guide” has been framed that provides recommendations to enable the successful application of three important geophysical methods in geomorphology and to help users avoid making serious mistakes. © 2007 Elsevier B.V. All rights reserved.

Keywords: Ground-penetrating radar; DC resistivity; Seismic refraction; Landforms; Internal structure

1. Introduction

Recently, the use of geophysical techniques has ⁎ Corresponding author. become increasingly important in many geomorpholog- E-mail addresses: [email protected] (L. Schrott), ical studies. Without the application of geophysics our [email protected] (O. Sass). knowledge about subsurface structures, especially over

0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.12.024 56 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 larger areas, remains extremely limited. During the last be adjusted to particular environmental conditions and decade the use of geophysical techniques has become a landforms. Studies that compare the application of various new and exciting tool for many geomorphologists. One interpretation tools and discuss the combined/composite reason for the increasing interest in geophysical field applications of geophysical field methods for a particular methods is certainly related to technical innovations as are still rare (Schrott et al., 2000; Schwamborn increased computer power and the availability of light- et al., 2002; Otto and Sass, 2006; Sass, 2006a,b). weight equipment allows for relatively user-friendly, Thus, the objectives of the present paper are: efficient and non-destructive data gathering. However, the correct handling of the geophysical instruments and (i) to show and assess the advances and limitations of subsequent data processing are still difficult tasks and the different geophysical methods with the focus on methods often require advanced mathematical treatment applied geomorphological research; for interpretation. It should be pointed out that without (ii) to demonstrate the application of these methods in close collaboration between geomorphologists and geo- different natural environments and for distinctive physicists the accurate and effective use of geophysical landform types, and techniques and their geophysical and geomorphological (iii) to illustrate the advantages of combined and interpretation are often very limited. In addition, the composite techniques leading to a set of recom- correct description and interpretation of geomorpholog- mendations for the application of field geophysics ical settings and thus, the choice of adequate and in geomorphology. meaningful field sites are not an easy task. In most studies and textbooks, interdisciplinary aspects As the paper focuses on the potential application of combining geophysics and geomorphology are poorly three geophysical methods in different environments, addressed. Many textbooks, in their description of the site descriptions have been reduced to a necessary geophysical surveying applications, focus on the explo- minimum. ration for fuels and mineral deposits, underground water supplies, engineering site and archaeological 2. Ground-penetrating radar (GPR) investigation (e.g. Reynolds, 1997; Kearey et al., 2002). Although several potential applications for geophysical 2.1. Principle and geomorphic context methods exist, many of them have not yet been fully integrated into geomorphological research. The most Ground-penetrating radar is a technique that uses high- common geophysical applications are currently focusing frequency electromagnetic to acquire information on permafrost mapping, sediment thickness determination on subsurface composition. The electromagnetic pulse is of talus slopes, block fields, alluvial fans and, increas- emitted from a transmitter antenna and propagates ingly, on the depth and internal structures of landslides through the subsurface at a velocity determined by the (Hecht, 2000; Tavkhelidse et al., 2000; Hauck, 2001; dielectric properties of the subsurface materials. The pulse Hoffmann and Schrott, 2002; Hauck and Vonder Mühll, is reflected by inhomogeneities and layer boundaries and 2003; Israil and Pachauri, 2003; Kneisel and Hauck, is received by a second antenna after a measured travel 2003; Schrott et al., 2003; Bichler et al., 2004; Sass et al., time. In order to calculate depth, wide-angle reflection and 2007-this issue). Other landforms such as karst and colluvia refraction (WARR) or common mid-point (CMP) mea- are comparatively rarely investigated (Hecht, 2003). surements have to be performed. These signal travel time Currently, the most common geophysical methods in measurements are made with a stepwise increase in the geomorphological research are ground-penetrating radar, distance between the two antennas. From the distance/ DC resistivity, and seismic refraction (Gilbert, 1999). Thus, travel time diagram, the propagation velocity of the radar the paper focuses on typical applications of these methods. waves in the subsurface can be derived. Each geophysical technique is based on the interpre- The common mode of GPR data collection is fixed- tation of contrasts in specific physical properties of the offset reflection profiling (Jol and Bristow, 2003). In this subsurface (e.g. dielectric constant, electrical conductiv- step-like procedure, the antennas are moved along a ity, density). The type of physical property to which a profile line and the measurement is repeated at discrete particular geophysical method responds determines and intervals resulting in a 2-D image of the subsurface. The limits the range of applications. As non-geophysicists possible working frequencies can range from 10 MHz to cannot generally be aware of all limitations and pitfalls, 1 GHz depending upon the aim of the investigation. there is a need to develop a set of the most suitable recipes Higher frequencies allow higher spatial resolution of the combining geophysical methods. These methods can then ground information, but lead to a lower penetration depth. L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 57

This has important implications in geomorphological high-resistive debris, a penetration of between 30 and applications. Knowledge about the geomorphological 60 m can be achieved (e.g. Smith and Jol, 1995). Sandy context (expected maximum depth and grain-size com- are also favourable for GPR measurements at position of a sediment body) is essential for choosing the depths of between 15 and 30 m. Due to the strong appropriate frequencies. A comprehensive “good practice attenuation in materials that have a high electrical guide” for the application of GPR in sediments is provided conductivity the penetration depth in wet, silty and/or by Jol and Bristow (2003). clayey sediments diminishes rapidly to, for example, less The maximum depth of investigation depends mainly than 5 m in (Doolittle and Collins, 1995); in clayey upon the dielectric constant (ε) and the electrical the application of GPR may be altogether impossible conductivity (σ) of the subsurface. A higher ground (see 1). This is, however, only a very rough guide, water and/or content (high ε, high σ) leads to a because the penetration depth depends upon the device stronger attenuation and, therefore, a markedly reduced and antenna frequency used. The vertical resolution of penetration depth. However, very pure GPR data is a function of frequency and propagation that have a low conductivity (e.g. glacial meltwater, velocity. With higher velocities, resolution decreases and bogs) are characterized by relatively low levels of GPR vice versa. As a rule of thumb, in the medium velocity signal loss. Again, geomorphological expertise about range (0.1 m/ns) the resolution is approximately 1 m possible water and clay layers can help to avoid using 25 MHz antennae, 0.25 m using 100 MHz and disappointing measurements. On dry and electrically 2.5 cm using 1 GHz.

