Featured Application This study highlights marine Distributed Acoustic Sensing (DAS) as a low-impact, spatially continuous technology for monitoring sub-seabed fluid and gas migration. In volcanic and hydrothermal settings, DAS enables long-term observation of degassing processes, providing high temporal resolution that complements geochemical and visual surveys. Furthermore, its sensitivity to bubble-related acoustic signals makes it a viable tool for offshore Carbon Capture and Storage (CCS), ensuring early detection of potential leaks. DAS offers a cost-effective, scalable, and non-invasive solution for environmental surveillance in both natural and engineered submarine systems. Submarine gas emissions represent a key expression of fluid migration processes in volcanic and hydrothermal marine environments and provide valuable analogues for monitoring strategies relevant to sub-seabed carbon storage. This study investigates the feasibility of using marine Distributed Acoustic Sensing (DAS) to detect natural CO2 bubble emissions in a shallow-water setting offshore Panarea (Aeolian Islands, Italy). A 1.1 km armored fiber-optic cable was deployed on the seabed and interrogated using two different DAS systems to acquire continuous passive acoustic data. The DAS recordings were complemented by controlled gas releases from scuba tanks to provide reference signals, as well as by independent high-resolution boomer seismic survey and side-scan sonar imaging to characterize the shallow subsurface and seabed morphology. The results show that DAS is sensitive to acoustic signals associated with both artificial and natural bubble emissions, despite the complex acoustic conditions typical of shallow marine environments. The integration of passive DAS monitoring with independent geophysical observations provides a robust framework for interpreting gas-related signals and seabed processes. These findings demonstrate that marine DAS represents a promising geophysical tool for monitoring of submarine volcanic-hydrothermal systems and offers important insights for the development of sub-seabed CO2 leakage detection in offshore CCS contexts.

Green hydrogen is one of the solutions in the European energy transition strategy towards Net Zero, despite the high production costs and associated risks.Underground hydrogen storage (UHS) can help mitigate energy security issues related to hydrogen production in foreign countries and seasonality of green hydrogen production from renewable wind and solar sources. UHS requires in-depth knowledge of the subsurface and long-term monitoring to minimise the risks associated with hydrogen storage.Here, we present FUSE (Open Infrastructure on Future Underground Hydrogen Storage), a partnership between OGS, the University of Trieste and the University of Udine, and funded by the Friuli Venezia Giulia Region (NE Italy). The project aims to create an open, integrated and distributed infrastructure designed to link academia and industry for the investigation, characterisation, and de-risking of potential UHS sites and accelerate white hydrogen exploration.Within the project we will integrate geophysical instrumentation with advanced laboratory facilities and multi-scale numerical modelling to characterize reservoir/caprock systems and assess potential hazards related to hydrogen injection, storage and extraction. The infrastructure includes:The acquisition of high-resolution imaging and monitoring equipment including borehole logging systems, seismic and geoelectric arrays, and optical DAS cable for monitoring purposes. This will enable both the characterisation of potential UHS sites and the continuous observation of pressure-induced changes and fluid migration patterns within the reservoir once hydrogen storage begins.The development of multi-platform remote sensing capabilities through the acquisition of airborne and drone-based magnetic and gravity systems to map subsurface heterogeneities and structural discontinuities.The enhancement of experimental petrophysical and fluid-dynamics laboratories to define hydrogen–rock–fluid interactions and processes. These facilities are essential to quantify the petrophysical properties affecting hydrogen containment and recovery and fluid migration within the reservoir/caprock system.The integration of predictive modelling software suites to derisk site selection and quantify fluid-dynamic processes in the subsurface.Furthermore, FUSE aims to provide new tools to support the emerging exploration of natural (white) hydrogen. Overall, FUSE will boost opportunities for research and industry realms in the identification of potential UHS sites and help de-risk future efforts aimed at initiating large scale hydrogen storage.

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.

