Seismic crosshole techniques are powerful tools to characterize the properties of near-surface aquifers. Knowledge of rock-physical relations at the field scale is essential for interpreting geophysical measurements. However, it remains difficult to extend the results of existing laboratory studies to the field scale due to the usage of different frequency ranges. To address this, we develop an experimental layout that successfully determines the dependency of gas saturation on seismic properties. Integrating geophysical measurements into a hydrogeologic research question allows us to prove the applicability of theoretical rock-physical concepts at the field scale, filling a gap in the discipline of hydrogeophysics. We use crosshole seismics to perform a time-lapse study on a gas injection experiment at the TestUM test site. With a controlled two-day gaseous [Formula: see text] injection at a depth of 17.5 m, we monitor the alteration of water saturation in the sediments over a period of 12 months, encompassing an observational depth of 8–13 m. The investigation contains an initial P-wave simulation followed by a data-based P-wave velocity analysis. Subsequently, we discuss different approaches to quantifying gas content changes by comparing Gassmann’s equation and the time-average relation. With the idea of patchy saturation, we discover that analyzing P-wave velocities in the subsurface is a suitable method for our experiment, resulting in a measurement accuracy of 0.2 vol%. We determine that our seismic crosshole setup is able to describe the relation of the rock’s elastic parameter on modified fluid properties at the field scale. With this method, we are able to quantify the relative water content changes in the subsurface.

Extensive mine dumps consisting of loosely deposited sands have been created as a result of open-pit lignite mining, with a risk of soil liquefaction under high water saturation and a corresponding initiating event. Soil compaction is one of the feasible methods for reducing the probability of liquefaction. For the monitoring of liquefaction events and the evaluation of compaction work, seismic survey methods with sensitivity to changes in soil saturation and structure may thus complement other methods. Compared to exploration methods for deep systems, the shallow subsurface presents some unique challenges. To this end, an open-source, customizable code based on Biot's theory was developed in the FEniCS library, which takes into account partial saturation and porosity dependence of stiffness, permeability, and other quantities. Following code verification, a comprehensive investigation of parameter studies is conducted, from which the effects of different factors on wave propagation characteristics were obtained. The numerical model was applied to simulate the expected changes in seismic response following soil compaction. Furthermore, the position of the high saturation area could be detected from the reflection and refraction P waves. The goal of this work is to provide an analysis framework for the assessment of compaction works and monitoring liquefiable soils in mine dumps under conditions of variable saturation due to rising groundwater tables.

Abstract. In complex geological settings, such as in hard-rock environments, Fresnel volume migration (FVM) has been successfully applied and found to deliver superior image quality compared to conventional imaging techniques. However, previous studies on FVM have mainly focused on obtaining kinematic seismic images, and the analysis of the migrated amplitudes has not received major attention. Therefore this study presents a method for constructing angle-domain common-image gathers (ADCIGs) and common-angle stacks from FVM, which can facilitate prestack amplitude analysis from the migrated seismic data in the angle-domain. These ADCIGs were constructed inside the migration loops using phase slowness vectors derived from traveltime gradient fields. We then tested this method on synthetic and field seismic data and investigated the reliability of the output for amplitude versus angle (AVA) analysis. The test results obtained showed that the AVA responses from the common-angle stacks resemble that of the input synthetic shot gather of migration relatively well, indicating the promising feasibility of AVA analysis from common-angle stacks. When implemented on field data acquired from a hard-rock environment, the proposed method can provide common-angle stacks with a higher signal-to-noise ratio and better reflection coherency compared to the common-angle stacks from the standard Kirchhoff prestack depth migration. This study extends the implementation of FVM toward amplitude analysis, which can help improve the feasibility of hard-rock characterization.

