Klaus Motschka

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.

Precipitation correction of airborne gamma-ray spectrometry data using monitoring profiles: methodology and case study

Variations of soil moisture content caused by precipitation often complicate the interpretation of airborne gamma-ray spectrometry data. This is particularly the case in repeated surveys designed to monitor the change of near surface abundances of radioactive elements or in large and time-consuming surveys. To counter this precipitation effect we propose a correction method based on repeated survey flights over a monitoring profile. Assuming that the weather and the soil conditions at the monitoring profile are representative for the survey area, the weather dependent effect of soil moisture can be observed and sufficiently corrected.

Advanced Airborne Electromagnetics for Capturing Hydrogeological Parameters Over the Coastal Karst System of Tulum, Mexico

The presented case study is part of a series of international research cooperations started in 2006 and still ongoing. The study area is located at the east coast of the Yucatan Peninsula, Mexico, south of the city of Tulum including the northernmost part of the Sian Ka’an Biosphere Reserve. This study focuses on the third aero-electromagnetic (AEM) survey in the area conducted in January 2015 by the Geological Survey of Austria and Amigos de Sian Ka’an and on results prepared in course the Swiss–Mexican–Austrian project ‘Xibalba’. AEM enables quick measurement of the distribution of electrical conductivity in the subsurface over difficult accessible terrain. By means of a common frequency-domain four-channel helicopter probe and application of newly developed data processing algorithms, signatures of karst conduits and aquifers could be resolved. It is shown that the methods are able to deliver crucial structural information about karst groundwater regimes with unique depth information compared to previous surveys.

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.

Airborne and Ground Geophysics for Modelling a Karstic Conduit System: New Results from the 2007-2011 Campaigns in Tulum

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.

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.