1,721,051 research outputs found
Comparison of Straight and Curved-Ray Surface Wave Tomography at Near-Surface Scale: a 3D Numerical Example
No full textSurface Wave Tomography (SWT) is used to build shear-wave velocity models. In some studies, it is assumed that surface waves propagation follows a straight line between the source and the receiver. This assumption might be violated in near-surface studies because of high level of complexity and lateral heterogeneity. In curved-ray SWT, the actual ray paths between every receiver couple are computed. Curved-ray SWT can increase the accuracy of the model and will increase the computational effort. It is important to investigate the gained model improvement together with the associated additional computational cost from curved-ray over straight-ray SWT for near-surface applications. We apply straight- and curved-ray SWT on a generated 3D synthetic dataset and compare the results in terms of accuracy and computational costs.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Applied Geophysics and Petrophysic
Joint Inversion of Surface Wave Tomography and Body Wave Tomography Applied to 2D Media
No full textSurface Wave Tomography is used to obtain a shear wave velocity model by inverting computed dispersion curves. Body Wave Tomography is used to obtain a longitudinal wave model through travel time inversion of picked first break travel times. Individual inversions suffer from various different limitations. Joint surface and body wave tomography inversion aims to reduce the limitations and produce better subsurface velocity models than either individual inversion. We integrate these two methods by inverting dispersion curves and first breaks simultaneously in a 2D joint inversion scheme. We propose a joint inversion algorithm in which Poisson’s ratio provides the physical link between the shear and longitudinal wave velocities. The joint inversion results show encouraging improvements compared with individual inversion results.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Applied Geophysics and Petrophysic
Early-Time GPR Signal Attributes to Estimate Soil Dielectric Permittivity: A Theoretical Study
No full textHigh-frequency electromagnetic (EM) surveys have
5 shown to be valuable techniques in the study of soil water content
6 due to the strong dependence of soil dielectric permittivity with
7 moisture content. This quantity can be determined by analyzing
8 the average value of the early-time instantaneous amplitude of
9 ground-penetrating radar (GPR) traces. We demonstrate the re10
liability of this approach to evaluate the shallow soil water content
11 variations from standard fixed-offset GPR data by simulating the
12 data over different likely EMsoil conditions. A linear dipolemodel
13 that uses a thin-wire approximation is assumed for the transmit14
ting and receiving antennas. The homogenous half-space model is
15 used to calculate the waveform instantaneous amplitude values av16
eraged over different time windows.We analyzed their correlation
17 with the soil surface dielectric parameters, and we found a clear
18 inverse linear dependence on the permittivity values. Moreover,
19 we evaluated how different kinds of noise affect this correlation,
20 and we determined the influence of the electrical conductivity on
21 the trace attributes. Finally, through a two-layered medium, we
22 estimated the effect on the GPR signal of a shallow reflector, we
23 analyzed how its presence can carry out inaccuracies in the soil
24 surface dielectric permittivity estimation, and we determine the
25 best time window to minimize these errors
DIRECT DATA TRANSFORM FOR EM SOUNDING INTERPRETATION
No full textOver the last years, new techniques are being researched in order directly estimate relevant geological properties of the rocks present in the subsurface without using an inversion process, this can be achieved by obtaining relationships between the data obtained by geophysical surveys and the data obtained in one place by well logging, core analysis or other techniques in which the actual physical properties of the rocks are measured. Using the apparent resistivity measurements from an MT survey and the resistivity measured from an exploratory well, we want to assess if it is possible to obtain a polynomial function that can be used to correct the misfit between the depth-apparent resistivity model obtained by MT surveys and the depth-resistivity model obtained by means of exploratory wells, and we want to assess if the polynomial expression obtained can be used to retrieve an accurate electrical model in nearby areas from the exploratory well in which only apparent resistivity measurements have been acquired
Inversion of Magnetotelluric Data into 1D Resistivity Models Via Cumulative Resistance Mapping
No full textInverting magnetotelluric (MT) surface data to obtain subsurface resistivity models is a complex and non-linear process where the solution is inherently non-unique. We present a fully data-driven method that enables the direct transformation of MT data into 1D resistivity models using cumulative resistance models. Our approach introduces a cumulative representation of a layered resistivity model, which, at each depth, integrates the effect of overlying layers into the subsurface model. We then establish a relationship between the real part of the TE mode of the MT data and its corresponding cumulative resistance model. Subsequently, we use this relationship to train a neural network that rescales MT data directly into cumulative resistance models. Once the resistance model is retrieved, a numerical derivative is applied to obtain the interval resistivity model, without any prior assumptions about the subsurface structure or resistivity distribution. This approach was validated using both synthetic and real MT datasets, introducing a new perspective for tackling the inversion problem
Direct 1D Resistivity Estimation from Data Rescaling Using Cumulative Resistance Models
No full textThis study presents a novel methodology to transform 1D resistivity data into layered resistivity models without prior information by using the concept of cumulative reference models. The proposed methodology involves deriving an error function that transforms apparent resistance measurements into a cumulative resistance, which is then transformed into a layered resistivity model. We applied the methodology to simulated data from various 1D models with different physical parameters, and the results demonstrate that our method can be used to directly transform the data into a layered resistivity model without requiring prior information. This methodology provides a valuable alternative to inversion methods when one local model is available and multiple measurements are available over an area with similar physical parameters. Furthermore, the retrieved rescaled model can be used as a reference model for the inversion process, reducing computational and economic costs. This study highlights the potential of cumulative reference models for subsurface characterization, providing a new paradigm to study the subsurface with increased efficiency
Civil Engineering Applications of Ground Penetrating Radar: Research Perspectives in COST Action TU1208
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