| 61. |
- Johansson, Sara, et al.
(author)
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An alternative methodology for the analysis of electrical resistivity data from a soil gas study
- 2011
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In: Geophysical Journal International. - 0956-540X. ; 186:2, s. 632-640
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Journal article (peer-reviewed)abstract
- The aim of this paper is to present an alternative method for the analysis of resistivity data. The methodology was developed during a study to evaluate if electrical resistivity can be used as a tool for analysing subsurface gas dynamics and gas emissions from landfills. The main assumption of this study was that variations in time of resistivity data correspond to variations in the relative amount of gas and water in the soil pores. Field measurements of electrical resistivity, static chamber gas flux and weather data were collected at a landfill in Helsingborg, Sweden. The resistivity survey arrangement consisted of nine lines each with 21 electrodes in an investigation area of 16 x20 m. The ABEM Lund Imaging System provided vertical and horizontal resistivity profiles every second hour. The data were inverted in Res3Dinv using L-1-norm-based optimization method with a standard least-squares formulation. Each horizontal soil layer was then represented as a linear interpolated raster model. Different areas underneath the gas flux measurement points were defined in the resistivity model of the uppermost soil layer, and the vertical extension of the zones could be followed at greater depths in deeper layer models. The average resistivity values of the defined areas were calculated and plotted on a time axis, to provide graphs of the variation in resistivity with time in a specific section of the ground. Residual variation of resistivity was calculated by subtracting the resistivity variations caused by the diurnal temperature variations from the measured resistivity data. The resulting residual resistivity graphs were compared with field data of soil moisture, precipitation, soil temperature and methane flux. The results of the study were qualitative, but promising indications of relationships between electrical resistivity and variations in the relative amount of gas and water in the soil pores were found. Even though more research and better data quality is necessary for verification of the results presented here, we conclude that this alternative methodology of working with resistivity data seems to be a valuable and flexible tool for this application.
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| 62. |
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| 63. |
- Johansson, Sara, et al.
(author)
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Spectral induced polarization of limestones: time domain field data, frequency domain laboratory data and physicochemical rock properties
- 2020
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 220:2, s. 928-950
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Journal article (peer-reviewed)abstract
- With advances in data acquisition and processing methods, spectral inversion of time domain (TD) induced polarization (IP) data is becoming more common. Geological interpretation of inverted spectral parameters, for instance Cole–Cole parameters, often relies on results from systematic laboratory measurements. These are most often carried out with frequency domain (FD) systems on sandstone samples. However, the two different methods of measuring the spectral IP response differ in both measurement technique and scale. One of the main objectives of this study is, thus, to perform a direct comparison of inverted spectral parameters from TD IP field data with FD IP spectra from laboratory measurements. To achieve this, field measurements were carried out before a ∼50-m-long rock core was drilled down along one of the measurement lines. Solid parts of the core were vacuum-sealed in plastic bags to preserve the natural groundwater in the samples, after which the core samples were measured with FD spectral IP in laboratory. The results showed that the inverted Cole–Cole parameters closest to the borehole were comparable to the IP spectra measured at the core samples, despite differences in measurement techniques and scale. The field site chosen for the investigation was a limestone succession spanning over a well-known lithological boundary (the Cretaceous–Palaeogene boundary). Little is known in previous research about varying spectral IP responses in limestones, and an additional objective of this study was, therefore, to investigate possible sources of these variations in the laboratory. The IP spectra were interpreted in light of measured lithological and physicochemical properties. The carbonate texture differed strongly across the Cretaceous–Palaeogene boundary from fine-grained calcareous mudstone (Cretaceous) to more well-lithified and coarse-grained wackestone and packstone (Palaeogene). Both laboratory and field spectral IP results showed that these differences cause a large shift in measured bulk conductivity across the boundary. Furthermore, carbonate mound structures with limestone grains consisting mainly of cylindrical bryozoan fragments could be identified in the inverted Cole–Cole parameters as anomalies with high relaxation times. A general conclusion of this work is that limestones can give rise to a wide range of spectral responses. The carbonate texture and the dominant shape of the fossil grains seem to have important control over the electrical properties of the material. A main conclusion is that the inverted Cole–Cole parameters from the field scale TD IP tomography were comparable to the magnitude and shape of FD IP spectra at low frequencies. This opens up large interpretational possibilities, as the comprehensive knowledge about relationships between lithological properties and IP spectra from laboratory research can be used for field data interpretation.
