In February 2024, the European Union published its Industrial Carbon Management Strategy, setting ambitious goals for carbon capture and storage (CCS), carbon capture and utilisation (CCU), and related technologies. Industrial decarbonisation will require a mix of solutions, CCUS, electrification, hydrogen and hydrogen-derived fuels, and energy efficiency, which are all dependent on affordable clean energy. Although carbon management technologies could contribute substantially to climate targets, their deployment has been slowed by technical barriers and public concerns. Sotacarbo has created a research centre dedicated to developing and testing carbon capture, utilisation, and storage technologies. Within this framework, the new Sotacarbo Fault Laboratory (SFL) was designed to investigate gas migration in faults and to test monitoring systems capable of detecting potential short- and long-term CO2 leakages. This paper presents a preliminary study, including seismic full-waveform simulations for time-lapse surveys before and after CO2 injection, and a suite of geophysical methods used to characterise the Matzaccara Fault within the Eocene Sulcis Basin. The results of the application of integrated geophysical methods support the selection of a safe and suitable injection-well location and demonstrate the value of these methods for detailed fault characterisation in CCUS applications.

The three-dimensional (3D) distributed acoustic sensing (DAS) vertical seismic profile (VSP) technique is an effective tool to characterize subsurface reservoirs, enabling the use of large and densely sampled borehole receiver arrays with many surface vibrator source points for onshore time-lapse monitoring. However, the processing of the DAS VSP signals for imaging purposes is based on a reliable wavefield separation, which may depend on the recognition and quality of the direct arrivals. To overcome this limitation for common-source gathers with poor signal-to- noise ratio or with interferences, we apply the dual-signal processing method, which allows us to estimate and separate the DAS wavefields by signals' combination without arrival picking. We present a case study of a 3D VSP DAS dataset recorded at a geothermal reservoir in Turkey, showing that the method, similar to a geophone and hydrophone combination, is robust and effective and can be advantageously integrated with the conventional processing. Supported by signal benchmarking, modelling and signal-to-noise ratio analysis, we treat common-source and common-receiver data. Our analysis shows the advantages and limitations of the proposed approach, valuable in the time-lapse perspective.

High temperatures in rocks lead to thermal cracking, whereby porosity and permeability increase and the P and S wave velocities as well as the Q (quality) factor decrease. In addition, the crack density increases and the crack aspect ratio decreases. There are many ultrasonic data in the literature, usually for dry, stiff rocks (granite, basalt) and ambient pressure, which can be processed and described using appropriate theories. The data were usually measured up to a temperature of about 600-800 degrees C before melting of the grain minerals. In this work, we use Gassmann equations based on Pride's dry-rock moduli and estimate the consolidation coefficient, mass density, wave velocities, quality factor and permeability as a function of temperature. Fluid substitution is then applied to determine the density and velocities of the rock saturated with steam and water under supercritical conditions. The equivalent inclusion average stress (EIAS) model is used in combination with the Zener model to model attenuation and estimate crack fraction, density and aspect ratio. We compare the results with the simplified approach of O'Connell and Budiansky to determine the crack density. The permeability mainly follows power laws as a function of porosity. The inversion method uses a simulated annealing algorithm.

CO2 capture and underground storage, combined with geothermal resource exploitation, are vital for future sustainable and renewable energy. The SUCCEED project explores the feasibility of re-injecting CO2 into geothermal fields to enhance production and store CO2 for climate change mitigation. This integration requires novel time-lapse monitoring approaches. At the Hellisheiði geothermal power plant in Iceland, seismic surveys utilizing conventional geophones and a permanent fiber-optic helically wound cable (HWC) for Distributed Acoustic Sensing (DAS) were designed to provide subsurface information and CO2 monitoring. This work details the feasibility study and active seismic acquisition of the baseline survey, focusing on optical fiber sensitivity, seismic modeling, acquisition parameters, source configurations, and quality control. Post-acquisition signal analysis using a novel electromagnetic vibrating source is discussed. The integrated analysis of datasets from co-located sensors improved quality-control performance and geophysical interpretation. The study demonstrates the advantages of using densely sampled DAS data in space by multichannel processing. This experimental work highlights the feasibility of using HWC DAS cables in active surface seismic surveys with an environmentally friendly electromagnetic source, providing also a unique case of joint signal analysis from different types of sensors in high-temperature geothermal areas for energy and CO2 storage monitoring in a time-lapse perspective.

As part of the Synergetic Utilisation of CO2 storage Coupled with geothermal EnErgy Deployment project, investigating CO2 reinjection with different seismic methods, both passive and active seismic surveys have been conducted at the geothermal power plant at Hellisheioi, Iceland. During the 2021 survey, two geophone lines recorded noise for a week. We process the passive-source data with seismic interferometry to image the subsurface structure around the CarbFix2 reinjection reservoir. To improve image quality, we perform an illumination analysis to select only noise panels dominated by body-wave energy. The results show that most noise panels are dominated by air-wave energy arriving from the direction of the power plant. We use panels with a near-vertical incidence to create a zero-offset image and a larger selection of body-wave-dominated panels to create virtual common-shot gathers. We process the gathers with a simple reflection seismology processing workflow to obtain stacked images. The zero-offset images show a relatively lower signal-to-noise ratio and only horizontal reflectors. The stacked images show slightly dipping reflectors and possibly lateral amplitude variations around the expected injection region. This could indicate a region of interest for future research into the reinjection reservoir.

