A. Römer

Geoelectrical monitoring: an innovative method to supplement landslide surveillance and early warning

Permanent geoelectrical monitoring, using the GEOMON4D instrumentation in combination with high resolution displacement monitoring by means of the D.M.S. system, was performed at two active landslide areas: Ampflwang/Hausruck in Austria, and Bagnaschino in Italy. These sites are part of the Austrian geoelectrical monitoring network, which currently comprises six permanently monitored landslides in Europe. Within the observation intervals, several displacement events, triggered by intense precipitation, were monitored and analysed. All of these events were preceded by a decrease of electric resistivity. The application of an innovative 4D inversion algorithm made it possible to investigate the potential processes which led to the triggering of these events. We conclude that resistivity monitoring can significantly help in the investigation of the causes of landslide reactivation. Since the results also contribute to the extrapolation of local displacement monitoring data to a larger scale, resistivity monitoring can definitely support decision-finding in emergencies.

Geoelectrical monitoring of frozen ground and permafrost in alpine areas: field studies and considerations towards an improved measuring technology

Processes that control permafrost warming in Alpine regions are still not completely understood. Recently, geoelectrical monitoring has emerged as a useful tool to investigate thawing and freezing processes. However, high resistive environments and harsh environmental conditions pose very unfavourable conditions for automated resistivity measurements. Based on the results of several test studies, an improved data acquisition system for geoelectrical monitoring of frozen soils was developed. Furthermore, the implementation of algorithms for statistical analysis of raw data time series led to a significant improvement in the reliability of inversion results. At two Alpine sites, namely Molltaler Glacier and Magnetkopfl/Kitzsteinhorn, the adapted system was tested at soil temperature conditions between 0°C and –12°C. Data was continuously collected at both locations over nearly a full seasonal cycle. The results showed an almost linear dependency of resistivity and temperature at values above –0.5°C. At lower temperatures, the relation was non-linear, indicating that the reduction of porosity due to the shrinking of connected brine channels was the dominating process that determined the value of resistivity. Based on the derived results, further improvements were suggested, especially for measurements at soil temperatures below –4.5°C as low injection currents make it extremely challenging to gather these.

Airborne geophysical mapping as an innovative methodology for landslide investigation: evaluation of results from the Gschliefgraben landslide, Austria

Abstract. In September 2009, a complex airborne geophysical survey was performed in the large landslide affected area of the Gschliefgraben valley, Upper Austria, in order to evaluate the applicability of this method for landslide detection and mapping. An evaluation of the results, including different remote-sensing and ground-based methods, proved that airborne geophysics, especially the airborne electromagnetic method, has a high potential for landslide investigation. This is due to its sensitivity to fluid and clay content and porosity, which are parameters showing characteristic values in landslide prone structures. Resistivity distributions in different depth levels as well as depth slices along selected profiles are presented and compared with ground geoelectrical profiles for the test area of Gschliefgraben. Further interesting results can be derived from the radiometric survey, whereas the naturally occurring radioisotopes 40K and 232Th, as well as the man-made nuclide 137Cs have been considered. While the content of potassium and thorium in the shallow subsurface layer is expressively related to the lithological composition, the distribution of caesium is mainly determined by mass wasting processes.

Spatial mapping of submerged cave systems by means of airborne electromagnetics: an emerging technology to support protection of endangered karst aquifers

Karst aquifers represent important but very vulnerable sources for water supply to a significant part of the Earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. In principle karst aquifers are characterized by the presence of two distinct flow domains: the limestone matrix fractures and the conduits. A flow model of karst aquifers requires detailed, spatially distributed information on the hydrologic characteristics of the aquifer and flow paths. Geophysical methods determining the distribution of the electrical resistivities within the subsurface could provide such information. An international scientific research project was initiated to explore the potential of airborne electromagnetic mapping for providing such innovative information for improving groundwater modelling of karst aquifers. The project study area is located in the Sian Ka’an Biosphere Reserve located in Yucatan, Mexico, a coastal wetland of international importance. As a first step ground geoelectric and ground electromagnetic measurements were performed in March 2006 to determine the electrical properties of the Sian Ka’an Biosphere Reserve subsurface environment. These results were used for 3D forward modelling to calculate the expected airborne electromagnetic response. Based on these promising results, an airborne pilot survey was performed in 2007 to evaluate the applicability of airborne electromagnetic methodology. This survey covers an area of 40 square kilometres above the well-mapped Ox Bel Ha cave system. The results showed that the signature of the cave system could be clearly detected. The pilot survey offered as well the chance to define the limits of current state-of-the-art airborne data acquisition and inversion. The study helped to define the needs for further developments and improvements to establish the frequency domain electromagnetic method as a practical karst exploration method.

