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- Volume 63, Issue 4, 2015
Geophysical Prospecting - 4 - Hard Rock Seismic imaging, Fri Jul 24 00:00:00 UTC 2015
4 - Hard Rock Seismic imaging, Fri Jul 24 00:00:00 UTC 2015
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Building a 3D model of lithological contacts and near‐mine structures in the Kevitsa mining and exploration site, Northern Finland: constraints from 2D and 3D reflection seismic data
Authors E. Koivisto, A. Malehmir, N. Hellqvist, T. Voipio and C. WijnsABSTRACTThe Kevitsa mafic–ultramafic intrusion in Northern Finland hosts a large, disseminated nickel–copper sulphide ore body. The Kevitsa intrusion is an active mining and exploration site, for which we have built a 3D model of the main lithological contacts and near‐mine structures in the area. To build the 3D model, 2D and 3D reflection seismic data have been used together with borehole data and geological map of the area. The Kevitsa reflection seismic data reveal the internal architecture of the Kevitsa intrusion and the surrounding units. For example, the seismic data have uncovered a previously unknown, deeper continuation of the Kevitsa intrusion. Improved 3D knowledge of the basal contact of the intrusion provides an exploration target for contact‐type mineralization. Within the intrusion, a limited area of strong reflections is observed in the data. This has been associated with discontinuous, smaller‐scale magmatic layering that is thought to control the extent of the Kevitsa main mineralization. Thus, our 3D model of the extents of the internal reflectors can provide a framework for near‐mine and deep exploration of the main type of mineralization in the area. In addition to exploration, the original purpose of the 3D seismic survey was geotechnical planning of the Kevitsa open‐pit mine. Accordingly, the 3D seismic data were used to create a 3D model of the subsurface structures, with a focus on the vicinity of the mine. The interpreted structures reveal a complex pattern of fault and fracture zones, some of which will be important for slope stability and operational planning of the final stages of the mine.
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Delineating structures controlling sandstone‐hosted base‐metal deposits using high‐resolution multicomponent seismic and radio‐magnetotelluric methods: a case study from Northern Sweden
ABSTRACTOver the past few decades seismic methods have increasingly been used for the exploration of mineral, geothermal, and groundwater resources. Nevertheless, there have only been a few cases demonstrating the advantages of multicomponent seismic data for these purposes. To illustrate some of the benefits of three‐component data, a test seismic survey, using 60 digital three‐component sensors spaced between 2 m and 4 m and assembled in a 160 m‐long prototype landstreamer, was carried out over shallow basement structures underlying mineralized horizons and over a magnetic lineament of unknown origin. Two different types of seismic sources, i.e., explosives and a sledgehammer, were used to survey an approximately 4 km‐long seismic profile. Radio‐magnetotelluric measurements were also carried out to provide constraints on the interpretation of the seismic data over a portion of the profile where explosive sources were used. Good quality seismic data were recorded on all three components, particularly when explosives were used as the seismic source. The vertical component data from the explosive sources image the top of the crystalline basement and its undulated/faulted surface at a depth of about 50 m–60 m. Supported by the radio‐magnetotelluric results, however, shallower reflections are observed in the horizontal component data, one of them steeply dipping and associated with the magnetic lineament. The vertical component sledgehammer data also clearly image the crystalline basement and its undulations, but significant shear‐wave signals are not present on the horizontal components. This study demonstrates that multicomponent seismic data can particularly be useful for providing information on shallow structures and in aiding mineral exploration where structural control on the mineralization is expected.
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Physical property analysis and preserved relative amplitude processed seismic imaging of volcanogenic massive sulfides—a case study from Neves–Corvo, Portugal
ABSTRACTNeves–Corvo is one of the biggest mining districts on the Portuguese side of the Iberian Pyrite Belt hosting six different lower Carboniferous copper, zinc, lead and tin orebodies including Lombador, Neves, Graça, Corvo, Zambujal, and Semblana. During the past 50 years, geological, geochemical, and geophysical methods were utilized in the exploration of volcanogenic massive sulfide deposits at Neves–Corvo. Electromagnetic, earth resistivity, and principally gravimetry methods played major roles in the geophysical exploration of the area. However, in 2011, as the exploration depth for volcanogenic massive sulfide mineralization became ever deeper, the surface reflection seismic technique was trialled.
