Jaroslava Pánisová

The calculation of building corrections in microgravity surveys using close range photogrammetry

Use of the microgravity technique for cavity detection in the exploration of historical buildings requires careful data acquisition and modern processing procedures. We have developed a new method for the calculation of building effects, where geodetic measurements and special photogrammetric software are used. In our new approach, a three-dimensional polyhedral model of an historical building is created from images using Eos System’s PhotoModeler Scanner software. A comparison of equations for the calculation of the gravitational effect of polyhedral bodies is presented on a simple test model. The methodology of microgravity data processing is demonstrated on a small Slovak church, where two crypts were successfully detected using microgravity and GPR techniques in summer 2009. We have shown that close range photogrammetry methods offer a possibility to improve the microgravity data processing procedure.

The use of microgravity technique in archaeology: A case study from the St. Nicolas Church in Pukanec, Slovakia

The use of microgravity technique in archaeology: A case study from the St. Nicolas Church in Pukanec, Slovakia The detection of subsurface cavities, such as crypts, cellars and tunnels, in churches and castles belongs to successful applications of the employment of surface gravity measurement techniques in archaeo-prospecting. The old historic building exploration requires using of non-invasive methods, and hence the microgravity technique is a proper candidate for this task. On a case study from the Roman-Catholic Church of St. Nicolas in the town Pukanec the results of using microgravity for detection and delineation of local density variations caused by a near-surface void are shown. The acquired negative anomaly in the residual Bouguer anomalies field suggested the presence of a possible void feature. Euler deconvolution and 3D modelling were used to estimate the depth and shape of the anomalous source. Additionally, measurements of the vertical gravity gradient on several stations were performed. We tested how the use of a downward continuation of gravity, utilizing the real vertical gravity gradient, influences the shape and amplitude of the final Bouguer anomaly map.

Chapter 7 – National Gravimetric Database of the Slovak Republic

Compilation of the Slovak gravimetric database with the actual amount of about 320,000 observation points is presented. Gravity data were collected during more than 50 years, which yields a very heterogeneous dataset, with large variations in the station coverage and processing methods. The regional gravimetric database (more than 212,000 points) was resumed in 2001. The compilation discussed herein (with more than 107,000 detailed gravity measurements) was made during 2011–14. Quality-control process and complete recalculation of the Bouguer anomalies is presented. Primary focus of this project was on a proper recalculation of the terrain corrections. New detected linear features in the Bouguer anomaly map were verified by the field measurements. A new software solution for reconstruction of the gravity acceleration values from the Bouguer anomaly map was developed for geodetic applications.

3D density modelling of Gemeric granites of the Western Carpathians

Abstract The position of the Gemeric Superunit within the Western Carpathians is unique due to the occurrence of the Lower Palaeozoic basement rocks together with the autochthonous Upper Palaeozoic cover. The Gemeric granites play one of the most important roles in the framework of the tectonic evolution of this mountain range. They can be observed in several small intrusions outcropping in the western and south-eastern parts of the Gemeric Superunit. Moreover, these granites are particularly interesting in terms of their mineralogy, petrology and ages. The comprehensive geological and geophysical research of the Gemeric granites can help us to better understand structures and tectonic evolution of the Western Carpathians. Therefore, a new and original 3D density model of the Gemeric granites was created by using the interactive geophysical program IGMAS. The results show clearly that the Gemeric granites represent the most significant upper crustal anomalous low-density body in the structure of the Gemeric Superunit. Their average thickness varies in the range of 5–8 km. The upper boundary of the Gemeric granites is much more rugged in comparison with the lower boundary. There are areas, where the granite body outcrops and/or is very close to the surface and places in which its upper boundary is deeper (on average 1 km in the north and 4–5 km in the south). While the depth of the lower boundary varies from 5–7 km in the north to 9–10 km in the south. The northern boundary of the Gemeric granites along the tectonic contact with the Rakovec and Klátov Groups (North Gemeric Units) was interpreted as very steep (almost vertical). The results of the 3D modelling show that the whole structure of the Gemeric Unit, not only the Gemeric granite itself, has an Alpine north-vergent nappe structure. Also, the model suggests that the Silicicum–Turnaicum and Meliaticum nappe units have been overthrusted onto the Golčatov Group.