The polymetallic, hydrothermal deposit of the Freiberg mining district in the southeastern part of Germany is characterised by ore veins that are framed by Proterozoic orthogneiss. The ore veins consist mainly of quarz, sulfides, carbonates, barite and flourite, which are associated with silver, lead and tin. Today the Freiberg University of Mining and Technology is operating the shafts Reiche Zeche and Alte Elisabeth for research and teaching purposes with altogether 14 km of accessible underground galleries. The mine together with the most prominent geological structures of the central mining district are included in a 3D digital model, which is used in this study to study seismic acquisition geometries that can help to image the shallow as well as the deeper parts of the ore-bearing veins. These veins with dip angles between 40° and 85° are represented by triangulated surfaces in the digital geological model. In order to import these surfaces into our seismic finite-difference simulation code, they have to be converted into bodies with a certain thickness and specific elastic properties in a first step. In a second step, these bodies with their properties have to be discretized on a hexahedral finite-difference grid with dimensions of 1000 m by 1000 m in the horizontal direction and 500 m in the vertical direction. Sources and receiver lines are placed on the surface along roads near the mine. A Ricker wavelet with a central frequency of 50 Hz is used as the source signature at all excitation points. Beside the surface receivers, additional receivers are situated in accessible galleries of the mine at three different depth levels of 100 m, 150 m and 220 m below the surface. Since previous mining activities followed primarily the ore veins, there are only few pilot-headings that cut through longer gneiss sections. Only these positions surrounded by gneiss are suitable for imaging the ore veins. Based on this geometry, a synthetic seismic data set is generated with our explicit finite-difference time-stepping scheme, which solves the acoustic wave equation with second order accurate finite-difference operators in space and time. The scheme is parallelised using a decomposition of the spatial finite-difference grid into subdomains and Message Passing Interface for the exchange of the wavefields between neighbouring subdomains. The resulting synthetic seismic shot gathers are used as input for Kirchhoff prestack depth migration as well as Fresnel volume migration in order to image the ore veins. Only a top mute to remove the direct waves and a time-dependent gain to correct the amplitude decay due to the geometrical spreading are applied to the data before the migration. The combination of surface and in-mine acquisition helps to improve the image of the deeper parts of the dipping ore veins. Considering the limitations for placing receivers in the mine, Fresnel volume migration as a focusing version of Kirchhoff prestack depth migration helps to avoid migration artefacts caused by this sparse and limited acquisition geometry.

In this paper we solve the forward scattering problem in the first order Born approximation as it applies to the multipole borehole acoustic measurement configuration. This configuration consists of a multipole source and an array of multipole receivers, both deployed on a mandrel (The tool), and where the latter is axially offset from the former by a certain distance. The tool is deployed in a fluid-filled borehole, penetrating the earth formations. Assuming an arbitrary elastodynamic scatterer away from the fluid-filled borehole, we derive explicit expressions for the far-field scattered wavefield in the borehole fluid. In principle, these expressions could be used as a fast forward operator in the development of a full waveform inversion technique. Subsequently, to further our understanding, we derive analytical expressions for the borehole fluid scattered pressure at 'small' source-receiver offsets, and where the scatterer is represented by a Heaviside step change in elastodynamic formation properties, having an arbitrary orientation relative to the borehole axis. Our expressions apply to tools having a multipole source and receiver of order n. Such a source/receiver consists of 2n sources/receivers, located on the circumference of the tool and azimuthally separated from one another by π/n radians. All sources may be fired with equal polarity or adjacent sources may be fired with alternate polarity. In the latter case we compare our analytical solution with 3-D finite difference simulations. We do this for the practically relevant dipole (n = 1) and quadrupole (n = 2) excitation, for the (specular) PP, SVSV and SHSH borehole acoustic scattered pressure contributions. We obtain excellent results as long as the elastodynamic contrast between scatterer and embedding is small (<30%).

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2020Logging-while-drilling cross-multipole measurements in anisotropic mediaAuthors: Christoph DemmlerTim GeeritsOlaf HellwigStefan BuskeChristoph DemmlerTU Bergakademie FreibergSearch for more papers by this author, Tim GeeritsBaker HughesSearch for more papers by this author, Olaf HellwigTU Bergakademie FreibergSearch for more papers by this author, and Stefan BuskeTU Bergakademie FreibergSearch for more papers by this authorhttps://doi.org/10.1190/segam2020-3426574.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail AbstractIn this paper, we start by summarizing the fundamental results that apply to the borehole acoustic pressure due to an alternate polarity double multipole excitation in an isotropic formation and we define the four-component cross multipole data matrix. Subsequently, we generalize the concept of HTI dipole shear wave splitting to higher order cylindrical modes (i.e., higher order than dipole) in a mathematically rigorous fashion. This results in a generalized formula for Alford rotation. We verify our results using finite-difference (FD) modeling, and we propose a new inversion method to extract HTI formation parameters (i.e., fast and slow formation shear slowness and associated polarization directions) from four-component crossquadrupole measurements.Presentation Date: Tuesday, October 13, 2020Session Start Time: 1:50 PMPresentation Time: 4:20 PMLocation: 351DPresentation Type: OralKeywords: anisotropy, borehole acoustics, logging while drilling, finite difference, HTIPermalink: https://doi.org/10.1190/segam2020-3426574.1FiguresReferencesRelatedDetails SEG Technical Program Expanded Abstracts 2020ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2020 Pages: 3887 publication data© 2020 Published in electronic format with permission by the Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 30 Sep 2020 CITATION INFORMATION Christoph Demmler, Tim Geerits, Olaf Hellwig, and Stefan Buske, (2020), "Logging-while-drilling cross-multipole measurements in anisotropic media," SEG Technical Program Expanded Abstracts : 400-404. https://doi.org/10.1190/segam2020-3426574.1 Plain-Language Summary Keywordsanisotropyborehole acousticslogging while drillingfinite differenceHTIPDF DownloadLoading ...

