Leonardo B. Monachesi

Seismic wave attenuation and dispersion due to wave-induced fluid flow in rocks with strong permeability fluctuations

Oscillatory fluid movements in heterogeneous porous rocks induced by seismic waves cause dissipation of wave field energy. The resulting seismic signature depends not only on the rock compressibility distribution, but also on a statistically averaged permeability. This so-called equivalent seismic permeability does not, however, coincide with the respective equivalent flow permeability. While this issue has been analyzed for one-dimensional (1D) media, the corresponding two-dimensional (2D) and three-dimensional (3D) cases remain unexplored. In this work, this topic is analyzed for 2D random medium realizations having strong permeability fluctuations. With this objective, oscillatory compressibility simulations based on the quasi-static poroelasticity equations are performed. Numerical analysis shows that strong permeability fluctuations diminish the magnitude of attenuation and velocity dispersion due to fluid flow, while the frequency range where these effects are significant gets broader. By comparing the acoustic responses obtained using different permeability averages, it is also shown that at very low frequencies the equivalent seismic permeability is similar to the equivalent flow permeability, while for very high frequencies this parameter approaches the arithmetic average of the permeability field. These seemingly generic findings have potentially important implications with regard to the estimation of equivalent flow permeability from seismic data.

An analytical study of seismoelectric signals produced by 1-D mesoscopic heterogeneities

The presence of mesoscopic heterogeneities in fluid-saturated porous rocks can produce measurable seismoelectric signals due to wave-induced fluid flow between regions of differing compressibility. The dependence of these signals on the petrophysical and structural characteristics of the probed rock mass remains largely unexplored. In this work, we derive an analytical solution to describe the seismoelectric response of a rock sample, containing a horizontal layer at its center, that is subjected to an oscillatory compressibility test. We then adapt this general solution to compute the seismoelectric signature of a particular case related to a sample that is permeated by a horizontal fracture located at its center. Analyses of the general and particular solutions are performed to study the impact of different petrophysical and structural parameters on the seismoelectric response. We find that the amplitude of the seismoelectric signal is directly proportional to the applied stress, to the Skempton coefficient contrast between the host rock and the layer, and to a weighted average of the effective excess charge of the two materials. Our results also demonstrate that the frequency at which the maximum electrical potential amplitude prevails does not depend on the applied stress or the Skempton coefficient contrast. In presence of strong permeability variations, this frequency is rather controlled by the permeability and thickness of the less permeable material. The results of this study thus indicate that seismoelectric measurements can potentially be used to estimate key mechanical and hydraulic rock properties of mesoscopic heterogeneities, such as compressibility, permeability, and fracture compliance.

A generalized effective anisotropic poroelastic model for periodically layered media accounting for both Biot's global and interlayer flows: Effective anisotropic poroelastic model

We present a generalized effective poroelastic model for periodically layered media in the mesoscopic scale range, which accounts for both Biot’s global and interlayer wave-induced fluid flow, as well as for the anisotropy associated with the layering. Correspondingly, it correctly predicts the existence of the fast and slow P-waves as well as quasi and pure S-waves. The proposed analytical model is validated through comparisons of the P-wave and S-wave phase velocity dispersion and attenuation characteristics with those inferred from a one-dimensional numerical solution of Biot’s poroelastic equations of motion. We also compare our model with the classical mesoscopic model of White for a range of scenarios. The results demonstrate that accounting for both wave-induced fluid flow mechanisms is essential when Biot’s global flow prevails at frequencies that are comparable or smallerwith respect to those governing interlayer flow. This is likely to be the case in media of high permeability, such as, for example, unconsolidated sediments, clean sandstones, karstic carbonates, or fractured rocks. Conversely, when interlayer flow occurs at smaller frequencies with respect to Biot’s global flow, the predictions of this model are in agreement with White’s model, which is based on quasi-static poroelasticity.

An Analytical Solution of Tide-Induced Head Fluctuations in an Inhomogeneous Coastal Aquifer

A new analytical solution for tide-induced head fluctuations in a confined aquifer with hydraulic conductivity that increases quadratically with the distance to the coastline is presented. To derive the analytical solution the following assumptions are made: the aquifer ends at the coastline and extends landward infinitely, the water flow is horizontal and obeys Darcy’s law, the specific storativity is constant, and the sea tide is described by a sinusoidal function. The proposed analytical solution has a relatively simple mathematical expression and it is used to estimate the hydraulic conductivity from experimental data. In general terms, it can be concluded that the quadratic heterogeneity produces dampened amplitudes near the coastline and a faster transmission of the tidal effect into the aquifer.

Seasonal Variability of Land Water Storage in South America Using GRACE Data

The objective of the present study is to analyze seasonal variability of land water storage in South America from GRACE data. High precision estimate of temporal variations in the Earth’s gravity are obtained using monthly Release-04 (RL04) gravity field coefficients provided by the Center for Space Research (CSR) of the University of Texas at Austin. Water mass anomalies, as equivalent height of water, are calculated based on the direct relationship between gravity and mass. To remove the effects of the noise observed in the equivalent-water thickness solutions at high harmonic degrees, an optimized smoothing technique is applied. Finally, temporal distributions of land water storage are compared to monthly mean precipitation data extracted from in-situ rain gauge records in order to identify, correlate and understand patterns of water movement at continental scale in South America.