PROfile Jean-Luc Mari and Geoffroy Paixach Geophysics in geothermal exploration A review
Geophysics in Geothermal Exploration A review
Geophysics in Geothermal Exploration A review Jean-Luc Mari and Geoffroy Paixach QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE
DOI: 10.1051/978-2-7598-3752-6 ISBN(ebook): 978-2-7598-3752-6 This book is published under Open Access Creative Commons License CC-BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/en/) allowing non-commercial use, distribution, reproduction of the text, via any medium, provided the source is cited. © EDP Sciences, 2025
5 Contents Prefaces 11 Foreword 13 J.-L. Mari and G. Paixach The authors 15 Introduction 25 J.L. Mari and G. Paixach The Role of Geophysics in Geothermal Energy 25 Glossary of Geothermal Energy 26 Geothermal Energy in France 28 Book content 28 Reference 29 Chapter 1 • Overview of the different geothermal systems: role of geophysics in exploration and production 31 G. Paixach, H. Traineau, F. Bugarel, E. Lasne and C. Maïlhol 1.1 What is geothermal energy? 31 1.2 What are the main geothermal systems? 37 1.3 The role of geophysics 43 References 49
6 Geophysics in Geothermal Exploration Chapter 2 • Surface geophysical methods 51 J.-L. Mari and G. Paixach 2.1 Physical properties of rocks and pore space properties 55 2.1.1 Porosity 57 2.1.2 Permeability 58 2.2 Geophysical methods 61 2.2.1 Gravity method 63 2.2.2 Magnetic method 68 2.2.3 Electrical and EM methods 69 2.2.4 Seismic methods 80 Conclusion 107 References 108 Chapter 3 • Borehole geophysical methods 115 J.L. Mari 3.1 Conventional logging methods 116 3.2 Hydrogeological methods 120 3.3 Full waveform acoustic methods 123 3.4 Borehole seismic method 129 Conclusion 140 References 141 Chapter 4 • Towards a revisited geothermal conceptual model in the Upper Rhine Graben 145 A. Genter, C. Baujard, C. Glaas and V. Maurer 4.1 Geothermal development in the Upper Rhine Graben 146 4.2 Evolution of the geothermal concept during the SsF adventure 148 4.3 Pre-exploration phase 153 4.4 Optimizing borehole design according to the geological knowledge of the reservoir 159 Conclusion and perspectives 160 Acknowledgments 161 References 161
7 Contents Chapter 5 • DEEP ERT/IP for geothermal exploration and de-risking 165 A. Rosselli, C. Truffert, F. Barsuglia, F. Fischanger, A. Coletti, G. Morelli and S. Del Ghianda 5.1 Context 165 5.2 Why electrical resistivity tomography is useful? 166 5.3 Deep electrical resistivity tomography for geothermal exploration – an Italian example 166 5.3.1 Unconventional ERT data acquisition 167 5.3.2 Acquisition methodology 169 5.3.3 Acquisition layout 170 5.3.4 Current transmissions 172 5.3.5 Quality control 173 5.3.6 Processing of resistivity and chargeability measurements 175 5.3.7 Results 177 Conclusion 180 Chapter 6 • The use of passive seismic methods for Geothermal exploration and monitoring 181 T. Kremer, J. M. Ars, T. Gaubert-Bastide, K. Khazraj and C. Voisin Introduction 181 6.1 Methods 185 6.1.1 Seismological analysis 185 6.1.2 Ambient noise seismic interferometry (ANSI) 189 6.1.3 Tomography 190 6.1.4 Monitoring 191 6.2 Passive seismic methods for geothermal exploration 193 6.2.1 Seismological analysis 194 6.2.2 Ambient noise seismic interferometry (ANSI) 204 6.2.3 Integration into the geothermal exploration workflow 206 6.3 Geothermal monitoring 208 6.3.1 Seismological analysis monitoring 208 6.3.2 Ambient noise seismic interferometry monitoring 210 Concluding remarks 214 References 216
8 Geophysics in Geothermal Exploration Chapter 7 • Seismic inversion and characterization applied to geothermal energy 223 R. Baillet, T. Chrest, T. Defreminville and E. Masse Introduction 223 7.1 Technical background 224 7.1.1 Seismic gathers and partial stacking 224 7.1.2 The subsurface as an isotropic elastic medium 225 7.1.3 Convolution and resolution 226 7.2 Seismic inversion 228 7.2.1 About seismic conditioning 228 7.2.2 Wavelet extraction and optimization 228 7.2.3 Construction of a low-frequency model 229 7.2.4 Performing a seismic inversion 230 7.3 Introduction to seismic characterization 232 7.3.1 Exploring well response through a petro-elastic model building 232 7.3.2 Seismic attributes related to faults and fractures 234 7.3.3 Characterization empowered by machine learning 235 7.4 Example: identification of lithology, good porosity and fractured areas through a seismic inversion study 237 Conclusions and perspectives 239 References 239 Chapter 8 • Seismic anisotropy applied to geothermal prospection 241 R. Baillet, N. Desgoutte, V.Thomas and J. Caudroit Introduction 241 8.1 Technical background 242 8.1.1 The HTI and VTI models for anisotropy models 242 8.1.2 Azimuthal stacking and required processing 243 8.2 Velocity versus Azimuth (VVAz): a shift detection methodology 244 8.3 Amplitude versus Azimuth (AVAz): an inversion methodology 245 8.4 Ellipse fitting on properties to estimate the anisotropy 245 8.5 From anisotropy to fracture attributes 247 8.6 Case study: Fracture characterization through azimuthal inversions to prospect the geothermal potential of Geneva basin 248 8.6.1 Processing, conditioning, shift detection 248 8.6.2 Model-based inversions 250 8.6.3 Results and way forward 251 Conclusions and perspectives 254 References 254