Table 1 Comparison of common field geophysical methods in geomorphology; examples of applications and some technical considerations and advice Geophysical method Geomorphological application Technical considerations and practical advice Ground-penetrating ▶Delineation of the boundaries of massive in ▶Small penetration depth in case of conductive near-surface layers radar (GPR) , and other periglacial phenomena ▶Determining the thickness of a permafrost layer ▶Less successful in, for example, wooded with many surface reflectors and in electrically conductive sediments (e.g. clay-rich) ▶Active layer thickness ▶Experience in data processing and interpretation is needed ▶Glacial ice thickness ▶Correlate radar data with geomorphic/geologic control and site observations such as exposures or logs ▶Delineation of aquifers ▶Fracture mapping within massive ▶Mapping of internal structures in sediment storage types (e.g. talus slopes, rock glaciers)

2-D DC resistivity ▶Determining sediment thickness, internal ▶Obtaining good electrical contact between electrodes and ground is sounding, 2-D DC differentiation and essential resistivity profiling ▶Depth and extension of bodies ▶Blocky and dry material is very problematic due to poor electrode coupling ▶Detection of massive ice or permafrost in rock ▶Experience in data inversion is needed for data processing. A priori glaciers, moraines and other periglacial phenomena information influences (improves) significantly your iterative modelling ▶Ice thickness ▶Differentiation between ice, air and special rock types can sometimes be difficult ▶Determining the altitudinal permafrost limit ▶Correlate resistivity data with geomorphic/geologic control and site observations such as exposures or borehole logs ▶Moisture distribution in rock walls ▶Seasonal variation in fluid content

Seismic refraction (2-D ▶Sediment thickness (talus, alluvial fans, etc.) ▶Number of geophones should be at least 12, with shots at every sounding, other receiver location tomography) ▶Detection of massive ice in rock glaciers, ▶Sledge hammer as source is sufficient for most shallow moraines and other periglacial phenomena applications (b30 m) ▶Differentiation between ice, air and special rock ▶Experience in data processing and inversion is needed for data types interpretation ▶Mapping active layer thickness ▶Correlate seismic data with geomorphic/geologic control and site observations such as exposures or borehole logs 58 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73

2.2. Advantages and disadvantages of GPR tree-line and where there is limited soil development. However, shallow subsurface investigations are also Even when low-frequency 25 and 50 MHz antennas are possible in fluvial deposits and even in , when the used, GPR still provides a better spatial resolution than electrical conductivity of the groundwater is low. standard geophysical techniques. The survey speed even in rough terrain is relatively high and several hundred metres 2.3. Examples of application per day are possible. Steep and rocky slopes limit the progress of the survey, because low-frequency antennas There is a broad range of successful applications of have large geometrical dimensions and are difficult to GPR in geomorphological studies. These include the handle. Recently developed, so-called, Rough Terrain detection of buried structures, assessment of internal Antennas (RTAs) may facilitate data gathering, however, sediment structures and estimation of depth to bedrock. conventional antennas are still necessary for CMP Various types of sediments have been investigated for measurements or small object detection. geomorphological purposes (Bristow et al., 2000). The available antenna frequencies allow for a broad The internal structures of floodplain deposits and variety of possible applications. However, the extremely deltaic sediments have been visualized by, for example, variable penetration depth requires careful assessment of Leclerc and Hickin (1997), Jol (1996) and Büker et al. the subsurface parameters in the study area in order to (1996). Buried fluvial channels have been detected by minimize the risk of error. The electrical conductivity of Roberts et al. (1997) and Loope et al. (2004).Themost the soil provides a rough indicator of potential target frequently used antenna frequency for comparative studies depth. From the authors' experience, the use of GPR is is 100 MHz. The penetration depth is dependant upon clay not promising if the soil resistivity is lower than ca. 50– and , but usually ranges from 10 to 20 m. 100 Ωm. Loose sediments in arctic and alpine areas have also The application of GPR is subject to further restric- been the subject of GPR measurements. Lønne and tions. As the reflectivity at a layer boundary is determined Lauritsen (1996) and Overgaard and Jakobsen (2001) by the contrast in the dielectrical properties of the have investigated internal deformation structures of push- subsurface units, no distinct reflection is found when moraines and Berthling et al. (2000) clearly detected this contrast is low. Small-scale spatial differences in internal structures as as the bedrock base of rock water content and/or grain-size composition may yield glaciers. The working groups used 50 and 100 MHz stronger reflections than the target of the investigation antennas and achieved a penetration depth of up to 30 m. (e.g. the bedrock surface). This problem may be overcome Sass and Wollny (2001) and Sass (2006a) achieved a by using the radar of the sediments for interpreta- penetration depth of up to 50 m on talus slopes using tion. Different sediment units and bedrock yield typical 25 MHz antennas. They found surface-parallel structures reflection patterns that can be derived from, for example, in the debris body and evidence for material at the reference profiles. The reconnaissance of these patterns base of the talus. Studies of sediment structures have also significantly facilitates the interpretation of the radargram. frequently been carried out in bogs. Völkel et al. (2001) A portion of the energy transmitted by an unshielded investigated the subsurface structure of buried periglacial antenna is emitted into the air and may be reflected by slope deposits, while e.g. Holden et al. (2002) determined features above the surface. These air reflections the position and depth of subsurface piping. cannot always be unequivocally distinguished from The application of GPR on landslides has repeatedly ground information and may severely affect the data been tested but with limited success. Wollny (1999) quality. This makes the application of GPR particularly investigated the near-surface moisture distribution at 10 difficult in wooded terrain where, depending on the landslide areas and gained valuable information from only frequency used, each tree may act as a single reflector, three sites. Bruno and Mariller (2000) and Wetzel et al. leading to very noisy or altogether useless data. Although (2006) used GPR for detecting the vertical extension of the effect of air wave reflections may be reduced using landslides but did not reach the slip surface even with the sophisticated filter algorithms (e.g. Van der Kruk and use of low-frequency antennas. However, internal slide Slob, 2004), measuring in forested areas is not advisable. structures such as rotational features can be mapped when The use of shielded antennas is only possible for higher the slide surface is comparatively dry (e.g. Sass et al. (this frequencies (N100 MHz). Taking into consideration the issue) in displaced limestone blocks). Bichler et al. (2004) major restrictions arising from “clayey or silty subsur- obtained very good results, distinguishing seven different face” and “wooded terrain” it is clear that GPR shows its facies of loose sediments. Performing GPR measurements potential particularly in arctic or alpine areas above the at landslide sites is only worthwhile when comparatively L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 59 coarse and dry deposits (debris, displaced blocks) 2.3.1. Case study: localization of buried structures superimpose the silty or clayey material of the slip The aim of this GPR application was to locate a surface. However, detailed assessment of the near-surface Roman buried under 1.5 to 3 m of peat and fluvial propagation velocity allows a rather accurate estimate of sediments in the Murnauer Moos, Upper Bavaria (Sass the content (Topp et al., 1980) which is of et al., 2004). This example primarily highlights the interest for landslide investigation and many more potential for archaeological studies (Fuchs and Hruska, geomorphological questions. 1996). However, from a geomorphological perspective Another possible field of application is the investiga- the Roman road can be used as a time marker for the tion of permafrost features. The active layer thickness has evolution of the overlaying peat and interbedded clay- been determined, for example, by Arcone et al. (1998) and rich sediments. The road had been examined at an Hinkel et al. (2001). Moorman et al. (2003) provided archaeological excavation nearby. Thus, the structure of instructive pictures of typical reflection patterns in frozen the road (logs lying on a layer) was known. The and unfrozen ground. Although the presence or non- objective of the measurements was the quick localiza- presence of permafrost is more difficult to establish with tion in the surroundings of the excavation. GPR than electrical resistivity techniques, GPR is As a result of the high water content of the peat, the superior in detecting spatially confined structures such propagation velocity derived from WARR measurements as ice wedges (Hinkel et al., 2001). The thickness and the was very low (0.45 m/ns). However, because of the low internal structure of ice (unfrozen water content, mineralization of the bog water the penetration depth cavities) have also been the target of many GPR using 200 MHz antennae was up to 5 m. The road was investigations (e.g. Moorman and Michel, 2000). quickly and clearly located in a number of cross profiles The investigation of quasi point-shaped or linear (Fig. 1). Thus, the measurements provided valuable buried structures in high resolution is the aim of many information on the straight-line structure. The cross- studies in the relatively new field of geo- profile presented (Fig. 1a) shows the radar reflection of that is closely related to geomorphology (Baker et al., the Roman road at a profile distance of between 3 and 7 m. 1997; Fuchs and Zöller, 2006). Leckebusch (2003) has The shallow depression in the middle of the road (as provided a detailed description of the GPR method for observed at the excavation site), caused by the weight of archaeological purposes with numerous examples. The the vehicles, can be clearly recognized. The longitudinal working frequencies for these applications are usually profile (Fig. 1b) illustrates the linear structure of the road. rather high (=200 MHz); the target depth is usually The main reflection is caused by the artificial gravel layer between 1 and 5 m. which obviously shows a distinct dielectrical contrast to

