Jakub Chromčák, Michal Grinč, Jaroslava Pánisová, Peter Vajda and Anna Kubová
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.
Peter Vajda, Pavol Zahorec Pavol, Juraj Papčo and Anna Kubová
We review here the gravitational effects on the temporal (time-lapse) gravity changes induced by the surface deformation (vertical displacements). We focus on two terms, one induced by the displacement of the benchmark (gravity station) in the ambient gravity field, and the other imposed by the attraction of the masses within the topographic deformation rind. The first term, coined often the Free Air Effect (FAE), is the product of the vertical gradient of gravity (VGG) and the vertical displacement of the benchmark. We examine the use of the vertical gradient of normal gravity, typically called the theoretical or normal Free Air Gradient (normal FAG), as a replacement for the true VGG in the FAE, as well as the contribution of the topography to the VGG. We compute a topographic correction to the normal FAG, to offer a better approximation of the VGG, and evaluate its size and shape (spatial behavior) for a volcanic study area selected as the Central Volcanic Complex (CVC) on Tenerife, where this correction reaches 77% of the normal FAG and varies rapidly with terrain. The second term, imposed by the attraction of the vertically displaced topo-masses, referred to here as the Topographic Deformation Effect (TDE) must be computed by numerical evaluation of the Newton volumetric integral. As the effect wanes off quickly with distance, a high resolution DEM is required for its evaluation. In practice this effect is often approximated by the planar or spherical Bouguer deformation effect (BDE). By a synthetic simulation at the CVC of Tenerife we show the difference between the rigorously evaluated TDE and its approximation by the planar BDE. The complete effect, coined here the Deformation Induced Topographic Effect (DITE) is the sum of FAE and TDE. Next we compare by means of synthetic simulations the DITE with two approximations of DITE typically used in practice: one amounting only to the first term in which the VGG is approximated by normal FAG, the other adopting a Bouguer corrected normal FAG (BCFAG).