9 Contents Chapter 9 • Defining high enthalpy geothermal drilling target with multi-physics integrated exploration program. Mayotte’s Petite-Terre Island case study 257 A. Stopin, C. Dezayes and T. Farlotti Introduction 257 9.1 Integration of magnetotelluric data 260 9.2 Electric profile integration 264 9.3 Gravimetric data integration 267 9.4 Final model 268 9.5 Choice of the drilling target 271 References 272 Chapter 10 • Feasibility of monitoring cold fronts of geothermal doublets using 4D active electromagnetic techniques – a field trial in the Dogger play in the Paris Basin 275 F. Dubois, A. Stopin, F. Bretaudeau and P. Wawrzyniak Introduction 276 10.1 Context 277 10.2 Acquisition 278 10.3 Receiver conception 281 10.4 Survey 282 10.5 Data processing 284 10.6 Detectability of the cold front 287 Conclusions 288 References 290 Synthesis 291 G. Paixach and J.L. Mari A range of geothermal systems 291 A range of geophysical techniques 292 Geophysics for geothermal systems 293 From resource exploration to drilling project de-risking and asset monitoring 295 Conclusion 297 J.L. Mari and G. Paixach
11 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE Prefaces An invitation in March 2024 to the scientific days of the “AGAP-Qualité” from Jean-Luc Mari and its president Michel Hayet made it possible to initiate interesting conversations between the applied geophysics community and the French administration on subsurface exploration. This book entirely dedicated to geophysical investigation applied to the characterization of geothermal resources is, I hope, a first step in the dissemination of knowledge and know-how for everyone, from practitioners to a broad public. We are all convinced that geothermal energy represents a promising renewable energy source, capable of meeting energy needs while reducing our carbon footprint. I sincerely thank all the authors of the technical chapters constituting this book with a special mention to Geoffroy Paixach and Jean-Luc Mari who, in their free time, made it possible to develop this work. Dear reader, I hope that, like me, you will have the pleasure of discovering this book and keeping it within reach. Jean-Claude Lecomte Geoscience expert Bureau des ressources énergétiques du sous-sol Direction Générale de l’Energie et du Climat Ministère de la transition écologique, de l’énergie, du climat et de la prévention des risques © EDP Sciences, 2025 DOI: https://doi.org/10.1051/978-2-7598-3752-6.c901
12 Geophysics in Geothermal Exploration It is with great pride and enthusiasm that I present this preface to the latest issue of Cahiers de l’AGAP. While the term “Cahiers” may evoke the idea of a lighter compilation, I believe, and I hope readers will agree, that the exceptional quality and depth of the contributions in this volume will serve as a valuable resource for professionals in the geothermal industry, supporting their future projects. State-of-the-art geophysics has become essential to accelerate the exploration and development of new geothermal projects. Geophysical methods, long established in fields such as petroleum exploration, mining, hydrogeology, and civil engineering, are now proving increasingly indispensable in the context of geothermal energy development. Geothermal energy is becoming increasingly important in today’s energy transition and is poised to play a growing role in the national, european, and global energy mix. On behalf of AGAP-Qualité, I extend my sincere gratitude to all the authors for their remarkable contributions, as well as to the coordinators, Jean-Luc Mari and Geoffroy Paixach, for their involvement in bringing this work to fruition. Michel Hayet Chairman of AGAP AGAP-Qualité Association for Quality in Applied Geophysics
13 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE Foreword J.-L. Mari and G. Paixach The book entitled “Geophysics in Geothermal exploration: a review” provides a practical guide on how to apply geophysical methods in geothermal exploration, illustrated with real-world field examples. These methods support resource exploration, the de-risking of drilling projects, and the ongoing monitoring of geothermal assets. The book begins with an introduction to geothermal energy systems. It then delves into geophysical methods, presenting the current state of knowledge and illustrating how electrical-electromagnetic and active-passive seismic methods can be combined into a Multiphysics approach for geothermal exploration. The book is published under Open-Source Creative Commons License CC-BYNC-ND allowing non-commercial use, distribution, reproduction of the text, via any medium, provided the source is cited. © EDP Sciences, 2025 DOI: https://doi.org/10.1051/978-2-7598-3752-6.c902
15 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE The authors Jean-Luc MARI A graduate of the Institut Physique du Globe Strasbourg and the IFP School (petroleum geosciences, major in geophysics in 1978), Jean-Luc Mari was employed by IFP Energies Nouvelles in 1979 as a research engineer in the Geophysics Department. Here he worked on several research projects, such as high-resolution seismic surveying, reservoir monitoring, and the development of borehole tools, in collaboration with industrial partners GdF-Suez, CGG, Total and ELF Aquitaine. In 1984, he was awarded a PhD in Astronomy and Celestial Mechanics by the Université Pierre et Marie Curie. In 1986, he was seconded to ELF Aquitaine where he worked on reservoir geophysics. He joined IFPEN in 1987 and was seconded to the Reservoir Department, where he studied, in particular, the benefits of using geophysical methods in horizontal wells. In 1994 he was appointed to the IFP School as a professor and obtained the accreditation to supervise earth science research (HDR). He was an expert in geophysics for IFP Energies Nouvelles. Jean-Luc, member of the EAGE, was an associate editor for Near Surface Geophysics. Currently retired from IFP Energies Nouvelles, Jean-Luc is an independent researcher and consultant in geophysics. He is a member of the board and of the accreditation committee for the Association for Quality in Applied Geophysics (AGAP -Association pour la Qualité en Géophysique Appliquée). An author and co-author of patents and numerous scientific articles, Jean-Luc Mari has also contributed to educational scientific books and has been involved in the design and development of tutorials and e-books. In 2010, he received a Knighthood from the Ordre des Palmes Académiques. © EDP Sciences, 2025 DOI: https://doi.org/10.1051/978-2-7598-3752-6.c903
