The characterization of contaminated sites can benefit from the supplementation of direct investigations with a set of less invasive and more extensive measurements. A combination of geophysical methods and direct push techniques for contaminated land characterization has been proposed within the EU FP7 project ModelPROBE and the affiliated project SoilCAM. In this paper, we present results of the investigations conducted at the Trecate field site (NW Italy), which was affected in 1994 by crude oil contamination. The less invasive investigations include ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and electromagnetic induction (EMI) surveys, together with direct push sampling and soil electrical conductivity (EC) logs. Many of the geophysical measurements were conducted in time-lapse mode in order to separate static and dynamic signals, the latter being linked to strong seasonal changes in water table elevations. The main challenge was to extract significant geophysical signals linked to contamination from the mix of geological and hydrological signals present at the site. The most significant aspects of this characterization are: (a) the geometrical link between the distribution of contamination and the site's heterogeneity, with particular regard to the presence of less permeable layers, as evidenced by the extensive surface geophysical measurements; and (b) the link between contamination and specific geophysical signals, particularly evident from cross-hole measurements. The extensive work conducted at the Trecate site shows how a combination of direct (e.g., chemical) and indirect (e.g., geophysical) investigations can lead to a comprehensive and solid understanding of a contaminated site's mechanisms.
We propose an efficient algorithm for modeling seismic plane-wave propagation in vertically heterogeneous viscoelastic media using a finite-difference time-domain (FDTD) technique. In the algorithm, the wave equation is rewritten for plane waves by applying a Radon transform to the 2D general wave equation. Arbitrary values of the quality factor for [Formula: see text]- and [Formula: see text]-waves ([Formula: see text] and [Formula: see text]) are incorporated into the wave equation via a generalized Zener body rheological model. An FDTD staggered-grid technique is used to numerically solve the derived plane-wave equations. The scheme uses a 1D grid that reduces computation time and memory requirements significantly more than corresponding 2D or 3D computations. Comparing the finite-difference solutions to their corresponding analytical results, we find that the methods are sufficiently accurate. The proposed algorithm is able to calculate synthetic waveforms efficiently and represent viscoelastic attenuation even in very attenuative media. The technique is then used to estimate the plane-wave responses of a sedimentary system to normal and inclined incident waves in the Kanto area of Japan via synthetic vertical seismic profiles.
The process of capturing carbon dioxide ([Formula: see text]) and injecting it into deep saline aquifers is becoming an important method to reduce future atmospheric emissions of [Formula: see text]. Key challenges facing carbon capture and storage (CCS) are the storage reservoir's size and safety. The storage size can be addressed by focusing on large saline aquifer reservoirs. The safety concern may be lower if [Formula: see text] is injected into depleted hydrocarbon reservoirs because their cap-rock integrity is already proven, but such capping systems are generally potentially compromised by poorly cemented abandoned wells, and compared to saline aquifers, their storage size is small. Therefore, the long-term focus of CCS is on saline aquifers. To reduce the corresponding risks, comprehensive long-term monitoring is inevitable.
S U M M A R YA scheme for the non-standard finite-difference method in the time-domain (NS-FDTD), 2-Box scheme, is proposed for elastic wave simulations in two dimensions (P-SV ). The method improves the accuracy and efficiently reduces grid dispersion and anisotropy. The proposed non-standard scheme is based on two main operations. The first operation replaces spatial grid spacing and time step by their frequency optimized counterparts, called the correction functions, and the second operation introduces an optimum grid stencil for the finite-difference operator of the 2-D Laplacian. The optimal stencil is obtained by introducing two optimization parameters estimated for a design frequency. Error analysis of the proposed scheme (2-Box scheme) shows that specifying the maximum frequency as the design frequency leads to a significant reduction of the grid dispersion over a wide frequency band. We derive the formulations of grid dispersion and stability condition for the scheme. The grid dispersion is investigated, and it is shown that the proposed scheme reduces not only the grid dispersion but also the grid anisotropy significantly. The grid dispersion is insensitive to the Poisson's ratio and size of the time step within the stability limit. Since the wide spatial stencil of the 2-Box scheme might become difficult to implement at the computational domain boundaries, two additional non-standard schemes-1-Box and 0-Box schemes-are also introduced. The 1-Box scheme uses narrower stencil than the 2-Box scheme, and the 0-Box scheme uses the same stencil as the standard FDTD. Numerical experiments of elastic wave propagation demonstrate the significant superiority of the proposed non-standard schemes over the commonly used standard one. With six grid spacings per minimum wavelength, the 2-Box and 1-Box schemes represent excellent results and the 0-Box scheme has higher accuracy than the standard scheme with seven grid spacings per minimum wavelength.The standard finite-difference in the time-domain (FDTD) is one of the most common techniques used for simulating elastic wave propagation. This technique is popular because it is simple and easy to programme.Various schemes based on the standard FDTD have been reported in geophysical publications. Among them, the staggered-grid scheme introduced by Madariaga (1976) has been a very popular method for modelling elastic wave propagation, both in exploration seismology and in earthquake seismology (e.g. Olsen et al. 2006). The staggered grid can be applied both to the displacement-stress (e.g. Moczo et al. 2002) and velocity-stress finite-difference schemes; however, the later is more common in seismic wave modelling. A velocity-stress finite-difference (FD) scheme, using the staggered grid, with second-order accuracy in space and time, was presented by Virieux (1984Virieux ( , 1986 for SH and P-SV wave propagation. His second-order scheme needs about 10 grid spacings per wavelength for correct modelling. Levander (1988) developed a staggered-grid scheme with fourth-order acc...
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