Stéphane Garambois

On the use of combined geophysical methods to assess water content and water conductivity of near-surface formations

Abstract We propose to deepen the interpretation of combined geophysical methods (georadar, seismic and electric) to assess physical properties characterizing the near-surface porous formations, especially the influence of water. Velocity analysis of multioffset georadar data are used together with seismic methods to estimate lateral and vertical ground water fluctuations. This enables us to identify transitions from non-saturated to fully saturated porous layers with certainty. Furthermore, the accurate knowledge of seismic velocities helps to estimate the porosity of the ground water formations. Finally, we show how the radar technique may be useful in solving the problem of trade-off between bulk resistivity, which is deduced from electrical measurements, and that of water ionic conductivity and water content. These theoretical considerations are illustrated using various measurements conducted at the same test site. Our interpretation is compared with a few laboratory measurements on water and soil samples. This study displays the impact of combined geophysical approaches for providing models of water and ionic transfers down to a depth of several metres.

Multiconfiguration GPR measurements for geometric fracture characterization in limestone cliffs (Alps)

Rock-mass fracturing is a key parameter in rock-fall hazard assessment. However, traditional geologic observations can provide information only about discontinuities at the surface. In this case study, detailed ground-penetrating-radar (GPR) measurements (with antennas of 50 MHz , 100 MHz , 200 MHz , and 400 MHz ) were conducted on a test site, using different acquisition configurations deployed on vertical cliff faces. Conventional 2D profile data, common-midpoint (CMP) survey data, and transmission data were acquired to evaluate the potential use of radar waves to characterize the geometry and properties of the major discontinuities (fractures) within a Mesozoic limestone massif. Results showed that the continuity and geometry (orientation and dip) of the major observed fractures, which are crucial parameters for assessing rock stability, can be obtained by combining vertical and horizontal profiles measured along the cliff. We used 100-MHz antennae and reached a maximum penetration of 20 m , which limi...

On the use of dispersive APVO GPR curves for thin-bed properties estimation: Theory and application to fracture characterization

The presence of a thin layer embedded in any formation creates complex reflection patterns caused by interferences within the thin bed. The generated reflectivity amplitude variations with offset have been increasingly used in seismic interpretation and more recently tested on ground-penetrating radar (GPR) data to characterize nonaqueous-phase liquid contaminants. Phase and frequency sensitivities of the reflected signals are generally not used, although they contain useful information. The present study aims to evaluate the potential of these combined properties to characterize a thin bed using GPR data acquired along a common-midpoint (CMP) survey, carried out to assess velocity variations in the ground. It has been restricted to the simple case of a thin bed embedded within a homogeneous formation, a situation often encountered in fractured media. Dispersive properties ofthe dielectric permittivity of investigated materials (homogeneous formation, thin bed) are described using a Jonscher parameterization, which permitted study of the dependency of amplitude and phase variation with offset (APVO) curves on frequency and thin-bed properties (filling nature, aperture). In the second part, we discuss and illustrate the validity of the thin-bed approximation as well as simplify assumptions and make necessary careful corrections to convert raw CMP data into dispersive APVO curves. Two different strategies are discussed to correct the data from propagation effects: a classical normal-moveout approach and an inverse method. Finally, the proposed methodology is applied to a CMP GPR data set acquired along a vertical cliff. It allowed us to extract the characteristics of a subvertical fracture with satisfying resolution and confidence. The study motivates interest to use dispersion dependency of the reflection coefficient variations for thin-bed characterization.

Three-dimensional magnetic resonance imaging for groundwater

The surface nuclear magnetic resonance method (SNMR) is an established geophysical tool routinely used for investigating one-dimensional (1D) and sometimes 2D subsurface water-saturated formations. We have expanded the tool by developing a 3D application. 3D-SNMR is a large-scale method that allows magnetic resonance imaging of groundwater down to about 80?m. Similar to most surface geophysical methods, 3D-SNMR has limited resolution, but it is effective for investigating water-saturated geological formations larger than several tens of meters. Because the performance of the method depends on variable survey conditions, we cannot estimate it in general. For demonstration purposes, we present an example of numerical modeling under fixed conditions. Results show that under certain conditions it is possible to detect a water volume as small as 500?m3 and the detection threshold depends on the ambient electromagnetic noise magnitude and on the location of the target volume relative to the SNMR loops. The 3D-SNMR method was used to investigate accumulated water within the T?te Rousse glacier (French Alps). Inversion of the field measurements made it possible to locate the principal reservoir in the central part of the glacier and estimate the volume of accumulated water. These results were verified by 20 boreholes installed after the 3D-SNMR results were obtained and by pumping water out of the glacier. Very good correspondence between the 3D-SNMR and borehole results was observed.