Submarine gas emissions represent a key expression of fluid migration processes in volcanic and hydrothermal marine environments and provide valuable analogues for monitoring strategies relevant to sub-seabed carbon storage. This study investigates the feasibility of using marine Distributed Acoustic Sensing (DAS) to detect natural CO₂ bubble emissions in a shallow-water setting offshore Panarea (Aeolian Islands, Italy). A 1.1 km armored fiber-optic cable was deployed on the seabed and interrogated using two different DAS systems to acquire continuous passive acoustic data. The DAS recordings were complemented by controlled gas releases from scuba tanks to provide reference signals, as well as by independent high-resolution boomer seismic survey and side-scan sonar imaging to characterize the shallow subsurface and seabed morphology. The results show that DAS is sensitive to acoustic signals associated with both artificial and natural bubble emissions, despite the complex acoustic conditions typical of shallow marine environments. The integration of passive DAS monitoring with independent geophysical observations provides a robust framework for interpreting gas-related signals and seabed processes. These findings demonstrate that marine DAS represents a promising geophysical tool for monitoring of submarine volcanic–hydrothermal systems and offers important insights for the development of sub-seabed CO₂ leakage detection in offshore CCS contexts.

Fibre-optic sensing technologies are rapidly transforming geophysical monitoring by enabling spatially dense, temporally continuous observations of seismic and acoustic wavefields in environments that are difficult to instrument with conventional sensors. In marine settings, Distributed Acoustic Sensing (DAS) applied to seabed fibre-optic cables offers new opportunities for low-impact monitoring of fluid and gas migration processes, which are fundamental both to volcanic–hydrothermal systems and to emerging offshore carbon capture and storage (CCS) applications.In this study, we investigate the feasibility of marine DAS for detecting natural and artificial CO₂ bubble emissions in a shallow-water volcanic environment offshore Panarea (Aeolian Islands, Italy). Panarea hosts the OGS NatLab Italy, part of ECCSEL-ERIC, thanks to its active submarine degassing associated with a hydrothermal system and therefore represents a natural laboratory and an analogue site for potential subseabed CO₂ leakage scenarios. A 1.1-km-long armored fibre-optic cable was deployed on the seabed and interrogated using two different DAS systems, providing continuous passive acoustic and seismic recordings. To support signal identification and interpretation, the DAS data were complemented by controlled gas releases from scuba tanks, by a High Resolution Seismic (boomer) survey and side-scan sonar imaging, to characterize seabed morphology and shallow subsurface structures along the cable route.The DAS recordings revealed acoustic signatures associated with both natural CO₂ bubble emissions and controlled artificial releases. Bubble-related signals were detected as localized, temporally variable acoustic responses along the fibre, demonstrating the sensitivity of DAS to gas-driven processes at the seabed. The integration of passive DAS monitoring with active seismic imaging techniques enabled a more robust interpretation of observed signals and seabed processes.From an Earth sciences perspective, these results demonstrate that marine DAS can serve as a low-impact, spatially continuous monitoring tool for submarine volcanic and hydrothermal systems, complementing traditional geochemical sampling and visual observations and offering new insights into the temporal variability of degassing activity. Beyond natural systems, the demonstrated capability of DAS to detect bubble-related acoustic signals has direct implications for offshore CCS, where early detection of CO₂ leakage is critical for storage integrity and environmental safety.Overall, this field-scale experiment highlights the potential of fibre-optic sensing to address key challenges in marine monitoring, and underscores the value of integrated approaches for studying fluid and gas migration processes.Acknowledgements:ECCSELLENT project (“Development of ECCSEL - R.I. ItaLian facilities: usEr access, services and loNg-Term sustainability”)ITINERIS - Italian Integrated Environmental Research Infrastructures System - Next Generation EU Mission 4, Component 2 - CUP B53C22002150006 - Project IR0000032 Panarea NatLab Italy: https://eccsel.eu/catalogue/facility/?id=124 ECCSEL: https://eccsel.eu/ References:Detection of CO2 emissions from Panarea seabed with Distributed Acoustic Sensing (DAS): a preliminary investigation. Meneghini et al. OGS report (2025).Marine Fiber-Optic Distributed Acoustic Sensing (DAS) for Monitoring Natural CO₂ Emissions: A Case Study from Panarea (Aeolian Islands, Italy). Bellezza et al. Upon submission to Applied Sciences (2026).