Fig. 1. Investigation of a buried Roman road using 200 MHz GPR profiles. Filters applied: DC-shift removal, bandpass frequency filter and runtime- dependant gain function. Above: cross profile, below: longitudinal profile. The road is recognizable from typical reflection patterns (see text) (Sass et al., 2004). 60 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 the surrounding peat. The V-shaped structures below profile, these lines represent reflectors dipping at an angle (e.g. at profile distances of between 10 and 14 and be- of between 60 to 70° to the south (left side). This agrees tween 80 and 100 ns) are the hyperbolic reflection patterns with the dipping of the gneiss–micaschist layers of the of singular logs lying at right angles to the course of adjacent rockwall. The occurrence of airborne (rockwall) the road. From this typical pattern the road can be un- reflections is improbable because of the position and equivocally distinguished from geological reflectors. angle of the reflectors, together with the low inclination of In a series of cross profiles, the road was located even the rockwall above. in positions where borings failed to detect the structure, probably due to the advanced rotting of the logs. 3. 1-D and 2-D DC resistivity However, alluvial layers near the surface considerably reduced the penetration depth due to high attenuation in 3.1. Principle and geomorphic context the clay. In the vicinity of some shallow alluvial cones, the penetration depth was reduced to less then 1 m Geoelectrical sounding provides a one-dimensional which made the detection of any structures impossible. vertical profile of the electrical resistivity distribution with depth. Resistivity measurements are conducted by 2.3.2. Case study: talus sediment thickness applying a constant current into the ground through two Fig. 2 shows the 50 MHz longitudinal section of a talus “current electrodes” and measuring the resulting voltage slope in the Kühtai area (Central Alps, Austria) at an differences at two “potential electrodes”. From the current elevation of between 2400 and 2600 m. The profile and voltage values, an apparent resistivity value is stretched from a moraine ridge at the foot of the talus calculated. 1-D surveys use only four electrodes. upwards (inclined approx. 30°) to the gneiss/mica-schist Generally, the two potential electrodes remain in position rockwall (inclined at 55 to 60°). In the upslope parts of the in the middle of the profile, while the current electrodes profile (approximately 70 to 130 m), the debris is rather move stepwise to both sides (Schlumberger array). The fine-grained (5 to 20 cm) and gradually gets coarser wider the electrodes are apart, the deeper the electrical towards the base of the slope, where boulder-sized clasts field penetrates into the ground. Thus, a depth profile prevail. under the approximate middle of the section is created. The bedrock surface is recognizable as a distinct This array has been widely used in geomorphic research reflector (highlighted by the thick dashed line) revealing a (Etzelmüller et al., 2003). 2-D arrays represent a further shallow basin (see Fig. 2). The part of the talus body close development of the 1-D technique, using 50 or more to the rockwall (between 60 and 110 m) is characterized electrodes at a time. A micro-controller unit automatically by surface-parallel, slightly undulated and partly over- switches between numerous electrode configurations, lapping reflection patterns probably indicating stacked thus creating a 2-D pseudosection of the subsurface. The sediment layers. No comparable layers can be observed inversion of the gathered data using sophisticated near the foot of the talus (0–60 m). This points to fluvial computer programs produces a 2-D section of the distribution that is restricted to the vicinity of the subsurface resistivity. The Res2Dinv-software provided moderately steep rock face. The maximum thickness of by Loke and Barker (1995) is the most commonly used in the debris is 12 m in the middle of the profile. In the deeper geosciences. The principle of operation is based on a subsurface, thin lines are visible which are slanting stepwise iteration process which tries to minimize the diagonally to the left. Considering the inclination of the deviation between the measured apparent resistivity and