16 Geophysics in Geothermal Exploration Geoffroy PAIXACH Geoffroy is a geophysicist with a degree from the IFP School, Paris. He began his career with an Oil and Gas operator at the Geoscience Research Center in London, focusing on seismic processing amplitude preservation. Over the years, Geoffroy has held various positions in reservoir characterization for Beicip-Franlab, a subsidiary of IFPEN. Geoffroy worked in the Marine CSEM industry and the seismic multi-client industry, developing offshore seismic acquisition programs in multiple countries. He co-founded and managed a startup specializing in real-time simulation for basin modeling in Houston. In 2025, he became the Chief Executive Officer of CFG Geothermal, a geothermal resource consulting and engineering services firm. Jean-Michel ARS Graduated from École Normale Supérieure d’Electricité et de Mécanique in 2012 and from Université Paul Sabatier in 2013. Jean-Michel Ars obtained a PhD from Université Bretagne Occidental in 2018 in the field of geophysics, focusing on multi-physics joint inversion applied to geothermal exploration. He was a post-doc for 3 years before becoming a geophysics researcher at Geolinks since 2022. He works primarily in the field of inverse methods for geophysical imaging but also studies seismic ambient noise as subsurface characterization and monitoring tools. Romain BAILLET Romain Baillet is a Geophysicist with an Engineer Degree (MSc) from the “École et Observatoire des Sciences de la Terre” school in Strasbourg, France. He joined BeicipFranlab in 2012 as a specialist in seismic inversion and characterization. Till then, he has been involved in many integrated or specialized studies for all energies, including geothermal activities. As an expert and project leader, he currently participates in the innovation of the methodologies proposed by Beicip-Franlab by designing and carrying out workflows including seismic data, to lower the risks in both prospection and production phases. As
17 The authors a product manager of the geophysical software InterWell, he improves and optimizes the workflows of seismic inversion, seismic characterization or time-depth conversion. either in terms of technology (e.g. Machine Learning) or customer needs (Performance, data type/transfer…). Clément BAUJARD Dr Clément Baujard is an experienced reservoir engineer with a strong background in flow, thermal exchanges and mechanical processes in fractured reservoir. He was graduated as a geoscience engineer in 2002 and obtained a PhD in quantitative hydrogeology in 2005 from Paris School of Mines. From 2006, he worked as a geothermal reservoir engineer in Switzerland and then joined geothermal industry at Électricité de Strasbourg Géothermie (ÉSG), a small company offering engineering services in deep geothermal energy in 2013. At ÉS-Géothermie, he is currently leading the subsurface engineering department. He contributed to many European research projects in relation with geothermal energy and he was in charge of several industrial projects in the Upper Rhine Graben dealing with deep geothermal energy and geothermal lithium. He is author or co-author of many international scientific publications Frédérik BUGAREL Graduating from Pierre et Marie Curie University (Paris VI) with a master’s degree in hydrogeology in 2004, Frédérik Bugarel developed his skills in drilling project engineering by working successively in the international oil industry, in the monitoring and safety of former French mining sites and in geothermal energy. Since 2023, he has held the position of Project Director at CFG Geothermal, a subsidiary of BRGM and BEICIP-FRANLAB, an engineering firm specializing in deep geothermal energy.
18 Geophysics in Geothermal Exploration Chrystel DEZAYES Chrystel Dezayes is a researcher at the French Geological Survey (BRGM) in the Geothermal Energy Department. Since her PhD in 1995, she is working about fracture network characterisation at the Soultz-sous-Forêts EGS site and elsewhere in the world. She joined BRGM in 2002 in the geothermal team as researcher. She continues her work about fractured deep geothermal reservoir and develop exploration methods for sedimentary basin context. She is also project leader for high temperature geothermal exploration in oversea islands and scientific coordinator for the geothermal energy and energy storage at BRGM. She has authored and co-authored about 40 articles in scientific journals and numerous others in international conferences. Frédéric DUBOIS Graduating from the École et Observatoire des Sciences de la Terre de Strasbourg in 2016, Frédéric Dubois is working at the BRGM (Bureau de recherches géologiques et minières) since 2020 as geophysicist specialized in electromagnetism. His expertise spans over the broad spectrum from magnetotelluric to intermediate frequency domain and through land and airborne controlled source electromagnetism. Thomas GAUBERT-BASTIDE Thomas Gaubert-Bastide, graduated from Université Grenoble Alpes in 2017, obtained a PhD in geophysics from both Université de Pau et des Pays de l’Adour and Université Grenoble Alpes in 2023, focusing on water-table monitoring through seismic interferometry. Since early 2023, he has been working as a geophysicist researcher at Geolinks. His main research is centered on the characterization and monitoring of subsurface physical processes using seismic noise analysis at various scales.
