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- Volume 66, Issue 9, 2018
Geophysical Prospecting - Volume 66, Issue 9, 2018
Volume 66, Issue 9, 2018
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Trace‐imposed stretch correction in Kirchhoff prestack time migration
Authors Qiancheng Liu and Jianfeng ZhangABSTRACTKirchhoff prestack time migration, which works trace‐by‐trace, remains appealing to academia and industry due to its robustness and efficiency. However, like the other prestack migration methods, Kirchhoff prestack time migration also suffers from the angle‐dependent stretching effect, which narrows the amplitude spectra of seismic data. This effect gets more severe as the incident angle increases. In this paper, we propose a novel approach, which attaches a prediction shaping filter to the Kirchhoff prestack time migration, to mitigate the stretching effect. Our approach takes advantage on the trace‐by‐trace implementation of the prestack time migration algorithm, without the output of the angle‐domain common‐imaging gather. Also, our method can cascade with Q‐compensation in prestack time migration. We demonstrate our method with both a numerical example and a field data example.
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Marchenko imaging by unidimensional deconvolution
Authors Mayara M. A. Matias, Reynam da C. Pestana and Joost van der NeutABSTRACTObtaining an accurate image of the subsurface still remains a great challenge for the seismic method. Migration algorithms aim mainly on positioning seismic events in complex geological contexts. Multiple reflections are typically not accounted for in this process, which can lead to the emergence of artefacts. In Marchenko imaging, we retrieve the complete up‐ and downgoing wavefields in the subsurface to construct an image without such artefacts. The quality of this image depends on the type of imaging condition that is applied. In this paper, we propose an imaging condition that is based on stabilized unidimensional deconvolution. This condition is computationally much cheaper than multidimensional deconvolution, which has been proposed for Marchenko imaging earlier. Two specific approaches are considered. In the first approach, we use the full up‐ and downgoing wavefields for deconvolution. Although this leads to balanced and relatively accurate amplitudes, the crosstalk is not completely removed. The second approach is to incorporate the initial focussing function in the deconvolution process, in such a way that the retrieval of crosstalk is avoided. We compare images with the results of the classical cross‐correlation imaging condition, which we apply to reverse‐time migrated wavefields and to the up‐ and downgoing wavefields that are retrieved by the Marchenko method.
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Prediction of sonic velocities from other borehole data: An example from the Kevitsa mine site, northern Finland
Authors Duy Thong Kieu, Anton Kepic and Maria Cornelia KitzigABSTRACTP‐wave and S‐wave velocities are vital parameters for the processing of seismic data and may be useful for geotechnical studies used in mine planning if such data were collected more often. Seismic velocity data from boreholes increase the robustness and accuracy of the images obtained by relatively costly seismic surface reflection surveys. However, sonic logs are rarely acquired in boreholes in‐and‐near base metal and precious metal mineral deposits until a seismic survey is planned, and only a few new holes are typically logged because the many hundreds of holes previously drilled are no longer accessible. If there are any pre‐existing petrophysical log data, then the data are likely to consist of density, magnetic susceptibility, resistivity and natural gamma logs. Thus, it would be of great benefit to be able to predict the velocities from other data that is more readily available.
In this work, we utilize fuzzy c‐means clustering to build a “fuzzy” relationship between sonic velocities and other petrophysical borehole data to predict P‐wave and S‐wave velocity. If boreholes with sonic data intersect most of the important geological units in the area of interest, then the cluster model developed may be applied to other boreholes that do not have sonic data, but do have other petrophysical data to be used for predicting the sonic logs. These predicted sonic logs may then be used to create a three‐dimensional volume of velocity with greater detail than would otherwise be created by the interpolation of measured sonic data from sparsely located holes.