Fig. 2. 50 MHz radargram of a talus slope in the Sellrain region, Austria. The inclination of the slope is approximately 30°. Filters applied: DC-shift removal, bandpass frequency filter and runtime-dependant gain function. L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 61 the simulated apparent resistivity values calculated from a orders of magnitude, depending for example upon water subsurface model. The 2-D section can also be conducted content and jointing. Thus, a measured resistivity cannot in a Schlumberger array, which provides particularly good be directly assigned to a certain substrate. resolution for lateral inhomogeneities. The Wenner array shows a good signal–noise ratio and is favourable for the 3.3. Examples of application detection of horizontal layers, while the Dipole–Dipole array is favourable for the delimitation of spatially Through the multitude of possible electrode arrays, confined objects in the shallow subsurface. For a 2-D resistivity is suitable for many very different tasks comprehensive description of these configurations we (Beauvais et al., 2003). However, the two main groups refer to geophysical textbooks (Milsom, 1996; Reynolds, of current applications in geomorphology comprise the 1997; Kearey et al., 2002). detection and characterization of permafrost and the The maximum amount of aprioriinformation on the investigation of landslides. Employing 2-D resistivity geomorphological context should be obtained (e.g. layer profiling for the detection of permafrost is highly thickness, bedrock type and expected resistivity value) advisable because of the very strong electrical contrast before starting the inversion of the raw data. The apriori between ice and almost all other common substrates. estimate (e.g. maximum resistivity value of an expected Evidence for frozen ground in permafrost areas using 1- type of sediment) can be set as a fixed parameter and helps D resistivity profiling has been found, for example, by to improve the model. The required information can be Assier et al. (1996) and Ishikawa and Hirakawa (2000). derived from test profiles of, for example, bedrock or The active layer thickness in mountain permafrost has sediment units of known composition. been assessed (e.g. by Gardaz, 1997), while Isaksen et al. (2000) have measured the thickness of the ice layer 3.2. Advantages and disadvantages of rock glaciers in . 2-D resistivity sections performed, for example, by Kneisel (2003) and Kneisel A great advantage of the method is the high variability and Hauck (2003) clearly show the lateral extension of of electrode spacing and configuration. The distances ice lenses in alpine rock glaciers and slopes. A between the electrodes can range from some centimetres to range of possible applications in permafrost areas has several tens or hundreds of meters, allowing a penetration recently been presented by Kneisel (2006). depth from decimetres to hundreds of meters. The Numerous papers have dealt with the assessment of electrode array (e.g. Schlumberger, Wenner, Dipole– the depth and the detection of the structures of Dipole) can be chosen according to the aim of the landslides. The target depth of the investigations can investigation. Almost no restrictions regarding topogra- be quite variable. The slip surface of a very shallow phy, subsurface features and apply. However, landslide was detected by Wetzel et al. (2006) using 2-D very dry or extremely blocky substrates are basically profiling to a depth of between 10 and 15 m, while unfavourable. Special measures have to be taken to Perrone et al. (2004) achieved a penetration depth of improve the electrode-to-ground coupling such as water- 80 m using survey lines of up to 600 m. Agnesi et al. ing of the electrodes or inserting them through wet (2005) reached a penetration depth of almost 200 m in a sponges. It is possible to insert electrodes into hard rock 1-D survey of a deep-seated landslide in Italy. Bichler using a hammer drill. However, the best results are et al. (2004) combined numerous 2-D profiles to a quasi generally obtained on loamy and relatively wet subsurface. three-dimensional picture of the landslide subsurface. Geoelectrical surveys always integrate the electrical Mauritsch et al. (2000), Godio and Bottino (2001) and properties of a certain volume of the subsurface. The Bichler et al. (2004) combined the geoelectric measure- extent of this volume increases in the deeper subsurface. ments with further methods (electromagnetic profiling, Thus, the accurate detection of sharp boundaries is seismic refraction) to improve the interpretation of the rarely possible especially in the deeper parts of a profile. combined data. Suzuki and Higashi (2001) demonstrat- An accurate assessment of depth to bedrock is only ed the effectiveness of 2-D resistivity for long-term possible where there is a very sharp resistivity contrast investigations monitoring the of rainfall into between overlying loose sediments and bedrock. a landslide body in numerous time slices. A further, significant problem is related to significant Kneisel (2003) provides some further examples for overlapping ranges of resistivities for different substrata. the investigation of depth to bedrock such as the According to textbook tables, the resistivity values of assessment of the thickness of aeolian sediments. Sass almost every subsurface unit (regardless if loose (2006a,b) and Otto and Sass (2006) applied Electrical sediment or bedrock) may cover a range of several Resistivity Tomography (ERT) to the measurement of 62 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73

Fig. 3. Small scale (electrode spacing 4 cm) 2-D geoelectric sections of a partially frozen rock outcrop in the Karwendel mountains (Germany). The red colours in the lower picture indicate frozen rock near the surface, while the deeper parts remain unfrozen (Sass, 2004).

