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- Volume 16, Issue 5, 2018
Near Surface Geophysics - Volume 16, Issue 5, 2018
Volume 16, Issue 5, 2018
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Seismic wave propagation in floating ice sheets – a comparison of numerical approaches and forward modelling
More LessABSTRACTSeismic data processing collected from arctic sea ice is often challenging. Large amplitudes of coherent surface waves interfere with the reflected seismic waves and can make the reflection data virtually unusable. In order to understand this noise, forward modelling of the full wavefield is a useful approach. I present and test a forward modelling workflow by comparing the modelling results with common numerical approaches. The main differences between numerical curves and modelling results depend on simplifications in the assumptions of the numerical approaches. Forward modelling is less dependent on rough assumptions and can give a more accurate understanding of the surface wave propagation and its interaction with reflected waves. Furthermore, a thin elastic layer with high velocity located between two low velocity layers, like sea ice, generates guided waves. The numerical approaches focus only on the flexural waves and ignore other wave types. The presented results demonstrate that forward modelling can provide improved detail in the generated wavefield. In addition, the forward modelling results in this work show that leaky Rayleigh and Scholte waves can exist simultaneously.
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Development of an adaptive multi‐method algorithm for automatic picking of first arrival times: application to near surface seismic data
Authors Amin Khalaf, Christian Camerlynck, Nicolas Florsch and Ana SchneiderABSTRACTAccurate picking of first‐arrival times is important in many seismic studies, particularly in seismic tomography and reservoirs or aquifers monitoring. Many techniques have been developed, mainly for seismological purposes, in order to pick first arrivals automatically or semi‐automatically. However, these techniques do not reach the accuracy required in shallow seismics due to the complexity of near‐surface structures and low signal‐to‐noise ratio. We propose here a new adaptive algorithm to automatically pick first arrival in near‐surface seismic data by combining three picking methods: multi‐nested windows, higher order statistics, and Akaike information criterion. They benefit from combining different properties of the signal in order to highlight first arrivals and finally to provide an efficient and robust automatic picking. This strategy mimics the human first‐break picking, where a global trend is first defined at the beginning of the picking procedure. The exact first breaks are then sought in the vicinity of each point suggested by this trend. Three successive phases are combined in a multistage algorithm, each of them characterizing a specific signal property. Within each phase, the potential picks and their error range are automatically assessed and sequentially used as prior constraints in the following phase picking. Since having realistic estimates of the error in picked traveltimes is crucial for seismic tomography, our adaptive algorithm automatically provides picked arrival times with their associated uncertainties. We demonstrate the accuracy and robustness of the implemented algorithm using synthetic, pseudo‐synthetic and real datasets that pose challenges to classical automatic pickers. A comparison of both manual and adaptive picking procedures demonstrates that our new scheme provides more reliable results even under different noisy conditions. All parameters of our multi‐method algorithm are self‐adaptive, thanks to the sequential integration of each sub‐algorithm results in the workflow. Hence, it is nearly a parameter‐free algorithm, which is straightforward to implement and demands low computational resources.
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Joint inversion of cross‐borehole P‐waves, horizontally and vertically polarized S‐waves: tomographic data for hydro‐geophysical site characterization
More LessABSTRACTP‐wave, as well as horizontally and vertically polarized S‐wave, tomographic data were collected between two borehole pairs. This enabled the joint‐inversion of the three datasets. By employing structural constraints, the S‐wave traveltimes were coupled to the more accurate P‐wave traveltimes during the inversion. Thereby, the traveltime and anisotropic artefacts, initially observed in the individually inverted S‐wave tomograms, were significantly reduced and the correlation with the borehole logs improved, while the resolution of the jointly inverted P‐wave tomogram was only marginally affected. The joint inversion proves successful in determining the S‐wave velocity distribution more accurately than individual inversions. In addition, the jointly inverted tomograms were used to detect aquifer heterogeneities, caused by differences in clay content, and to distinguish areas of relatively high effective pressure. Comparison of the jointly inverted S‐wave tomograms suggests the effect of S‐wave anisotropy, which showed substantial velocity differences of approximately −10% to +10%. The anisotropy may have been caused by the presence of water‐filled pores, micro‐cracks and preferred mineral alignment (mainly clay) in the media.
