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

Contributions to Geophysics and Geodesy

Kušnirák

Wilken

et al. 2019

This contribution is focused on a common utilization of microgravimetry (very precise and detailed gravimetry) and geoeletrical methods (ground penetrating radar and electric resistivity tomography) in the detection of subsurface cavities in non-destructive archaeological prospection. Both methods can separately detect such kind of subsurface objects, but their complementary and at the same time an eliminating aspect can be very helpful in the interpretation of archaeogeophysical datasets. These properties were shown in various published case-studies. Here we present some more typical examples. Beside this, we present here for a first time an application of the electric resistivity tomography in the interior of a building (a church) in Slovakia. We also demonstrate an example with an extremely small acquisition step in microgravity as a trial for the detection of cavities with very small dimensions -in this case small separated spaces for coffins as a part of the detected crypt (so called columbarium). Unfortunately, these cavities were too small to be reliably detected by the microgravity method. We have tried the well-known 3D Euler deconvolution method to obtain usable depth estimates from the acquired anomalous gravity field. Results from this method were in the majority of cases plausible (sometimes little bit too shallow), when compared with the results from the ground penetrating radar.In one selected example, the 3D Euler solutions were too deep and in the present stage of study we cannot well explain this situation. In general, all presented results support an important role of common combination of several geophysical methods, when searching for subsurface cavities in non-destructive archaeological prospection.

Section: Third Case-study: Microgravimetric and Gpr Research Of The Isupporting

confidence: 85%

“…1.5 m). Our experiences from the applications of the 3D Euler deconvolution method are consistent with this result (Pašteka et al, 2011). This crypt was not verified by means of any archaeological excavation or video-inspection.…”

Section: First Case-study: Microgravimetric and Gpr Research Of The Isupporting

confidence: 84%

Effective combination of microgravimetry and geoelectrical methods in the detection of subsurface cavities in archaeological prospection – selected case-studies from Slovakia

Contributions to Geophysics and Geodesy

Kušnirák

Wilken

et al. 2019

“…Very similar results are produced from the method of stable downward continuation, based on the Tikhonov regularization algorithm (Pašteka, Karcol, Kušnirák, & Mojzeš, 2012; Pašteka, Kušnirák, & Karcol, 2018; Tikhonov, Glasko, Litvinenko, & Melichov, 1968). Using the analysis of special Norm‐functions we can estimate the maximum depth of stable downward continuation, which estimates the source position (Pašteka et al, 2011). In this case the situation differs from the theoretical position.…”

Section: Visualization Of Results and Their Interpretationmentioning

confidence: 99%

Microgravity method in archaeological prospection: methodical comments on selected case studies from crypt and tomb detection

Archaeological Prospection

Pánisová

Zahorec

et al. 2020

Detailed and precise measurement of the Earth's gravity field (microgravity method) can be effectively used for the detection and quantification of subsurface voids and/or cavities. There exist a variety of successful applications of the microgravity method in near surface geophysics, namely in geotechnical, environmental and archaeological prospection. Using state-of-the-art 'microgal' relative gravity meters, cavities of several metres in each dimension (positioned at a similar depth) can be detected and interpreted. Such objects produce negative anomalies with amplitudes of several tens of microGals (1 microGal = 10 −8 m s −2). This contribution is focused on a methodological overview of the most important acquisition and processing steps in archaeological microgravimetry. In the processing of acquired gravimetrical data into a Bouguer anomaly, the so-called building correction plays an important role, because the gravitational effect of building masses can produce false, usually negative anomalies. Several selected methods for quantitative interpretation are presented, these are based on depth estimation and density modelling. These interpretation methods give satisfactory results in the case of these type of negative anomalies that are caused by subsurface cavities. Microgravimetry can obtain good support from electromagnetic and electrical methods, mainly from ground penetrating radar and electrical resistivity tomography, respectively. Finally, we present successful case-studies of microgravimetrical detection of crypts in various churches from the Middle Ages and period of Modern History, surveyed during recent decades in Slovakia and Czechia.

“…Appearance and disappearance of the local minimum can be used for the source depth estimation in the TRDC method -vanishing of this local minimum during downward continuation to larger depth levels points to the depth of the source (e.g. Tikhonov et al, 1968;Glasko et al, 1970;Pašteka et al, 2011Pašteka et al, , 2012. We can say in other words: when we enter with the continuation depth into a non-harmonic space (into the sources), method cannot find a correct solution (local minimum vanishes).…”

Section: Tikhonov's Regularization Approach In Stable Downward Continmentioning

confidence: 99%

Matlab tool REGCONT2: effective source depth estimation by means of Tikhonov’s regularized downwards continuation of potential fields

Contributions to Geophysics and Geodesy

Kušnirák

Karcol

2018

Transformation based on downward continuation of potential fields is an important tool in their interpretation – depths of shallowest important sources can be determined by means of stable downward continuation algorithms. We analyse here selected properties of one from these algorithms (based on Tikhonov’s regularization approach) from the scope of two most important discretization parameters – dimensions of the areal coverage of the interpreted field and the sampling interval size. Estimation of the source depth is based on the analysis of computed LP-norms for various continuation depths. A typical local minimum of these norms disappears at the source depth. We show on several synthetic bodies (sphere, horizontal cylinder, vertical rod) and also real-world data-sets (results from a magnetic survey for unexploded ordnance detection) that there is a need for relatively large surroundings around the interpreted anomalies. Beside of this also the sampling step plays its important role – grids with finer sampling steps give better interpretation results, when using this downward continuation method. From this point of view, this method is more suitable for the interpretation of objects in near surface and mining geophysics (anomalies from cavities, unexploded ordnance objects and ore bodies). Anomalies should be well developed and separable, and densely sampled. When this is not valid, several algorithms of interpolation and extrapolation (grid padding methods) can improve the interpretation properties of studied downward continuation method.