talus thickness. A rather unusual application is the very 3.3.2. Case study: internal structure of a landslide small-scale monitoring of water movement during The Schliersberg slope is situated at the northern edge freezing, as performed by Sass (2004). of the Bavarian Alps. Flysch and Ultrahelvetikum rock series prevail (sandstone and claystone) which are very 3.3.1. Case study: detection of frozen rock prone to landslide processes. The most prominent 2-D resistivity profiling was used for the monitoring geological feature is the overthrust of the Flysch of freeze–thaw cycles in different rock types at various nappe (here represented by comparatively stable - test sites using 50 small electrodes inserted neatly into stone) on the underlying clayey Ultrahelvetikum series. drilled holes with a spacing of 4 cm (Sass, 2003, 2004). The slope is rather steep in the area of outcropping The profile line extended diagonally over a jointed sandstone (up to 40°) and only moderately inclined in the limestone outcrop. During the night, temperatures clayey rock series (20–25°). A very active combined dropped to −4.0 °C at the surface and −2.6 °C 15 cm landslide has developed in the Ultrahelvetikum series beneath the surface. 2-D resistivity measurements were which has caused considerable damage to the spruce carried out hourly using the Wenner array. forest and forestry . The slope beside the active slide Fig. 3a shows the resistivity profile of the unfrozen shows obvious signs of former rotational slide movements rock. In Fig. 3b, the frozen areas of the rock are visible on the surface such as ridges and damming wetness in as confined patches of very high resistivity (≥15 kΩm). hollows but is obviously stable today. Extensive 2-D In the course of freezing, near-surface ice formations resistivity profiling was carried out in order to highlight occurred over the entire survey line, while the water the structure and depth of the slide. The longitudinal content in the deeper parts of the rock slowly increased profile presented in Fig. 4 runs parallel to the active slide due to displaced pore water. The comparatively high at the currently stable slope, while the cross profile RMS error of 15.9% in the inversion model is caused by extends across both the inactive and active zones. the extremely sharp resistivity contrasts in the vicinity of The contrast between the electrical properties of the frozen areas. A minimum temperature of −2.6 °C these series is clearly to be seen in the upper part of the was reached at a depth of 15 cm at 06:30. However, the longitudinal profile. The Flysch overthrust fault dips entire rock body below approximately 10 cm remained steeply to the south. Further down the slope the structure unfrozen, which points to supercooling due to hydro- of a multiple rotational slide can be recognized. The static pressure under the superficial ice formation. The landslide blocks consist of displaced, partly mingled measurement highlights the generation of hydraulic sandstone and mudstone. The slip surface is at the top of pressure as a possible agent of frost . the native mudstone bedrock. A step in the longitudinal L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 63

Fig. 4. Longitudinal and cross sections of the Schliersberg landslide in the Bavarian Alps (Germany). The longitudinal profile highlights the rotational structure of the slide, while the cross profile shows the difference between the active and inactive areas of the slope. profile of the slope (64–80 m) is obviously due to a The propagation of seismic waves through layered resistant layer of the Ultrahelvetikum bedrock. The ground is determined by the reflection and refraction of cross profile shows the comparatively dry, inactive the waves at the interface between different layers. When rotational blocks on the left, while the active combined the wave reaches the interface some energy of the wave is landslide on the right consists of wet, clayey substrate refracted into the deeper layer, while the reflected wave and displays a very low resistivity. The conchiform slip transmits the energy back into the overburden layer. The surface is highlighted by the white dashed line. A angle of incidence of the reflected wave is equal to the displaced rotated block is embedded in the landslide angle of reflection, independent of the seismic velocities near the surface. The 2-D profiles aid the recognition of of the layers. However the angle of refraction follows the type of movement and clearly show the zones of Snell's law: rotational and translational features possibly associated h = h ¼ = with changing depths to the slip surface. sin i sin r V1 V2 where θ is the angle of incidence, θ is the angle of 4. Seismic refraction i r refraction and V1 and V2 the seismic velocities of the upper and lower layer respectively. In the case of critical 4.1. Principle and geomorphic context refraction, where θi =θc and θr =90° the refracted wave travels parallel to the interface with a velocity V .The The principle of seismic refraction is based on elastic 2 critical refraction is given by the ratio of the velocities V waves travelling through different subsurface materials, 1 and V : such as sand, gravel, and bedrock, at different velocities. 