Initially, elastic property measurements were employed on numerous core samples to determine the seismic properties of the major formations of Neves–Corvo. The contrast in acoustic impedance values derived from these measurements showed that there should be a significant difference in the seismic response of mineralization relative to the surrounding host rocks. Based on this, a high‐resolution 3D seismic survey was acquired over the Neves–Corvo mine and its southeastern extension in order to image known deep volcanogenic massive sulfide mineralization to validate the seismic reflection technique and to potentially identify new mineralization targets. As a result, the Semblana and Lombador deposits were successfully imaged, along with key lithological contacts and geologic structures. Additionally, copper sulfide extensions south of Semblana were discovered. Unfortunately, all of the high‐priority targets that were identified from the seismic data were subsequently drilled and many of them found to be non‐economic.
In order to overcome the non‐uniqueness of the original seismic data, full‐waveform sonic and pseudo‐logs were used to model different interfaces and calibrate the seismic data. These results indicated that preserved relative amplitude processing might be of importance to help reduce the ambiguity in direct detection of volcanogenic massive sulfide based on seismic amplitude anomalies. The customized relative amplitude processing of a sub‐dataset over the Semblana deposit was then performed. The newly obtained seismic cube was calibrated with existing drillholes, and a volumetric interpretation was performed by utilizing amplitude‐based geobodies. Eventually, superior target zonation and precision for the subsequent deep drilling campaign was achieved with the revised interpretation, clearly showing that the high priority targets originally identified from the legacy data would not have passed the targeting criteria in the reprocessed data due to their relatively weak amplitude response. The results obtained from this study inspired the subsequent reprocessing of the full seismic dataset.
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3D seismic imaging of the Lalor volcanogenic massive sulphide deposit, Manitoba, Canada
Authors G. Bellefleur, E. Schetselaar, D. White, K. Miah and P. DueckABSTRACTA three‐component three‐dimensional seismic data set was acquired over the Lalor volcanogenic massive sulphide deposit near Snow Lake, Manitoba, Canada, to assess the reflectivity of the ore and further validate the potential of three‐dimensional reflection seismic methods for deep mineral exploration. The Lalor deposit was chosen as a test site as it provided an intact, well‐characterized 25‐Mt‐deep ore deposit with a rich catalogue of geological and geophysical data, as well as extensive drill‐core and drill‐hole geophysical and geological logs. An analysis of physical rock properties from borehole logging data indicates that massive sulphides associated with the zinc‐rich zones could produce prominent reflections, whereas acoustic impedances of zones with disseminated gold do not sufficiently differ from the impedances of the host rocks to produce reflections. The interpretation of the seismic data is constrained with a detailed three‐dimensional lithofacies model constructed from the categorical kriging of 15 lithological units identified in borehole intersections. Processing of the seismic data included prestack dip‐moveout and poststack time migration. Final images reveal some strong reflections associated with the zinc‐rich massive sulphide zones. The most prominent reflection results from the constructive interference of thin and closely spaced massive sulphide zones and felsic–mafic volcanic rock contacts above and below the mineralization. Contacts between felsic and mafic volcanic rocks, including those that were hydrothermally altered and subsequently metamorphosed, produced prominent and continuous reflections that are used to map the main architecture of the footwall rocks. At depth, a series of continuous and conformable reflections indicate the general geometry of the volcanic sequences in the area of the three‐dimensional seismic survey.
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Feasibility of virtual source reflection seismology using interferometry for mineral exploration: A test study in the Lalor Lake volcanogenic massive sulphide mining area, Manitoba, Canada
Authors Saeid Cheraghi, James A. Craven and Gilles BellefleurABSTRACTApproximately 300 hours of ambient noise data were recorded on a grid of receivers covering an area of 4 km2 over the Lalor Mine, Canada, to test the capability of seismic interferometry to image ore deposits in the crystalline rock environment. Underground mining activities create the main source of ambient noise in the area. Alongside the ambient noise survey, a larger three‐dimensional active‐source seismic survey was also acquired and used to evaluate the interferometry results. Power spectral density calculations show random ambient noise with a frequency range of 2 Hz–35 Hz. A beamforming analysis identified body waves arriving from the west–northwest (pointing towards the mine) and surface waves propagating from the northeast. The calculated virtual shot gathers retrieved by cross‐correlating ambient noise at all receivers were processed following both two‐dimensional and three‐dimensional approaches using a sequence similar to the one applied to the active‐source three‐dimensional data. The dip‐moveout stacked section reveals a number of events similar to those observed on the processed active seismic sections. In particular, the passive seismic interferometry method is capable to partly image shallowly dipping reflections but did not produce convincing images of steeply dipping reflections. Dip‐moveout stacked sections obtained with different cross‐correlation time windows indicate that the strength and number of reflections generally increase with longer noise records. However, a few reflections at depth show reduced coherency with longer noise time windows. The passive seismic interferometry results over the Lalor mining area are encouraging, but image quality of the passive survey is lower than the acquired active three‐dimensional survey at the area. Future ambient noise surveys with longer offsets, shorter receiver spacing, and wider azimuth distribution are needed in crystalline rock environment to address the potential of the method for mineral exploration.