Our methodology was tested on a dataset from the Kevitsa Ni‐Cu‐PGE deposit in northern Finland. The dataset includes five boreholes with wireline logs of P‐wave velocity, S‐wave velocity, density, natural gamma, magnetic susceptibility and resistivity that were used for cluster analysis. The best combination of input data for the training section was chosen by trial and error, but differences in the misfit between the various training datasets were not particularly significant. Our results show that the fuzzy c‐means method can predict sonic velocities from other borehole data very well, and the fuzzy c‐means method works better than using multiple linear‐regression fitting. The predicted P‐wave velocity data are of sufficient quality to robustly add low‐frequency information for seismic impedance inversion and should provide better velocity models for accurate depth conversion of seismic reflection data.
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Seismic deconvolution and inversion with erratic data
Authors Ronghuo Dai, Cheng Yin, Shasha Yang and Fanchang ZhangABSTRACTIf there are some erratic data (e.g. outliers), which may arise from measurement error, or other reasons, in seismic data, the seismic deconvolution and inversion need to be implemented in a way that minimizes their effects. However, the deconvolution and inversion methods based on L2‐norm misfit function are highly sensitive to these erratic seismic observations. As an alternative, L1‐norm misfit functions are more robust and erratic‐resistant. In order to find the solution of the inverse problem constrained by an L1‐norm misfit function, an iteratively re‐weighted least squares algorithm is used frequently. However, it is relatively time consuming. In this paper, we propose a new method based on the sparse signal representation theory. The overcomplete dictionary used for the sparse representation of seismic data with erratic data is composed of two bases: a wavelet basis used for representing the seismic data to implement deconvolution and a Dirac basis used for representing the erratic data. In addition, at the stage of seismic inversion after deconvolution, total variation and a priori model are used as the regularization constraint terms to estimate inversion results with a blocky and laterally continuous structure. The new method is successfully tested on the noisy synthetic seismic data with erratic data. Finally, the proposed method is performed on a real seismic data section, and the inversion results are reasonable, i.e. consistent with the geologic structure of the original seismic data. Compared to the conventional sparse deconvolution and inversion method, the proposed method not only eliminates the effect of outliers, but also has highly improved computational efficiency.
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A method for deghosting of data recorded with a streamer of arbitrary shape in rough sea conditions
Authors Mikhail Denisov, Anton Egorov and Alexander BurtsevABSTRACTDeghosting of pressure‐only data has become a routine in marine seismic processing. Most existing techniques suffer from noise susceptibility or excessive simplification of the used ghost model, which leads to difficulties in removing the ghost waves. The algorithm presented in this paper is based on the wavefield extrapolation theory, and is capable of taking into account arbitrary streamer shapes and rough sea surfaces. The computations are performed in the frequency domain and come down to solving systems of linear equations. Regularization and data‐adaptive statistical optimization of the parameters prevent noise amplification. We describe the theory of the method and test it against synthetic and field datasets with different streamer shapes for both rough and flat sea surfaces.
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Efficient dip‐angle angle‐domain common‐image gather estimation using Poynting vector in acoustic reverse time migration and its application in noise suppression
Authors Qiancheng Liu and Jianfeng ZhangABSTRACTIn reverse time migration, we can produce angle‐domain common‐imaging gathers in either the scattering‐angle domain or the dip‐angle domain. The latter, although not used as widely as the former, still provides a unique view to look into seismic imaging. The reverse time migration dip‐angle angle‐domain common‐image gather can be computed via the subsurface offset extension, which is a bit costly in storage. We here obtain dip‐angle angle‐domain common‐imaging gathers efficiently in acoustic reverse time migration by using the normalized Poynting vectors. Poynting vector, an indicator of the directional energy flux, is easy to compute during wavefield propagation. Similar to the subsurface‐offset method, our method also has dip‐angle angle‐domain common‐imaging gathers of blocky horizontal coherence. In the theory of local semblance analysis, the signal with better horizontal coherence has a higher semblance score, and vice versa. Based on the estimated semblance scores, we can thus design a specularity filter to preserve valid signals and suppress noises. We validate our method with two data sets. Both the Marmousi data and the real data show that our method works effectively in eliminating noises and enhancing resolution in the acoustic reverse time migration imaging.