2 The denser the material, the faster the waves travel. A sin hc1;2 ¼ V1=V2: prerequisite for the successful application is that each successive underlying layer of sediment or bedrock must Since sin θc1,2 ≤1 it follows that V1 has to be smaller increase in density and therefore, velocity. Two types of than V2. Therefore the prerequisite in refraction seismics seismic waves can travel through the subsurface: longitu- is an increasing seismic velocity with depth. The critical dinal or primary (P) waves which are characterized by refracted wave that travels along the interface produces deformation parallel to the direction of wave propagation, oscillation stresses, which in turn generate upward and transverse or secondary (S) waves where particle moving waves, known as head waves. Since the motion takes place at right angles to the direction of wave propagation velocity V2 is faster within the lower layer, propagation. at a certain distance from the shotpoint (crossover 64 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 distance) these head waves reach the surface (geophones) More powerful sources (e.g. drop weights, explosives) faster than the direct wave providing the first arrivals at must be used for longer surveys with larger penetration the geophones. In seismic refraction studies, it is the first depths. To apply seismic surveys in geomorphic studies it arrivals of the P waves that are utilized. is highly recommended to use instruments with several The seismic velocity of P waves is dependent on the single light units rather than to use an all-in-one (heavy) elastic modulus and the density, ρ, of the material, seismograph. through which the seismic waves propagate. Based on the raypath geometry described above and the 4.2. Advantages and disadvantages structure of the layered underground, it follows that the travel times for the seismic waves can be used to obtain The accurate first onset detection of seismic waves is certain time–distance plots. In the case of a layered very often a difficult task. In coarse grained surface underground with planar interfaces, the first arrival times lie conditions (i.e. on talus slopes and debris cones) and on a number of clearly defined straight-line segments. The hummocky (i.e. on rock glaciers or rockfall number of the segments corresponds to the number of the deposits) difficulties may arise from the poor coupling of underground layers. The slope of the straight line segments geophones to the surface. On talus slopes with larger is proportional to the reciprocal velocity of the layers. block sizes directly on the talus surface, it is advisable to Travel time anomalies are caused by irregularities of the improve the coupling of the geophones by removing the interfaces and varying velocities within the different layers. upper layer of larger boulders and pushing the spike of the The arrival of a seismic wave is detected by geophones, geophone into fine-grained material underneath. Cou- which are placed firmly in the ground using a spacing of pling to larger boulders or to compact bedrock may be between 1 and 5 m. For example, 24 geophones (with a 24 established by drilling into the rock and plunging the channel seismograph) combined with a spacing of 4 m spike within the drill hole. results in a spread of 92 m. If the expected depths of the In the vicinity of torrents and under strong or interfaces are shallower than 30 m the geophone spacing rainfall, the detection of seismic waves may be impossible may be reduced (i.e. between 1 and 3 m) to gain a higher due to strong noise in the seismic record. Special attention resolution record of the underground. To detect a layer should be drawn to the obtained velocities of materials within the time–distance plot the geophone spacing must which are common in particular landforms (e.g. talus be smaller than the difference between two following deposit, till, ). The large range of observed crossover distances. At the crossover distance the direct velocity values, spanning from ∼400 m/s (loose debris) wave is overtaken by a refracted wave. Beyond this offset up to 6500 m/s (compact rocks) is generally conducive to distance the first arrival is always a refracted wave. In the application of refraction seismics, as large velocity refraction surveying, recording ranges are chosen to be contrasts between the underlying materials are necessary. sufficiently large enough to ensure that the crossover However, ranges of P wave velocities for rocks and distance is well exceeded in order to detect a sufficient sediments can overlap significantly. For instance, seismic number of first arrivals of refracted waves. In cases of thin velocities of dolomite and limestone can vary between subsurface layers, where the distance between crossover 2000 and 6500 m/s depending on the of fracturing points is small, a narrow geophone spacing (between 1 and and weathering, while, for example, glacial sediments 3 m) is necessary. This in turn may result in a lower compacted by overlying ice can possibly range from 1500 penetration depth due to a reduced total spread. to 3500 m/s. Consequently, there is no unambiguous The most common seismic source in geomorpholog- relationship between certain subsurface units and the ical studies is a 5 kg sledge hammer which generates linked P wave velocities. As a result, it is not always seismic waves by hitting a metal plate placed on the possible to identify subsurface materials simply based on ground. To improve the signal to noise ratio the sledge seismic velocities. To differentiate glacial till (without ice) hammer “shot” is usually repeated several times (typically from frozen ground or solid rock, cross checks with other between 5 and 10 times) at the same source spot. The geophysical methods and/or evidence from borehole resulting seismograms of each shot are stacked (summed) logging are needed. Generally, landforms consisting of manually or automatically to obtain a single seismogram similar sediments but deposited by different geomorphic per shot location. processes cannot be distinguished (Hoffmann and Generally, a sledge hammer is sufficient to measure the Schrott, 2003). Another very common disadvantage in first arrivals along offset distances of approx. 50 m and seismic refraction is the “hidden layer” problem, which seldom more than 90 m. This corresponds to penetration may lead to incorrect interpretations (Pullan and Hunter, depths of 10 to 30 m and is suitable for many landforms. 1990; Hecht, 2001). The hidden layer is caused by a L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 65