Time-lapse seismic imaging using regularized full-waveform inversion with a prior model: which strategy?

Full-waveform inversion is an appealing technique for time-lapse imaging, especially when prior model information is included into the inversion workflow. Once the baseline reconstruction is achieved, several strategies can be used to assess the physical parameter changes, such as parallel difference (two separate inversions of baseline and monitor data sets), sequential difference (inversion of the monitor data set starting from the recovered baseline model) and double-difference (inversion of the difference data starting from the recovered baseline model) strategies. Using syntheticMarmousi data sets, we investigate which strategy should be adopted to obtain more robust and more accurate time-lapse velocity changes in noise-free and noisy environments. This synthetic application demonstrates that the double-difference strategy provides the more robust time-lapse result. In addition, we propose a target-oriented time-lapse imaging using regularized full-waveform inversion including a prior model and model weighting, if the prior information exists on the location of expected variations. This scheme applies strong prior model constraints outside of the expected areas of timelapse changes and relatively less prior constraints in the time-lapse target zones. In application of this process to the Marmousi model data set, the local resolution analysis performed with spike tests shows that the target-oriented inversion prevents the occurrence of artefacts outside the target areas, which could contaminate and compromise the reconstruction of the effective time-lapse changes, especially when using the sequential difference strategy. In a strongly noisy case, the target-oriented prior model weighting ensures the same behaviour for both time-lapse strategies, the double-difference and the sequential difference strategies and leads to a more robust reconstruction of the weak time-lapse changes. The double-difference strategy can deliver more accurate time-lapse variation since it can focus to invert the difference data. However, the double-difference strategy requires a preprocessing step on data sets such as time-lapse binning to have a similar source/receiver location between two surveys, while the sequential difference needs less this requirement. If we have prior information about the area of changes, the target-oriented sequential difference strategy can be an alternative and can provide the same robust result as the doubledifference strategy.

Wave propagation in heterogeneous porous media formulated in the frequency-space domain using a discontinuous Galerkin method

Biphasic media with a dynamic interaction between fluid and solid phases must be taken into account to accurately describe seismic wave amplitudes in subsurface and reservoir geophysical applications. Consequently, the modeling of the wave propagation in heteregeneous porous media, which includes the frequency-dependent phenomena of the fluidsolid interaction, is considered for 2D geometries. From the Biot-Gassmann theory, we have deduced the discrete linear system in the frequency domain for a discontinuous finite-element method, known as the nodal discontinuous Galerkin method. Solving this system in the frequency domain allows accurate modeling of the Biot wave in the diffusive/propagative regimes, enhancing the importance of frequency effects. Because we had to consider finite numerical models, we implemented perfectly matched layer techniques. We found that waves are efficiently absorbed at the model boundaries, and that the discretization of the medium should follow the same rules as in the elastodynamic case, that is, 10 grids per minimum wavelength for a P0 interpolation order. The grid spreading of the sources, which could be stresses or forces applied on either the solid phase or the fluid phase, did not show any additional difficulties compared to the elastic problem. For a flat interface separating two media, we compared the numerical solution and a semianalytic solution obtained by a reflectivity method in the three regimes where the Biot wave is propagative, diffusive/propagative, and diffusive. In all cases, fluid-solid interactions were reconstructed accurately, proving that attenuation and dispersion of the waves were correctly accounted for. In addition to this validation in layered media, we have explored the capacities of modeling complex wave propagation in a laterally heterogeneous porous medium related to steam injection in a sand reservoir and the seismic response associated to a fluid substitution.

