David Ottowitz

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.

Simultaneous Inversion of Resistivity Structure and Electrode Locations in ERT

Accurate coordinates of electrodes are essential in ERT, and electrode mislocation or inaccurate information on electrode positions inescapably results in crucial distortions of the subsurface images. This problem would be more probable in crosshole ERT due to lack of borehole deviation data or to their inaccuracy. ERT monitoring especially performed in an area where ground deformations are expected such as in landslide monitoring may also suffer from this problem owing to the electrode movements over time. To provide a fundamental way to solve these problems, a new inversion algorithm was developed so that resistivity distribution and electrode coordinates are simultaneously optimized. We performed numerical experiments to examine the performance of the developed algorithm. The numerical experiments showed that x coordinates can be recovered close to the true ones while the inverted z coordinates have higher errors. Even though the true coordinates cannot be precisely calculated, we were able to calculate the distribution of resistivity which was comparable to the inversion results based on the true electrode coordinates. Finally, the performance of the proposed algorithm is highlighted in two field applications: a surface ERT survey at a landslide-prone are and a crosshole ERT survey.

Numerical Modelling of Hydrological Parameters for an Enhanced Interpretation of ERT Monitoring Data

Open image in new window Electrical Resistivity Tomography (ERT) has become a standard application for evaluation of structural and hydraulic properties of landslides. The technical improvement during the last few years made the method of ERT also attractive for long-term monitoring in term of regularly repeated measurements as newly developed 4-D inversion routines can assure the comparability of different ERT measurements. The interpretation of the achieved data is typically carried out qualitatively on a visual basis. In the framework of the LAMOND project (Long-Term Landslide Monitoring for Understanding of Underlying Dynamic Processes as Basis for an End-User Focused Early Warning) the abilities of numerical optimization and finite element modeling are conducted to close the gap between ERT observations and a dynamic, hydrological process model. So far, only one site could be examined and a workflow for numerical optimization of Archies law has been developed. These results will be the basis for a finite element process model which will deliver basic insights and help in decision-making for early warning systems.

Pechgraben Landslide: Evaluation of Geophysical/Geotechnical Methods in Terms of Remediation Support

Information from the application of geophysical and geotechnical methods are essential for decision makers to initiate appropriate emergency or remediation measures in case of a catastrophic landslide event. This paper summarizes all the applied measuring and monitoring methods that accompanied the catastrophic landslide event of Pechgraben (Upper Austria) in 2013 as well as the three years after the completion of major parts of the remediation work. Beside scientific aspects, the focus of the geophysical/geotechnical investigations was put on the support of decision makers. To improve the efficiency of the support for future similar landslide events, it is necessary to evaluate the applied methods in terms of their information content concerning the emergency and remediation strategy. We ended up with the conclusion that there are a few key methods providing essential information for decision makers during different time phases of the landslide event. However, for a detailed understanding of the landslide behavior and the ongoing subsurface processes, a combination of different methods is required.

Monitoring im Bereich der Rutschung Pechgraben (Oberösterreich): Methodik und Ergebnisse

ZusammenfassungDer folgende Artikel beinhaltet eine Zusammenfassung der Monitoring-Methoden, die während der Reaktivierung der Rutschung Pechgraben (OÖ) angewandt wurden bzw. nach wie vor angewandt werden. Die Methoden umfassen die regelmäßige Einmessung eines dichten Punktnetzes mittels GNSS, wiederholte Befliegungen mittels Laserscan zur Bestimmung des Massenverlustes bzw. -zuwachses, Oktokopterbefliegungen für die photogrammetrische Bilddatenauswertung sowie verschiedene geophysikalische Methoden (Geoelektrik, Inklinometer, Bodenfeuchte und Bodentemperatur). Der Schwerpunkt des Artikels liegt auf der exemplarischen Darstellung einzelner Ergebnisse, um die Anwendungsmöglichkeiten der jeweiligen Monitoring-Methode im Bereich von Hangrutschungen zu zeigen.AbstractThis article summarises the application of different monitoring methods during the reactivation of the Pechgraben landslide (Upper Austria). The applied methods (some of them are still acquiring data) incorporated repeated GNSS measurements of several defined benchmarks, repeated laser scan flights for the determination of mass losses or accumulation, octocopter flights for photogrammetric data analysis as well as several geophysical methods (geoelectrics, inclinometer, soil moisture and soil temperature). The focus in this article is put on exemplary evaluation of particular results to demonstrate the application spectrum of the respective monitoring methods in landslide areas.

Geophysics for the Determination of Hydrological Parameters of Karst Systems in Yucatan, Mexico

In the karstic system of Yucatan, airborne geophysics as well as ground geophysics and groundwater levelling, which were performed between 2006 and 2013, led to the generation of a general groundwater model of the area between the town of Tulum and the biosphere reserve of Sian Ka’an. However, for calibration of the model, significant parameters, e.g. the geometry of the cave system, estimates on flow velocity, as well as knowledge on the hydrological properties of the karst matrix are still missing, a necessity to derive reliable modelling results. Therefore, within a small part of the conduit system, the subsurface structure was determined in detail, applying surface geoelectrical measurements of different configurations, hole-to-surface measurements, borehole logging, drilling and tracer tests. Additionally, a low cost method based on laser scanning was developed and tested to derive a 3D model of the cave shape, which was used for calibration purpose. Results coming from different methods are compared and interpreted to derive advanced knowledge about hydrological parameters of the karst system of Yucatan.

Geophysics in the Context of Disaster Mitigation: Results from the Landslide at Pechgraben, Austria

In early June 2013 a large landslide was triggered at the area of Pechgraben, situated in a mountainous area of Upper Austria. Only several hours after the onset of the first movements, a geophysical investigation program was started to support the planning of the disaster response actions as well as the design of a follow up early warning system. The geophysical methods included geoelectric profiling, geoelectrical monitoring, borehole logging, GPS, inclinometric, piezometric, soil humidity and optical monitoring as well as airborne geophysics. Within this paper results of different methods are given and evaluated on their role within disaster mitigation.