Fig. 5. (A) Seismograms of forward and reverse shots and intercept models of the refraction sounding on a talus cone (velocities of refractors derived from the slopes); (B) Intercept time model (straight line) and wavefront inversion model of the above mentioned sounding; (C) Refractors of the network ray tracing analysis and tomography using the same data set. The colours indicate the velocities (red b300 m s−1,blue300–800 m s−1, green/ yellow 800–1200 m s−1,orangeN1200 m s−1). 66 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 sandwiched layer with lower velocities between higher idated rockfall deposits probably have similar veloci- velocity units of refracted first P wave arrivals. Although a ties. Thus, the main objective of this case study is the hidden layer produces head waves it does not give rise to assessment of depths to bedrock and of the variation of clear first arrivals. Pullan and Hunter (1990) report an values obtained from different interpretation tools. One underestimation of bedrock of up to 25% due to a hidden of the most interesting findings of this case study is the layer. varying depth to bedrock (or perhaps highly consoli- dated till) with regard to the interpretation tools (see 4.3. Example of application Fig. 5). The mean depths of the refractors based on the most sophisticated network ray tracing model are 1.6 m 4.3.1. Case study: coalescing talus cone (first refractor) and 10 m (second refractor), whereas the Seismic refraction was carried out on a moderately less reliable intercept method gives mean depths of 6 steep (21°) talus cone in an alpine valley in the Bavarian and 27 m for the first and second refractor, respectively. Alps. For the survey, a 24-channel Bison Galileo In this case, the sole application of the intercept method seismic system and a 5 kg sledge hammer, as an impact would have caused a serious error in the data source, were used. The applied geophone spacing was interpretation. 3 m resulting in a profile length of 69 m. The intercept Based on the P-wave velocities of the model layers time method, the wavefront inversion method in using the above mentioned methods and on geomorphic combination with network ray tracing and the seismic evidence of some exposures (loose debris on top and tomography method were applied in order to interpret rockfall and debris-flow deposits underneath) on the the seismic data and to show the differences among flanks of the coalescing talus cone we interpret the model themselves (Hoffmann and Schrott, 2003). The intercept shown in Fig. 5 as follows: time method was only applied to forward and reverse shots, resulting in the apparent velocities and intercept First layer: very loose limestone debris accumulated on times of the different layers. top of the debris cone. Wavefront inversion (WFI) modelling was per- Second layer: most likely rockfall and debris flow formed using the following steps: (i) reading of travel material with a higher degree of and a times of first arrivals of the direct and refracted P waves, higher content of fines (compared to first layer). (ii) combining travel times of each record to a single Third layer: probably bedrock (limestone). Remnants of travel-time curve, assignment of the travel times to the morainic deposits, however, are also possible. To corresponding layers in the subsurface model, and (iii) distinguish between fractured bedrock and till an inversion of the travel times using the wavefront additional geophysical method (GPR or DC resistivity) inversion algorithm. should be applied. The velocity of P waves as a physical For network ray tracing a start model (typically an property is in this case not sufficient to enable existing WFI model) was used to calculate synthetic interpretation of the third layer. The alternative travel times and tomography analysis was applied to give assumption of till would result in a greater depth to automatic of synthetic travel time data to real bedrock. data. Tomography analysis allows the modelling of a heterogeneous subsurface structure with a minimum of 5. Combining and adjusting geophysical methods to knowledge about it. Therefore, we used tomography geomorphological problems models as background information for the modification of the WFI models in order to obtain a better The various geophysical field methods rely on the correspondence of the synthetic and measured travel evaluation of different physical properties and it is, times. A detailed description of the sophisticated therefore, essential that the most appropriate technique is analysis of the subsurface structure including wavefront applied to a given geomorphological problem (Milsom, inversion method, network ray tracing and refraction 1996). Seismic refraction is sensitive to density contrasts tomography is described in Hoffmann and Schrott and, thus, suitable for differentiating loose sediments and (2003). bedrock. With GPR it is possible to acquire information It is likely that the investigated landform consists of on internal sediment structures and thus, it can be a mixture of different types of sediment sources successfully used to delimitate sediment units of (rockfall deposits, moraine). From the seismic data different origin. 2-D resistivity is highly appropriate for alone, it is impossible to differentiate the internal investigations in permafrost environments and can structure because debris covered moraine and consol- deliver high-resolution subsurface data in loamy and/or L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 67 wooded terrain whereas GPR frequently fails to deliver problems it is strongly advised that two or more any data at all. geophysical methods are used whenever possible. Thus, each of the geophysical methods presented has a great potential for aiding clarification of geomorpholog- 5.1. Composite application on talus slopes ical problems (Table 2). Nonetheless, every method has its drawbacks and limitations. The main reason for incom- In the small Tegelberg cirque in the northern plete or false conclusions lies in a lack of contrast between European Alps, a relict talus slope is interlocked with the physical properties of the subsurface layers, particu- sediments at the cirque floor. The aim of the larly between compacted or water-saturated sediments investigations was to assess the depth to bedrock and to and bedrock. With regard to the common problem of detect further internal structures within the talus body in differentiating loose debris from the bedrock base, it can order to provide an estimate of the rate of Holocene be stated that (1) there might be no distinct dielectrical backweathering. Three geophysical techniques were contrast and thus, no radar reflection at the bedrock applied. Fig. 6 shows the combined interpretation of the surface, (2) the electrical resistivity of debris can vary by results (Sass, 2006b). orders of magnitude, which means that contrasts within the sediment body may exceed and mask the contrast to – The 25 and 50 MHz GPR measurements were the bedrock, and (3) the seismic velocities of compacted performed using a RAMAC GPR (MALÅ Geosys- sediment such as basal moraine may overlap the velocity tems). The profiles show a distinct reflector between range of bedrock, thus producing potential errors in the the debris and the silty and clayey deposits of the detection of the sediment base. To overcome these cirque floor. A weaker reflector, that was traced upwards, was initially thought to mark the bedrock surface (white dashed line in Fig. 6A). The upper part of the debris body showed two different reflection Table 2 Common geophysical methods in geomorphological applications and patterns: an upper layer characterized by surface- qualitative assessment in terms of suitable detection of thickness and/ parallel stratification and an unbedded layer below or internal structure (separated by dotted line in Fig. 6A). The interface Ground- 2-D DC 2-D seismic between these units was marked in the lower part of penetrating resistivity refraction the talus by a distinct reflector. radar – The 2-D resistivity measurements (GeoTom device, Physical property Dielectrical Resistivity Elastic 88 electrodes, spacing 2 m) also displayed a distinct properties moduli, contrast between talus and cirque floor (Fig. 6B). density Near the middle of the profile, a weak resistivity Geomorphic context contrast at a depth of approximately 15 m was Talus slopes, debris cones ++ a/++ b ±/+ ++ a/± b assumed (white dashed line) and this matches the Block fields +/+ ±/o +/o GPR reflector mentioned above. However, the 2-D Alluvial fans, floodplain +/++ +/+ ++/± resistivity method was at the limit of its penetration Colluvial +/+o +/o ±/– – c c depth. Landslides /± +/++ ±/± – Karst features ± o o Seismic refraction measurements revealed that the P Permafrost lenses + ++ ± wave velocities near the supposed talus base were Active layer thickness ± ++ ++ still much too low (800 to 2000 m/s) to indicate Permafrost occurrence ++++ dolostone bedrock. The transition to higher velocities (widespread) (3500 m/s) was found almost 10 m deeper than Rock glaciers ±/+ ±/++ ± d Rock/soil moisture ±+– supposed. distribution Note: the indication of suitable application may be incorrect under Thus, it was concluded that a thick layer of basal specific or extreme local conditions (e.g. wet, dry and blocky). moraine forms the base of the talus. According to field (++ very recommended, + recommended, ± may be used but not evidence and to the stacked radar facies, the Holocene necessarily the best approach, o has not been widely used or developed debris is mainly due to episodic mudflow activity; it was – for geomorphic application, unsuitable). marked off from the GPR reflection patterns at an a Thickness, depth-to-bedrock. b Internal structure/distribution. average depth of 7 m. Within the accessible depth range c Only suitable in dry sediments. of between 7 and 10 m, a number of percussion drillings d Unsuitable on active rock glaciers with a permafrost body. successfully verified the geophysical results (Fig. 6C). 68 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73