Summary Geothermal energy projects vary significantly in the temperature and target depths of the thermal energy source. The industry seeks for cost-effective, continuous monitoring solutions to maximise operational efficiency and safety. Distributed fibre optic sensing (DFOS) solutions provide flexible, multi-parameter measurements for the exploration and exploitation of the full range of geothermal resources, from shallow borehole, ground source heat to hydrothermal geothermal projects and Enhanced Geothermal Systems (EGS). Permanently installed integrated fibre optic sensing-based solution offers reliable, long-term geothermal reservoir monitoring and helps operators comply with legal requirements. The sensing element, a single fibre optic cable, either installed in shallow heat exchanger wells or cemented behind casing to the reservoir depth, can provide simultaneous and continuous measurements including temperature, seismic, microseismic, flow distribution, and strain. Here we present an overview of case studies of DFOS monitoring for a range of geothermal applications.

As part of the Synergetic Utilisation of CO 2 $_2$ storage Coupled with geothermal EnErgy Deployment project, investigating CO 2 $_2$ reinjection with different seismic methods, both passive and active seismic surveys have been conducted at the geothermal power plant at Hellisheiði, Iceland. During the 2021 survey, two geophone lines recorded noise for a week. We process the passive-source data with seismic interferometry to image the subsurface structure around the CarbFix2 reinjection reservoir. To improve image quality, we perform an illumination analysis to select only noise panels dominated by body-wave energy. The results show that most noise panels are dominated by air-wave energy arriving from the direction of the power plant. We use panels with a near-vertical incidence to create a zero-offset image and a larger selection of body-wave-dominated panels to create virtual common-shot gathers. We process the gathers with a simple reflection seismology processing workflow to obtain stacked images. The zero-offset images show a relatively lower signal-to-noise ratio and only horizontal reflectors. The stacked images show slightly dipping reflectors and possibly lateral amplitude variations around the expected injection region. This could indicate a region of interest for future research into the reinjection reservoir.

Geothermal power production may result in significant CO2 emissions as part of the produced steam. CO2 capture, utilisation, subsurface storage (CCUS) and developments to exploit geothermal resources are focal points for future clean and renewable energy strategies. The Synergetic Utilisation of CO2 Storage Coupled with Geothermal Energy Deployment (SUCCEED) project aims to demonstrate the feasibility of using produced CO2 for re-injection in the geothermal field to improve geothermal performance, while also storing the CO2 as an action for climate change mitigation. Our study has the aim to develop innovative reservoir-monitoring technologies via active-source seismic data acquisition using a novel electric seismic vibrator source and permanently installed helically wound cable (HWC) fibre-optic distributed acoustic sensing (DAS) system. Implemented together with auxiliary multi-component (3C and 2C) geophone receiver arrays, this approach gave us the opportunity to compare and cross-validate the results using wavefields from different acquisition systems. We present the results of the baseline survey of a time-lapse monitoring project at the Hellisheiði geothermal field in Iceland. We perform tomographic inversion and multichannel seismic processing to investigate both the shallower and the deeper basaltic rocks targets. The wavefield analysis is supported by seismic modelling. The HWC DAS and the geophone-stacked sections show good consistency, highlighting the same reflection zones. The comparison of the new DAS technology with the well-known standard geophone acquisition proves the effectiveness and reliability of using broadside sensitivity HWC DAS in surface monitoring applications.

Summary We present the approach and initial QC results of the dual-signal processing of VSP data acquired using semipermanent DAS technology during a baseline survey of a CO2 injection- monitoring project in the Kizildere (Turkey) geothermal-production reservoir. The data were recorded in the framework of the SUCCEED project in two wells using a high-sensitivity engineered fibre with the cable suspended in the vertical cased wells. The source was a new electric seismic vibrator operated at the surface with a 3D configuration, supported with measurements on two bi-axial geophone lines. Good-quality VSP results were obtained during the initial QC performed by in-field and remote control and from the prompt data processing after the survey acquisition. The VSP-data processing takes advantage of the dual-field separation method effectively applied with the DAS well data densely sampled every 1 m. This approach enabled us to quickly separate up- going and down-going VSP wavefields. This technique does not require first-arrival picking, which is advantageous for processing extensive 3D-VSP datasets. The results from sample VSP revealed the reflection information contained in the data, relevant for target characterization. This analysis demonstrates the potential of the dataset for carbonate- reservoir monitoring purposes, to be compared in the future with time-lapse measurements.

