Marcus Gurk

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.

Multi-dimensional Interpretation of Radiomagnetotelluric and Transient Electromagnetic Data to Study Active Faults in the Mygdonian Basin, Northern Greece

The Mygdonian Basin is located about 45 km NE of the city center of Thessaloniki in northern Greece. We conducted an electromagnetic (EM) survey that covers parts of the epicenter area between Lake Volvi and Lake Langada of the local 1978 earthquake that caused structural damage in the city center of Thessaloniki. Ambient noise measurements in the basin strongly suggest a complex 3-D tectonic setting. Hence, near-surface electromagnetic measurements were carried out to map the local fault pattern in the research area. We conducted radiomagnetotelluric (RMT) and transient electromagnetic (TEM) measurements along eight profiles, giving a total number of 443 RMT and 107 TEM soundings to study the spatial electrical conductivity distribution. In this context, RMT data are used to study the shallow conductivity structure down to about 35 m, whereas the TEM data explore the conductivity distribution down to a depth of 200 m. The 1-D and 2-D interpretation of RMT and TEM data indicate a local fault pattern in the survey area that was also studied by 3-D modeling of the RMT data.

Joint inversion of long-offset and central-loop transient electromagnetic data: Application to a mud volcano exploration in Perekishkul, Azerbaijan

Mud volcanism is commonly observed in Azerbaijan and the surrounding South Caspian Basin. This natural phenomenon is very similar to magmatic volcanoes but differs in one considerable aspect: Magmatic volcanoes are generally the result of ascending molten rock within the Earth’s crust, whereas mud volcanoes are characterised by expelling mixtures of water, mud, and gas. The majority of mud volcanoes have been observed on ocean floors or in deep sedimentary basins, such as those found in Azerbaijan. Furthermore, their occurrences in Azerbaijan are generally closely associated with hydrocarbon reservoirs and are therefore of immense economic and geological interest. The broadside long-offset transient electromagnetic method and the central-loop transient electromagnetic method were applied to study the inner structure of such mud volcanoes and to determine the depth of a resistive geological formation that is predicted to contain the majority of the hydrocarbon reservoirs in the survey area. One-dimensional joint inversion of central-loop and long-offset transient electromagnetic data was performed using the inversion schemes of Occam and Marquardt. By using the joint inversion models, a subsurface resistivity structure ranging from the surface to a depth of approximately 7 km was determined. Along a profile running perpendicular to the assumed strike direction, lateral resistivity variations could only be determined in the shallow depth range using the transient electromagnetic data. An attempt to resolve further two-dimensional/threedimensional resistivity structures, representing possible mud migration paths at large depths using the long-offset transient electromagnetic data, failed. Moreover, the joint inversion models led to ambiguous results regarding the depth and resistivity of the hydrocarbon target formation due to poor resolution at great depths (>5 km). Thus, 1D/2D modelling studies were subsequently performed to investigate the influence of the resistive terminating half-space on the measured long-offset transient electromagnetic data. The 1D joint inversion models were utilised as starting models for both the 1D and 2D modelling studies. The results tend to show that a resistive terminating half-space, implying the presence of the target formation, is the favourable geological setting. Furthermore, the 2D modelling study aimed to fit all measured long-offset transient

Exploration of the Near-surface Structure of Mud Volcanoes using Electromagnetic Techniques: A Case Study from Perekishkul, Azerbaijan

ABSTRACT The general aim of this study is, for the first time, to investigate the inner structure of mud volcanoes in Azerbaijan using several near-surface electromagnetic methods. The central-loop transient electromagnetic (TEM) method and the radiomagnetotelluric (RMT) method were applied on mud volcanoes near Perekishkul, Azerbaijan. These methods complement each other in terms of depth of investigation. In particular, the RMT method generally resolves the very shallow resistivity structures (<10 m) directly beneath the mud volcanoes, whereas the TEM method provides a resolution of up to 100- to 150-m depth. The obtained data sets were processed and interpreted individually. The TEM data were interpreted by conventional 1-D inversion algorithms, i.e., Occams and Marquardt inversion, and by a quasi-2-D laterally constrained inversion. The RMT data were interpreted by 2-D inversion. Subsequently, a combined model regarding the sediments and corresponding resistivities for the area around the mud volcano...

RMT and TEM Measurements on an Active Fault in Northern Greece

To improve the seismic wave propagation model it is vital to know about site effects, e.g. the geotectonic properties of the area such as the top‐of‐basement, vertical tectonic boundaries (faults and basement fracturation) and the geothermal regime. Therefore, we carried out near surface EM (Electromagnetic) studies to understand the distribution of the active faulting and the top of basement structure of this particular area. The RMT (Radiomagnetotelluric) and TEM (Transient electromagnetic) measurements were carried out on eight profiles, 440 RMT and 104 TEM soundings were realized. The inverted RMT and TEM data show generally a four layer model. The layers are indicated as metamorphic and sediment rocks, which are in detail: marly silty sand with gravel (>>100 Ωm), marly silty sand with clay (50–100 Ωm), sandy clay (30–50 Ωm) and silty sand (10–30 Ωm) with varying thickness. Due to the high resistivity of the top layer, the skin depths of the RMT soundings are around 35 m. The TEM data gives detail inf...

Mineralogical characterization of the intraseam layers of Lofoi lignite deposits in Florina basin (western Makedonia, northwest Greece)

ABSTRACT The mineralogical composition of intraseam layers from Lofoi lignite deposits (northwest Greece) is the subject of the present study. The samples were examined by means of X-ray diffraction (XRD), thermo-gravimetric (TG/DTG) and differential thermal analysis (DTA), and Fourier transform infrared (FT-IR) spectrometry. The clay minerals prevail in most samples, with illite-muscovite being the dominant phase, and kaolinite and chlorite being the other major clay components. No smectite was found. Quartz and feldspars, dominate in two cases. The studied materials are characterized as clays to clayey sands, showing significant similarities with the intraseam layers of the adjacent Achlada lignite deposits.