New achievements in developing a high speed geoelectrical monitoring system for landslide monitoring

Landslides are one of the major threats to human settlements and infrastructure, causing over time enormous human suffering and property losses than any other geological hazard. Especially in alpine regions, population pressure has prompted settlement to more extreme areas, which are, due to their geological settings, vulnerable to landslides. Therefore much effort has to be centred on risk detection, risk reduction and development of timely waming systems to prevent future loss of life and property.

Assessment of Shallow Interflow Velocities in Alpine Catchments for the Improvement of Hydrological Modelling

Advective long-duration rain events play an important role in the development of floods. A realistic depiction of interflow processes is not possible with the most customary precipitation/runoff (P/R) models. To increase knowledge about interflow processes in long-term rainfall events and improve the data situation for hydrological modelling, field experiments were performed in five Austrian catchments. These catchments differ from one another in precipitation characteristics, land cover, land use, pedological and geological situation. Long-term irrigation experiments on large plots combined with the insertion of a salt tracer (LiCl or NaCl) on smaller plots and geoelectrical measurements were used to assess the water movement in the soil and underlying substrate. This study has made significant contributions to (i) improving the design of measuring interflow on the plot and the hillslope-scale, (ii) improving the knowledge of bandwidths of shallow interflow velocities for typical substrates in the Eastern Alps and (iii) facilitating the regionalisation of data acquired at the local level, to the catchment or the regional scale.

Results of Geoelectrical Monitoring of Landslides Collected by the SafeLand/TEMPEL Network

Based on the promising results from a pilot monitoring project the GEOMON4D geoelectrical monitoring system was applied to several landslide monitoring tests sites, where also permanent monitoring data of displacement and hydrology are available to investigate the correlation of electrical parameters with other data. This monitoring network was set up in frame of the FP7 project SafeLand and the project TEMPEL (Austrian Science Fund) starting from October 2009 on. Analysis of the first monitoring results confirmed the potential of geoelectrical parameters as additional indicators to be applied within future early warning systems of landslides.

Geoelectric Monitoring to Investigate Landslide Dynamics

Although the monitoring of temporal electrical resistivity changes has undergone an intensive boom within the past few years, there is still a lack of applications on landslides, allowing performing an evaluation of the methodology for monitoring and early warning. Therefore after the development of specialized equipment for geoelectric monitoring, a network of several monitoring systems was installed on several landslides in different geological environment to evaluate the applicability of the methodology. Results from two of the sites are discussed. At the site of Ampflwang geoelectrical monitoring could successfully monitor subsurface dynamics whereas at the test site of Ancona, several problems due to the low signal to noise ration were encountered. However based on these results it can be concluded that resistivity monitoring can help to map the subsurface processed, that accompany the triggering of a landslide.

Karst Mapping Using Airborne Electromagnetics – Multi – Layer Inversion to Derive Spatially Distributed Parameter

Karst aquifers represent important but very vulnerable sources for water supply to a significant part of the earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. However a flow model for karst aquifers requires detailed, spatially distributed information on the flow- relevant characteristics of the subsurface. Methods determining the distribution of the electrical resistivities within the subsurface could provide such information. To explore the potential of airborne electromagnetic mapping for providing such innovative input information, the international scientific research initiative XPLORE was initiated. Within this paper, successful approaches to derive the subsurface cave structure and to map the depth of the halocline using multi-layer 1D inversion of frequency domain electromagnetic data are presented.

SNMR test measurements in Austria

In December 1997 the NMR- method (NUMIS-equipment from IRIS-instruments) was tested the first time in Austria for groundwater research. NMR soundings were carried out in two different areas of Austria. The locations were chosen at sites where other geophysical methods, especially geoelectric multielectrode profiles, had been used before, to correlate the results with the findings of these methods. Unfortunately at location Pulkau the proton resonance frequency, calculated from the measured local magnetic field, was a multiple of the 50 Hz frequency. Therefore no reliable measurement was possible in that area due to the high background noise. In the area of Marz a total of 9 NMR soundings were carried out using both square loop and eight-shaped-loop. At two locations also no measurements were possible because of the high background noise of 6000 to 60000 nV. Only at the location “Marzer Kogel” reliable measurements were possible at some points, e.g the Paulinerkloster. In this area an intensive geoelectric research program was carried out for groundwater research purposes the year before. The results of the NMR sounding at the location Paulinerkloster are shown in fig.1. Here only the eight-shaped-loop gave good results. The inversion results show a constant water content of 20% between 12 and 50m. Two months before a multielectrode geoelectric profile was measured crossing the location of the NMR-sounding. The results are shown in fig.2. At the location of the NMR-sounding a high resistive area down to about 80 m can be seen. The results of both methods were so promising that in the summer of 1998 a borehole was made there. The results of the drilling fully confirmed the findings of the geophysical research. As the quantity and quality of water is sufficient, a local water supply station will be planned for the near future.