| 1. |
- Andersson, Fredrik, et al.
(author)
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Directional interpolation of multicomponent data
- 2017
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In: Geophysical Prospecting. - : Wiley. - 0016-8025. ; 65:5, s. 1246-1263
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Journal article (peer-reviewed)abstract
- A method for interpolation of multicomponent streamer data based on using the local directionality structure is presented. The derivative components are used to estimate a vector field that locally describes the direction with the least variability. Given this vector field, the interpolation can be phrased in terms of the solution of a partial differential equation that describes how energy is transported between regions of missing data. The approach can be efficiently implemented using readily available routines for computer graphics. The method is robust to noise in the measurements and particularly towards high levels of low-frequent noise that is present in the derivative components of the multicomponent streamer data.
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| 2. |
- Andersson, Fredrik, et al.
(author)
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Extended structure tensors for multiple directionality estimation
- 2013
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In: Geophysical Prospecting. - : Wiley. - 1365-2478 .- 0016-8025. ; 61:6, s. 1135-1149
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Journal article (peer-reviewed)abstract
- Standard structure tensors provide a robust way of directionality estimation of waves (or edges) but only for the case when they do not intersect. In this work, a structure tensor extension using a one-way wave equation is proposed as a tool for estimating directionality in seismic data and images in the presence of conflicting dips. Detection of two intersecting waves is possible in a two-dimensional case. In three dimensions both two and three intersecting waves can be detected. Moreover, a method for directionality filtering using the estimated directions is proposed. This method makes use of the ideas of a one-way wave equation but can be applied to generic images not related to wave propagation.
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| 3. |
- Balestrini, Florencia, et al.
(author)
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Improved target illumination at Ludvika mines of Sweden through seismic-interferometric surface-wave suppression
- 2020
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In: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 68:1, s. 200-213
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Journal article (peer-reviewed)abstract
- In mineral exploration, new methods to improve the delineation of ore deposits at depth are in demand. For this purpose, increasing the signal-to-noise ratio through suitable data processing is an important requirement. Seismic reflection methods have proven to be useful to image mineral deposits. However, in most hard rock environments, surface waves constitute the most undesirable source-generated or ambient noise in the data that, especially given their typical broadband nature, often mask the events of interest like body-wave reflections and diffractions. In this study, we show the efficacy of a two-step procedure to suppress surface waves in an active-source reflection seismic dataset acquired in the Ludvika mining area of Sweden. First, we use seismic interferometry to estimate the surface-wave energy between receivers, given that they are the most energetic arrivals in the dataset. Second, we adaptively subtract the retrieved surface waves from the original shot gathers, checking the quality of the unveiled reflections. We see that several reflections, judged to be from the mineralization zone, are enhanced and better visualized after this two-step procedure. Our comparison with results from frequency-wavenumber filtering verifies the effectiveness of our scheme, since the presence of linear artefacts is reduced. The results are encouraging, as they open up new possibilities for denoising hard rock seismic data and, in particular, for imaging of deep mineral deposits using seismic reflections. This approach is purely data driven and does not require significant judgment on the dip and frequency content of present surface waves, which often vary from place to place.
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| 4. |
- Bastani, Mehrdad, et al.
(author)
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A radio magnetotelluric study to evaluate the extents of a limestone quarry in Estonia
- 2013
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In: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 61:3, s. 678-687
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Journal article (peer-reviewed)abstract
- Electromagnetic signals from distant radio transmitters in the frequency range 15250 kHz were measured to model an electrical resistivity structure beneath 7 profiles in the vicinity of the Karinu limestone quarry in Estonia with the aim to map the extent of the economically exploitable limestone. The resistivity models from a 2D inversion of determinant resistivity and phase values using an Occam type of regularization contained reasonably accurate information about the geometry, namely depth to the top and the bottom of the target high-resistivity limestone. The resistivity models correlated well with existing geological evidences as well as information from closely located boreholes. However, the sharp lithological boundaries seen in the boreholes were not resolved exactly in the resistivity models. This is probably because of the smoothing regularization used in the inversion process. Combined use of borehole data together with resistivity models resulted in two major geological interpretations; a) towards the western part of the existing limestone quarry there is a NNW to NS striking fault, covered by post-glacial sediments, b) a potential cost-effective exploitable area containing high quality highly resistive limestone is located south of the existing quarry. This case study shows the applicability of the reasonably fast radio magnetotelluric (RMT) method for the exploration of near-surface resources.
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| 5. |
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| 6. |
- Beiki, Majid, et al.