The PITOP geophysical test site, operated by the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) in north-eastern Italy, provides a unique experimental environment for testing seismic acquisition technologies under realistic field conditions. Covering ~22,000 m², PITOP was established to support the development and validation of geophysical methods and instrumentation in both surface and borehole installations. Here, we evaluate PITOP’s potential for Distributed Acoustic Sensing (DAS) experiments, focusing on small-scale seismic measurements relevant to urban settings and engineering applications. Five boreholes with distinct purposes and instrumentation are available at the PITOP site, including a water well (PITOP1), two 400-m-deep wells associated with geosteering research (PITOP2 and PITOP3), a 150-m-deep borehole permanently equipped with optical fibre for DAS measurements (PITOP4), and a recently drilled well dedicated to geoelectrical surveys (PITOP5). The site also hosts a surface-deployed fibre-optic cable, containing both linear and helicoidal fibers, and about 20 3C seismic nodes. Finally, several seismic sources are available, which are a borehole Sparker Pulse, suitable for crosshole VSP configurations, and two surface vibratory sources, the IVI MiniVib T-2500, which can generate sweeps in the 10–550 Hz frequency range, and the ElViS VII vibrator, designed for frequencies between 20 and 220 Hz.We conducted three dedicated experiments: (i) cross-hole measurements with sources in PITOP3 at depths of 10, 50, 75, and 100 m, and DAS recording in PITOP4; (ii) a vertical seismic profiling (VSP) survey using the MiniVib source close to the well head with DAS recording in PITOP4; and (iii) recordings of the seismic wavefield generated by P- and S-wave vibratory sources using surface DAS arrays in linear and helicoidal configurations, together with co-located 3D geophones for comparison.DAS data were acquired with multiple gauge lengths and acquisition settings. The resulting datasets enable a systematic evaluation of acquisition parameters selection and highlight processing strategies required for different DAS configurations. They provide a valuable basis for assessing optimal DAS acquisition strategies for small-scale seismic applications and for defining processing workflows adapted to diverse source and receiver geometries.The present study is being carried out within the framework of the USES2 project, which receives funding from the EUROPEAN RESEARCH EXECUTIVE AGENCY (REA) under the Marie Skłodowska-Curie grant agreement No 101072599.This research has been supported by the Interdepartmental Research Center for Cultural Heritage CIBA (University of Padova) with the World Class Research Infrastructure (WCRI) SYCURI—SYnergic strategies for CUltural heritage at RIsk, funded by the University of Padova.

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.

The OGS PITOP geophysical testing site, located in northeastern Italy, serves as a cutting-edge facility for developing and testing geophysical methods, technologies, and tools under realistic conditions. Covering an area of 22,000 m², PITOP is equipped with instrumented wells, permanent and mobile data acquisition systems, and active seismic and geoelectric sources. This infrastructure provides a unique environment for advancing subsurface characterization techniques.The PITOP facility includes five wells (PITOP1-PITOP5) with depths ranging from 150 to 423 m. PITOP2 is equipped with 30 permanently installed three-axial geophones, while PITOP4 houses a Distributed Acoustic Sensing (DAS) fiber optic system. In March 2024, the addition of PITOP5 enabled a series of innovative experiments using the drill bit as a seismic source. These experiments had three primary objectives: (1) enhancing subsurface knowledge of the area, (2) testing new seismic instrumentation, and (3) performing integrated analyses using multidisciplinary methodologies.During the PITOP5 drilling process, receivers were deployed both at the surface and in wells enabling cross-well tests acquired while drilling. The Seismic While Drilling (SWD) technique utilizes the drill bit as the seismic source. Surface receivers included standard geophones and triaxial nodes, arranged symmetrically around PITOP5.In addition to SWD, Vibroseis was employed as an active seismic source, complementing the drill bit experiments. This enabled a comparison of results from different seismic energization methods, further enhancing the robustness of the dataset.Preliminary seismic data analysis revealed the presence of shallow/medium depth reflectors.Complementary to the seismic investigations, the PITOP5 upgrade included geoelectrical instrumentation. An array of electrodes was installed and cemented within the PITOP5 well to a depth of 260 m, allowing for advanced geoelectrical analyses and interdisciplinary investigations of the subsurface including the seismic methods.Overall, a key feature of the PITOP upgrade is its capability for integrated geophysical experiments. During the drilling of PITOP5, surface seismic, SWD, and cross-hole experiments were conducted using diverse sources and acquisition systems. Data processing and integration are ongoing, with the aim of combining seismic and electrical information to provide a comprehensive understanding of subsurface structures.The PITOP testing site represents a significant advancement for the scientific and technical community, offering a versatile platform for multidisciplinary studies. Its capabilities are particularly relevant for projects related to CO2 and energy storage, both of which are critical for climate change mitigation and the transition to sustainable energy systems.With its state-of-the-art infrastructure and recent upgrades, PITOP is positioned as a leading facility for applied geophysics. The site not only facilitates the development of innovative geophysical techniques but also fosters collaboration among researchers, paving the way for future advancements in geosciences.References: Geophysical exploration case histories at the geophysical test site PITOP - a key facility in the ECCSEL-ERIC consortium: an overview. Bellezza et al., Bulletin of Geophysics and Oceanography, 2025

