Tina Wunderlich

Application of 2D Fourier filtering for elimination of stripe noise in side-scan sonar mosaics

Different types of along-track stripe noise are common artifacts in side-scan sonar mosaics, hampering both visual image quality and automatic seabed classification attempts. In this study, a 2D Fourier domain slope filter is applied to remove stripes from side-scan sonar mosaics. This technique has the advantage of using only two simple tuning parameters, namely, the slope angle of the stripe noise, and a filtering width. Furthermore, the image processing tool presented here can be applied to non-optimally processed side-scan mosaics without the need of reprocessing the original data. The performance of the filter technique is demonstrated by two datasets in which stripe noise, including nadir stripes, has been successfully removed. To one of the filtered dataset a simple unsupervised seabed classification was then applied, leading to a much improved result with only little misclassification due to residual stripe noise.

Comparison of seismic activity for Llaima and Villarrica volcanoes prior to and after the Maule 2010 earthquake

Llaima and Villarrica are two of the most active volcanoes in the Chilean Southern Volcanic Zone and presently show contrasting types of activity. Llaima is a closed vent edifice with fumarolic activity, while Villarrica has an open vent with a lava lake, continuous degassing and tremor activity. This study is focused on characterizing the relationships between volcanic and seismic activity in the months before and after the 2010 M8.8 Maule earthquake, which was located in NNW direction from the volcanoes. Time series for tremors, long-period and volcano-tectonic events were obtained from the catalogue of the Volcanic Observatory of the Southern Andes (OVDAS) and from the SFB 574 temporary volcanic network. An increase in the amount of tremor activity, long-period events and degassing rates was observed at Villarrica weeks before the mainshock and continued at a high level also after it. This increase in activity is interpreted to be caused by enhanced magma influx at depth and may be unrelated to the Maule event. In Llaima, an increase in the volcano-tectonic activity was observed directly after the earthquake. The simultaneous post-earthquake activity at both volcanoes is consistent with a structural adjustment response. Since this enhanced activity lasted for more than a year, we suggest that it is related to a medium-term change in the static stress. Thus, the Maule earthquake may have affected both volcanoes, but did not trigger eruptions, from which we assume that none of the volcanoes were in a critical state.

Absorption and frequency shift of GPR signals in sandy and silty soils: empirical relations between quality factor Q, complex permittivity and clay and water contents

The arrival time, shape and amplitude of a reflected GPR wavelet depend mainly on the real and imaginary parts of dielectrical permittivity and electrical conductivity. All these parameters are strongly affected by the water and clay content of soils. From a comparison of the amplitude and shape of the direct and reflected wavelets the quality factor Q can be determined independently as an additional soil parameter. To investigate the relationships between the influencing factors, we conducted high-frequency GPR reflection measurements on soil samples with varying clay and water contents. The Q factor, derived from the spectral ratio method, is found to be between 5–15 for our samples that range from about 5–90% in sand content, 5–65% in silt content, 3–63% in clay content and 0–37% in pore water content. A multivariate non-linear empirical function linking water and clay content to Q agrees well with the observations. We found that the ratio of the real and imaginary parts of the dielectric permittivity depends basically on the clay content. Therefore, it may become possible to determine the clay content of soils in situ from a combination of GPR and conductivity measurements. The Q values determined from spectral ratios are confirmed by synthetic radargrams and agree with the observed decrease of the central frequency of GPR reflections caused by absorption.

Case history: integrated geophysical survey at Katarínka Monastery (Slovakia)

Katarinka (St. Catherine) is the ruin of an abandoned Franciscan monastery from the early 17th century located in the western Small Carpathians in Slovakia. Historical sources and paintings suggest that, beside the remains of the monastery that are still visible, a circle of eight chapels, a pilgrim’s hospice, a cemetery, and garden terraces originally surrounded the main building of the monastery. From 2009 to 2012, geophysical campaigns were performed to find evidences and positions of remains of these buildings of the monastery campus. An initial magnetic overview survey revealed multiple local accumulations of disordered dipole anomalies. Since these accumulations did not allow a structural interpretation, ground penetrating radar measurements were conducted. The ground penetrating radar results clearly showed wall structures beneath almost all magnetic anomaly accumulations. In between the remains of the monastery main building, ground penetrating radar and electrical resistivity tomography were performed at different areas that were difficult to access because of a strong cover of vegetation and steep topography.

