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.

In seismic exploration, it is very important to consider the presence of anisotropy in order to image the subsurface correctly. The knowledge of anisotropic parameters leads to more precise characterization of reservoir, fracture density and flow paths. However, conventional geophysical methods do not directly measure these parameters, and it is useful to have a method to estimate them from seismic data. In the weak transverse isotropy approximation, the fields to be considered are the vertical and horizontal velocity components and the Thomsen parameters epsilon$\rm \epsilon$ and delta$\rm \delta$. We present a method for estimating the anisotropic Thomsen parameters in the presence of weak vertical transverse isotropy using P-wave traveltime tomography based on anisotropic ray tracing. Depending on the available information, we propose different approaches to retrieve the unknowns. A conventional three-dimensional traveltime tomography algorithm has been extended to include anisotropic ray tracing and using the algebraic reconstruction technique or modified simultaneous iterative reconstruction technique to retrieve the unknowns. We test the method on synthetic examples for the inversion of transmitted and reflected traveltimes, and we evaluate the sensitivity of the tomographic results to the available information. Furthermore, we also consider the case of tilted transverse isotropy in a seismic reflection example.

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.

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.

There are basins with proven hydrocarbon and geothermal sources across western Anatolia. Only a few moderate-scale tomography studies investigate their velocity structure and stratigraphical architecture. We present the first 3-D Neogene velocity-depth model of the Gulf of Izmir down to -1.7 km. The aim is displaying a better portrait of the 3-D stratigraphical architecture, velocity structure of the Gulf of Izmir, and constitute a solid foundation for future studies. We provide the first 3-D view of the complex Late-Miocene basement topography representing an unconformity surface. It consists of basins and ridges separating the Miocene basement (SSU3) and overlying Plio-Quaternary deposits (SSU2 and SSU1). Four N-S oriented volcanic ridges accommodate offshore Uzun Island, Karaburun, and Foca. They border a basin with a thickness of -720 m in the mid-central and a deeper one with -1400 m in the northernmost sector of the gulf. The 3-D P-wave traveltime tomography results indicate prominent lateral and vertical velocity variations at different depths (-300-800 m). The lowvelocity zones (1600 = Vp = 1850 m/s) are interrupted laterally by the high-velocity zones (2150 = Vp = 2350 m/s) that corresponds to the squeezed sediments within Plio-Quaternary basins deformed by the N-S trending strike-slip faults of Karaburun Fault Zone. The same faults also deform the volcanic basement highs identified by the high velocities (>2600 m/s). The thick depocenter with low-velocity anomalies in the outer gulf suggests the gas/fluid containing sediments. Their controlling faults are likely responsible for the circulation of gas/fluid and heat transfer from a deeper source making the region target for exploration activities.

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.

In some geological settings, such as thick sedimentary basins, the deeper layers may also significantly contribute to the amplification of the ground motion during an earthquake, particularly at low frequencies. To properly quantify the influence of the deeper geology on the seismic ground motion at the surface, the use of indirect geophysical exploration techniques is particularly suitable, as it can produce realistic representations of the subsoil structure over a large range of depths. This paper focuses on the acquisition and processing of combined shear- (S) and compressional-wave (P) seismic reflection data to obtain an accurate geological/geophysical model with reliable seismic velocities for local-scale seismic site response studies, as needed for hazard assessment and calculation of earthquake scenarios. As an application case, we focus on a microzonation study performed for the Cavezzo municipality in Northern Italy. The investigated site is located in the epicentral area of the 2012 Emilia earthquake in the Po Valley, which is a typical deep sediment-filled basin. Specifically, in this study we were able to: (1) confirm the advantages of reflection seismic prospecting for soft-sediment basin characterization; (2) evaluate the resolution of S- and P-wave signals and their depth of penetration; (3) improve the knowledge on the geological structure of the Cavezzo area by measuring S- and P-wave interval velocities in Plio-Quaternary formations.