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The application of variable‐frequency directional seismic wave technology under complex geological conditions
Authors Yonggao Yue, Tao Jiang and Qi ZhouABSTRACTMonofrequency directional seismic wave technology can be used to manipulate the phases of seismic waves, thereby focusing seismic signals into beams and strengthening the energy in the main beam direction. However, it is unknown whether a variable‐frequency signal can form a directional seismic wave, and the applicable conditions for utilizing this technology have not yet been reported. In this paper, we analysed the variable‐frequency directional seismic wave theory and methodology through ray tracing and the seismic wave equation. Based on the variable‐frequency directional seismic wave principle, the effects of directional parameters on the main beam direction and half‐power beamwidth were discussed in detail. In addition, the effectiveness of the variable‐frequency directional seismic wave method in several complicated velocity models was discussed. Numerical tests with the variable‐frequency directional seismic wave technique indicated that the energy intensity in the main beam direction increased by two to six times and the signal‐to‐noise ratio of the targets signal increased by 0.60–0.84 dB. Additionally, we used variable‐frequency directional seismic wave technology to analyse a shallow geological structure with a 9‐vibrator array in an area of Jilin Province. The field experiment results revealed that the near‐surface exploration depth increased from 60 to 240 m. Therefore, under complex geological conditions, the variable‐frequency directional seismic wave technology can improve the target signal at little cost.
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Using surface‐downhole ERT for detecting contaminants in deep aquifers due to exploitation of oil reservoirs
Authors V. Grünhut, M.V. Bongiovanni and A. OsellaABSTRACTThe exploitation of a hydrocarbon reservoir, whether conventional or unconventional, may cause environmental damages. Even though sink wells used by oil companies are located much deeper than the aquifers, they may eventually contaminate the water with brine leakage, oil spill or other polluting agents, such as chemicals used in hydraulic fracturing. This makes vulnerability studies extremely important. In this work we aim to detect and characterize contamination of an aquifer that lies above an oil reservoir, at approximately 250 m depth, using electric current injected in the ground surface and voltage sensors installed in the well. At that depth, it is not usual to use downhole measurements to monitor the contaminated aquifer. This gave us the motivation to check if it was possible to identify the contamination of a part of the aquifer using surface‐downhole measurements. In order to achieve this we have designed an electrical model of the reservoir including the aquifer with either a conductive or resistive contaminant such as saline brine, oil spill or CO2, and have numerically simulated the forward and inverse geoelectrical responses. Numerical results show that the detection of contamination is possible only with a surface‐downhole configuration. Our results illustrate the advantage of using the electrical resistivity tomography configuration in order to detect and characterize the contaminated deep aquifers.
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A new far‐field low‐frequency electromagnetic method for measurement of temporal variations in subsurface electrical conductivity averaged over a transect
ABSTRACTThe new far‐field low‐frequency electromagnetic method used the electromagnetic ground wave from distant radio transmitters at low frequencies to estimate temporal variations of factors affecting subsurface electrical conductivity averaged along the propagation path between either a transmitter and a receiver or two receivers that are in line with a transmitter. Phase and phase difference between two receivers depend on three factors that influence the changes in electrical conductivity: soil moisture, depth to the groundwater table and soil temperature. This dependence was investigated by simulations and evaluated by an experiment. The measurement layout was based on simulating ground wave propagation over a layered subsurface using the surface impedance method and the Sommerfeld ground wave attenuation function. A three‐layer model for the subsurface was used, which includes a soil layer, an unsaturated vadose zone and a saturated groundwater zone. The results of simulations at a frequency of 77.5 kHz showed that the phase of the ground wave is strongly influenced by natural variation of the above‐mentioned three factors; 77.5 kHz is the carrier frequency of the Normal Time Service Germany (DCF77) in Mainflingen/Germany, that was chosen as a source of the low‐frequency radio waves used in the experiment. Over a 2‐year measurement period, the amplitude and phase of the ground wave were recorded with two receivers, one 70 km and the other 110 km away from the transmitter. Additionally, phase difference between the two receivers was calculated. In situ observations of soil moisture, depth to the groundwater table, and soil temperature along the transects under investigation were used to estimate phase and phase difference dependencies. Multiple regression analysis of the measured phase and phase difference revealed a strong dependence on the depth to the groundwater table and on soil temperature, whereas the impact of soil moisture on the phase and phase difference was found to be very low. Conversely, the relations obtained can be used to estimate the variation of the depth to the groundwater table, if the phase at a given frequency and the soil temperature information are available.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)