Experiences from microgravity and GPR surveys for subsurface cavities detection – case studies from SW- and central Slovakia

Combination of microgravity and GPR method constrains each other and help to detect subsurface cavities in a very effective way. Several examples are presented, some of the data-set were acquired during common summer schools between Kiel University and Comenius University in Bratislava

On Gravimetric Detection of Thin Elongated Sources Using the Growth Inversion Approach

Thin elongated sources, such as dykes, sills, chimneys, inclined sheets, etc., often encountered in volcano gravimetric studies, pose great challenges to gravity inversion methods based on model exploration and growing sources bodies. The Growth inversion approach tested here is based on partitioning the subsurface into right-rectangular cells and populating the cells with differential densities in an iterative weighted mixed adjustment process, in which the minimization of the data misfit is balanced by forcing the growing subsurface density distribution into compact source bodies. How the Growth inversion can cope with thin elongated sources is the subject of our study. We use synthetic spatiotemporal gravity changes caused by simulated sources placed in three real volcanic settings. Our case studies demonstrate the benefits and limitations of the Growth inversion as applied to sparse and noisy gravity change data generated by thin elongated sources. Such sources cannot be reproduced by Growth accurately. They are imaged with smaller density contrasts, as much thicker, with exaggerated volume. Despite this drawback, the Growth inversion can provide useful information on several source parameters even for thin elongated sources, such as the position (including depth), the orientation, the length, and the mass, which is a key factor in volcano gravimetry. Since the density contrast of a source is not determined by the inversion, but preset by the user to run the inversion process, it cannot be used to specify the nature of the source process. The interpretation must be assisted by external constraints such as structural or tectonic controls, or volcanological context. Synthetic modeling and Growth inversions, such as those presented here, can serve also for optimizing the volcano monitoring gravimetric network design. We conclude that the Growth inversion methodology may, in principle, prove useful even for the detection of thin elongated sources of high density contrast by providing useful information on their position, shape (except for thickness) and mass, despite the strong ambiguity in determining their differential density and volume. However, this yielded information may be severely compromised in reality by the sparsity and noise of the interpreted gravity data.This work was partially supported by the Slovak Research and Development Agency under the contract (project) No. APVV-19-0150 (acronym ALCABA), by the VEGA grant agency under projects No. 2/0002/23 and No. 2/0100/20, and under the ERA.net program ERA.MIN-2 project “Deposit-to-Regional Scale Exploration (acronym D-Rex). A.G.C. and J.F. were supported by the Spanish Agencia Estatal de Investigación (https://doi.org/10.13039/501100011033) grant RTI2018-093874-B-I00 (DEEP-MAPS) and grant G2HOTSPOTS (PID2021-122142OB-I00) from the MCIN/AEI/10.13039/501100011033/FEDER, UE. This work represents a contribution to CSIC Thematic Interdisciplinary Platform VOLCAN. The first author thanks Richard Almond of Geophysical Software Solutions P/L for providing the Potent software package and assistance for the forward computations in the case studies free of charge. We thank the two reviewers, Peter Lelievre and the anonymous reviewer for their thorough and constructive reviews that helped improving the presented work significantly.Peer reviewe