Integrated geophysical analysis using different methods along with a priori information from wells, is a proven approach to investigate the geology and the petro-physical characteristics of subsoil. We collected seismic and geoelectric data in an area located on the Adriatic coast in North-Eastern Italy, aimed at characterizing the quaternary sediments and the shallow geological structures. Compressional and shear-wave data provided information about geometry and velocity of the quaternary sedimentary succession, while geoelectric data provided information about the resistivity in the shallower formation, which strongly depends on the presence of groundwater (brine) and on its salinity. Clustering analysis allowed us to subdivide the study area into subdomains showing similar values of resistivity and compressional- and shear-wave velocity, enabling for a better interpretation of the processed seismic sections. Then, we calculated the petro-physical properties of the investigated sediments, i.e., brine saturation and resistivity, porosity, and clay content, for each cluster. This inverse problem involves rock-physics theories and an optimization algorithm based on the simulated annealing global-search method. The results, validated using borehole stratigraphy, provided information about the salty water wedge intrusion.

We present two case studies of the application of seismic surveys to estimate the elastic properties of soil and rock in the shallow subsurface. The two sites present very different geological characteristics. The first test site is a town on the Croatian coast, not far from the city of Split, built on hard rock, where we acquired three seismic lines. The second site is located in the outskirts of the city of Ferrara, in Italy, in an alluvial plain, where two lines were acquired. In both sites, for detailed characterization, we acquired surface-, compressional- and shear-waves, further distinguishing the latter between horizontally (SH) and vertically (SV) polarized wavefields. We processed the data by performing a Multichannel Analysis of Surface Waves to compute a preliminary one-dimensional shear wave velocity profile. Then, we performed first-break tomography to compute P-, SH- and SV-velocity profiles. Such unusual acquisition allowed us to compute not only basic engineering parameters such as the equivalent shear-wave velocity of the first 30 m of subsurface (VS30) from the SH profiles but also other useful parameters such as the VP/VS and estimate the anisotropy of the medium thanks to the VSV/VSH. Given the level of detail of the results and their engineering value, we conclude that the method of investigation we applied in the two test sites is a valuable tool for characterizing the shallow subsurface.

Summary We present the application of an innovative survey technique for in-depth characterisation of the elastic properties of the shallow subsurface. The novelty of the method lies in the fact that we acquired along the same line surface-, as well as P-, SH- and SV-waves. First, we perform multichannel analysis of surface waves to estimate a preliminary 1D VS profile, which served as reference for further analyses. Then, we performed first-break tomography on both lines, for all wavefields, obtaining velocity profiles. Thanks to the unusual acquisition, from these profiles we are able to compute not only basic engineering parameters like the VS30, but also geotechnical parameters like VP / VSH and VSV / VSH (i.e. estimate the anisotropy of the medium). We then perform a cluster analysis based on the VP / VS and map the sediments accordingly, obtaining further useful information like the position of the water table.

The acquisition of S-wave seismic data, separating S H (orthogonal to the seismic line) and S V (parallel to the seismic line) wavefields on the same seismic line, is not so frequent. However, this type of acquisition allows obtaining important information on the investigated area; in particular the anisotropy of the sediments, which can be detected from the differences between the corresponding two velocity fields. In this work, we analysed the S V - and S H -wave data in order to answer two questions: how can we estimate anisotropy from the comparison of the velocities of the S V and S H wavefields? What information can be obtained about the dip and strike angles of the anisotropic layers from this analysis? For this purpose, we used the travel time tomography to obtain the two velocity fields ( V SV and V SH ) and to exploit the computed ray paths from which we can know the directions of each ray segment crossing the model, associated with each V S component. We, then, used them, together with the V SV / V SH values obtained from the tomography, to estimate the anisotropy and the orientation of the anisotropic layer, defined by the slope (dip) and the azimuth angle (strike). Furthermore, we tried to define a possible relationship between the V SV and V SH components with the anisotropy parameters, the direction of the rays and the geometry of the anisotropic layer.