A 10.5 km(2) 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.

A primary focus of geothermal seismic imaging is to map dipping faults and fracture zones that control rock permeability and fluid flow. Vertical seismic profiling (VSP) is therefore a most valuable means to image the immediate surroundings of an existing borehole to guide, for example, the placing of new boreholes to optimize production from known faults and fractures. We simulated 2D and 3D acoustic synthetic seismic data and processed it through to pre-stack depth migration to optimize VSP survey layouts for mapping moderately to steeply dipping fracture zones within possible basement geothermal reservoirs. Our VSP survey optimization procedure for sequentially selecting source locations to define the area where source points are best located for optimal imaging makes use of a cross-correlation statistic, by which a subset of migrated shot gathers is compared with a target or reference image from a comprehensive set of source gathers. In geothermal exploration at established sites, it is reasonable to assume that sufficient à priori information is available to construct such a target image. We generally obtained good results with a relatively small number of optimally chosen source positions distributed over an ideal source location area for different fracture zone scenarios (different dips, azimuths, and distances from the surveying borehole). Adding further sources outside the optimal source area did not necessarily improve the results, but rather resulted in image distortions. It was found that fracture zones located at borehole-receiver depths and laterally offset from the borehole by 300m can be imaged reliably for a range of the different dips, but more source positions and large offsets between sources and the borehole are required for imaging steeply dipping interfaces. When such features cross-cut the borehole, they are particularly difficult to image. For fracture zones with different azimuths, 3D effects are observed. Far offset source positions contribute less to the image quality as fracture zone azimuth increases. Our optimization methodology is best suited for designing future field surveys with a favorable benefit-cost ratio in areas with significant à priori knowledge. Moreover, our optimization workflow is valuable for selecting useful subsets of acquired data for optimum target-oriented processing.

Single-well imaging using borehole acoustic array data aims at producing a high-resolution image of geologic structures away from the borehole. However, due to the limited number of receivers and sometimes poor signal-to-noise ratio of reflected arrivals, standard migration methods can produce severe artifacts in migration results. These artifacts, such as migration smiles, deteriorate the overall image quality, they might overshadow weaker reflections and, last but not least, they might be misinterpreted as real structures. To overcome these problems, we use a recently developed pre-stack Coherence-Based Migration (CBM), which is a focusing extension of the standard Kirchhoff-type migration. CBM is able to effectively reduce migration artifacts and random, uncorrelated noise and therefore to improve the resulting image quality. The application to field data clearly demonstrates that the CBM method delivers a better quality image than the standard Kirchhoff-type migration. Presentation Date: Tuesday, October 18, 2016 Start Time: 9:15:00 AM Location: 141 Presentation Type: ORAL

3D geological underground models are often presented by vector data, such as triangulated networks representing boundaries of geological bodies and geological structures. Since models are to be used for numerical simulations based on the finite difference method, they have to be converted into a representation discretizing the full volume of the model into hexahedral cells. Often the simulations require a high grid resolution and are done using parallel computing. The storage of such a high-resolution raster model would require a large amount of storage space and it is difficult to create such a model using the standard geomodelling packages. Since the raster representation is only required for the calculation, but not for the geometry description, we present an algorithm and concept for rasterizing geological models on the fly for the use in finite difference codes that are parallelized by domain decomposition. As a proof of concept we implemented a rasterizer library and integrated it into seismic simulation software that is run as parallel code on a UNIX cluster using the Message Passing Interface. We can thus run the simulation with realistic and complicated surface-based geological models that are created using 3D geomodelling software, instead of using a simplified representation of the geological subsurface using mathematical functions or geometric primitives. We tested this set-up using an example model that we provide along with the implemented library.

ABSTRACTWe 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.

ABSTRACTWe 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.