19 The authors Albert GENTER Graduating from University of Orléans with a PhD in Geology in 1989, Albert Genter carried out research on the Soultz-sous-Forêts pioneered geothermal project. After his PhD, he was employed by BRGM, working on Enhanced Geothermal System (EGS). At BRGM, he was successively a junior geologist, R&D project manager, then geo-energy unit manager and finally deputy manager of the geothermal division. In 2007, he joined the Heat Mining industrial Group as the scientific manager of the EGS Soultz site. He was responsible for a geoscientific program for monitoring this geothermal plant producing electricity. From 2014 to now, he joined geothermal industry at Électricité de Strasbourg Géothermie (ÉSG) as deputy general manager. He was, for example, the main coordinator of the European Horizon 2020 MEET project from 2018 to 2022. Since 2025, he is retired but still involved in geothermal energy as an independent consultant. Albert was elected at the board directors of the International Geothermal Association (IGA) from 2013 to 2020. He was also elected at the board of directors of the French Geothermal Association (AFPG) from 2022 to 2024. He is author or co-authors of about 70 international scientific publications. He has also contributed to educational purposes by providing academic lectures in universities (Mainz, Neuchâtel, Paris, Orléans, Strasbourg) and in High Schools (Mines Nancy, École de Géologie de Nancy, IFP School). During his career, he was examiner or supervisor of about 30 students (PhD, master) and 2 accreditations to supervise earth sciences research. He got two best presentation awards at the geothermal US conference GRC in 2009 and 2014. He got also an award from the WING (Women in Geothermal) association. Carole GLAAS Dr Carole Glaas is a junior geologist with an expertise in fractured reservoirs from the Upper Rhine Graben (URG). She was graduated in 2021 with a PhD from French Universities of Strasbourg and Poitiers. She did her PhD in relation with Électricité de Strasbourg Géothermie (ÉSG). She focussed on the crystalline part of geothermal wells drilled in fractured reservoirs located in the URG. During her PhD, she investigated hydrothermal alteration minerals like clay minerals as well as the fractured reservoirs from a structural point of view. At ÉSG, she was in charge of geological studies and well log interpretation
20 Geophysics in Geothermal Exploration for a geothermal well drilled South of Strasbourg and contributed to the well testing operations. Recruited at ÉSG in 2021, as a junior geologist, she contributed to many national and EU research projects dealing with deep geothermal energy and geothermal lithium. She is author or co-author of many international scientific publications Kaoutar KHAZRAJ Kaoutar Khazraj graduated with an engineering diploma from the École Nationale Supérieure d’Électricité et de Mécanique in Nancy in 2015, specializing in hydrodynamic reservoir engineering. She also obtained research master’s degree in Petroleum Geosciences and Reservoir Engineering from Université de Lorraine in 2015. In 2024, she completed her PhD in geophysics at CY Cergy Paris Université. Her doctoral research focused on developing an innovative hybrid full-waveform inversion method for borehole seismic data to improve inversion results for under-determined seismic problems. Since June 2024, she has been a geophysical researcher at Geolinks, specializing in inverse methods and the development of numerical algorithms for geophysical imaging. Thomas KREMER Thomas Kremer received a PhD degree received in 2015 from Institut de Physique du Globe de Paris. He is now a 13-year-old experienced research geophysicist specialized in geophysical monitoring applications with. His research work initiated with the use of electromagnetics methods for the monitoring of CCUS projects. He also developed innovative methods for the monitoring and characterization of hydrogeological systems. Since 2020, he has been managing the R&D department at Geolinks services, an innovative company that promotes the development of passive seismic methods for multiple subsurface monitoring applications, such as hydrogeological characterization, geotechnical risk assessment, geothermal exploration, and CCUS and UGS projects monitoring.
21 The authors Éric LASNE Geologist with a PhD in hydrogeology, Éric Lasne has held project management and management positions at Compagnie Française de Géothermie since 1992. As such, he has extensive knowledge of the challenges of the subsoil in terms of all the applications and uses of geothermal resources. On the technical side, Éric Lasne has worked on several low-energy geothermal projects in France (Dogger in the Paris Basin, Aquitaine Basin, Alsace, Centre Region, etc.) and for electricity generation in the French overseas territories (Guadeloupe, Martinique, La Réunion) and abroad (Azores – Portugal, Dominica, Indonesia, China, Russia, etc.). Involved in the development of CFG’s activities, he is also active in professional organizations dedicated to the promotion and development of geothermal energy. A representative of one of the founding members of AFPG, Éric Lasne is also a member of the geothermal energy committee of the French Renewable Energy Union (SER). Vincent MAURER Dr Vincent Maurer is a senior geophysicist specialized in seismology with skills in design and installation of seismic networks for monitoring induced micro-seismicity. He was graduated in 2009 with a PhD on geophysics at ETH-Zürich in Switzerland. From 2010 to now, he worked for geothermal industry at Électricité de Strasbourg Géothermie (ÉSG) as a geophysicist and is responsible for the seismic monitoring of the Soultz-sous-Forêts and the Rittershoffen geothermal plants. In parallel, he is involved in many geophysical surveys for geothermal exploration in Alsace: 2D/3D seismic acquisition, 3D MT/CSEM survey, aeromagnetic and gravity campaigns, and passive seismology. He also contributed to the interpretation of the geophysical acquisition to design well trajectories. At ÉSG, he is currently involved in several industrial geothermal projects in Northern Alsace. He contributed to national R&D projects and EU research projects dealing with geothermal energy. He is author or co-author of many international scientific publications.
