Distributed acoustic sensing (DAS) that uses optical fibres as sensing units is attracting increasing interest for micro-seismic monitoring in geothermal projects. Standard optical fibres provide one-component measurements along the fibre and this pose challenges in determining certain characteristics of the source, such as its azimuth and its full moment tensor. Full source characteristics can be obtained via offset downhole measurements and/or measurements from horizontal well sections but these come with substantial extra costs. This paper proposes a single-well dual-cable DAS configuration to reduce the need for drilling additional wells or sections, where two DAS cables are assumed to be positioned within a single vertical well at opposite sides of the well. Synthetic DAS signals are generated by an open-source code that assumes plane-layered media and are used to study the feasibility of the dual-cable DAS for localising a seismic source and resolving its moment tensor. A localisation procedure is presented, and a sensitivity analysis of localisation accuracy is conducted with respect to source parameters and noise levels. In addition, an analysis is performed to assess the resolvability of the moment tensor components from the dual-cable DAS configuration. Results suggest the source location can be fully determined, yet low signal-to-noise ratio and azimuth close to 0∘ (North, aligned with the two cables) lead to a decrease in accuracy. The full moment tensor can be resolved only if the epicentral distance is 5 m or less, while non-double-couple components can be reliably resolved with an epicentral distance up to 20 m, showing improvement compared to installations with a single cable. Consequently, near-borehole failures, regardless of the source mechanisms, can be characterised within an epicentral distance of 5 m. With epicentral distance increasing, resolvability of the mix-mode failures is reduced first, followed by the resolvability of the pure shear or tensile failures, which depends on the azimuth. Overall, the results demonstrate that a single-well dual-cable configuration has the potential for monitoring and understanding near-borehole micro-seismic events induced during geothermal reinjection and stimulation operations.

Achieving net zero in greenhouse gases emissions attributed to human activities depends on the transition to renewable energy resources. Low-enthalpy geothermal systems, characterized by their widespread geographical distribution and suitability for direct heating applications, represent a promising alternative to fossil fuels. However, maintaining the long-term efficiency and economic viability of such geothermal reservoirs requires the development of new methods to monitor subtle variations in their properties, particularly those induced by temperature changes during energy extraction and reinjection.
This thesis evaluates the feasibility and advances the methodology of two key geophysical approaches for reservoir monitoring: the controlled-source electromagnetic (CSEM) method and the full waveform inversion (FWI) of seismic data. The research is grounded in two study areas: the Delft campus geothermal project in the Netherlands and the Munich geothermal project in Germany.
A feasibility study of CSEM monitoring was carried out on the Delft site to assess its sensitivity to subtle resistivity variations corresponding to temperature changes in the reservoir. Surface-to-borehole CSEM survey configuration was modeled to optimize source frequency and offset, with results demonstrating the detectability of a 4 Ω・m resistivity increase calculated for a 25 ◦C temperature drop in the Delft Sandstone reservoir. The study systematically analyzed the impacts of environmental disturbances—random noise, repeatability errors, seasonal near-surface temperature fluctuations, and the presence of steel-cased wells—on the performance of CSEM monitoring data. It was shown that a careful survey design and adequate source parameters allow CSEM monitoring, which is robust against most undesired effects, although steel casings require careful consideration due to their strong field attenuation within a radius of 100 m for a frequency of 1 Hz.
For high-resolution seismic characterization, the thesis develops and validates a novel sequential FWI approach for reconstructing high-resolution models of P-wave velocity and impedance from vertical seismic profiling (VSP) data. The method incorporates traveltime tomography for starting models and introduces a temporal phase resemblance step to improve convergence and mitigate phase error propagation in impedance inversion. Inversion experiments of synthetic data demonstrate that this approach enables the detection of impedance variations greater than 2 %, directly linked to temperature-driven reservoir changes. Field application to baseline VSP data at the Munich geothermal site confirms the robustness of the approach. A comparative analysis of distributed acoustic sensing (DAS) and conventional geophone-based FWI of P-wave velocity further elucidates the operational benefits and challenges of fiber-optic deployments inside the casing for characterization of geothermal reservoirs.
The results presented in this thesis establish CSEM and advanced seismic FWI as promising and complementary tools for noninvasive monitoring of low-enthalpy geothermal reservoirs. The work concludes with a discussion of current limitations, practical considerations for field deployment, and recommendations for future research.

