Pascale Sénéchal

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.

Interpretation of reflection attributes in a 3-D GPR survey at Vallée d’Ossau, western Pyrenees, France

To study the attributes of reflected radar waves, a 3-D ground‐penetrating radar (GPR) data set was recorded, processed with ProMAX 3-D seismic processing software, and analyzed with Stratimagic 3-D seismic interpretation software. The zone studied was an old glacial valley (Vallee d’Ossau, western Pyrenees) where fluvio‐glacial deposits (8–10 m thick) overlay a karstic limestone bedrock. Reflections were picked to 250 ns (9 m depth with velocity υ = 0.07 m/ns). Analysis of the isochron map of the bedrock surface reflector showed north‐south undulations, corresponding to the displacement of the old glacier, together with sharp discontinuities, interpreted as local faults or karstic zones. Amplitude variations along the reflector were interpreted as being from water saturation and the nearby north‐south alignment of springs at the surface. In the fluvio‐glacial deposits, paths of old channels were also imaged using amplitude analysis. The radar‐wave amplitude attenuation was also characterized within the d...

Ck-reconstruction of surfaces from partial data

In this paper, we study the problem of constructing a smooth approximant of a surface defined by the equation z = f(x1, x2), the data being a finite set of patches on this surface. This problem occurs, for example, after geophysical processing such as migration of time-maps or depth-maps. The usual algorithms to solve this problem are picking points on the patches to get Lagranges data or trying to get local junctions on patches. But the first method does not use the continuous aspect of the data and the second one does not perform well to get a global regular approximant (C1 or more). As an approximant of f, a discrete smoothing spline belonging to a suitable piecewise polynomial space is proposed. The originality of the method consists in the fidelity criterion used to fit the data, which takes into account their particular aspect (surfaces patches): the idea is to define a function that minimizes the volume located between the data patches and the function, and which is globally Ck. We first demonstrate the new method on a theoretical aspect and numerical results on real data are given.

Geometrical and physical parameter comparison between GPR data and other geophysical data

Ground penetrating radar, 2D electric and refraction seismic methods are used to retrieve informations on geometrical, hydrogeological and chemical parameters in fluvial deposits. The results obtained with these methods give consistent informations on geometrical parameters. Indeed, the GPR interval velocities, electrical resistivities and the refraction seismic models show clearly three horizontal layers. Moreover, physical parameters are deduced from GPR and electrical data. Water content estimations of the medium are obtained from GPR data by comparing three different dielectric permittivity models for water-based porous materials: (1) an empirical model, (2) a semi-empirical fully-saturated model based on mixture theory and (3) a fully-saturated model based on the effective medium theory. Qualitative informations on water saturation are also deduced by combining two of these models. Finally, by combining water content estimations obtained from GPR interval velocities and conductivity measurements, a quantitative estimation of the salinity content of the medium is obtained. These studies show concrete applications in fluids flow and ground contamination monitoring.

Microstructural characterization of biobased carbon foam by means of X-ray microtomography and compared to conventional techniques

The objective of this work is to compare three techniques for characterizing the morphology of porous bio-based carbon foam, namely mercury intrusion porosimetry, scanning electron microscopy and X-ray microtomography. The first two techniques are commonly used in this field, while X-ray microtomography is a more recent characterization technique derived from medical tomography. Our study demonstrates the added value of the latter technique, especially concerning the evaluation of porosity, cell size distributions and cell connectivity. In this study we have shown that the material have a total porosity of around 90% with a cell size of less than 100 microns.

Spectral Analysis of Seismoelectric Signal for Water Saturation Monitoring

The seismo-electromagnetic method consists of measuring electric and magnetic fields (Bordes et al. 2006, 2008) induced by seismic wave propagation from electrokinetic coupling which are sensitive to the pore fluid properties. Even if the role of water content was studied at low frequencies (electrofiltration), the effect of saturation on seismoelectric is poorly understood. In this study, we performed laboratory experiments in order to monitor seismoelectric signals as a function of water saturation in sand (99% silica). First analysis on direct P waves in time domain seems to show a frequency dependence of the seismic and seismoelectric signal (Senechal et al., 2010). In this further discussion, a spectral analysis by Continuous Wavelet Transform (CWT) is proposed to study frequency content variations occuring during imbibition and/or drainage of the sand.

Seismic Characterization and Monitoring of Fresh Concrete Using Acoustic Waves

This study focuses on the seismic characteristics and behaviour of fresh concrete (water-cement ratio >2) designed for consolidation of soils, which was not really studied in the past. The propagation of acoustic waves, which constitute a potential non-destructive imaging tool in the future, was monitored and characterized using a specifically-designed experimental device during eight hours in the laboratory, to assess the evolution of the mixed material as a function of its maturity. This study highlights the very low values of P wave velocities in such a material, as well as the corresponding S waves velocities, their associated large attenuation and dispersion in the [0.5-6 kHz] frequency range. It has been found that attenuation remains the lowest when concrete mixture is younger than 90 minutes and that no velocity dispersion was detected. The study of velocity and attenuation at early age shows that the concrete has a fluid mixture with grains in suspension, and presents an elastic behaviour which stays poorly compressible during the first eight hours.

Characterization of agricultural contaminant transport using ground-penetrating radar and electrical data

In this on-going study, a combination of Ground Penetrating Radar (GPR) and electrical surveys is used to characterize the physico-chemical properties of soil, in particular those concerning lithology variations, water content, porosity and salinity in agricultural zones. These investigations will be completed by independent chemical analyses of soil and water. The experiments are conducted on fields of intensive corn culture (4 x 2 km) where the water table is 1.5 to 5 m deep, in a geological context of alluvial deposits. Analyses of water from various wells in the study area show high concentrations of nitrate coming from the chemical fertilizers spread on the soil surface. Globally, values increase from south to north and can be explained by the general direction of flow in the water table aquifer. In this on-going study, GPR and electrical data are recorded at two sites located along a north-south line and seasonal variations will be monitored for twelve months. Preliminary results which are presented here, give structural information concerning the subsurface and show vertical flow of mineralized water (from the surface to the aquifer) providing evidence of transport of dissolved contaminants to the water table aquifer. This study demonstrates the potential of non-destructive geophysical methods for providing information on hydric and solute transfer in the ground and monitoring soil contamination.