22 Geophysics in Geothermal Exploration Alberto ROSSELLI PhD in geophysics from the University of Lausanne, Switzerland. In 2001 he started a postdoc at the Laboratoire de Géophysique of the École Polytechnique de Montréal (Québec). In 2010 Alberto Rosselli was employed by PGS as a seismic navigator. In 2012 he started to work as offshore and onshore client representative for Total EP and ENI. He has been employed as Party Chief by Geo2X. In 2021 he co-founded GEG Experts and he’s currently General Director of the company. Alexandre STOPIN Alexandre Stopin is an experienced geophysicist currently working at BRGM (Bureau de Recherches Géologiques et Minières). His expertise spans geophysical exploration, geothermal resource assessment, and geological CO₂ storage. He has contributed to several European projects and is responsible for the geophysical acquisition and processing of the Geoscan project. Earlier in his career, he worked at Shell Global Solutions International BV, where he led a team specialized in advanced seismic methods, including Elastic Full Waveform Inversion (FWI), to tackle complex subsurface imaging challenges. Beyond his research contributions, Alexandre actively shares his findings through scientific publications and international conferences, promoting sustainable energy solutions and innovative geophysical techniques. Hervé TRAINEAU Hervé Traineau is a senior geologist and geothermal expert. He got a PhD in Petrology-Volcanology at the University of Paris 11 in 1982. He began his career in 1981 as a geologist at the BRGM. Assigned to the Geothermal Department, he participated in numerous geothermal exploration campaigns in the West Indies (Haiti, Santo Domingo, Martinique, Guadeloupe, Dominica). From 1983 to 1997, he was assigned to the Institut Mixte de Recherches Géothermiques and then to the BRGM Research Department, where he participated in various applied research programs and exploration
23 The authors campaigns in the fields of high-enthalpy geothermal energy. He also participated in the European geothermal research program “Roches Chaudes Sèches” in SoultzSous-Forêts (Alsace). In 1998, Hervé Traineau joined CFG (Compagnie Française de Géothermie) where he was particularly involved in development projects for power generation in the volcanic islands of the Caribbean (Guadeloupe, Martinique, Dominica). Between 2000 and 2015, he actively participated in the development of the field and then in the O&M activities of the Bouillante geothermal power plant (belonging to Géothermie Bouillante). At the same time, he carried out expertise and due diligences on various high-temperature geothermal fields in the Caribbean and around the world (Chile, Indonesia, Italy, Nevada...). Between 2017 and 2020, he worked as a well site geologist during the drilling of two deep geothermal wells (>5 km) in the Rhine Graben. Catherine TRUFFERT Holding a PhD in Geophysics issued by Marie Curie University France, she worked for more than twenty years in the French Geological Survey, BRGM. Catherine has served IRIS Instruments as the CEO since March 2017, managing day-to-day operations and working in research and innovation with electrical engineers and geophysicists. In 2024, Catherine received the insignia of Chevalier de la Légion d’Honneur. Christophe VOISIN Graduating for the École Normale Supérieure de Lyon in 1996, Christophe Voisin received a Ph.D. in the fields of geophysics and applied mathematics focused on earthquake physics at Université Grenoble Alpes in 2000. He has been a CNRS researcher since 2002 located at the Institut des Sciences de la Terre (ISTerre). Among different topics, he developed the field of environmental seismology (using ambient seismic noise). In 2018, he developed a new patented approach to seismic noise correlations aiming at capturing the intrinsic attenuation linked to the presence of fluids. Christophe Voisin has been the scientific advisor for Geolinks since 2021.
25 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE Introduction J.L. Mari and G. Paixach The Role of Geophysics in Geothermal Energy Geothermal energy represents one of the most promising paths to a more sustainable energy future. It offers a reliable, renewable source of heat and power while significantly contributing to global decarbonization efforts. However, harnessing geothermal energy is anything but straightforward. Geological uncertainties, high exploration and drilling costs, and regulatory hurdles create significant risks that can limit project development. This is where geophysics becomes indispensable, acting as the “eyes and ears” of engineers in the subsurface world. This book explores the critical role of geophysics throughout the lifecycle of geothermal projects – from initial exploration and feasibility studies to reservoir management and long-term monitoring. By employing advanced geophysical techniques, project developers can reduce uncertainties, identify optimal drilling locations, and minimize costly mistakes. The objective of geophysicists is to transform measurements of the subsurface into actionable insights, enabling engineers to unlock the Earth’s geothermal potential with greater confidence and precision. We invite the geophysical community to take on the challenge of innovating and collaborating to advance geothermal energy. Developing new technologies, refining multi-physics approaches, and monitoring are essential steps toward mitigating risks and optimizing resource extraction. Yet, progress in geophysics alone is not enough. The responsibility for advancing geothermal energy must be shared. © EDP Sciences, 2025 DOI: https://doi.org/10.1051/978-2-7598-3752-6.c904
26 Geophysics in Geothermal Exploration Project developers, institutional stakeholders, and policymakers also have a critical role to play in enabling the success of geothermal projects. Investment in the acquisition and interpretation of geophysical data is vital for de-risking exploration and maximizing project efficiency. Without sufficient geophysical data, engineers and decision-makers are effectively navigating in the dark, increasing the likelihood of costly errors and missed opportunities. By prioritizing geophysical studies and integrating their findings into project planning, stakeholders can significantly enhance the success rate of geothermal developments. Glossary of Geothermal Energy Definition and Sources Geothermal energy leverages the Earth’s internal heat, originating from radioactive decay and residual heat from planetary formation. This heat transfers through conduction, convection, and radiation, creating a geothermal gradient where temperature increases with depth. Variations in the geothermal gradient arise due to subsurface geological differences. Harnessing Geothermal Energy Despite geothermal heat at the surface being minimal (0.06 W/m²), subsurface temperatures stabilize below 10–20 meters, enabling surface applications like geothermal heat pumps. Deeper geothermal energy is tapped for direct heating (50 °C and above) or power generation (150 °C and above), with temperaturedependent applications ranging from district heating to electricity production. Applications and Impact Geothermal systems provide sustainable, year-round heat, contributing to electricity generation and industrial processes with minimal emissions. As heating accounts for 50% of global energy consumption, geothermal energy supports decarbonization efforts and reduces greenhouse gases. Installed capacity has grown globally, reaching over 16 GW for power and expanding direct-use heating applications. Technological Advancements and Challenges Advances in geophysics and drilling have expanded geothermal capabilities, but challenges like regulatory barriers, high drilling costs, and geological risks hinder growth. Enhanced Geothermal Systems (EGS) and closed-loop systems offer solutions in regions lacking natural hydrothermal resources, unlocking untapped geothermal potential. Geothermal Systems Geothermal resources manifest naturally (e.g., hot springs, geysers) or via engineered systems: • Direct Use: Low-temperature fluids (30–80 °C) for heating and agriculture. • Electricity Generation: High-temperature reservoirs (150 °C and above).
