Robert Supper

The reuse of vintage datasets which were acquired in the 20th century can pose challenges for modern geophysical modeling due to missing detailed preprocessing information, significant uncertainties, or lack of precise tracking, etc. Nevertheless, they are often the only available datasets in a target region. We explore here the potential of such vintage airborne geophysical datasets (magnetics, AEM, radiometrics) to detect the location and dip direction of geological faults, using a non-modeling interpretation approach based on multiple GIS tools. We apply our approach in a geologically well-known region where four different types of faults are mapped. The applicability of the tools used in this study depend on the geological setting of each fault and is evaluated based on the comparison with geological and—where available—with modeling data. In general, the GIS tools, especially used on a combination of datasets, show reliable results concerning the location and strike of faults, and even seem to be able to predict the dip direction of a fault.

We describe here the results of the characterization of subsurface structures in an area of the south-eastern edge of the Bohemian Massif, in Austria by high-resolution geophysical survey techniques and advanced analysis methods of potential fields. The employed methods included potential field multiscale techniques for source-edge location and characterization of sources at depth. Our results confirmed the presence of already known structures: the location of the Diendorf Fault and the Moldanubian Shearzone are clearly recognized in the data at the same location as on the geological maps, even where the Diendorf fault is covered with sediments of the Molasse Basin. In addition, we detected several geological contacts between different rock types in the Bohemian Massif west of the Diendorf Fault. From our results, we were also able to quickly identify and image, without a priori information, previously unknown structures, such as faults with-depth-to-the top of about 500 m and magmatic intrusions about 400 m deep.

We have developed a new protocol for measuring direct current (DC) resistivity data, in which self-potential (SP) data were measured immediately prior to obtaining the resistivity data. Based on this SP measurement, we have defined two different resistances: the forward resistance (i.e., a normalized potential difference caused by forward current injection) and the backward resistance (i.e., a normalized potential difference caused by backward current injection). This has allowed us to quantify distortions in the DC resistivity potential field caused by all unknown mechanisms, including ambient noises. In accordance with the proposed measurement protocol, we further devised a data-weighting method in a resistivity inversion for calculating more reasonable subsurface structures, and the method was based on the forward and backward resistances. Through inversion experiments using field resistivity data acquired by the proposed protocol, we have determined the validity and effectiveness of the proposed method.

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.

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.

In many cases landslide and landslide related phenomena are abetted or triggered by hydrological processes. Since variations in soil humidity and subsurface water content are reflected in time-variable subsurface resistivities, the geoelectric method could be an appropriate methodology to monitor such phenomena. Therefore in 2002 the Geological Survey of Austria started to develop a complex geoelectrical system optimized for monitoring applications. This equipment now fulfills the requirements of permanent maintenance access, rapid and reliable data acquisition and robust design and automated data transfer and processing. Within a follow-up test phase of several years, the system was optimized for long-time field operation. For instance a lightning protection device for multi-core cables had to be developed as well as durable cable configuration had to be sorted out. The system was applied to observe the mudflow of Rindberg in the area of Sibratsgfäll. There a correlation of increased occurrences of geoelectric and self potential anomalies were detected at times characterized by larger movement rates, recorded by time-lapse GPS measurements. Furthermore the geoelectric methodology was also tested for observation of permafrost phenomena at the observatory of Sonnblick at an altitude of 3106 m parallel to depth dependent temperature monitoring. A first test phase of the system showed that due to the predominant high subsurface resistivities several adoptions to the GEOMON^4D system had to be developed. Amongst others, current injection had to be changed from applying a constant voltage to applying a constant current flow to compensate for polarization effects.

In early 2009 a complex airborne geophysical survey, comprising magnetic, electromagnetic and gamma ray measurements, was conducted to investigate the shallow geological structure at the Island of Socorro, Mexico, in order to derive indications on possible groundwater resources. For the airborne survey was complemented by ground geoelectrical measurements to further detail the airborne results at selected profiles. A total area of 200 square kilometers was covered in early 2009 by complex airborne measurements using an average line spacing of 100 m. Since on-site pre-processing of the electromagnetic data already showed clear indication on groundwater-relevant structures, the layout of the geoelectric profiles could be optimized according to these findings. As a result advanced knowledge on the shallow subsurface structure of the island could be derived. This outcome allowed setting up a research plan for further groundwater exploration measures of the island. However, apart from search for groundwater, the data provides a valuable data base to be interpreted within further investigation in the area of volcanic research and natural protection.

After a large landslide event in Sibratsgfäll/Austria several exploration methods were evaluated on their applicability to investigate and monitor landslide areas. The resulting optimised strategy consists of the combined application of airborne electromagnetics, ground geoelectrical measurements and geoelectrical monitoring combined with hydrological and geological mapping and geotechnical modelling. Interdisciplinary communication and discussion was the primary key to assess this complicated hazard situation.