ALBERT

Description: ABSTRACT We present the results of a 3D seismic survey acquired near the city of Schneeberg in the western Erzgebirge (Germany). The main objective of this survey was to use reflection seismic exploration methods to image a major fault zone in crystalline rock, which could serve as a geothermal reservoir at a target depth of about 5 km–6 km with expected temperatures between 160°C–180°C. For this purpose, a high-resolution 3D Vibroseis survey was performed in late 2012 covering an area of about 10 km × 12 km. The 3D survey was complemented by a wide-angle seismic survey for obtaining velocity information from greater depths using explosives along ten profile lines radially centred at the target area. The region itself is dominated by the northwest-southeast striking Gera-Jáchymov fault system and the southwest–northeast striking Lössnitz–Zwönitz syncline. The main geological features in the survey area are well known from intensive mining activities down to a depth of about 2 km. The seismic investigations aimed at imaging the partly steeply dipping fault branches at greater depths, in particular a dominant steeply northeast dipping fault (Roter Kamm) in the central part of the survey area. In addition to this main structure, the Gera–Jáchymov fault zone consists of a series of steeply southwest dipping conjugate faults. For imaging these structures, we used a focusing pre-stack depth migration technique, where the wavefield coherency at neighbouring receivers is used for weighting the amplitudes during migration. This method delivers a clear, focused image of the 3D structures within the target area. A 3D velocity model for depth imaging was obtained by first-arrival tomography of the wide-angle survey data. With this approach, we were able to image several pronounced structures interpreted as faults within the crystalline rock units, which partly reach the target depth where the temperatures for a geothermal usage would be sufficient. In general, the results show a complex three-dimensional image of the geological structures with different reflection characteristics, which can serve as a basis for a detailed characterization of the potential deep geothermal reservoir.

Description: ABSTRACT This work represents a case study concerning the application of reflection seismic imaging methods in the context of geothermal exploration. Our goal is to obtain accurate structural images of a geothermal active area in southern Tuscany. These images will be required in subsequent studies as the input for geological model building and numerical simulation of the heat transport and fluid flow. The target region exhibits great geologic complexity, including strong velocity contrasts, lateral near-surface inhomogeneities, fracture zones, and significant topography. Those features are typical for a volcanic hard-rock environment and pose significant challenges to conventional seismic imaging methodology. Therefore, we apply a sophisticated and robust depth imaging workflow to previously acquired surface seismic data. Within our workflow, we focus on estimating the seismic velocities of the predominant rock units and subsequently carry out Kirchhoff pre-stack depth migration and Fresnel volume migration to obtain high-resolution images of the subsurface. Our results demonstrate that the applied methodology provides a valueable tool for imaging in a complex environment such as a volcano-geothermal area. In detail, the resulting reflector images show the main horizons that delineate the Tuscan sedimentary rocks in the target region. The images from standard Kirchhoff migration can be significantly enhanced by utilizing Fresnel volume migration, which eliminates migration artefacts and provides a better result. Moreover, we obtain the migration velocities and depths for an important regional reflector, known as the K-horizon, which is of major interest for geothermal characterization.

Description: ABSTRACT We present an approach for analysing seismic reflections from faults in a crystalline hard rock environment. We analysed 3D seismic reflection data for geothermal reservoir characterization acquired in the Erzgebirge Region, Germany. The seismic image derived from this data set revealed two main features: a less pronounced reflector corresponding to a steeply dipping major fault zone Roter Kamm and a group of pronounced reflectors attributed to the existence of conjugate mineralized faults. We analysed these reflections in the pre-stack data to characterize the nature and origin of reflectivity. This was done by extracting the corresponding waveforms from the raw data and carefully pre-processing them, including amplitude correction for geometrical spreading and signal-to-noise enhancement. Reflection coefficients were derived from the pre-processed shot gathers by comparing the amplitudes of the reflected and direct waves. Synthetic waveform modelling using the reflectivity method has been performed for several model families consisting of one-dimensional velocity–depth functions with varying velocities, densities, and thicknesses of the layers. A comparison of the modelled and observed waveforms revealed that a reflection coefficient of 0.18 for the conjugate mineralized faults can be explained by single layers with high impedance contrast and a thickness between 30 m and 40 m, whereas the reflection from the Roter Kamm fault zone with a reflection coefficient of −0.23 requires a model consisting of several low-velocity layers with a total thickness of up to 100 m embedded in a high-velocity background model. These results are in accordance with the geological interpretation of these reflectors. However, the characteristics of these reflections vary significantly within the investigation area, both in terms of the reflection coefficient and the waveform, which is also in agreement with the general lateral variation of fault zone characteristics known from tectonic investigations such as geological mapping of outcrops and fabric analysis.

Description: ABSTRACT The seismic K‐Horizon is the key to gaining understanding on the deep supercritical geothermal rocks in Southern Tuscany. The K‐Horizon is hosted in metamorphic rocks, which cause strong seismic wavefield scattering resulting in a poor signal‐to‐noise ratio. Our study aims to reveal high‐resolution seismic images of the K‐Horizon below a geothermal field in Southern Tuscany, using an advanced three‐dimensional seismic depth imaging approach. The key seismic pre‐processing steps in the time domain include muting a large amount of persistent noise based on the statistical analysis of the seismic amplitudes, and tomostatics technique to correct for static effects. We carried out seismic depth imaging using Kirchhoff Pre‐Stack Depth Migration and Fresnel Volume Migration techniques. Each migration technique was tested with constant and heterogeneous three‐dimensional velocity models. Due to the difficulties in determining emergent angles for this low signal‐to‐noise ratio data set, the migration results with the heterogeneous three‐dimensional velocity model show less coherent reflections compared to the migration results using the constant velocity model. Both velocity models however lead to relatively the same structure and depth of the K‐Horizon, indicating the similarity of the average velocities along the wave propagation paths in both velocity models. With both velocity models Fresnel Volume Migration yields the K‐Horizon with better reflection coherency and higher signal‐to‐noise ratio than standard Kirchhoff Pre‐Stack Depth Migration. Nevertheless, both migration techniques have been able to reveal the K‐Horizon with relatively high resolution and provide a reliable basis for geothermal rock characterization as well as steering of the first geothermal well penetrating the K‐Horizon.