27 Introduction • Heat Pumps: Stable subsurface temperatures (10–16 °C) for efficient building heating and cooling. Geothermal Resources • Conventional Hydrothermal Systems: These systems involve naturally occurring hot water or steam reservoirs. They are typically used in volcanic or highgeothermal-gradient areas and are well-suited for electricity generation. • Non-Conventional Systems (Enhanced Geothermal Systems and Closed Loop). In regions lacking natural hydrothermal reservoirs, EGS can artificially create or enhance pathways in hot dry rock/low permeability rocks for water to circulate, picking up heat for use at the surface. Closed-loop systems involve circulating a working fluid through pipes underground without any interaction with natural groundwater, making them potentially viable and after a complete economic assessment in a broad range of geological environments. Hydrothermal Play Conventional hydrothermal systems exploration requires four key elements: a heat source (e.g., magmatic activity or geothermal gradients), a porous and permeable reservoir for fluid storage, a circulating fluid to transfer heat, and a caprock to trap fluids. Geophysical methods can contribute to the assessment of these components. Derisking subsurface elements • Identifying subsurface structures: For instance, faults, fractures, and geological boundaries between different geological formations are important to characterize. • Mapping temperature distribution: Mapping the temperature distribution underground and monitoring its variation over time allows engineers to target regions with sufficient heat for effective geothermal energy production. • Characterizing reservoirs: It is important to have insights into rock types and properties away from the wellbore to estimate the size, depth, porosity, permeability and productive thickness of the geothermal reservoir. • Characterizing geothermal fluids: Identifying fluid pathways, assessing fluid properties, tracking thermal and cold fronts in the reservoir are important to address success. • Fault activity assessment: Avoiding active faults minimizes the risk of induced seismicity and other drilling complications, enhancing operational safety. • Real-time well steering: Need reassurance on optimal drilling trajectories by guiding wells toward the targeted zones.
28 Geophysics in Geothermal Exploration Geothermal Energy in France Here’s a summary of the geothermal energy landscape in France from the 2023 report from the AFPG (2023), including upcoming projects, current operations, and both deep and shallow geothermal energy contributions: • Upcoming Projects: – 22 geothermal research permits granted for mainland France, – 7 research permits were issued for geothermal exploration in French overseas territories. • Heat Production: – 79 deep geothermal operations are currently active in France, – 1 million people benefit from geothermal heating in the country, – deep geothermal operations generate 2.05 TWh of heat energy annually. • Geothermal lithium extraction: several projects in Upper Rhine Graben are underway. • Power Generation: Two geothermal power plants are operational: – Bouillante, Guadeloupe: 15.5 MW capacity, – Soultz-sous-Forêts, Alsace: 1.7 MW capacity. • Shallow Geothermal Energy: – over 205, 300 shallow geothermal installations provide heating and cooling, – shallow geothermal systems contribute 4.58 TWh of heating and cooling energy annually from near-surface resources. This range of geothermal initiatives highlights France’s commitment to leveraging both deep and shallow geothermal energy for sustainable heating, cooling, and power generation. Book content After an introduction on geothermal energy and an overview of the different geothermal systems (chapter 1), the book focuses on geophysical methods. Chapters 2 and 3 give the current state of knowledge respectively in surface methods (gravity, magnetic, electrical – EM and seismic methods) and borehole methods (conventional logging, hydrogeological measurements, full waveform acoustics, VSP). Fundamentals of each method are described in basic words and illustrated with field examples, notably geothermal examples. The reader is invited to refer to the selected papers or books listed in the references for detailed information on each method. Chapter 4 describes the integrated approach that led to the establishment of the geothermal model in the Upper Rhine Graben. It shows how the occurrence of fractured reservoirs characterized by natural brine circulations with fractured zones
29 Introduction obliged developers to adapt geophysical exploration methods, geophysical well logging strategies as well as technical well design for reaching geothermal targets. Chapters 5 to 10 illustrate the use of geophysical methods for geothermal exploration and monitoring, with the following topics: • ERT-IP for geothermal exploration and de-risking, • The use of passive seismic methods for geothermal exploration and monitoring, • Seismic inversion and characterization applied to geothermal energy, • Seismic anisotropy applied to geothermal prospection, • Feasibility of monitoring cold fronts of geothermal doublets using 4D active electromagnetic techniques – a field trial in the Dogger play in the Paris Basin, • Defining high enthalpy geothermal drilling target with multi-physics integrated exploration program. Mayotte’s Petite-Terre Island case study. This book serves as both a guide and a call to action. It highlights the value of geophysical methods in building a sustainable energy future and emphasizes the need for collaboration across disciplines and sectors. Geophysics is not just a tool; it is the bridge between the subsurface’s hidden secrets and the engineers striving to harness them. Together, by investing in and advancing geophysical science, we can overcome the challenges of geothermal energy and unlock its full potential. Reference AFPG (2023) La géothermie en France, Étude de filière 2023, 6e édition.