Tracking temperature changes by measuring the resulting resistivity changes inside low-enthalpy reservoirs is crucial to avoid early thermal breakthroughs and maintain sustainable energy production. The controlled-source electromagnetic method (CSEM) allows for the estimation of sub-surface resistivity. However, it has not yet been proven that the CSEM can monitor the subtle resistivity changes typical of low-enthalpy reservoirs. In this paper, we present a feasibility study considering the CSEM monitoring of 4–8 Ω·m resistivity changes in a deep low-enthalpy reservoir model, as part of the Delft University of Technology (TU Delft) campus geothermal project. We consider the use of a surface-to-borehole CSEM for the detection of resistivity changes in a simplified model of the TU Delft campus reservoir. We investigate the sensitivity of CSEM data to disk-shaped resistivity changes with a radius of 300, 600, 900, or 1200 m at return temperatures equal to 25, 30, …, 50 °C. We test the robustness of CSEM monitoring against various undesired effects, such as random noise, survey repeatability errors, and steel-cased wells. The modelled differences in the electric field suggest that they are sufficient for the successful CSEM detection of resistivity changes in the low-enthalpy reservoir. The difference in monitoring data increases when increasing the resistivity change radius from 300 to 1200 m or from 4 to 8 Ω·m. Furthermore, all considered changes lead to differences that would be detectable in CSEM data impacted by undesired effects. The obtained results indicate that the CSEM could be a promising geophysical tool for the monitoring of small resistivity changes in low-enthalpy reservoirs, which would be beneficial for geothermal energy production.

Natural gas hydrates production tests over the last two decades has sown that production is not without risks. Indirect effects in the sedimentary rocks of phase changes are changes in porosity, permeability, and saturation. From a field production test site, porosity changes in the range of 15% to 19% and saturation from 5% to 60% were reported. Monitoring is in principle possible using an electromagnetic survey with a downhole vertical electric source and a horizontal electric field receiver on the seafloor. Computed model responses over a wide frequency range and for many depth locations of an electric current source show that both changes can be detected. Best detectability occurs when the current source is below the reservoir layer in case of changes differences can be detected above, inside and below the reservoir layer at frequencyies below 10 Hz. At a source operating frequency of 0.1 Hz maximum response difference between the two values in saturation occur when the source is 20 m above the top of the reservoir layer unil 100 m below the bottom. Only below the top of the reservoir there is almost no difference in the electric field amplitude between the two saturation levels below 10 Hz.

Delft geothermal project (DAPwell) is a planned geothermal well doublet, where relatively cold water is going to be injected through one well into a low enthalpy geothermal reservoir to produce hot water from the other well. The volume of the cold water around the injection well will increase over time and, in the end, result in a thermal breakthrough. Thus, it is essential to trace the time-lapse change in the volume of the cold water to monitor the DAPwell efficiently. The invaded reservoir volume by the cold water is associated with a decrease in the pore fluid temperature and salinity. This increases the electrical resistivity of the geothermal reservoir, where the cold front is located. Hence, estimating the time-lapse change in the electrical resistivity of the geothermal reservoir can be used to identify the distribution of the cold water. From a theoretical point of view, the controlled-source electromagnetic (CSEM) method can be used to get information about the change in the electrical resistivity within the geothermal reservoir. In this study, we investigate the feasibility of monitoring a geoelectric model of the DAPwell using land CSEM forward modelling. The optimal survey design is investigated as well as the influence of cold water volumetric changes on the time-lapse electric field response. The impact of measurements undesired effects on time-lapse CSEM response is analysed and then synthesized. A subsurface model of the DAPwell is illuminated by a horizontal electric dipole source, which emits a sinusoidal field with several frequencies. Based on the numerical experiments, surface measurements do not pick up sufficient time-lapse signal to use them for field applications. On the other hand, the difference in the z-component of the electric field, determined along a depth section, allows for a successful detection of the electrical resistivity changes within the geothermal reservoir. The correlation between the spatial distribution of the cold water and the difference in time-lapse electric field responses is clarified. Finally, it is noticed that the difference in time-lapse signal is measurable in the presence of the different sources of noise.

Difficulties in detecting and characterising shallow objects close the surface with seismic shear waves are often problematic because of dominant surface waves. By sequencing a specific combination of two data driven processing steps followed by diffraction tomography can overcome these problems. Small scattering objects become visible in the final image that can have importance of the understanding of subsurface locations, such as areas of archaeological interest. On the other hand, deep changes in the electric resistivity on land are often problematic to detect and especially to monitor time-lapse change over long periods of time. The usual electrodes slowly erode and vanish. Geothermal heat production environments often lead to changes in the resistivity between in-situ water-filled formations and cooler injected water-filled formations of less than one order of magnitude. A dedicated set of capacitively coupled electrode could overcome to erosion problem. When placed in a well with composite casing, these could be used in measurements of much enhanced detectability. In that case it is necessary to have electrodes in a zone from below to above the target layer. By changing the source offset at the surface, optimal measurements can be done to detect the small and deep changes in resistivity.