The Los Humeros volcanic complex is a geothermal field in Mexico that is now being further investigated as a candidate for the development of a super-hot geothermal system, as it has a temperature of over 350 ∘C. The European Horizon 2020 project GEMex (2016–2020) worked in this context aiming to characterize the geological and geophysical aspects of the area to assess the feasibility of a super-hot geothermal system. We describe the results of time and depth processing and interpretation of legacy 2D reflection seismic lines acquired in 1998 to study the deep structures of the caldera in the Trans-Mexican Volcanic Belt. The study is relevant for the geophysical characterization of the subsurface in this area, where no vertical seismic profiles are available but only two short sonic well logs. The interpretation of selected seismic horizons in the seismic depth profiles was first determined by the local geological model of the curved caldera and then calibrated by the stratigraphy of the boreholes and the matching of the 2D seismic lines crossing points. The processing outcomes are depth-migrated profiles, the corresponding P-wave velocities and the updated geological models. The main results are the representation of the basement and the identification of the structural make-up of the main units with some of the main faults of the collapsed caldera. Our results provide new insights into the subsurface geometry of volcanic collapses and the geothermal field contained within them at the caldera scale.

Chapter 7 (SWD acquisition: operational aspects) – Onshore and offshore acquisition design, use of seismic arrays and refraction profiles to record SWD data and quality-control procedures are the arguments of this chapter. Use of 3D onshore with modern areal receiver grids and of offshore acquisition systems with 4C nodes are new aspects addressed in this chapter.

Chapter 3 (General theory: drill-bit seismic waves)—describes the drilling energy in terms of drilling parameters and the drill-bit signal production. Here, we develop the analysis of the energy balance in the drilling process, including the near-field vibrations produced by drilling bits and the radiated wavefields.

Summary In the ACT Consortium funded project SUCCEED, researchers study the potential for monitoring the process of (re-)injecting produced and captured CO2 into the Hellisheiði geothermal field for the aid of enhancing geothermal deployment as well as permanently storing CO2 through mineralization. The Hellisheiði site provides an excellent opportunity for demonstrating an innovative seismic monitoring technique. Prior to conducting an active-source monitoring survey, we perform acoustic transmission measurements, on Hellisheiði rock samples, at field-representative stress conditions to obtain the seismic-response characteristics of all present formations. Subsequently, we use the acquired velocity data as an input for simulating 2D seismic surveys using a subsurface model representing the Hellisheiði site. Results show that the impact of increasing depth, i.e., stress, on seismic velocities is most apparent for the porous basalt layers due to their relatively large portion of open pore space, allowing for substantial compaction, increasing their bulk density and thus velocity. The poorly-consolidated hyaloclastites reveal a negligible effect of increasing depth on their velocity as the material already reached its maximum compaction at low stresses, thus at shallow depths. Comparison of synthetic and field geophone data reveal that the velocity profiles have to be updated for the shallow depths in the model.

Chapter 1 (Introduction and overview) – introduces seismic-while-drilling with an overview of the conventional borehole seismic methods. This chapter gives a short description of the history of seismic while drilling. It summarizes the results of this method, with a review of the existing approaches and perspectives including recent improvements.

Chapter 9 (Processing of signal and noise RVSP fields) – describes the processing of signal and noise wavefields. Repeatability of the source and selective stacking are analyzed by drilling parameters. Noise removal is discussed using orthogonalization analysis or independent pilot-signal separation. The main steps of wavefield processing of reverse VSP data, data quality control and cross-correlogram migration are further subjects of this chapter. This chapter was expanded to include a new description on the coherency-classification methods and with the introduction of a new section on SWD and seismic interferometry (SI), with a comparison of the different redatuming concepts, and of the processing and interpretation aspects in these approaches.

Chapter 8 (Preprocessing of SWD data) – describes preprocessing of the seismic-while-drilling data, i.e., the production of interpretable seismograms. Preprocessing includes correlation, stacking, time correction, phase correction and deconvolution. Modeling of drill-string waves is used to interpret the delays of the pilot signals in the correlations. In this chapter, significant extensions have been made in the computation of the propagation matrix for downhole signal and noise in the drill string, analysis of drill-string multiples in downhole pilot data, downhole pilot-delay synchronization and, last but not least, the estimation of the drill-bit SWD signature by the downhole ground force. An overview of new SWD results obtained with downhole pilot signal is given.

Chapter 6 (SWD acquisition: tools and systems) – describes the tools used for the acquisition of drill-bit while-drilling data. Sensor technology and specifications, acquisition systems, joint use of surface recording lines, acquisition procedures by drilling-parameters control are described here. The discussion includes the use of acceleration and strain dual sensors in the drill string. Spatial sampling of SWD data is a part of the arguments of this chapter. Evolution of the technology used for downhole measurements, including ground force; modern downhole transmission; descriptions and comparisons of new SWD systems, including controlled downhole sources are new aspects addressed in this chapter.