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| 64. |
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| 65. |
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| 66. |
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| 67. |
- JOKAT, W, et al.
(author)
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NEW INSIGHTS INTO THE EVOLUTION OF THE LOMONOSOV RIDGE AND THE EURASIAN BASIN
- 1995
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In: GEOPHYSICAL JOURNAL INTERNATIONAL. - : BLACKWELL SCIENCE LTD. - 0956-540X. ; 122:2, s. 378-392
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Journal article (other academic/artistic)abstract
- The international expedition ARCTIC'91 has yielded new results concerning the evolution of the Eurasian Basin. Seismic reflection data were acquired with a total line length of 1500km in the Eurasian Basin. The signals imaged the whole sedimentary cover d
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| 68. |
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| 69. |
- Joud, Mehdi S. Shafiei, et al.
(author)
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Use of GRACE Data to Detect the Present Land Uplift Rate in Fennoscandia
- 2017
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 209:2, s. 909-922
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Journal article (peer-reviewed)abstract
- After more than 13 years of GRACE monthly data, the determined secular trend of gravity field variation can be used to study the regions of glacial isostatic adjustment (GIA). Here we focus on Fennoscandia where long-term terrestrial and high-quality GPS data are available, and we study the monthly GRACE data from three analysis centres. We present a new approximate formula to convert the secular trend of the GRACE gravity change to the land uplift rate without making assumptions of the ice load history. The question is whether the GRACE-derived land uplift rate by our method is related to GIA. A suitable post-processing method for the GRACE data is selected based on weighted RMS differences with the GPS data. The study reveals that none of the assumed periodic changes of the GRACE gravity field is significant in the estimation of the secular trend, and they can, therefore, be neglected. Finally, the GRACE-derived land uplift rates are obtained using the selected post-processing method, and they are compared with GPS land uplift rate data. The GPS stations with significant differences were marked using a statistical significance test. The smallest RMS difference (1.0 mm/a) was obtained by using GRACE data from the University of Texas.
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| 70. |
- Ju, Ma, et al.
(author)
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Moment tensor inversion with three-dimensional sensor configuration of mining induced seismicity (Kiruna mine, Sweden)
- 2018
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In: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 213:3, s. 2147-2160
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Journal article (peer-reviewed)abstract
- Mining induced seismicity is an undesired consequence of mining operations, which poses significant hazard to miners and infrastructures and requires an accurate analysis of the rupture process. Seismic moment tensors of mining-induced events help to understand the nature of mining-induced seismicity by providing information about the relationship between the mining, stress redistribution and instabilities in the rock mass. In this work, we adapt and test a waveform-based inversion method on high frequency data recorded by a dense underground seismic system in one of the largest underground mines in the world (Kiruna mine, Sweden). A stable algorithm for moment tensor inversion for comparatively small mining induced earthquakes, resolving both the double-couple and full moment tensor with high frequency data, is very challenging. Moreover, the application to underground mining system requires accounting for the 3-D geometry of the monitoring system. We construct a Green's function database using a homogeneous velocity model, but assuming a 3-D distribution of potential sources and receivers. We first perform a set of moment tensor inversions using synthetic data to test the effects of different factors on moment tensor inversion stability and source parameters accuracy, including the network spatial coverage, the number of sensors and the signal-tonoise ratio. The influence of the accuracy of the input source parameters on the inversion results is also tested. Those tests show that an accurate selection of the inversion parameters allows resolving the moment tensor also in the presence of realistic seismic noise conditions. Finally, the moment tensor inversion methodology is applied to eight events chosen from mining block #33/34 at Kiruna mine. Source parameters including scalar moment, magnitude, double-couple, compensated linear vector dipole and isotropic contributions as well as the strike, dip and rake configurations of the double-couple term were obtained. The orientations of the nodal planes of the double-couple component in most cases vary from NNW to NNE with a dip along the ore body or in the opposite direction.
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