The Gulf of Trieste (GT), northeastern Adriatic, sits at the rigid edge of the Adria microplate representing the foreland of the adjoining Meso-Cenozoic External Dinarides and Southeastern Alps. The Adria-Eurasia plate boundary extends along the GT eastern coastline, depicted by the Karst Thrust, outer ramp of the SW-verging Dinarides. This separates the Cretaceous-Paleogene carbonates outcropping hundreds of meters on the Karst Plateau (hanging-wall) from their counterparts buried in the eastern gulf. Although the thrust has no evidence of historical and instrumental seismicity, a detailed seismic velocity field is needed to quantify reliable geometries in the foredeep, embedded in a tectonically active area where Adria is moving NNW-ward. Availability of a newly acquired marine multichannel seismic dataset, allowed us to provide the first well-constrained 3D P-wave velocity and depth model in the footwall of the Karst Thrust. Two iterative techniques, traveltime reflection tomography and depth seismic imaging, were applied on the data surveying the Dinaric foredeep. Our findings provide mean velocity values of 1700 m/s for the Quaternary sediments, 2900 m/s for the upper Eocene flysch turbidites, 4500 m/s for the lower flysch, 5000 m/s for the upper carbonates. The maximum flysch unit thickness results in about 1500 m and the top carbonates depth reaches about 1600 m below sea level, 1.7 km offshore Trieste, revealing the thrust is responsible for a minimum 1600-1800 m vertical throw. This study provides benefits for Adria geodynamic models and give new constraints for the geological and tectonic setting assessment, in a region settled over a currently active continental margin.

Seismic surveys allow estimating lithological parameters, as P-wave velocity and anelastic absorption, which can detect the presence of fracture and fluids in the geological formations. Recently, a new method has been proposed for high-resolution imaging of anelastic absorption, which combines a macro-model from seismic tomography with a micro-model obtained by the pre-stack depth migration of a seismic attribute, i.e., the instantaneous frequency. As a result, we can get a broadband image that provides clues about the presence of saturating fluids. When the saturation changes sharply, as for gas reservoirs with an impermeable caprock, the acoustic impedance contrast produces “bright spots” because of the resulting high reflectivity at its top. When the fluid content changes smoothly, the anelastic absorption becomes a good detector, as fluid-filled formations absorb more seismic energy than hard rocks. We apply this method for imaging the anelastic absorption in a regional seismic survey acquired by OGS in the Gulf of Trieste (northern Adriatic Sea, Italy).

We carried out new geological, morphotectonic, geophysical and paleoseismological investigations on the Meduno Thrust that belongs to the Pliocene-Quaternary front of the eastern Southern Alps in Friuli (NE Italy). The study area is located in the Carnic Prealps, where a series of alluvial terraces, linked to both climatic and tectonic pulses characterises the lower reach of the Meduna Valley. In correspondence of the oblique ramp of the Meduno Thrust, the Late Pleistocene Rivalunga terrace shows a set of scarps perpendicular to the Meduno valley, often modified by human activity. In order to reconstruct the tectonic setting of the area and identify the location for digging paleoseismological trenches, integrated geophysical investigations including electrical resistivity tomography, seismic refraction and reflection, ground penetrating radar and surface wave analyses (HVSR, ReMi and MASW), were carried out across the scarps of the Rivalunga terrace. Geophysical surveys pinpointed that in correspondence of the oblique ramp, stress is accommodated by a transpressive thrust system involving all the seismo-stratigraphic horizons apart from the ploughed soil. Trenching illustrated the Meduno Thrust movements during Late Pleistocene-Holocene. Trenches exhibited both shear planes and extrados fracturing, showing deformed alluvial and colluvial units. C-14 datings of the colluvial units show that the most recent fault movements occurred after 1360 CE and 1670 CE. The age of the deformed stratigraphic units compared with the earthquakes listed in current catalogues, suggests that the 1776 earthquake (Mw 5.8, Io = 8-9 MCS) could represent the last seismic event linked to the Meduno thrust activity. This study provided new quantitative constraints improving seismic hazard assessment for Carnic prealpine area.