Attenuation of GPR waves in soil samples based on reflection measurements

We investigated the influence of clay and water content on the geoelectrical and GPR properties of various soil samples. The GPR measurements were analyzed not only regarding wave velocity but also with respect to amplitude decay. The attenuation of the GPR waves is quantified in terms of the quality factor Q that can be determined from a comparison of the amplitudes of the direct and reflected waves. We applied the spectral ratio method known from seismic data processing to determine the Q-factor and confirmed the obtained Q-values by foreward wavelet modeling. The investigated soil samples showed variations in the clay contents of 2–60 % and in the volumetric water content from 0 to 35 %. The corresponding quality factors vary between 5 and 15 for high and low clay and water contents, respectively.

Pedophysical Study of Electrical Properties and Water Content, Laboratory and Field Experiments

Within the iSOIL project we are developing a mobile multisensor platform to gain fast, accurate areal information of soils at the field scale. The measured geophysical attributes (e.g. electric, electromagnetic) need to be linked to soil parameters (e.g. clay and water content) via geophysical pedotransfer functions, GPTFs. These could either be empirical (e.g. Archie and Topp et al. Equations) or physically-based (e.g. mixing and effective medium models). Our interdisciplinary activities aim to develop robust GPTFs using diverse laboratory and in situ experiments. In laboratory experiments we investigated the influence of water content on di-/electrical measurements. The dried samples collected from different sites and depths were uniformly saturated with rain water in steps of ~2% pore volume. For each step we measured georadar reflections and geoelectrical resistivity as well as water content and temperature. Also independent permittivity measurements were conducted to validate GPTFs results. The geophysical results were complemented by a suite of standard soil parameters determined on the samples. From these lab and field data the relationship of water content with respect to both the dielectric permittivity and electric resistivity are established. Different empirical and mixing models are fitted to the data and discussed.

iSOIL – Integration of Geophysical Technologies in Measuring Platforms

We present work that was done within the EU-level project iSOIL “Interactions between soil related sciences – Linking geophysics, soil science and digital soil mapping”. iSOIL focuses on improving and developing fast and reliable mapping approaches of soil properties, soil functions, and soil degradation threats. This requires the improvement and integration of advanced soil sampling approaches, geophysical and spectroscopic measurement techniques, as well as pedometric and pedophysical approaches. Integration of geophysical technologies in measuring platforms allow a rapid and economical feasible mapping of large areas with complementary parameters. Application of techniques and their combinations will improve to a great extent the significance of the resulting digital maps. Performance tests and a quantitative data evaluation, e.g. of electromagnetical data, are necessary for further integration and field application of mobile geophysical platforms. The data quality needs to be analyzed with respect to technical parameters of instruments, survey speed and possible interference between sensors and towing vehicle. An experiment was designed as first step towards a efficient mobile sensing of electromagnetical data. Results of this first experiment are presented here to illustrate challenges of iSOIL concerning measuring technologies. Furthermore we present data of different sensor types resulting in information about subsurface heterogeneities.

Geophysical Survey for Crypt Detection in the Church of Virgin St. Mary’s Birth in Horné Krškany, Slovakia

Geophysical methods are an essential part of non-destructive prospecting methods in archaeology. Geophysical measurements for crypt detection were performed in the interior of the Church of Virgin St. Mary’s Birth in Horne Krskany during the International course on ArchaeoGeophysics INCA 2009 (European Union LLP-Erasmus programme). The church is a valuable representative of Slovak historical heritage. Ground-penetrating radar (GPR) and microgravity techniques independently confirmed the presence of two crypts in the nave. Obtained results are very expressive by the character of the deformation of geophysical fields. Recommendations for video inspection or archaeological opening were also given to the archaeological site-excavators.