31 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE 1 Overview of the different geothermal systems: role of geophysics in exploration and production G. Paixach, H. Traineau, F. Bugarel, E. Lasne and C. Maïlhol 1.1 What is geothermal energy? Geothermal energy is all about tapping into and utilizing the Earth’s internal heat, which comes from two primary sources: the decay of radioactive elements and the residual heat left over from the planet’s formation billions of years ago. Heat is a form of energy associated with the movement of particles within matter. Heat can be transferred in three ways: conduction, convection, and radiation. Heat © EDP Sciences, 2025 DOI: https://doi.org/10.1051/978-2-7598-3752-6.c001
32 Geophysics in Geothermal Exploration naturally flows from areas of higher temperature to areas of lower temperature. These processes create a transfer of heat from the Earth’s hot interior to its cooler surface where we live, resulting in what’s known as a geothermal gradient, which is a measure of how temperature increases with depth (Figure 1.1). In general, the deeper we go into the Earth’s crust, the hotter it gets. On average, temperatures rise by roughly 25–30 °C for every kilometer below the surface, but this geothermal gradient varies significantly depending on location due to the underlying geology. Figure 1.1 From Kolawole and Evenick (2021). (a) Typical averaged (and simplified) temperature profile of the Earth, showing the variation of temperature with depth (modified after Mckenzie and Bickle, 1988; Boehler, 1996). Inset in panel (a) shows a zoom-in of the non-zero curvature of slope of temperature(T)- depth(y) profile, in which the local slope is defined by Fourier’s law, and for a constant thermal conductivity (k), heat flow (q) is a function of depth, y, q = q(y) (modified after Turcotte and Schubert, 2002). (b) Schematic representation of variation in geothermal gradient with depth (Z) as a function of k in sedimentary sequences (after Chapman et al., 1984). (c) Cartoon showing the crustal and lithospheric structures of the Earth with the primary sources of geogenic heat (after Evenick, 2019). (a) (b) (c)
33 1. Overview of the different geothermal systems: role of geophysics Variation in measured heat flow at the surface of the Earth (Figure 1.2) highlights significant lateral differences across regions, suggesting a strong influence of subsurface characteristics. Indeed, the subsurface is far from being homogenous. This heat is stored in rocks and reservoirs of water deep underground. Different rock types, fractures, water presence, and other geological features, particularly related to tectonic plate activity, play a role in how heat is stored and transferred underground. Figure 1.2 From Davies (2013). Map of heat flow measurement points. Geothermal energy is all about harnessing the natural heat generated beneath the Earth’s surface. However, harvesting geothermal heat directly from the Earth’s surface is challenging because on average, the natural geothermal heat flux reaching the Earth’s surface is only about 0.06 watts per square meter, which is a tiny amount compared to solar power, which delivers around 200 watts per square meter on a sunny day. While the Earth’s surface temperature is highly influenced by atmospheric conditions, fluctuating with daily and seasonal changes, this effect diminishes rapidly just a few meters below the ground. After descending around 10 to 20 meters, the Earth’s temperature becomes nearly constant throughout the year, insulated from surface weather variations. This stable temperature zone is primarily influenced by the geothermal gradient. This stable subsurface temperature zone is crucial for surface geothermal applications, as it provides a reliable, year-round source of heat for geothermal heat pumps. To access deep geothermal energy for more energy-intensive applications like direct heating or power generation, we must drill deeper into the Earth’s crust, where temperatures are significantly higher. For direct heating applications, temperatures typically need to reach between 50 °C and above. At these depths, geothermal fluids can be used directly for district heating, greenhouse heating, aquaculture, and industrial processes. For electricity generation, however, much higher temperatures,
34 Geophysics in Geothermal Exploration generally above 150 °C, are required to produce steam or vaporize a working fluid that drives turbines. The diagram displayed in Figure 1.3 highlights how different temperature levels of geothermal fluids are suited for various direct-use applications. It showcases the versatility of geothermal energy, illustrating how it can be utilized for both power generation and numerous direct heat applications, depending on the resource temperature. At the high end of the temperature spectrum, above 150 °C, geothermal fluids are typically used for electricity generation through dry steam, flash steam, or binary cycle power plants. Moving down in temperature, between 100 °C and 150 °C, geothermal fluids can be used in processes like drying, industrial heating, and chemical extraction. At lower temperatures, around 50 to 100 °C, geothermal fluids are ideal for district heating, aquaculture, greenhouse heating, and various agricultural applications. Finally, even low-temperature geothermal fluids, between 20 and 50 °C, have applications in bathing, balneology, and heat pump systems for residential heating and cooling. Figure 1.3 Modified Lindal diagram about possible usage of geothermal fluids (from Kaczmarczyk et al., 2020; based on Gudmundsson et al., 1985; Operacz and Chowaniec, 2018).
35 1. Overview of the different geothermal systems: role of geophysics Heating has always been essential to human societies, forming the backbone of daily life and industrial activities. Today, heating and cooling account for a substantial portion of global energy consumption worldwide. According to the International Energy Agency (IEA), heating alone, used for residential, commercial, and industrial purposes, accounts for about 50% of final energy consumption globally (Figure 1.4). In colder climates, space heating for homes and buildings is a major energy expense, especially during winter, and accounts for nearly 40% of energy demand in the building sector. Meanwhile, the need for cooling is rapidly increasing, especially in warmer regions, where air conditioning and refrigeration demand has soared over the last few decades. Given the immense need for heating and cooling, geothermal energy presents a powerful, sustainable alternative, as it can provide constant, low-emission heat for both buildings and industry, helping to meet this demand while reducing greenhouse gas emissions. Figure 1.4 Heat Roadmap Europe (2019), Heating and Cooling facts and figures. By tapping into this steady, abundant heat source, geothermal systems can produce electricity, provide direct heating, and power industrial processes with minimal environmental impact. Geothermal energy offers a unique advantage in the push to decarbonize societies because it provides a constant, reliable power supply independent of weather conditions. The installed capacity for geothermal heat and power generation has seen a steady increase, with global geothermal power capacity exceeding 16 gigawatts in recent years (Figure 1.5) and direct-use heating capacity growing even faster (Figure 1.6), especially for district heating, greenhouses, and aquaculture. As of the latest trends, geothermal heating is expanding quickly in regions with abundant lowto-medium temperature resources, while geothermal power plants continue to rise in areas with high-temperature resources. In recent years, advancements in geophysics, hydrogeology, and drilling technology, and have expanded the potential of geothermal energy.
