Ulrich Polom

Geophysical assessment and geotechnical investigation of quick-clay landslides – a Swedish case study

We present a preliminary assessment of the potential utility of various geophysical measurements carried out over a quick-clay landslide site in south-west Sweden. The multidisciplinary approach includes active P- and S-wave seismic investigations, including 2D and 3D reflection and refraction surveys, passive single and 3C seismic surveys, electrical resistivity tomography and electromagnetic surveys including controlled-source and radio-magnetotellurics, ground-penetrating radar and potential field studies. The P-wave and particularly S-wave reflection seismic data show a highresolution image of bedrock topography and the stratigraphy of a 100 m thick sequence of sediments that lies on top, which include lightly consolidated quick-clays. Of particular interest is the identification of a layer of relatively coarse-grained material between 10–20 m below the ground surface. Geotechnical investigations indicate that most but not all quick-clays at the site are located above this layer. Further studies are required to determine the importance of their relationship and whether the coarse-grained layer may have had a role in triggering quick-clay landslides in the region. Geoelectrical and electromagnetic methods provide high-resolution images of the unconsolidated subsurface and particularly the normal and leached clays. Radio-magnetotelluric methods proved valuable near the river where traditional geoelectrical methods failed to provide sufficient depth coverage. The study shows that geophysical data are able to image major subsurface structures associated with quick-clay landslides.

The Middle Pleistocene tunnel valley at Schöningen as a Paleolithic archive

Schöningen represents one of the key sites for Lower Paleolithic archaeology in central Europe, where a Middle to Late Pleistocene sedimentary succession, locally up to 45 m thick, has been preserved in an Elsterian tunnel valley. After deglaciation, the tunnel valley remained underfilled and provided the accommodation space for Holsteinian interglacial deposition and also kept the artifact-bearing strata below base level for subsequent erosion. The Holsteinian (MIS 9) succession consists of laterally and vertically stacked lacustrine delta systems, which were controlled by repeated lake-level changes. In the face of changing climatic and environmental conditions the long-lived interglacial lake provided an attractive site for animals and early humans. Artifacts were deposited on the subaerial delta plain and became embedded during lake-level rise. Although the area was considerably affected by erosion and glacitectonic deformation during the subsequent Saalian glaciation, the artifact-bearing Holsteinian strata were preserved in the deeper part of the tunnel valley. Tunnel valleys should be regarded as potential archives for interglacial deposits, which may contain important Paleolithic sites. Tunnel valleys may provide accommodation space and also have a high preservation potential. Interglacial lakes situated within underfilled tunnel valleys represented attractive sites for animals and early human hunter-gatherers.

Influence of ice crystal anisotropy on seismic velocity analysis

Abstract In 2010 a reflection seismic survey was carried out on the Alpine glacier Colle Gnifetti. The processed and depth-converted data could be compared to a nearby ice core, drilled almost to the bed. Comparisons showed that the depth of the P-wave bed reflection was too shallow, while the depth of the SH-wave bed reflection fitted the ice-core length well. We are now able to explain the major part of these differences using the existing crystal orientations of the ice at Colle Gnifetti. We calculate anisotropic velocities for P- and SH-waves that are usually picked for stacking and compare them with zero-offset velocities needed for the depth conversion. Here we take the firn pack at Colle Gnifetti into account for P- and S-wave analysis. To incorporate the S-wave analysis we first derive a new equation for the relationship between density and S-wave velocity from diving waves. We show that anisotropic fabrics observed at Colle Gnifetti introduce a difference of only 1% between stacking and depth-conversion velocities for the SH-wave, but 7% for the P-wave. We suggest that this difference in stacking and depth-conversion velocity for the P-wave can be used to derive information about the existing anisotropy by combining our seismic data with, for example, radar data.

First glacier-vibroseismic experiment - results from cold firn of Colle Gnifetti

In the summer of 2010, a small shallow reflection seismic experiment was carried out on the firncovered cold glacier of Colle Gnifetti, Monte Rosa group, Swiss/Italian Alps. At this site, the physical properties of ice are comparable to polar conditions, which is why this site is often used for methodological tests. The experiment at 4500 m elevation was designed to explore the scope of shallow vibroseis for seismic targets within and below the glacier. A small ELVIS vibrator system was used to generate shear waves and compression waves for SH- and P-wave receiver setups of two profiles. The resulting sections clearly show a boundary from ice to rock around 60 m and deeper structures below the glacier. The deepest features are estimated to be 150 m for the SH-waves and 220 m for the P-waves. Reflections could be detected also within the ice overburden, which are preliminarily interpreted as a change of density in the upper 30 m and possibly crystal orientation fabric in the ice column. Furthermore, elastic parameters could be derived from seismic velocities, due to clear basement reflections. The results of this unique experiment enable new insights into the internal structure of ice masses and open a promising new investigation method for sub-ice structures and properties, such as basal sediments.

Joint interpretation of explosive and vibroseismic surveys on cold firn for the investigation of ice properties

Abstract Two seismic surveys were carried out on the high-altitude glacier saddle, Colle Gnifetti, Monte Rosa, Italy/Switzerland. Explosive and vibroseismic sources were tested to explore the best way to generate seismic waves to deduce shallow and intermediate properties (<100 m) of firn and ice. The explosive source (SISSY) excites strong surface and diving waves, degrading data quality for processing; no englacial reflections besides the noisy bed reflector are visible. However, the strong diving waves are analyzed to derive the density distribution of the firn pack, yielding results similar to a nearby ice core. The vibrator source (ElViS), used in both P- and SH-wave modes, produces detectable laterally coherent reflections within the firn and ice column. We compare these with ice-core and radar data. The SH-wave data are particularly useful in providing detailed, high-resolution information on firn and ice stratigraphy. Our analyses demonstrate the potential of seismic methods to determine physical properties of firn and ice, particularly density and potentially also crystal-orientation fabric.

