1,721,007 research outputs found
Multi-Offset Phase Analysis of Surface Wave Data (MOPA)
No full textInaccuracy in the shear wave velocity profile inverted from surface wave data manifests from both modelling error and data uncertainty. An alternative method for dispersion curve evaluation by weighted linear regression of phase-offset data can be applied to both equispaced and non-equispaced data for objective identification of these often overlooked error sources. From field data, near-field effects are noted to at most half a wavelength and lateral discontinuities identified by marked changes in wavenumber with offset. Transition frequencies to dominant higher modes appear lower than when identified from standard plane-wave transform methods. Effects can be discriminated by their frequency, position or offset dependence. When a non-corrupt dispersion curve is extracted, the errors are up to 5% at low frequency. Through theoretical Gaussian error propagation analysis, the resulting shear wave velocity profile shows up to 18% uncertainty at dept
THE BUFFON'S NEEDLE PROBLEM AND THE DESIGN OF A GEOPHYSICAL SURVEY
No full textA logical procedure for designing a geophysical survey when sampling an area with a regular grid can be summarized as follows: model the expected anomaly, estimate the expected noise level, estimate the area of the anomaly above the noise level, choose the spacing, in both the x- and y-directions, of the measurement grid. This last step can be approached according to two main strategies: either when applying the sampling theorem to the shortest dimension of the anomaly or when using a coarser grid, leaving a more complete definition of the anomaly to a later fitting. When following this second option, it can be constructive to estimate the probability of intercepting a given anomaly with a specific segment of profile and a given profile spacing. This latter procedure is analysed by considering a rectangle approximating the plane projection of the anomaly shape and taking into account various ratios between the grid spacing and the rectangle sides. The formulae for estimating the probability of intersecting a given anomaly with a given segment of a given profile spacing are calculated. To demonstrate the accuracy of the results, a Monte-Carlo simulation on a synthetic magnetic map was performed, obtaining, for different ratios between the sides, segment length and profile interval, an agreement better than 0.1% with the analytical formulae
Guide d'onda GPR multistrato per la stima del contenuto idrico negli strati superficiali del sottosuolo
No full textAnalysis of borehole guided waves for geotechnical application
Full text linkThe reliability of Stoneley waves (SWs) is discussed for the characterisation of the mechanical properties of soft and hard rock in borehole seismic techniques using source on the surface and hydrophones as receivers. The SWs propagate along the fluid-filled borehole; the propagation is affected by the mechanical and hydraulic properties of the fluid and the surrounding medium. At low frequencies, in a non-diffusive medium (impermeable formation), the wave velocity depends on the density, the wave velocity of the fluid and the shear modulus of the formation. The models adopted to infer the wave velocity in elastic formations in uncased and cased boreholes are discussed. We discuss two examples to check the discrepancies between the theoretical and the experimental evidence. The presence of casing in soft rock greatly reduces the sensitivity of the SW propagation to the mechanical properties of the medium. In hard rock, the scattering of the primary wavefields could be adopted to detect the presence of rock mass discontinuities (fractures). © 2007 - OGS
Multilayer ground-penetrating radar guided waves in shallow soil layers for estimating soil water content
No full textThe knowledge of moisture-content changes in shallow soil layers has important environmental implications, and ground-penetrating radar (GPR) used in surface-to-surface configuration has been used increasingly to quickly image soil moisture content over large areas. The technique is based on measuring direct GPR wave velocity in the ground. However, in the presence of shallow and thin low-velocity soil layers, dispersive guided GPR waves are generated and the direct ground wave is not identifiable as a simple arrival. Under such conditions, the dispersion relation of guided waves can be estimated from field. data and then inverted to obtain the properties of the guiding layers. This approach is applied to a mountain slope with a 1-m soil cover where repeated measurements over time, inverted by conceptualizing the soil as a single guiding layer, lead to estimates of velocity and thickness varying over time. Varying soil thickness clearly is not a plausible physical process. To remove this problem, we develop a multilayer GPR waveguide model. We first assess, using a Monte Carlo sensitivity analysis, the model error arising from using a single-layer forward model to invert data generated by a multilayer waveguide. The single-layer model always underestimates the total soil thickness because the inversion is sensitive mainly to the layer with the lowest velocity (the wettest layer). We then use a multilayer forward model to invert the actual field data. By constraining the total soil thickness, we still manage to invert accurately only for velocity and thickness of the wettest layer, leaving uncertainty about the position of such a layer in the layer sequence. We conclude that these inversion equivalence problems cannot be neglected when guided GPR data are used to estimate time-lapse moisture content in shallow soils
- …
