Mining of Mineral Deposits

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Reliability control of mapping dataset on small subsurface geothermal gradient, a case study in the Zagreb geothermal field, Croatia

Marija Jurilj1, Ivica Pavičić1, Tomislav Malvić1

1Faculty of Mining, Geology and Petroleum Engineering University of Zagreb, Zagreb, Croatia


Min. miner. depos. 2025, 19(2):1-9


https://doi.org/10.33271/mining19.02.001

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      ABSTRACT

      Purpose. The research aims to use interpolation methods for mapping, such as Inverse Distance Weighting (IDW), as well as normality tests on geothermal gradient data from the Zagreb geothermal field (ZGF) in the Croatian part of Pannonian Basin System (CPBS).

      Methods. The IDW method is applied to original small (8 values) and artificial added (45 values) datasets. The IDW method and a comparison of three geothermal gradient maps (8, 45 and 53 data, respectively) are applied for the ZGF breccia-fractured carbonate reservoir. The maps are compared visually and using mean square error / root mean square error (MSE / RMSE). Dataset is tested on normal distribution (Shapiro-Wilk and Kolmogorov-Smirnov tests, Q-Q plot).

      Findings. The insufficient amount of data is the main shortcoming for any subsurface reservoir characterisation. The IDW method has successfully outlined the main reservoir geothermal gradient zones. Increasing dataset with artificial values sampled onto original map (8 values) showed that starting dataset is enough reliable for basic reservoir characterisation. Further reservoir development should be based not on numerous new wells, but on the development of existing wells, including new trajectories and more precise determination of drainage radius, capacity and temperature decline over time.

      Originality. For the first time IDW, supplemented with normality tests and artificial sampling based on original small datasets, is applied as a development method in the geothermal reservoir of the CPBS area.

      Practical implications. This research is a necessary step in determining the future planning of geothermal reservoir development in the Zagreb urban area. This can be primary or additional approach for a similar reservoir with a small sample, while for a reservoir with a large sample it can show the meaningfulness of choosing an interpolation method.

      Keywords: Croatia Pannonian Basin System, Zagreb, geothermal field, Inverse Distance Weighting (IDW), formal normality tests, Q-Q plot, geothermal gradient, small dataset


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