Shear-wave Reflection-seismic Pilot Study at the UNIS CO2 Lab site, Longyearbyen, Svalbard

As part of the world’s needs for CO2-injection test sites, the city of Longyearbyen in Svalbard is an interesting location for testing technologies related to carbon capture and storage (CCS) in a vulnerable arctic environment, being a closed energy system with a coal-fuelled power plant. Therefore, the University Centre in Svalbard (UNIS) established the UNIS CO2 Lab site a few km away of Longyearbyen. The local geological structures appear suited for storing CO2 at about 600-900 m depth and injection tests are carried out. Monitoring micro-seismicity during and after injection is important, but to properly analyse micro-seismicity, a good velocity model for both P- and S-wave is necessary. The top 100-m of the site, including permafrost, is however difficult to assess. In an attempt to improve the actual velocity model near the surface, a pilot study of S-wave reflection seismic was carried out in 2012 and is reported here. Despite numerous noise sources, including wind and strong surface waves, a profile acquired on a nearby filled road showed promising results, indicating very low S-wave velocity values down to 200 m/s, thus giving a much better image of the top 70-m than P-wave seismic earlier acquired. Lessons learned are given too.

Seismic Tomography and Monitoring in Underground Structures: Developments in the Freiberg Reiche Zeche Underground Lab (Freiberg, Germany) and Their Application in Underground Construction (SOUND)

The construction of large tunnels and underground infrastructures faces increasingly large dimensions and complex geological conditions. Under these conditions, exploration techniques are needed which enable for a detection of potentially hazardous structures during construction. Seismic sensors, integrated into rock anchors, and small seismic signal sources using defined pneumatic impulses or sweep signals generated by magnetostrictive actuators are the components of an exploration system which can be easily integrated into different types of underground excavation work and which can also be deployed for the long-term monitoring of already existing tunnels or caverns. However, for a continuous acquisition of seismic signals during tunnel excavation, the strong and broadband signal generated by a tunnel boring machine (TBM) may be used as a continuously operating source. Within the collaborative project SOUND, the seismic equipment at the Underground Lab of the Reiche Zeche Research Mine in Freiberg (Germany) has been used for a tomographic monitoring study during the excavation of an inclined gallery. A synthetic, but realistic seismic data set was simulated using a randomly heterogeneous velocity model which can be regarded as a realistic prototype of the velocity distribution in the real Gneiss block. The simulated acquisition geometry has been derived from the actual source and receiver point distribution in the Underground Laboratory. It can be shown that the analysis of the modelled seismic data by full waveform inversion (FWI) was able to reveal the lateral heterogeneity of the velocity model with significantly higher resolution compared to traveltime tomography of the direct P-wave arrivals. The analysis of field data from the Underground Laboratory has shown that there are complex interactions in close vicinity to the receiver location, and before FWI can be applied to this real data set, source and receiver dependant signatures need to be removed by inversion and deconvolution. A further field experiment, performed during gallery excavation in the Underground Laboratory, has shown that the setup of seismic receivers in rock anchors and a sparse array of adaptive vibro-sources is able to detect subtle changes in seismic wave propagation related to stress changes due to the excavation of an inclined gallery. After the deployment in the Underground Laboratory, a field survey was carried out on a tunnel construction site. A broadband seismic data set, using the tunnel boring machine could be acquired providing a basis for high resolution imaging of structures ahead of the construction site and geotechnical characterization of the imaged volume.

Shear wave reflection seismics yields subsurfacedissolution and subrosion patterns: application to theGhor Al-Haditha sinkhole site, Dead Sea, Jordan

Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline in the Dead Sea level. Since different structural models of the upper 50 m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development. For this purpose, high-frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor AlHaditha area along several crossing and parallel profiles with a total length of 1.8 and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200 m at a high resolution and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area. The most surprising result of the survey is the absence of evidence of a thick (> 2–10 m) compacted salt layer formerly suggested to lie at ca. 35–40 m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0 and 200 m depth. Furthermore, the underground section of areas affected by sinkholes is characterized by highly scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulfate, and carbonate minerals as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines and are consequently destabilized and mobilized by both dissolution and physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall, this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings.

Combination of 2D Shear Wave Reflection Seismics and Travel Time Analysis of Borehole Geophone Data for the Investigation of a Sinkhole Area

downhole receivers recording direct waves, enables analyzing of seismic wave propagation and velocities in more detail and beyond 2D. Therefore, the experiment setup will be further extended in future. The presented method shows the potential to locate instable zones in a sinkhole area. In our further research we propose to evaluate the suitability of the method for the time lapse monitoring of changes in the seismic wave propagation, which could be related to subrosion processes.

Sinkhole Imaging and Identification of Fractures with S H -wave Reflection Seismic

Introduction Subrosion, the underground leaching of rocks, requires the presence of soluble rocks (e.g., evaporites), water (e.g., groundwater), and fractures or faults enabling water flow through the subsurface to form cavities (Smyth, 1913; Martinez et al., 1998). Different kinds of structures can evolve, and the two main types are (1) sinkholes and (2) depressions (for a detailed classification see Waltham et al. (2005); Gutierrez et al. (2008)). Subrosion is a natural process, but it can be influenced by, e.g., manipulation of the aquifer system (Bell, 1988) and extraction of saline water (Getchell & Muller, 1995). Reflection seismic (e.g., Steeples et al., 1986) delivers a high-resolution image of the underground for a detailed characterization of the subsurface structures, especially using shear-waves, because the near-subsurface surrounding sinkholes often consists of loose sediments and strongly fractured, and therefore not compacted rocks (Krawczyk et al., 2012; Wadas et al., 2016).