The noncondensable gases in most geothermal resources include CO2 and smaller amounts of other gases. Currently, the worldwide geothermal power is a small sector within the energy industry, and CO2 emissions related to the utilisation of geothermal resources are consequently small. In some countries, however, such as Turkey and Iceland, geothermal energy production contributes significantly to their energy budget, and their CO2 emissions are relatively significant. SUCCEED is a targeted innovation and research project, which aims to investigate the reinjection of CO2 produced at geothermal power production sites and develop, test, and demonstrate at field scale innovative measurement, monitoring and verification (MMV) technologies that can be used in most CO2 geological storage projects. The project is carried out at two operating geothermal energy production sites, the Kizildere geothermal field in Turkey and the CarbFix project site at the Hellisheioi geothermal field in Iceland. Together with a brief description of the project, this paper presents the details of the two field sites and the progress made in seismic velocity characterisation and modelling relevant to the Kizildere geothermal field in Turkey.

Additional information regarding methods (Reflection seismic processing, Drill-site measurements, Core-log-seismic correlations, Spatial Velocity calculations, and Reflection Tomography model) and regional stratigraphy descriptions, as well as detailed considerations regarding the opal distribution and depth.

The ongoing tectonism in the Western Anatolia creates N-S extension and counter-clockwise rotational motion along the right-lateral North Anatolian fault (NAF) and left-lateral East Anatolian Fault (EAF). This continental extension creates predominantly E-W extending onshore grabens rarely NE to SW and NW to SE trending onshore/offshore grabens characterised by the intense seismic activity, high heat flow associated with volcanism, crustal thinning and geothermal systems. Our study area, the gulf of İzmir, has an “L” shape composing of an E-W oriented inner bay from İzmir to Urla and incompatibly NNW-SSE oriented outer bay between offshore Foça and Karaburun. It is located at the intersection of the E-W oriented onshore Gediz Graben and NE-SW oriented onshore Bakırçay graben. Geophysical evidence for fluid discharge and subsurface gas-associated structures such as gas chimneys, pockmarks, mud diapirs and acoustic turbidity zones have been detected in the inner and outer parts of the Gulf of İzmir by the previous studies. For this reason, the Gulf of İzmir and the adjacent onshore grabens are areas of great interest for further study of the region. In this study, the 3-D stratigraphic architecture (up to 1.5 km) and the Upper Miocene-Pliocene depositional settings of the Gulf of İzmir reconstructed by reflection tomography for the first time. Three seismic stratigraphic units, labelled SSU1, SSU2 and SSU3 from bottom to top, were identified by their bounding unconformity surfaces (H1-H5). We have subdivided unit SSU1 into three subunits named SSU1c-SSU1a. The acoustic basement associated with SSU3 is likely tied to the Lower-Middle Miocene Yuntdağ Volcanics consisting of tuffs, sandstones, limestones and volcanics. The upper surface of SSU3 (horizon H5) is marked as a major regional unconformity representing a basin-ridge morphology. The first rocks deposited on top of acoustic basement (SSU2) correspond to the sandstones, limestones, volcanics and shales of the Bozköy Formation and the limestones of the Ularca Formation, dating from the Late Miocene to the Pliocene. The top of SSU2 (horizon H4) is interpreted as another unconformity and is correlated with the Pliocene unconformity. Above that, part of the Bayramiç Formation (SSU1c) is dated as Quaternary, consisting of conglomerates at the base overlain by sandstones and shales above. On top of the SSU1c are two further sub-units of the Bayramiç Formation separated by horizons H3 and H2. SSU1b consists of a similar sequence of conglomerates, sandstones and shales; SSU1a consists of Quaternary sandstones. Following the tomographic analysis, the isopach map of the Plio-Quaternary sediment fills was derived from the depth of interpreted horizons calculated using tomographic interval velocities. According to the isopach map of the sedimentary fills, thickness abruptly decreasing from NW to SE. The maximum thickness of total sedimentary succession is ~1400 m in the NW, whereas the thickness decreases through the west, east (up to ~450 m) and the southeastern flank of the basin, reaching ~150 m forming a ridge. A few local lateral velocity variations were identified within the Plio-Quaternary sedimentary succession associated with faults, fluid escape and shallow gas occurrences or a combination of these.