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Mode‐converted volcanogenic massive sulphide ore lens reflections in vertical seismic profiles from Flin Flon, Manitoba, Canada
Authors D.M. Melanson, D.J. White, C. Samson, G. Bellefleur, E. Schetselaar and D.R. SchmittABSTRACTIn October 2006, three‐component zero‐offset vertical seismic profile data were acquired from a deviated well in the Flin Flon mining camp in Manitoba, Canada, using a dynamite source. These vertical seismic profile data were processed to reveal reflections originating from the 85.5 Mt Flin Flon‐Callinan‐777 volcanogenic massive sulphide ore system. From drill records, mine plans, surficial maps, and seismic data, 3D voxel models of the local geology and known ore zones were built, which were then used in 3D finite‐difference modelled simulations of the vertical seismic profile surveys. The number of geological units partitioning the model was incrementally increased to study the effects of the massive sulphide ore and the major rock units on the seismic response. The simulations and field data were jointly visualized, and reflections originating at some of the known ore zones were identified. These reflections were observed in each of the three components in both the real field and the forward modelled data and indicate a strong mode‐converted component of the reflected wavefield.
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Uranium mineralization indicators from seismic and well log data in the Shea Creek area at the southern margin of the Carswell impact structure, Athabasca Basin, Canada
Authors Z. Hajnal, E. Takacs, B. Pandit and I.R. AnnesleyABSTRACTThe primary economic uranium (U) mineralization indicators in the Athabasca Basin are the reactivated prominent basement deformation zones, basement alteration halos, and the overlying sandstone alteration halos enveloping the deposit. High‐resolution seismic surveys, from the Shea Creek area, are investigated to assess the potential of the surface seismic reflection technique for detecting these key mineralization markers. Four seismic profiles identify ductile to highly brittle deformation features of the basement. One of the brittle deformation features is a southwest‐dipping prominent reactivated shear zone comparable to basement structures hosting other known deposits in the basin. The Carswell meteor impact structure is also a conspicuous component of the Shea Creek regional structural regime. The seismic sections identify several features of the impact structure, including recognition that the dimensions of this structure are significantly larger than those estimated by previous geological studies. This larger area includes both the Cluff Lake mine and the Shea Creek U deposits, and it reveals that the basement was considerably disturbed by the impact and suggests the reactivation of the southwest‐dipping shear zone. Complex trace analysis of the seismic sections provides detailed information about the lateral changes in the reflectivity properties of the unconformity interval, which is another key zone of interest for uranium exploration. Based mainly on velocity information, several sub‐zones of clay alteration and silicification were identified within prominent fracture depth intervals: both in the sandstone and the basement. Thus, this study documents that these primary mineral indicators can be detected by the seismic method. Cross‐plots of velocity versus density of well log data identify changes in lithological units such as unaltered basement, fractured/altered basement, unaltered sandstone, clay alterations in sandstone, and silicification in sandstone.
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Integrated interpretation of 3D seismic data to enhance the detection of the gold‐bearing reef: Mponeng Gold mine, Witwatersrand Basin (South Africa)
Authors Musa Manzi, Gordon Cooper, Alireza Malehmir, Raymond Durrheim and Zamaswazi NkosiABSTRACTWe present an integrated approach to the seismic interpretation of one of the world's deepest gold ore body (Carbon Leader Reef) using three‐dimensional seismic data, ultrasonic velocity measurements at elevated stresses, and modified instantaneous attribute analysis. Seismic wave velocities of the drill‐core samples (quartzite, shale, and conglomeratic reef) from the mine are sensitive to uniaxial stress changes, i.e., they slowly increase with increasing pressure until they reach maximum value at ∼25 MPa. For all the samples, seismic velocities are constant above 25 MPa, indicating a possible closure of microcracks at stress corresponding to 1.0 km–1.5 km. A reflection coefficient of 0.02 computed between hanging wall and footwall quartzites of the Carbon Leader Reef ore body suggests that it may be difficult to obtain a strong seismic reflection at their interface. Our modified seismic attribute algorithm, on the other hand, shows that the detection of the lateral continuity of the Carbon Leader Reef reflector can significantly be improved by sharpening the seismic traces. Three‐dimensional seismic data reveal that faults with throws greater than 25 m that offset the Carbon Leader Reef can clearly be seen. Faults with throws less than 25 m but greater than 2‐m throw were identified through horizon‐based attribute analysis, while most dykes and sills with thickness less than 25 m were invisible. The detection of the lateral continuity of the Carbon Leader Reef reflector and its depth position is greatly improved by integrating the modified instantaneous attributes with controls from borehole observations. The three‐dimensional visualization and effective interpretation of the Carbon Leader Reef horizon shows a host of structurally complex ore body blocks that may impact future shaft positioning and reduce its associated risks.