36 Geophysics in Geothermal Exploration Figure 1.5 From IRENA and IGA (2023). Estimated installed geothermal electricity capacity, by region, 2021. Figure 1.6 IRENA and IGA (2023). Estimated geothermal heating and cooling installed capacity, by region, 2020. In the renewable energy sector, there is competition between technologies like solar, wind, biomass, and geothermal for investment, policy support, and market share. Solar and wind energy have surged in deployment due to their rapid advancements, decreasing costs, and modular nature, making them accessible and scalable across a wide range of locations (Figure 1.7). In contrast, geothermal energy faces distinct challenges that can slow its expansion despite its potential for stable, baseload power or direct heat usage. One of the significant hurdles is the regulatory landscape: accessing deep geothermal resources requires extensive permits and regulatory compliance due to their subsurface nature. Another challenge for geothermal energy deployment is the inherent risk and complexity of drilling deep into the Earth to access high-temperature resources. Deep drilling is costly and carries geological risks, including the lack of targeted
37 1. Overview of the different geothermal systems: role of geophysics geothermal fluids or the possibility of triggering seismic activity. Current drilling technologies also have limitations, as they can only reach certain depths before technical constraints and costs become prohibitive. This restriction means that vast geothermal potential, notably for power generation remains untapped. Addressing these challenges requires continued advancements in drilling technologies, risk mitigation strategies, investment, and regulatory support, all of which would help make geothermal a more prominent player in the global renewable energy mix. Figure 1.7 Renewable energy consumption and shares of heat demand in selected regions, 2022 (left), and global increases in renewable energy consumption, 2017-2028 (right). Source: World Energy Outlook 2023 (IEA Report, 2023). 1.2 What are the main geothermal systems? In nature, geothermal activity is well known through phenomena like geysers, hot springs, and fumaroles, where the Earth’s internal heat escapes to the surface. These features form in geologically active areas, such as near tectonic plate boundaries or volcanic zones, where heat is channeled through fractures in the Earth’s crust, bringing hot water or steam to the surface. Historically, these natural hot water sources have been harnessed by people for bathing, cooking, and warming homes, using the naturally occurring thermal energy produced deep within the Earth. The first deliberate attempt to generate power from geothermal energy was made in 1904 in Larderello, Italy, where the French engineer François Jacques de Larderel used steam from a geothermal well to generate electricity. Since then, geothermal technology has evolved significantly, with modern techniques now
38 Geophysics in Geothermal Exploration allowing us to drill deep into the Earth and access high-temperature geothermal reservoirs. Today, geothermal power plants can produce electricity by tapping into hot water, two-phase or steam reservoirs, while enhanced geothermal systems (EGS) create artificial reservoirs by injecting water into low permeability rocks to generate hot geothermal fluids. Additionally, ground-source heat pumps make it possible to use stable temperatures just below the Earth’s surface for efficient heating and cooling in residential and commercial buildings. These advancements allow us to make use of geothermal energy far beyond natural manifestations, making it a sustainable and reliable source of heat and power. However, it’s essential to recognize that geothermal energy is not a one-size-fits-all resource. We can classify geothermal systems based on the intended usage, the fluid or geological context involved, and even the energy production design. Classifying by Usage • Direct Use of Hot Water: This is one of the oldest and most straightforward uses of geothermal energy, in which naturally heated water (30–80 °C) from geothermal springs or wells is used for heating buildings, agricultural greenhouses, aquaculture ponds, and industrial processes. • Electricity Generation: Higher temperatures, typically above 150 °C, are required to produce electricity. In these systems, steam from geothermal reservoirs drives turbines connected to generators. These are commonly used in areas with high geothermal activity, like volcanic regions. • Geothermal Heat Pumps (GHPs): GHPs leverage stable ground temperatures (10–16 °C) found a few meters below the surface to provide efficient heating and cooling for buildings. This technology is widely applicable and doesn’t require high temperatures. Classifying by Geological Settings • Shallow Geothermal Systems: This involves tapping into the moderate temperatures found at shallow depths, typically up to a few hundred meters, to power geothermal heat pumps. • Sedimentary Basin Systems: In regions with porous/fractured/karstified sedimentary layers, geothermal reservoirs of hot water can be found at moderate depths, often used for direct heating or low-temperature electricity production. • Volcanic Systems: High-temperature geothermal reservoirs in volcanic regions are ideal for electricity generation. Countries like Indonesia and New Zealand are renowned for tapping volcanic geothermal resources for power. • Rift and Fault Zones: In areas where tectonic plates pull apart or fracture, crust is thinner and heat flow is higher than usual promoting geothermal reservoir development in conjunction with volcanic activity. • Fractured Granite and Crystalline Rock: Some geothermal resources are found in fractured hard rock, where engineered geothermal systems (EGS) create or enhance pathways for water to circulate and absorb heat.
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