(author)
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Estimating magnetic dike parameters using a non-linear constrained inversion technique : an example from the Sarna area, west central Sweden
- 2012
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In: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 60:3, s. 526-538
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Journal article (peer-reviewed)abstract
- In this paper, we describe a non-linear constrained inversion technique for 2D interpretation of high resolution magnetic field data along flight lines using a simple dike model. We first estimate the strike direction of a quasi 2D structure based on the eigenvector corresponding to the minimum eigenvalue of the pseudogravity gradient tensor derived from gridded, low-pass filtered magnetic field anomalies, assuming that the magnetization direction is known. Then the measured magnetic field can be transformed into the strike coordinate system and all magnetic dike parameters horizontal position, depth to the top, dip angle, width and susceptibility contrast can be estimated by non-linear least squares inversion of the high resolution magnetic field data along the flight lines. We use the Levenberg-Marquardt algorithm together with the trust-region-reflective method enabling users to define inequality constraints on model parameters such that the estimated parameters are always in a trust region. Assuming that the maximum of the calculated gzz (vertical gradient of the pseudogravity field) is approximately located above the causative body, data points enclosed by a window, along the profile, centred at the maximum of gzz are used in the inversion scheme for estimating the dike parameters. The size of the window is increased until it exceeds a predefined limit. Then the solution corresponding to the minimum data fit error is chosen as the most reliable one. Using synthetic data we study the effect of random noise and interfering sources on the estimated models and we apply our method to a new aeromagnetic data set from the Sarna area, west central Sweden including constraints from laboratory measurements on rock samples from the area.
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| 7. |
- BELAY, R, et al.
(author)
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ONE-DIMENSIONAL NONLINEAR INVERSION OF MAGNETOTELLURIC DATA - THE IMPORTANCE OF DATA ERRORS
- 1995
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In: GEOPHYSICAL PROSPECTING. - : BLACKWELL SCIENCE LTD. - 0016-8025. ; 43:7, s. 905-918
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Journal article (other academic/artistic)abstract
- Inversion of noisy magnetotelluric data over a horizontally stratified earth has no unique solution. Instead, the resistivities and thicknesses of the layers can only be specified to lie within certain bounds at a particular confidence level. The signific
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| 8. |
- Belay, R., et al.
(author)
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One‐dimensional non‐linear inversion of magnetotelluric data : the importance of data errorrs
- 1995
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In: Geophysical Prospecting. - : Wiley-Blackwell. - 0016-8025 .- 1365-2478. ; 43:7, s. 905-918
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Journal article (peer-reviewed)abstract
- Inversion of noisy magnetotelluric data over a horizontally stratified earth has no unique solution. Instead, the resistivities and thicknesses of the layers can only be specified to lie within certain bounds at a particular confidence level. The significance of having correctly estimated data errors when calculating the parameters and parameter bounds is discussed. Emphasis is put on the very corrunon problem of not being able to obtain models which can be considered acceptable with a reasonable certainty from a statistical point of view. A method is presented to obtain a pragmatic data error description by adjustment of wrongly estimated data errors while keeping the suppression of 1D data features at a minimum. A comparison is made between calculation of the parameter bounds based on approximating the confidence surface by hyperellipsoids and a calculation based on a full non‐linear most‐squares analysis. In general it is found that the approximation works very well when the confidence limit is small. However, significant deviations are found in some cases.
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| 9. |
- Braunig, Lena, et al.
(author)
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Seismic depth imaging of iron-oxide deposits and their host rocks in the Ludvika mining area of central Sweden
- 2020
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In: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 68:1, s. 24-43
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Journal article (peer-reviewed)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 data set 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 data set 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 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.
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| 10. |
- Brown, R.J., et al.
(author)
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Weighted vertical stacking in crustal seismic reflection studies on the Canadian shield
- 1977
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In: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 25:2, s. 251-268
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Journal article (peer-reviewed)abstract
- Seismic reflection methods are being developed at the University of Manitoba to aid in determining fine crustal structures in the Precambrian of Manitoba and northwestern Ontario. Present-day environmental concern as well as mineshaft conditions necessitate the detonation of several smaller charges repeated, say, I times and followed by ''vertical'' stacking. To obtain the familiar ..sqrt..I improvement in signal-to-noise (S : N) amplitude ratio applying the straight-sum (SS) method, one assumes, among other things, that both S : N ratio and signal variance are the same on all traces. Dropping these assumptions, as we must for our data, it becomes necessary to apply weighting coefficients to optimize the S : N ratio of the stacked trace. We still assume the signal shapes to be the same for repeated shots, so for the jth trace on the record of the ith shot we model the time series as: t/sub ij/ = a/sub i/(s/sub j/ + n/sub ij/); where a/sub i/ is a scaling factor. The proper weights w/sub i/ are then shown to be proportional to sigma/sub si//sigma/sup 2//sub ni/ where sigma/sup 2/ is variance, or to ..gamma../sub i//a/sub i/ where ..gamma../sub i/ is S : N power ratio. Applying the weighted-stack (WS) method gives S : N amplitude ratios which are, on average, 55% of the optimal ratios expected from WS theory compared with only 24% for the SS method. The 45% shortfall in WS performance is ascribed mainly to trace-alignment (or time-delay) errors. Varying noise levels on individual traces, slight dissimilarity of signal shape, and correlated noise may also contribute to a lesser extent (in decreasing order of significance). This WS method appears to strike a good practical balance between S : N improvement and processing efficiency.
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