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.

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 Late Miocene deposits in the Sorbas Basin (Spain) have been of an extreme importance in the understanding of the Messinian Salinity Crisis (MSC) events (5.97-5.33 Ma). They consist of four formations. The pre-crisis Abad marls topped by the evaporitic Yesares gypsum member, followed by two non-evaporitic units known as the Sorbas and Zorreras members. Those deposits have been widely explored and studied thanks to the numerous outcropping sections in the basin. The ‘SaltGiant’ European Training Network held a training school in October 2021 in the Sorbas Basin, where four boreholes (named SG0, 1, 2 and 3) covering most of the Messinian Salinity Crisis sequence, were drilled, cored and logged in this context along an overall thickness of about 175 m. The drillings took place inside and in the vicinity of the Torralba gypsum mine. It allowed for the first time in the scientific non-industrial domain, access to a continuous and non-outcropping succession of the Messinian deposits in the Sorbas basin. In addition to the recovered cores, borehole geophysical data were obtained from the four holes and digital images of the area were collected with a drone. Prior to the drilling, an OBO (Outcrop / Behind Outcrop) workflow was followed, which will allow integrating the outcrop and subsurface data by combining the 3D geometry of geobodies with geophysical information. Optical borehole wall images provide mm-scale images of the borehole walls, highlighting the sedimentological and structural characteristics of the deposits. Downhole geophysical measurements included acoustic velocity, electrical resistivity and natural spectral gamma ray, which allowed determining the petrophysical characteristics of the penetrated lithologies. In addition to the petrophysical logs, a Vertical Seismic Profiling was performed in holes SG2 and SG3, including a multi-offset VSP survey in hole SG3. The petrophysical characterization of the Messinian deposits will provide a reference case study for the lithologic characterization of MSC deposits in the subsurface elsewhere. VSP analysis provided an in-field preliminary seismic velocity evaluation in the encountered formations. Preliminary results confirm the astronomical precession-driven cyclicity observed elsewhere in the Messinian gypsum. Further processing and analyses of the large amount of acquired data will lead to identifying the astronomical and possibly higher-frequency cyclicity in the post-evaporitic deposits in the Sorbas member.