Validation of sensitivity and reliability of GPR and microgravity detection of underground cavities in complex urban settings: Test case of a cellar

Abstract We test here the feasibility of ground-penetrating radar (GPR) and microgravity methods in identifying underground voids, such as cellars, tunnels, abandoned mine-workings, etc., in complex urban conditions. For this purpose, we selected a cellar located under a private lot in a residential quarter of the town of Senec in Western Slovakia, which was discovered by chance when a small sinkhole developed on the yard just two meters away from the house. The size of our survey area was limited 1) by the presence of a technical room built at the back of the yard with a staircase leading to the garden, and 2) by the small width of the lot. Therefore the geophysical survey was carried out only in the backyard of the lot as we were not permitted to measure on neighbouring estates. The results from the GPR measurements obtained by the GSSI SIR-3000 system with 400 MHz antenna were visualized in the form of 2D radargrams with the corresponding transformed velocity model of studied cross-sections. Only the profiles running over the pavement next to the house yielded interpretable data because the local geological situation and the regular watering of the lawn covering prevailingly the backyard caused significant attenuation of the emitted GPR signal. The Bouguer gravity map is dominated by a distinctive negative anomaly indicating the presence of a shallow underground void. The quantitative interpretation by means of Euler deconvolution was utilized to validate the depth of the center and location of the cellar. Comparison with the gravitational effect of the cellar model calculated in the in-house program Polygrav shows a quite good correlation between the modelled and observed fields. Only a part of the aerial extent of the anomaly could be traced by the used geophysical methods due to accessibility issues. Nevertheless, the test cellar was successfully detected and interpreted by both methods, thus confirming their applicability in similar environmental and geotechnical applications, even in complex urban conditions.

On inverting gravity changes with the harmonic inversion method: Teide (Tenerife) case study

Abstract Here we investigate the applicability of the harmonic inversion method to time-lapse gravity changes observed in volcanic areas. We carry out our study on gravity changes occured over the period of 2004–2005 during the unrest of the Central Volcanic Complex on Tenerife, Canary Islands. The harmonic inversion method is unique in that it calculates the solution of the form of compact homogeneous source bodies via the mediating 3-harmonic function called quasigravitation. The latter is defined in the whole subsurface domain and it is a linear integral transformation of the surface gravity field. At the beginning the seeds of the future source bodies are introduced: these are quasi-spherical bodies located at the extrema of the quasigravitation (calculated from the input gravity data) and their differential densities are free parameters preselected by the interpreter. In the following automatic iterative process the source bodies change their size and shape according to the local values of quasigravitation (calculated in each iterative step from the residual surface gravity field); the process stops when the residual surface gravity field is sufficiently small. In the case of inverting temporal gravity changes, the source bodies represent the volumetric domains of temporal mass-density changes. The focus of the presented work is to investigate the dependence of the size and shape of the found source bodies on their differential densities. We do not aim here (yet) at interpreting the found solutions in terms of volcanic processes associated with intruding or rejuvenating magma and/or migrating volatiles.

Detecting Shallow Medieval Features in the Church of St. George, Slovakia

In the framework of OPIS-Operational Programme Information Society co-funded by the European Regional Development Fund (ERDF) the most important monuments of Slovak cultural heritage are recorded using modern geodetic techniques. More than two hundreds objects, e.g. historical bridges, cemeteries, sacral buildings, folk architecture reservations, rock-cut dwellings, have been documented so far. The Church of St. George was recorded using laser scanning combined with very precise geodetic measurements. We present here the results of microgravity and GPR surveys carried out in the nave of this church in order to delineate the position of burial vaults beneath the floor. In microgravity data processing we focused on the calculation of the building correction. For this purpose a detailed polyhedral model of the church was created. Integrated interpretation of geophysical results has confirmed the presence of two medieval crypts predicted from historical archives. Moreover, the remains of the oldest Romanesque construction stage of the church were successfully detected by both methods.

New Method for Calculation of the Building Effect Correction in Microgravity Technique

Microgravity surveys for archaeological applications conducted in the interior of the buildings require careful data acquisition and processing. We have developed modern, effective and fast data processing procedure for calculation of the building effect correction in microgravity technique. Polyhedral body of historical building is created from images in Eos System’s PhotoModeler Scanner software which is based on close range photogrammetry principles. Comparison of equations for calculation of the gravitational effect of polygonal bodies is presented on a simple test model. The case study from small Slovak church shows that close range photogrammetry offers a possibility to improve microgravity data processing procedures. Our new method provides high accuracy of calculated building effect corrections, thus improving the ability of microgravity technique to detect more difficult identifiable cavities which are situated in a close vicinity to man-made structures.

Depth Estimation of Microgravity Anomalies Sources by Means of Regularized Downward Continuation and Euler Deconvolution

A powerful toll in the estimation of potential field source depths is given by the analytical downward continuation of the measured field - down to the depth of the first important shallow sources. On the other hand, analytical downward continuation is an highly instable problem and one effective way for its solution is Tikhonov regularization. Combination with the Derivative Euler Deconvolution can effectively help in the estimation of the depths to the centres of researched near-surface microgravity anomaly sources. This was presented on one selected synthetic model studies and one real data application. In some situations the estimations from Euler deconvolution are deeper, in some shallower, on the present we are not able to explain this aspect. Experiences with the regularized downward continuation show its very low dependence on grid extent and the grid cells sizes. Derivative Euler Deconvolution has showed large sensitivity to the precise evaluation of the initial vertical derivative – it has to be smoothed or damped in the case of real data interpretation (where noise and acquisition errors are present).

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.