This paper summarizes part of an ongoing feasibility study that investigates the possible use of the full elastic Born approximation in multipole borehole acoustics. As a first step we exclude the fluid-filled borehole with the motivation that one or two wavelengths away from the fluid-filled borehole, radiating borehole mode amplitudes (e.g., Stoneley wave, formation dipole wave, etc.) are small compared to body wave amplitudes (P-, SV- and SH-waves). Consequently, for scatterers one or two wavelengths away from the fluid-filled borehole, it suffices to only consider their interaction with body waves.In this paper we apply the contrast-source stress-velocity forward scattering (integral equation) formulation for solid configurations in its first order (Born-) approximation (De Hoop, 1995) assuming a multipole force source excitation in a zero-offset configuration. To scrutinize the validity of the Born approximation, we consider the simplest type of scatterer, i.e., one characterized by a (Heaviside) step function change in one or more of the contrast (perturbation) parameters and we derive analytic zero-offset formulas for the scattered wave particle velocity and displacement in both the space-frequency and space-time domain, respectively. We assume the scatterer to be located in the far-field. More complicated layered configurations can easily be derived by superposition of the given solution types. Explicit results are given for the dipole and quadrupole excitation, where the former is allowed to have an arbitrary orientation relative to the scatterer and where the latter one is located in a plane perpendicular to the scatterer. In the time domain it is shown, how the scattered wave field decomposes in a specular and diffuse wave field (two terms borrowed from 'Optics'), where the former contribution vanishes in the absence of an imaging condition and where the latter is always present. For the dipole case, we subject our results to a sensitivity analysis with respect to the three independent perturbation parameters (i.e., density and two compliance parameters) and we compare these results to a full waveform benchmark code that has implemented the reflectivity method (Kennett, 1983), for a 'horizontally' stratified elastic medium. This allowed us to pinpoint the root cause of the observed (small) differences. As it turns out these deviations could be traced back to the inaccurateness of the first order Born scattering coefficients. An additional confirmation of this fact is provided through a comparison between the zero-offset scattering coefficients and the corresponding Zoeppritz reflection coefficients. Most notably, it was found that the PP first order scattering coefficient needs a higher than quadratic correction in two of the three independent perturbation parameters, i.e., the two compliance parameters, delta Lambda and delta M. With respect to the density perturbation parameter (delta rho) the PP scattering coefficient correction is quadratic with respect to this perturbation parameter, as is to be expected for a first order approximation. Moreover, also the SS first order scattering coefficient only needs a quadratic correction with respect to its associated perturbation parameters, i.e., delta rho and delta M.Finally, we give a brief outline on how to numerically implement the Born approximation (employing arbitrary offsets) in a configuration where a source-receiver pair is moving continuously relative to the 'contrast' (Geology), as is the case in borehole acoustic applications. (c) 2012 Elsevier B.V. All rights reserved.

In recent years the emphasis in acoustic logging has been shifting from the wireline to the Logging While Drilling (LWD) environment, the latter being far different from the former in that both tool rigidity and tool radius are considerably greater. In this paper we present a generic mathematical formulation for the multipole borehole acoustic measurement (alternate and equal polarity case), including a detailed analysis on the effects of multipole source amplitude imbalances. It is shown that source imbalance induced mode contaminants have excitation amplitudes that are scaled by the sum of the relative source imbalances between diametrically opposed sources. Furthermore it is shown that for mode contaminants with odd modal number there can be a significant offset in the associated directivity pattern, even at low levels of source imbalance. However, it is also shown that source imbalance induced mode contaminants can be completely eliminated if the multipole source is accompanied with a 'vertically' (but not azimuthally) offset multipole receiver. Mathematically, it is demonstrated (for a centered tool) that a polarity weighted stack of these multipole receivers completely eliminates the source imbalance induced mode contaminants. Excitation amplitudes and phase slowness of borehole guided modes are presented for the most common excitation regimes, i.e., monopole, dipole, quadrupole and the hexapole excitation, the latter one showing to have the advantage of a higher formation shear cut-off frequency than the quadrupole mode. Special emphasis will be on the analysis of the dipole excitation and the differences that occur due to variations in tool rigidity, tool diameter and (isotropic) formation properties with the resulting conclusion that a formation flexural mode is not observable as a result of a LWD dipole excitation (this opposed to its dipole wireline counterpart). The guided mode excitation amplitudes are calculated as residues using a Laurent series expansion. This (unconventional) way of calculating the residue has the advantage that it is independent of the pole order, does not require the numerical evaluation of derivatives with respect to the vertical wavenumber and allows for an accurate and efficient FFT implementation.