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<p>Geothermal zones and hot mineral springs are primarily associated with magmatism along the Eurasian Continental Margin. In our study, we examine the correlation between geothermal anomalies observed in Bulgarian territory and the magnetic anomalies produced by magmatic and metamorphic bodies. Specifically, if these magnetic sources are of relatively recent origin, they could contribute to heating the surrounding geological environment, thereby influencing geothermal patterns. We use geological and hydrological information, along with temperature distribution at depth, to clarify the geological environment. Next, we apply geophysical data processing and direct inversion techniques to compute the total field modulus and Euler solutions, refining the interpretation of magnetic sources and enabling a more precise evaluation of subsurface magnetic structures and their potential impact on geothermal activity. A thorough analysis of the geomagnetic field, including over forty described anomalies, is presented for the examined territory. The correlation between geothermal and magnetic anomalies is confirmed using the Chi-square test. The null hypothesis is rejected, and the zones that significantly contributed to this result are outlined. Our study identifies seven regions with the strongest correlations in terms of temperature and magnetic anomalies: (1) A broad area in the western part of the Moesian platform near Kozloduy, characterized by increased temperatures and moderate magnetic anomalies; (2) A well-defined area east of Sofia, the capital city, which features high temperatures and intense magnetic anomalies, as well as numerous hot springs; (3) The region north of Blagoevgrad, located at the foothills of the Rila Mountains, where the hottest spring, Sapareva Banya, can be found; (4) The lower course of the Struma River, near the town of Petrich, where higher temperatures coincide with a distinct group of magnetic anomalies caused by granite bodies locally enriched in ferromagnetic iron minerals; (5) A local area near Velingrad exhibiting high temperatures alongside magmatic bodies, set in a complex tectonic environment; (6) A confined zone to the north of Dospat, where rhyolites are exposed along a significant fault line; (7) A broad zone with the highest temperatures (exceeding 100&#xa0;°C at a depth of 1,000&#xa0;m) that aligns with magnetic anomalies from extensive outcrops of Precambrian metamorphic rocks. We analyze these relationships in the context of identified hydrogeological zones, providing a detailed examination of the spatial distribution of geothermal and magnetic features. Furthermore, we explore the correlation between these anomalies and the depth to the Curie point, as inferred from magnetic data, to better understand the thermal and magnetic structure of the region.</p> Graphical Abstract <p></p> <p>The present study aims to identify the spatial distribution of geothermal zones in Bulgaria that coincide with the appearance of magnetic anomalies. To delineate geothermal anomalies, we use temperatures at a depth of 1000&#xa0;m below the surface, obtained from temperature logs of wells. Magnetic anomalies are calculated from the vertical component of the anomalous geomagnetic field across Bulgaria. Additionally, we support our interpretation with geological information and hydrogeological zoning of mineral waters, including the distribution of hot springs. Two types of analysis are performed: (1) geophysical data processing in terms of magnitude calculation and the direct inverse method of Euler deconvolution, which outlines the magnetic sources, and (2) statistical examination using a Chi-square test.</p> <p>A uniform grid with over 200,000 cells is generated, with each cell containing geothermal and magnetic data organized by classes (levels). A contingency table is compiled that records the frequency of each combination of geothermal and magnetic classes. The Chi-square test results indicate a highly significant statistical correlation between geothermal and magnetic anomalies. Consequently, we reject the null hypothesis and conclude that geothermal activity and magnetic variations are closely linked, possibly due to shared geological structures, heat-altered magnetism, or fault systems. The cells that contribute most to this correlation are identified and discussed.</p>

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Geothermal Sources Detected by Magnetic Anomalies and their Relation with Hot Springs and Geology: A Case Study from Bulgaria

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年9月30日

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  • Petya Trifonova,
  • Metodi Metodiev,
  • Dilyana Hristova,
  • Krasya Kolcheva

摘要

Geothermal zones and hot mineral springs are primarily associated with magmatism along the Eurasian Continental Margin. In our study, we examine the correlation between geothermal anomalies observed in Bulgarian territory and the magnetic anomalies produced by magmatic and metamorphic bodies. Specifically, if these magnetic sources are of relatively recent origin, they could contribute to heating the surrounding geological environment, thereby influencing geothermal patterns. We use geological and hydrological information, along with temperature distribution at depth, to clarify the geological environment. Next, we apply geophysical data processing and direct inversion techniques to compute the total field modulus and Euler solutions, refining the interpretation of magnetic sources and enabling a more precise evaluation of subsurface magnetic structures and their potential impact on geothermal activity. A thorough analysis of the geomagnetic field, including over forty described anomalies, is presented for the examined territory. The correlation between geothermal and magnetic anomalies is confirmed using the Chi-square test. The null hypothesis is rejected, and the zones that significantly contributed to this result are outlined. Our study identifies seven regions with the strongest correlations in terms of temperature and magnetic anomalies: (1) A broad area in the western part of the Moesian platform near Kozloduy, characterized by increased temperatures and moderate magnetic anomalies; (2) A well-defined area east of Sofia, the capital city, which features high temperatures and intense magnetic anomalies, as well as numerous hot springs; (3) The region north of Blagoevgrad, located at the foothills of the Rila Mountains, where the hottest spring, Sapareva Banya, can be found; (4) The lower course of the Struma River, near the town of Petrich, where higher temperatures coincide with a distinct group of magnetic anomalies caused by granite bodies locally enriched in ferromagnetic iron minerals; (5) A local area near Velingrad exhibiting high temperatures alongside magmatic bodies, set in a complex tectonic environment; (6) A confined zone to the north of Dospat, where rhyolites are exposed along a significant fault line; (7) A broad zone with the highest temperatures (exceeding 100 °C at a depth of 1,000 m) that aligns with magnetic anomalies from extensive outcrops of Precambrian metamorphic rocks. We analyze these relationships in the context of identified hydrogeological zones, providing a detailed examination of the spatial distribution of geothermal and magnetic features. Furthermore, we explore the correlation between these anomalies and the depth to the Curie point, as inferred from magnetic data, to better understand the thermal and magnetic structure of the region.

Graphical Abstract

The present study aims to identify the spatial distribution of geothermal zones in Bulgaria that coincide with the appearance of magnetic anomalies. To delineate geothermal anomalies, we use temperatures at a depth of 1000 m below the surface, obtained from temperature logs of wells. Magnetic anomalies are calculated from the vertical component of the anomalous geomagnetic field across Bulgaria. Additionally, we support our interpretation with geological information and hydrogeological zoning of mineral waters, including the distribution of hot springs. Two types of analysis are performed: (1) geophysical data processing in terms of magnitude calculation and the direct inverse method of Euler deconvolution, which outlines the magnetic sources, and (2) statistical examination using a Chi-square test.

A uniform grid with over 200,000 cells is generated, with each cell containing geothermal and magnetic data organized by classes (levels). A contingency table is compiled that records the frequency of each combination of geothermal and magnetic classes. The Chi-square test results indicate a highly significant statistical correlation between geothermal and magnetic anomalies. Consequently, we reject the null hypothesis and conclude that geothermal activity and magnetic variations are closely linked, possibly due to shared geological structures, heat-altered magnetism, or fault systems. The cells that contribute most to this correlation are identified and discussed.