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Estimation of grain elasticity properties from ultrasonic measurements on dry granular pack
Authors Mahyar Madadi, Zubair Ahmed, Andrej Bona, Maxim Lebedev and Boris GurevichABSTRACTThe elastic properties of crystalline rocks can be estimated from ultrasonic measurements on the powders of crushed rocks produced by the drilling process. To determine the elastic properties of grains from properties of powder packs, we study the dependence of their ultrasonic wave velocities on pressure. From this dependency, using the Hertz–Mindlin theory, we can calculate the effective ratio of the grain shear modulus to one minus the Poisson ratio. The Hertz–Mindlin theory requires the knowledge of grain coordination number as a function of pressure, which can be obtained using an empirical relation based on published numerical simulations. Previous work has shown that this approach gives an accurate prediction of the effective bulk modulus of glass beads but produces a significant discrepancy for sand. This discrepancy may be attributed to the angularity of sand grains. To overcome this problem, we introduce a shape factor into the empirical relation for the coordination number. This new shape factor allows us to reconcile the rock physics model with laboratory measurements. We show that the shape factor varies from 1 to 2.5 for different grain shape angularity and sorting (grain size distributions). The modified theory allows us to estimate a combination of elasticity parameters of the grains from the measured dependence of P‐wave velocity in the pack on the pressure.
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Evaluation of groundwater potential in Kabirwala area, Pakistan: A case study by using geophysical, geochemical and pump data
Authors Muhammad Hasan, Yanjun Shang, Gulraiz Akhter and Weijun JinABSTRACTKnowledge of hydraulic parameters (hydraulic conductivity and transmissivity) is essential for the delineation of groundwater potential zones. Conventionally, these parameters are measured using pumping tests carried out on boreholes. However, pumping tests are costly, labour intensive and require a considerable amount of equipment. The integration of geophysical methods with pumping tests provides efficient and cost‐effective alternative to calculate hydraulic parameters. Fifty electrical resistivity soundings were carried out in the study area using Schlumberger inter‐electrode configuration to obtain hydraulic characteristics that are estimated through the pumping tests. To apply this approach successfully, sufficient number of boreholes are used. Part of the boreholes, in which pumping tests were carried out, is used for both to constrain resistivity inversions and to establish the empirical relationship between the interpreted geophysical and hydraulic parameters. The rest of the boreholes without pumping tests are still used for constraining the inversions. Initially, aquifer parameters were measured using pumping tests at 12 water wells. Afterwards, transmissivity (T) and hydraulic conductivity (K) were correlated with transverse resistance (Tr) and the bulk resistivity (ρo) of the aquifer at other sites where pumping tests had not been conducted. In this way, the entire study area was covered to assess the groundwater reserves. The hydraulic properties obtained by the geophysical method fit pretty well to both the pumping and physicochemical data of the investigated area. The integrated study reveals five layers (i.e. topsoil, clay, clay sand, sand and gravel sand) and three potential zones (i.e. high, medium and low potential aquifer zones) with specific ranges of T, K, Tr and ρo. The results suggest that, in case of sparse well data, the aquifer parameters can be estimated using the relations depending on the specifications of the area.
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Exploration of Iron ore deposits in Patagonia. Insights from gravity, magnetic and SP modelling
Authors Rodolfo Christiansen, José Kostadinoff, Julia Bouhier and Patricia MartinezABSTRACTThe Sierra Grande region in northern Patagonia is considered the largest iron ore reserve in Argentina; however, the extension of the non‐outcropping deposits as well as the depth of the basins that contain them remains unknown. Utilizing 3D litho‐constrained inversion of gravity and magnetic data, we delimited an area with good prospects for iron ore deposits. In this region, high‐resolution magnetic and self‐potential profiles were acquired over the most important anomalies. Correlating both methodologies, it was possible to specify the possible existence of iron oxides (martite–hematite) in the form of 2D inclined sheets.