Description: ABSTRACT The development of cost‐effective and environmentally acceptable geophysical methods for the exploration of mineral resources is a challenging task. Seismic methods have the potential to delineate the mineral deposits at greater depths with sufficiently high resolution. In hardrock environments, which typically host the majority of metallic mineral deposits, seismic depth‐imaging workflows are challenged by steeply dipping structures, strong heterogeneity and the related wavefield scattering in the overburden as well as the often limited signal‐to‐noise ratio of the acquired data. In this study, we have developed a workflow for imaging a major iron‐oxide deposit at its accurate position in depth domain while simultaneously characterizing the near‐surface glacial overburden including surrounding structures like crossing faults at high resolution. Our workflow has successfully been showcased on a 2D surface seismic legacy dataset from the Ludvika mining area in central Sweden acquired in 2016. We applied focusing prestack depth‐imaging techniques to obtain a clear and well‐resolved image of the mineralization down to over 1000 m depth. In order to account for the shallow low‐velocity layer within the depth‐imaging algorithm, we carefully derived a migration velocity model through an integrative approach. This comprised the incorporation of the tomographic near‐surface model, the extension of the velocities down to the main reflectors based on borehole information and conventional semblance analysis. In the final step, the evaluation and update of the velocities by investigation of common image gathers for the main target reflectors were used. Although for our dataset the reflections from the mineralization show a strong coherency and continuity in the seismic section, reflective structures in a hardrock environment are typically less continuous. In order to image the internal structure of the mineralization and decipher the surrounding structures, we applied the concept of Reflection Image Spectroscopy (RIS) to the data, which allows the imaging of wavelength‐specific characteristics within the reflective body. As a result, conjugate crossing faults around the mineralization can directly be imaged in a low‐frequency band while the internal structure was obtained within the high‐frequency bands. This article is protected by copyright. All rights reserved

Description: The plate-bounding Alpine Fault in New Zealand is a 850 km long transpressive continental fault zone that is late in its earthquake cycle. We have acquired and processed reflection seismic data to image the subsurface around the main drill site of the Deep Fault Drilling Project (DFDP-2). The resulting velocity models and seismic images of the upper 5 km show complex subsurface structures around the Alpine Fault zone. The most prominent feature is a strong reflector at depths of 1.5-2.2 km with an apparent dip of 48° to the southeast below the DFDP-2 borehole, which we assume to be the main trace of the Alpine Fault. Above the main reflector, parallel reflectors suggest the presence of a ∼600 m wide damage zone. Additionally, subparallel reflectors are imaged that we interpret as secondary branches of the main fault zone. Conjugate faults imaged by the data show the complexity of the subsurface. The derived P-wave velocity model reveals a 300-600 m thick sedimentary layer with velocities of ∼2.3 km/s above a schist basement with velocities of 4.5-5.5 km/s. A low-velocity layer can be observed within the basement at 0.8-2 km depth. A small-scale low-velocity anomaly appears at the top of the basement that can be correlated to the fault zone. The results provide a reliable basis for a seismic characterization of the DFDP-2 drill site that can be used for further structural and geological investigations of the architecture of the Alpine Fault in this area.

Description: A 10.5 km2 3D seismic survey was acquired over the Kylylahti mine area (Outokumpu mineral district, eastern Finland) as a part of the COGITO-MIN (COst-effective Geophysical Imaging Techniques for supporting Ongoing MINeral exploration in Europe) project, which aimed at the development of cost-effective geophysical imaging methods for mineral exploration. The cost-effectiveness in our case was related to the fact that an active-source 3D seismic survey was accomplished by using the receiver spread originally designed for a 3D passive survey. The 3D array recorded Vibroseis and dynamite shots from an active-source 2D seismic survey, from a vertical seismic profiling experiment survey, as well as some additional “random” Vibroseis and dynamite shots made to complement the 3D source distribution. The resulting 3D survey was characterized by irregular shooting geometry and relatively large receiver intervals (50 m). Using this dataset, we evaluate the effectiveness of the standard time-imaging approach (post-stack and pre-stack time migration) compared to depth imaging (standard and specialized Kirchhoff pre-stack depth migration, KPreSDM). Standard time-domain processing and imaging failed to convincingly portray the first ~1500 m of the subsurface, which was the primary interest of the survey. With a standard KPreSDM, we managed to obtain a good image of the base of the Kylylahti formation bordering the extent of the mineralization-hosting Outokumpu assemblage rocks, but otherwise the image was very noisy in the shallower section. The specialized KPreSDM approach (i.e., coherency-based Fresnel volume migration) resulted in a much cleaner image of the shallow, steeply dipping events, as well as some additional deeper reflectors, possibly representing repetition of the contact between the Outokumpu assemblage and the surrounding Kalevian metasediments at depth.