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Enhancing 3D post‐stack seismic data acquired in hardrock environment using 2D curvelet transform
Authors A. Górszczyk, M. Malinowski and G. BellefleurABSTRACTSeismic data acquired in hardrock environment pose a special challenge for processing. Frequent lack of clear coherent events hinders imaging and interpretation. Additional difficulty arises from the presence of significant amount of cultural noise associated with production and processing of ore, which often remains in the processed, stacked data. Motivated by those challenges, we developed an efficient workflow of denoising 3D post‐stack seismic data by using 2D discrete curvelet transform aimed at improving signal‐to‐noise ratio of the data. Our approach is based on the adjustment of the thresholds according to scales and angles in the curvelet domain, making parameterization flexible. We demonstrate effectiveness of our method using 3D post‐stack volumes from the three different mining camps in Canada, which were characterized by variable data quality. Remarkable signal enhancement, confirmed by the improvements in the mean signal‐to‐noise ratio of the dataset, is obtained not only due to random energy attenuation but also by removal of certain features corrupting the data (e.g., acquisition footprint). Comparison with the F‐X/F‐XY deconvolution results shows the superiority of our algorithm in respect to signal enhancement, signal preservation, and amount of the removed noise. Imaged structures, even if initially dominated by random energy, are easier to follow after curvelet denoising and enhanced for interpretation. Therefore, our approach can significantly reduce interpretation uncertainties when dealing with the seismic data acquired in the hardrock environment.
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Seismic imaging through the volcanic rocks of the Snake River Plain: insights from Project Hotspot
Authors Lee M. Liberty, Douglas R. Schmitt and John W. ShervaisABSTRACTHotspot: The Snake River Geothermal Drilling Project was undertaken to better understand geothermal systems across the Snake River Plain volcanic province. A series of surface and borehole seismic profiles were obtained to provide insights into volcanic stratigraphy and test the capabilities of engineering‐scale seismic imaging in such terranes. The Kimberly site drilled through 1.9 km of mostly rhyolite, with thin sedimentary interbeds in the upper part of the section. The Kimama site drilled through 1.9 km of mostly basalt with sedimentary interbeds at ∼200 m depth and 1700 m depth. The Mountain Home site contained numerous sediment and volcanic rock layers. Downhole and surface vibroseis seismic results suggest sedimentary interbeds at depth correspond with low‐velocity, high‐temperature zones that relate to reflections on seismic profiles. Our results suggest that eruption flow volumes can be estimated and flow boundaries can be imaged with surface seismic methods using relatively high‐fold and wide‐angle coverage. High‐frequency attenuation is observed at all sites, and this deficit may be countered by acquisition design and a focus on signal processing steps. Separation of surface and body waves was obtained by muting, and the potential for large static effects was identified and addressed in processing. An accurate velocity model and lithology contacts derived from borehole information improved the confidence of our seismic interpretations.
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3D fracture networks in the crystalline upper crust ‐ A new seismic model of the Continental Deep Drilling Site (South Germany)
Authors Eva Szalaiová, Katja Iwanowski‐Strahser and Wolfgang RabbelABSTRACTBased on three‐dimensional seismic reflection data, we present the first comprehensive three‐dimensional model of the fault and fracture inventory of the crystalline upper crust penetrated by the Continental Superdeep Drillhole (Kontinentale Tiefborung, Oberpfalz, Southeast Germany). The investigated volume spans ∼19×19 km2 down to 16‐km depth. It can be regarded as a typical example of metamorphic crust that has undergone numerous phases of ductile and brittle deformation since the start of the Variscan orogeny. We developed an automated workflow for identifying and quantifying the crustal fracture network in order to prepare a basis for an assessment of fluid pathways and geothermal potential of crystalline crust. The workflow comprises the following steps: determination of (i) a three‐dimensional model of major faults by structural tensor analysis and (ii) a three‐dimensional model of middle‐scale fractures by log‐Gabor filtering and image processing; (iii) validation of results by comparison with geophysical borehole data, with independent seismic and seismicity data; and (iv) definition of a three‐dimensional fracture density function serving for a statistical assessment of fracture connectivity and “relative permeability.” This assessment is based on probabilistic fractionation and percolation theories. The derived three‐dimensional distribution of “relative permeability” may be used as a kernel function for inverting hydraulic permeability from hydrothermal field experiments. By comparing borehole and three‐dimensional seismic data, we could confirm that the faults and fractures of the Kontinentale Tiefbohrung area follow a fractal law that is consistent with Turcotte's fractionation model. An important conclusion from this is that upscaling and downscaling between one‐dimensional borehole and three‐dimensional seismic data appear possible. The corresponding one‐, two‐, and three‐dimensional fractal dimension are about 0.8, 1.9, and 2.8, respectively.
