Summary The demand for different mineral raw materials increases from one year to the next, and the same applies to graphite. Especially due to the increasing demand in the course of electromobility. In order to secure the supply for the next years, it is always evaluated whether former mining sites can be reactivated, which is also the case in the investigated measuring area in Amstall (Lower Austria). In the 1980s, Austria was one of the largest graphite exporters worldwide, especially in Lower Austria. Since graphite is very conductive, several geophysical exploration methods are suitable for this purpose. A comparison between the ERT, IP and SP was attempted and subsequently correlated. The SP anomalies were used as a guide to plan the ERT/IP profiles. The certain areas were further investigated based on the geoelectrical measurements by means of geochemical investigations. Based on the results of the measurement methods, different graphite occurrences could be confirmed.

Summary The Department of Geophysics of the Geological Survey of Austria has developed 20 years ago its first geoelectrical monitoring instrument, now since recent years the third resistivity meter, the GEOMON4D-IP, is in operation. In contrast to commercially available instruments, we have among others, the possibility to access the full raw data, fast measurement times (4000 quadrupoles/hour for resistivity measurement) and easy reparability. The main difference to the former resistivity meter (GEOMON4D) is the ability for time domain IP measurements, which was one of the most important reasons for the development of the GEOMON4D-IP. The highlight of the instrument is that the user can define the measuring cycle (sequence of current injection, current cut off, time intervals for data recording, etc.) without any limitations. This allows a measuring site dependent optimization of the measuring cycle to ensure the highest possible data quality. The newly developed instrument uses a constant current source with a maximum current injection of 800 mA with a maximum voltage of 400 V. The voltage measurement between the potential electrodes has a maximum range of ± 80 V, which should be sufficient even for very high resistive subsurface conditions.

Summary The Hallstatt area in Austria has a very long history of salt mining, which was frequently interrupted by large landslides. Landslide material filled the prehistoric underground mining chambers and therefore conserved their dimensions. Within several geoelectric measurement campaigns, data from ground surface measurements and from geoelectric profiles in 2 mining tunnels from the 18th century were collected, resulting in a 3D data set of about 80,000 data points. The aim was to track a mining chamber of the Early Iron Age, which has been filled with landslide material. The already existing reconstruction of this mining chamber is based on several archeological excavation points and an interpolation in between. A collapsed mining shaft known from laser scan data has been confirmed beforehand with surface ERT measurements. Due to the high resistive salt rock and the hardness and dryness of the intruded landslide material, the Early Iron Age mining chamber could not be differentiated sufficiently. Instead, results could pinpoint that the mining chambers are following the area of the highest electrical resistivity, which is determined as “Haselgebirge” in geological maps. Therefore, indications of the location of the Early Iron Age mining chamber, where no archaeological finds exists until now, could be given.

We have developed algorithms of ERT monitoring data for filtering outliers and accurately evaluating data reliabilities. The filtering and editing methods are designed to provide qualified data to actual inversion process, while the evaluated reliabilities are to weight the data actually inverted. The parameters used in the data editing and in the weighting factor determination are automatically optimized by taking account of the statistical characteristics and spatio-temporal behaviours of the monitoring data. Consequently, various kinds of data filtering and quality evaluations are to be made in automatic and objective manner with little manual manipulations of the data and parameters. The performance and effectiveness of the developed algorithms are examined and demonstrated with the field data monitored at the Gresten test site, Austria.

Landslides developed on clay-rich slopes are controlled by the soil water regime and the groundwater circulation. Spatially-distributed and high frequency observations of these hydrological processes are important for improving our understanding and prediction of landslide triggering. This work presents observed changes in electrical resistivity monitored at the Super-Sauze clayey landslide with the GEOMON4D resistivity instrument installed permanently on-site for a period of one year. A methodological framework for processing the raw measurement is proposed. It includes the filtering of the resistivity dataset, the correction of the effects of non-hydrological factors (sensitivity of the device, sensitivity to soil temperature and fluid conductivity, presence of fissures in the topsoil) on the filtered resistivity values. The interpretation is based on a statistical analysis to define possible relationships between the rainfall characteristics, the soil hydrological observations and the soil electrical resistivity response. During the monitoring period, no significant relationships between the electrical response and the measured hydrological parameters are evidenced. We discuss the limitations of the method due to the effect of heat exchange between the groundwater, the vadose zone water and the rainwater that hides the variations of resistivity due to variations of the soil water content. We demonstrate that despite the absence of hydrogeophysical information for the vadose zone, the sensitivity of electrical resistivity monitoring to temperature variations allows imaging water fluxes in the saturated zone and highlighting the existence of matrix and preferential flows that does not occur at the same time and for the same duration. We conclude on the necessity to combine electrical resistivity measurements with distributed soil temperature measurements.

This study is part of a series of international research cooperations which commenced in 2007 and are still ongoing. The study area is located on the east coast of the Yucatan Peninsula, Mexico, and comprises the northernmost part of the Sian Ka’an biosphere reserve, a coastal wetland of international importance, as well as the city of Tulum in the state of Quintana Roo, and part of the second largest barrier reef in the world some 300 metres to one kilometre off shore. Two airborne surveys, conducted in 2007 and 2008 by the Geological Survey of Austria, covered an area of some 200 square kilometres, including the well-known Ox Bel Ha cave system, already mapped by exploration divers. In order to get additional ground truth data and input for the hydrological model, extended ground geophysical campaigns have been conducted annually. The first processing of the airborne electromagnetic (AEM) data revealed not only a clear signature from known caves but also the image of a vast, unexplored, hidden conduit network. However, lateral and depth resolution was limited due to measurement drift and noise as well the specific behaviour of the applied inversion technique. Newly developed algorithms for processing AEM data and inversion results have improved the signal-to-noise ratio significantly and enabled the imaging of well-defined structures in the underground. Therefore, the AEM method is now capable of quickly delivering crucial structural information of karst-water regimes in difficult-to-access areas with unique depth information compared to previous studies.

In this work we adopted an array optimization scheme for a special geoelectrical monitoring instrument for which no standard geoelectrical arrays exists. Optimized measurements are selected on the basis of their Jacobian matrix values. Application is demonstrated though model and real examples from a landslide site in Austria. Synthetic and real tests demonstrated that though measurement optimization it is possible to reduce the number of collected data without reducing the overall geoelectrical imaging quality. This is a very important for reducing both the power consumption as well the time-lapse data processing time.

The individual abstracts for this session are available to read in the PDF.

Summary 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.

Summary 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.

SUMMARY 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.

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.

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 aquifers represent important sources for water supply to a significant part of the earth’s population. For sustainable use of these resources, development of management tools based on numerical groundwater models is required. A flow model of karst aquifers requires spatially distributed information on its characteristic flow domains. Methods determining the distribution of the electrical resistivity within the subsurface could provide such information. To explore the potential of airborne electromagnetic (AEM) mapping for providing such information to groundwater modelling of karst aquifers, the international project XPLORE was initiated. The project is carried out around the Sian Ka’an Biosphere Reserve, located near Tulum, Mexico. Airborne surveys were performed in 2007 and 2008 to prove the basic applicability of the AEM method. The results show that the signature of the cave system can be clearly detected by AEM mapping. Additionally, for better coverage of ground truth and calibration of the hydrological model, three extended ground geophysical campaigns have been conducted in 2009-2011 comprising geoelectrics, GPS-water level measurements, GPR, and borehole geophysics. The airborne data as well as mapped caves were used to generate a numerical ground water model of the karst system.