The Balaruc hydrothermal system is fed both from surrounding karstic carbonates with fresh water outpouring in the nearby Thau lagoon with the Vise source, and at depth along deep regional faults with local springs up to 50°C. This hydrothermal system was cored and logged in 2020-2021 down to 765m depth at Balaruc-les-Bains, 200m to the NE of the Vise source. During the project, the Vise source underwent a reversal in November 2020, with the Thau lagoon salty water being drained deep into the subsurface, shedding light into the complex processes affecting the Balaruc hydrothermal system. The DEM’EAUX THAU project is aiming at a better understanding of this complex geological and hydrological system for a more sustainable use of this resource. A set of downhole geophysical data and borehole wall images was recorded from near surface to 756m depth in 4 vertical holes, only a few meters apart at surface. While mm-scale images reveal the detailed geological structure, petrophysical data (acoustic velocities, electrical resistivity and natural gamma) contribute to better define the penetrated structure, yielding porosity and permeability. Acoustic velocities provide a base to analyze the vertical (VSP) and walk-away seismic profiles shot to replace these holes in the regional geological structure. In turn, core petrophysical measurements are being made to support these analyses and, in particular, to provide a dm-scale description of the subsurface pore fluid salinity. In addition, the physical and chemical properties of the borehole fluid were characterized with an Idronaute probe, showing the impact of the Vise source reversal from measurements before and after. Similarly, electrical resistivity profiles were recorded over time and during downhole pumping tests, emphasizing the hydraulic vertical connectivity. On that basis, pore fluids dynamics are being described from time-lapse downhole logging measurements and the emplacement of permanent downhole geophysical observatories consisting in (i) an optical fiber for temperature and (ii) a flute for electrical resistivity of the formation. In the future, a second optical fiber already in place will be used for acoustic probing of subsurface fluid flow from Digital Acoustic Scanning (DAS).

The problem of localizing small (relative to wavelength) scatterers by diffractions to enhance their use in identifying small-scale details in a seismic image is extremely important in shallow exploration to identify interesting features such as frac-tures, caves, and faults. The conventional approach based on seis-mic reflection is limited in resolution by the Rayleigh criterion. In certain acquisition geometries, such as crosswell surveys aimed at obtaining high-resolution signals, the availability of suit-able data sets for effective migration depends on the spatial extent of the available source and receiver data intervals. With the aim of overcoming the resolution limits of seismic reflection, we study the detectability, response, and location of meter-and possibly submeter-dimension carbonate concretions (septaria) in the Boom Clay Formation (potential host rocks for radioactive waste disposal) by diffraction analysis of high-frequency signals. We investigate diffraction wavefields by signal separation, focusing, and high-resolution coherency analysis using the multiple signal classification method and semblance. The investigation is per-formed for two different surveys in Belgium, a shallow and high-resolution reverse vertical seismic profile (RVSP) and a near-offset crosswell application at Kruibeke and ON-MOL-2 sites, respectively. The data analysis is supported by synthetic wavefield modeling. The multioffset RVSP provides the appro-priate geometry to observe and investigate the septaria diffrac-tions from depth and the surface. The crosswell approach, calibrated using synthetic data in the analysis of wavefield pat-terns in 2D, indicates promising imaging results with field data of a selected diffraction zone in the interwell area.

Seismic-while-drilling (SWD) by drill-bit source has been successfully used in the past few decades and is proven using variable configurations in onshore applications. The method creates a reverse vertical seismic profile (RVSP) data set from surface sensors deployed as arrays in the proximity of the monitored wells. The typical application makes use of rig-pilot reference (pilot) sensors at the top of the drill string and also downhole. This approach provides while-drilling checkshots as well as multioffset RVSP for 2D and 3D imaging around the well and prediction ahead of the bit. For logistical (sensor deployment) and cost (rig time related to technical installation) reasons, the conventional drill-bit SWD application is typically much easier onshore than offshore. We have developed a novel approach that uses a network of passive-monitoring sea-bottom nodes predeployed for microseismic monitoring to simultaneously and effectively record offshore SWD data. We study the results of a pilot test in which we passively monitored the drilling of an appraisal well at the Wisting discovery in the Barents Sea with an ocean-bottom cable deployed temporarily around the drilling rig. The continuous passive recording of vibration signals emitted during the drilling of the well provides the SWD data set, which is treated as an RVSP. The study is performed without rig-pilot signal. The results are compared with legacy data and demonstrate the effectiveness of the approach and point to future applications for real-time monitoring of the drilling progress, in terms of geosteering the drill bit and predicting formation properties ahead of the bit by reflection imaging.