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Seismic depth imaging of a geothermal system in Southern Tuscany
Authors Marko Riedel, Cora Dutsch, Catherine Alexandrakis, Ivano Dini, Simonetta Ciuffi and Stefan BuskeABSTRACTThis 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.
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3D seismic survey for geothermal exploration in crystalline rocks in Saxony, Germany
Authors Ewald Lüschen, Sascha Görne, Hartwig von Hartmann, Rüdiger Thomas and Rüdiger SchulzABSTRACTA 3D seismic survey was recorded in 2012 to explore a petrothermal reservoir in a late‐Variscan granitic pluton within the Erzgebirge (Ore Mountains) in Saxony, Germany. The main objective was to test this area in the context of the Enhanced Geothermal System concept and to test the 3D seismic technique as an exploration tool. The intention and challenge are to image and characterize potentially permeable fracture zones at target depths of 5–6 km, with temperatures above 150 °C. Unconventional methods were applied for field acquisition and data processing. The vibroseis technique was used in the core experiment, accompanied by a special explosive seismic experiment. Field acquisition was characterized by severe noise conditions and a highly irregular layout. These conditions required extensive preprocessing and data conditioning. The imaging started with conventional Common Midpoint processing for quality control and for a first reference. Better images were obtained by Common Reflection Surface processing with subsequent post‐stack time migration. Prestack time migration was also used for comparison. Outstanding results were obtained by the ‘operator‐oriented’ version of the Common Reflection Surface technique. A rich repertoire of structures within the granite pluton was imaged, including steeply dipping fault zones and conjugate faults. Images and indications of fracture and crack porosity of a prominent fault zone provide the background to define an optimum drill path. This is considered as the next stage for a possible geothermal plant, if a decision is taken to drill a research well in the future. The 3D seismic reflection technique was shown to be an indispensable tool for geothermal exploration, even in crystalline basement rocks.
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Application of the 3D common‐reflection‐surface stack workflow in a crystalline rock environment
Authors K. A. Ahmed, B. Schwarz and D. GajewskiABSTRACTSeismic data from crystalline or hardrock environments usually exhibit a poor signal‐to‐noise ratio due to low impedance contrasts in the subsurface. Moreover, instead of continuous reflections, we observe a lot of steeply dipping events resembling parts of diffractions. The conventional seismic processing (common midpoint stack and dip moveout) is not ideally suited for imaging such type of data. Common‐reflection‐surface stack processing considers more traces during the stack than common midpoint processing, and the resulting image displays a better signal‐to‐noise ratio. In the last decade, the common‐reflection‐surface stack method was established as a powerful tool to provide improved images, especially for low‐fold or noise‐contaminated data. The common‐reflection‐surface stack and all attributes linked to it are obtained using a coherence‐based automatic data‐driven optimization procedure. In this work we applied the common‐reflection‐surface stack workflow to 3D crystalline rock seismic data, which were acquired near Schneeberg, Germany, for geothermal exploration. The common‐reflection‐surface stack itself provided an image of good signal‐to‐noise ratio. However, for data from environments with low acoustic impedance and poor velocity information, coherence, which is automatically obtained in the optimization procedure, provides an alternative way to image the subsurface. Despite the reduced resolution, for these data, the coherence image provided the best results for an initial analysis. Utilized as a weight, the coherence attribute can be used to further improve the quality of the stack. By combining the benefits of a decreased noise level with the high‐resolution and high‐interference properties of waveforms, we argue that these results may provide the best images in an entirely data‐driven processing workflow for the Schneeberg data.
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Three‐dimensional focused seismic imaging for geothermal exploration in crystalline rock near Schneeberg, Germany
Authors Felix Hloušek, Olaf Hellwig and Stefan BuskeABSTRACTWe 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.
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Characterization of seismic reflections from faults in a crystalline environment, Schneeberg, Germany
Authors Lena Schreiter, Felix Hloušek, Olaf Hellwig and Stefan BuskeABSTRACTWe 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.
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