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Three‐dimensional magnetotelluric responses for arbitrary electrically anisotropic media and a practical application
Authors Guo Yu, Qibin Xiao, Guoze Zhao and Man LiABSTRACTElectrical anisotropy in earth media increases the complexity of magnetotelluric responses. Magnetotelluric models based on anisotropic media must be developed to fully understand observed data. This paper presents a three‐dimensional algorithm for calculating magnetotelluric responses of arbitrary anisotropic media in the frequency domain. Using a staggered‐grid finite difference method, the model space is discretized into rectangular blocks with electric fields on the edges of each block and the magnetic fields normal to the faces of each block. The electric field Helmholtz vector equation that considers a full 3 × 3 conductivity tensor is calculated numerically under two orthogonal polarizations. In calculating the boundary values on the four sides of the three‐dimensional anisotropic model, we adopt different procedures for calculating the two‐dimensional responses on the sides in the x and y directions. The responses for a layered anisotropic model and a three‐dimensional isotropic model calculated with this algorithm are compared with the corresponding analytical and numerical solutions, respectively. The comparisons show that the algorithm's approximations are highly precise for a wide frequency band. A typical two‐dimensional anisotropic model and a general three‐dimensional anisotropic model were also constructed, and their responses were calculated. These anisotropic models have ordinary structures but can produce phase rolling out of quadrant magnetotelluric responses, which indicates that considering electrical anisotropy may improve our interpretation of observed data. Using this algorithm, we can model the observed data from the northern Qaidam Basin in northern Tibet, where ultrahigh‐pressure metamorphic rocks are exposed along an old suture, and seismic anisotropy was indicated in neighbouring areas. The phase tensors of the magnetotelluric sites at this location show large skew angles, and the corresponding phase splits are distinct in the off‐diagonal impedance elements. Although the isotropic three‐dimensional electrical structure can model the profile data well, the structure shows a sequence of conductive and resistive bodies in the mid‐lower crust of the north Qaidam Basin, which is very spatially inhomogeneous, and a simple intrinsic anisotropic body can also produce similar surficial responses. Using the three‐dimensional anisotropic algorithm, we found an equivalent anisotropic replacement for this area. The results of the three‐dimensional anisotropy modelling of the magnetotelluric data from the northern Tibetan Plateau show the valuable applicability of the three‐dimensional anisotropic algorithm in testing the qualitative presumption of electrical anisotropy.
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Gravity signature and physical properties of copper deposit in the Curaçá Valley, northern Bahia/Brazil – case study
More LessABSTRACTThis paper aims at analysing the application of the gravimetric method in the search for copper ore in the Valley of Curaçá River in northern Bahia, Brazil. The area where this study was carried out is known as Angico Farm, one of the claims of Caraíba S.A., a copper producer in the northern Bahia, Brazil. There are 18 drill holes at the Angico Farm target, drilled in order to investigate the mineralizations in depth. We have obtained information such as geographic coordinates and chemical results from the company in order to test the geophysical response and the correlation with geology. A 3D inverse gravimetric model was generated in order to verify the validity of the method in exploring for copper ore associated with hydrothermally altered mafic and ultramafic. Both mafic/ultramafic rocks and copper ore present high density, therefore the gravity method may not be effective for identification. We have shown, however, that copper ore from the Curaçá Valley presents a fairly good gravity response, and 3D inverse mathematical model pointed out a well‐delimited copper orebody in the regions where drill holes intersect the ore and coincide with the positive gravity anomaly. The ore contents were overlapped on cross‐sections of density extracted from the inverse model and such information helped us to check out the consistency of the gravimetric method in mapping and modelling mineralized bodies associated with mineral occurrence. Additionally, magnetic susceptibility and gammaspectrometric data were acquired along 18 drillcores to investigate their possible correlation with orebodies.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 67 (2019)
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Volume 66 (2018)
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