Fig. 6. Combined application of GPR (25 and 50 MHz), 2-D resistivity, seismic refraction and percussion core drillings on a talus slope at the Tegelberg, Bavarian Alps, Germany (Sass, 2006b). A: GPR section; B: 2-D resistivity section; C: combined interpretation with seismic refraction and drilling results.

5.2. Combined application on a block field process (inversion technique). No constraints were manually applied to the inversions. A combined application of seismic refraction and DC The 2-D interpretation of the seismic refraction data resistivity sounding using 24-channel Bison equipment was performed with the generalized reciprocal method and an Abem Terrameter respectively, was carried out on a (GRM), which is a direct inversion technique which uses basaltic block field in a German upland near Bonn. A travel-time data from both forward and reverse shots geophone spacing of 3 m was applied resulting in a profile (Palmer, 1981). This provides the geometry of sub-surface length of 69 m. For the 1-D resistivity sounding we refractors (Fig. 7). In order to compare the two data sets an applied the symmetrical Schlumberger configuration with identical profile location was used. Geophones and a total profile length of 100 m (AB/2=50 m). From the electrodes were placed parallel to contour lines in the measurements, field curves were established by plotting centre of the steep (30°) block field. The determination of the apparent resistivity against the distance between the the overall depth of the block field was of major interest. current electrode and the profile centre (AB/2) on a log– The seismic refraction reflects two refractors at a depth of log scale (see Fig. 8). In this study the profiles were approximately 2 and 14 m on average (Fig. 7). Along the interpreted using the software RESIXPlus (®Interpex profile the second refractor varies between 10 and 16 m. Lim). For both soundings a 3-layer model was assumed According to the obtained P wave velocities of based on the synthetic curve which describes the N2800 ms−1 it was assumed that this was the depth-to- resistivity variation with depth. The algorithm then bedrock. This coincides well with the DC resistivity data calculates a curve giving the best fit of the synthetic showing, at similar depths, a significant increase in the curve to the field data (see dots in Fig. 8) using an iterative apparent resistivity with typical values of between 103 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 69

Fig. 7. Results of 2-D seismic refraction of a block field in a German upland. Travel time curves and refractors generated with the General Reciprocal Method (Palmer, 1981). and 104 Ωm for basalt (Fig. 8). With regard to the upper depth in the data from the survey in Summer. In Winter, layers and internal structure of the block field comple- however, the resistivity is significantly higher at the mentary results were obtained from both methods. uppermost 2 m which may reflect an existing seasonal Whereas the refraction data show a first refractor at a frozen layer and also dryer conditions (Fig. 8). An depth of approximately 2 m, that is evident from an interesting feature is the drastic decrease in resistivity in increase in velocity of the P waves of more than 500 ms−1, both soundings just above the bedrock. This is probably the resistivity shows only a slight increase at the same caused by larger interstitials between the basalt blocks,

Fig. 8. Apparent resistivity curves and depth models obtained from a profile carried out on a block field (see Fig. 7 and text). 70 L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 interflows and accumulated downwashed clay minerals sufficient accuracy with the combination of several which may lead to very low resistivities. This interpre- methods. Furthermore, a multi-method approach allows tation, however, remains speculative and requires further for cross-checking of results and determination of the field evidence. In particular a two-dimensional sounding suitable methods for achieving valuable and reliable would help to improve the interpretation. Both methods results in a particular environment. delivered similar results for estimations of the depths to The consideration of some general recommendations bedrock. (“best practice guide”) may help to avoid incorrect and unsuccessful geophysical applications and interpreta- 6. Conclusions tions in geomorphic research and to improve signifi- cantly the accuracy of gathered data: Geophysical techniques have rendered it possible to obtain many new and stimulating solutions for 1. Before using field geophysics, attempt to obtain as geomorphological problems during the last decade. much information as possible about expected subsur- Geophysics, as used by geomorphologists, should face conditions such as lithology, soil thickness, become a common tool within geomorphology, but groundwater, and depth-to-bedrock as this may should not marginalise the geomorphological question. significantly influence the measurement strategy. For Thus, there is a need for studies coupling the GPR, DC resistivity and seismic refraction techniques geophysical background with geomorphic approaches the information may influence the choice of antenna in different natural environments and for different frequency, electrode array and seismic refraction landforms. respectively. A number of studies have provided new insights in 2. For reliable interpretation of survey results, available many landform and process related topics, for example, knowledge of, for example, depth of a particular active layer, internal structure of rock glaciers and talus sediment, hollows, ice lenses and depth of ground- slopes, depth and extension of landslide bodies and water at geomorphological and geological sites is sediment thickness. As previously pointed out, each essential. This a priori information may significantly technique is sensitive to contrasts in certain physical improve post-processing in terms of creating ade- properties which in turn reflect different sediment quate starting models or defining boundary condi- characteristics (see Table 2). Ground-penetrating radar tions such as expected depth to bedrock and type of has been successfully applied in permafrost studies, bedrock). depth to bedrock, and for soil moisture measurement, 3. Always ask the question: Is the geophysical model but is particularly suitable for assessment of internal geomorphologically and/or geologically plausible? sedimentary structures (especially in sediment units 4. Do not strictly adhere to the measurement strategy. with size fractions larger than silt). In these environ- Whenever possible, vary the applied frequency or ments, radar wave reflectivity is determined by contrasts electrode configuration. The results are often surprising. in water content that are directly influenced by grain size 5. Never trust in a single method. Combine — whenever composition. possible — two or three methods. Cross-checking is 2-D resistivity measurements are particularly prom- essential to increase the trustworthiness of the data ising in areas where there are strong contrasts in interpretation. electrical resistivity such as permafrost and landslides. 6. A geophysical method never provides a unique It is probably best suited for isolated bodies such as solution to a particular geomorphic situation, but may permafrost (Kneisel et al., 2000) or ice lenses and help to improve the interpretation. landslide structures. Sledge hammer seismic refraction is 7. Check and validate results with, where possible, particularly recommended for shallow applications investigations of , exposures, borehole–logs or (b30 m) in environments with mostly horizontal bedding drillings. and strong differences in P wave velocities such as dry 8. If there are not any exposures at the study site (which talus slopes with loose debris above bedrock. may be, in fact, the very reason for your application Further investigations using integrated approaches are of geophysics), try to obtain test profiles under more required. Only an appropriate combination of differing or less known subsurface conditions somewhere in methods allows for much more sophisticated data the vicinity of the site. Knowing the physical interpretation. Attention should be paid, not only to the properties or reflection patterns of typical sediments thickness, but also to the internal structure of loose in your study area will significantly facilitate your deposits — a task that can only be can be achieved in interpretation. L. Schrott, O. Sass / Geomorphology 93 (2008) 55–73 71

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