Deep fluids can facilitate rupture of slow-moving giant landslides as a result of stress transfer and frictional weakening

Landslides accommodate slow, aseismic slip and fast, seismic rupture, which are sensitive to fluid pressures and rock frictional properties. The study of strain partitioning in the Sechilienne landslide (France) provides a unique insight into this sensitivity. Here we show with hydromechanical modeling that a significant part of the observed landslide motions and associated seismicity may be caused by poroelastic strain below the landslide, induced by groundwater table variations. In the unstable volume near the surface, calculated strain and rupture may be controlled by stress transfer and friction weakening above the phreatic zone and reproduce well high-motion zone characteristics measured by geodesy and geophysics. The key model parameters are friction weakening and the position of groundwater level, which is sufficiently constrained by field data to support the physical validity of the model. These results are of importance for the understanding of surface strain evolution under weak forcing.

Origin of the outburst flood from Glacier de Tete Rousse in 1892 (Mont Blanc area, France)

Extensive field measurements and historical data have been used to re-analyse the cause of the outburst flood from Glacier de Tete Rousse that devastated the village of Saint-Gervais-Le Fayet, French Alps in 1892, causing 175 fatalities. The origin of this disaster was the rupture of an intraglacial cavity in Glacier de Tete Rousse that released 200 000 m 3 of water and ice. All previous studies have concluded that the intraglacial cavity was formed from a crevasse that was filled and enlarged by meltwater. The re-analysis presented here suggests that the reservoir of the upper cavity did not originate as an enlarging crevasse. The origin of the meltwater reservoir was more likely a supraglacial lake formed before 1878 during a period of negative mass balance. Following a period of positive mass balance after 1878, the lake was hidden until the outburst flood of 1892. This means that such hazards may be detected by checking regularly for the formation of a lake on the surface of the glacier before it is hidden.

Modelling Seismic Wave Propagation for Geophysical Imaging

The Earth is an heterogeneous complex media from the mineral composition scale (10−6m) to the global scale ( 106m). The reconstruction of its structure is a quite challenging problem because sampling methodologies are mainly indirect as potential methods (Gunther et al., 2006; Rucker et al., 2006), diffusive methods (Cognon, 1971; Druskin & Knizhnerman, 1988; Goldman & Stover, 1983; Hohmann, 1988; Kuo & Cho, 1980; Oristaglio & Hohmann, 1984) or propagation methods (Alterman & Karal, 1968; Bolt & Smith, 1976; Dablain, 1986; Kelly et al., 1976; Levander, 1988; Marfurt, 1984; Virieux, 1986). Seismic waves belong to the last category. We shall concentrate in this chapter on the forward problem which will be at the heart of any inverse problem for imaging the Earth. The forward problem is dedicated to the estimation of seismic wavefields when one knows the medium properties while the inverse problem is devoted to the estimation of medium properties from recorded seismic wavefields.

Sensitivity Analysis of Time-lapse Images Obtained By Differential Waveform Inversion With Respect to Reference Model

SUMMARYFor monitoring purposes, one of the promising techniques ded-icated to assess physical properties changes in target regionsis the differential waveform inversion, both in the acoustic andelastic cases. A central question of this technique regards thechoice of the reference model. One solution could be the useof the reconstructed baseline image provided through the stan-dard Full Waveform Inversion (FWI) procedure of initial data.However, how the accuracy of the baseline reconstructed imagewill affect the precision of further time-lapse images is of cru-cial importance. Here, we present a sensitivity analysis of time-lapse images obtained from differential inversion, with respectto various reference models. Density, P- and S-wave velocitychanges could be converted into fluid property changes thanksto an empirical downscaling relationship. For accurate estima-tion of fluid parameter changes, the construction of highly re-solved time-lapse images presenting acceptable errors is a keyissue for the downscaling procedure. We illustrate on a specificsynthetic example that the sensitivity analysis over the refer-ence model variation provides linear convergence towards thetime-lapse image obtained when using the exact baseline. Anaccurate baseline reconstruction is essential and could benefitfrom other data collected for monitoring purposes.INTRODUCTIONFWI is a data fitting procedure aiming to develop high resolu-tion quantitative images of the subsurface, through the extrac-tion of the full information content of the seismic data (Taran-tola, 1984). Beside the exploration application, the FWI methodcan be also used for monitoring applications, such as oil and gasreservoirs, steam injection, CO