1,721,082 research outputs found
Multimode multioffset phase analysis of surface waves, a new approach to extend MOPA to higher modes
Full text linkMultioffset phase analysis (MOPA) is a fairly recent technique for evaluating seismic surface wave dispersion and estimating the presence of lateral variations. The main limitation of MOPA is that it is based on the assumption of one predominant mode, usually the fundamental mode, in the wave propagation. However, MOPA can be extended (at least) to the two-mode case: this new technique will be called multimode MOPA (MMMOPA). The method employs both amplitude and phase spectral information. The analysis is performed for each frequency independently. The presence of two modes causes the amplitude to have an oscillating behaviour as a function of offset (beats): the spatial period of the oscillating amplitude is identified, amplitude maxima and minima are extracted, and the local wavenumber is computed via linear regression. The resulting multimodal dispersion curve is consequently derived. Model uncertainties can be estimated by propagating the experimental phase and amplitude error variances through the different steps of the analysis all the way to the final phase velocities. An algorithm running the process in an automatic way has been implemented and tested on both synthetic and real data, with success. This is the base for future developments that, in the MOPA framework, can take into account rapid lateral velocity variations within the same acquisition window and estimate the modal absorption, for the estimation of the damping ratio, even in the presence of multimode surface wave propagation
A study of geomechanical effects on time-lapse seismics
No full textTime-lapse seismics is known to be a very effective monitoring technique for the subsurface fluid movement and saturation changes, as well as for geomechanical phenomena [Snieder et al., 2007]. The integration of seismic and reservoir engineering is now becoming state-of-the-art [Boutte, 2007] while the number of applications is steadily increasing [Staples et al., 2006]. Among the future challenges to the use of time-lapse seismics is the integration with geomechanics [Landrà ̧, 2006]. The improvement of time-lapse seismic technology [e.g. Tang et al., 2007, Aarre et al., 2007] allows for better and more accurate data acquisition, that in turn allows to "see" effects previously difficult to detect. The effects of geomechanics on time-lapse seismic data have been described in detail by a number of publications [Hatchell and Bourne, 2005; Sayers and Schutjens, 2007; Cox and Hatchell, 2008; Kristiansen and Plischke, 2010]. The overall impact of reservoir exploitation on the changes in seismic response includes the following aspects: (1) Fluid saturation effects, that are based upon: (a) dependence of density on fluid saturation; and (b) dependence of bulk moduli on fluid saturation (Gassmann, 1951). This is the key effect sought in time-lapse seismics, as it allows remote monitoring of the fluid migration in the reservoir. Mainly, two effects are sought in data hopefully depending on the above saturation changes, i.e.: - time shifts, i.e. changes in reflector location in time as a consequence of changes in velocity, and mainly: - impedance changes, i.e. reflectivity changes, as impedance is the product of velocity and density, both changing with fluid content. (2) Pressure (effective stress) effect: this is the first, well known geomechanical effect, often referred to in the literature as pressure effect, but it is actually a dependence on effective stress. It is generally observed that the velocity decrease is very strong in presence of effective stress decrease (expansion), while velocity increase is relatively mild under stress increase (compaction) [e.g. Hatchell and Bourne, 2005]. This asymmetric behaviour is often explained in terms of crack opening under stress release conditions. In addition to the above well-known effects, other important time-lapse phenomena depending on geomechanics have been highlighted in the recent past, i.e.: (3) Seismic velocity changes in the overburden: in presence of significant arching and stress redistribution above the reservoir, a large region can undergo large velocity drops in presence of effective stress decreases. This effect can have a strong effect on time-shifts, with possible 3D (lens) effects causing also lateral time-shifts [Cox and Hatchell, 2008], as well as on changes in apparent impedance contrasts caused by wavelet interference phenomena. In this contribution we present an approach that can take into account all phenomena described above, by integrating the results of a reservoir model, of a geomechanical model and of a full-waveform seismic model - see Figure 1. This integrated approach can lead to a calibration of both the reservoir and the geomechanical model also on the basis of the time-lapse seismic data. A key role is taken by a suitable constitutive model linking stress changes/strains to seismic velocity changes, allowing for a different non-symmetric behaviour in compaction and dilation. (Fiture presented) We tested the proposed procedure on a synthetic model (Figure 2) where the reservoir is assumed to be fully depleted. The velocity model was constructed applying a simplified relationship (Calvert, 2005) to the effective stress field computed with an elasto-plastic geomechanical model (Isamgeo). The difference in velocity between base and monitor is shown in Figure 3, highlighting the complex spatial pattern resulting from stress redistribution. The Sem2Dpack seismic software (Ampuero, 2008) was then used to simulate the base and monitor seismic acquisitions. Examples of waveform snapshots are shown in Figure 4, highlighting amplitude and time differences resulting from geomechanical deformation
MONITORAGGIO ELETTRICO AD ALTA RISOLUZIONE PER LO STUDIO DELLE PROPRIETA' DI TRASPORTO IN UN ACQUIFERO SUPERFICIALE
No full textResistivity Tomography of a traces test experiment and local hydraulic properties assessment from data
No full textAging of oil/gas-bearing Sediments their compressibilità and subsidence
No full textThe in situ stiffness and apparent maximum preconsolidation stress of many soils and sediments
appear to be higher than in the laboratory tests. We seek to verify experimentally whether this also holds for
deep marine sediments. We also discuss an alternative explanation for this effect to the classical one, implying
the sample damage during coring. We test numerically the explanation, suggesting possible unaccounted changes
in stiffness, occurring in sediments in situ when subjected to aging, or secondary compression for geological
scale time periods. Results of ‘‘aging tests’’ on sandy and clayey sediments are presented, involving secondary
compression at the constant in situ stress level, during which strain develops together with other changes in
properties. Only two weeks of aging produced a notable increase in the apparent maximum preconsolidation
stress and in the stiffness below it, and above the in situ stress. A framework for a mathematical model is
proposed, based on the supposition that during aging the sediment develops a secondary microstructure through
reactions of local dissolution/precipitation of less stable minerals
Surface Waves Back-Scattering Analysis of a Vertical Surface Crack
No full textThe existence of heterogeneities within the near sub-surface becomes a potential hazard to subsurface exploitation activities. Consequently, accurately identifying and characterizing these discontinuities are vital for effective threat mitigation. In this regard, investigating the backscattering of surface waves generated by such obstacles emerges as a critical approach for locating and understanding these heterogeneities. This study focuses on analysing the presence of surface vertical cracks within a homogeneous and layered half-space. Through the analysis of the back-scattering coefficient, a consistent behaviour is identified, and dimensionless expressions can be determined, which can be extrapolated to any type of crack and frequency
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