Ludovic Baron

Hydrogeophysical characterization of transport processes in fractured rock by combining push-pull and single-hole ground penetrating radar experiments

The in situ characterization of transport processes in fractured media is particularly challenging due to the considerable spatial uncertainty on tracer pathways and dominant controlling processes, such as dispersion, channeling, trapping, matrix diffusion, ambient and density driven flows. We attempted to reduce this uncertainty by coupling push-pull tracer experiments with single-hole ground penetrating radar (GPR) time-lapse imaging. The experiments involved different injection fractures, chaser volumes and resting times, and were performed at the fractured rock research site of Ploemeur in France (H+ network, hplus.ore.fr/en). For the GPR acquisitions, we used both fixed and moving antenna setups in a borehole that was isolated with a flexible liner. During the fixed-antenna experiment, time-varying GPR reflections allowed us to track the spatial and temporal dynamics of the tracer during the push-pull experiment. During the moving antenna experiments, we clearly imaged the dominant fractures in which tracer transport took place, fractures in which the tracer was trapped for longer time periods, and the spatial extent of the tracer distribution (up to 8 m) at different times. This demonstrated the existence of strongly channelized flow in the first few meters and radial flow at greater distances. By varying the resting time of a given experiment, we identified regions affected by density-driven and ambient flow. These experiments open up new perspectives for coupled hydrogeophysical inversion aimed at understanding transport phenomena in fractured rock formations.

3-D density structure and geological evolution of Stromboli volcano (Aeolian Islands, Italy) inferred from land-based and sea-surface gravity data

We present the first density model of Stromboli volcano (Aeolian Islands, Italy) obtained by simultaneously inverting land-based (543) and sea-surface (327) relative gravity data. Modern positioning technology, a 1 × 1 m digital elevation model, and a 15 × 15 m bathymetric model made it possible to obtain a detailed 3-D density model through an iteratively reweighted smoothness-constrained least-squares inversion that explained the land-based gravity data to 0.09 mGal and the sea-surface data to 5 mGal. Our inverse formulation avoids introducing any assumptions about density magnitudes. At 125 m depth from the land surface, the inferred mean density of the island is 2380 kg m^−3, with corresponding 2.5 and 97.5 percentiles of 2200 and 2530 kg m^−3. This density range covers the rock densities of new and previously published samples of Paleostromboli I, Vancori, Neostromboli and San Bartolo lava flows. High-density anomalies in the central and southern part of the island can be related to two main degassing faults crossing the island (N41 and N64) that are interpreted as preferential regions of dyke intrusions. In addition, two low-density anomalies are found in the northeastern part and in the summit area of the island. These anomalies seem to be geographically related with past paroxysmal explosive phreato-magmatic events that have played important roles in the evolution of Stromboli Island by forming the Scari caldera and the Neostromboli crater, respectively.

Constraints on the Permeability Structure of Alluvial Aquifers From the Poro-Elastic Inversion of Multifrequency P-Wave Sonic Velocity Logs

We have explored the possibility of obtaining first-order permeability estimates for saturated alluvial sediments based on the poro-elastic interpretation of the P-wave velocity dispersion inferred from sonic logs. Modern sonic logging tools designed for environmental and engineering applications allow one for P-wave velocity measurements at multiple emitter frequencies over a bandwidth covering 5 to 10 octaves. Methodological considerations indicate that, for saturated unconsolidated sediments in the silt to sand range and typical emitter frequencies ranging from approximately 1 to 30 kHz, the observable velocity dispersion should be sufficiently pronounced to allow one for reliable first-order estimations of the permeability structure. The corresponding predictions have been tested on and verified for a borehole penetrating a typical surficial alluvial aquifer. In addition to multifrequency sonic logs, a comprehensive suite of nuclear and electrical logs, an S-wave log, a litholog, and a limited number laboratory measurements of the permeability from retrieved core material were also available. This complementary information was found to be essential for parameterizing the poro-elastic inversion procedure and for assessing the uncertainty and internal consistency of corresponding permeability estimates. Our results indicate that the thus obtained permeability estimates are largely consistent with those expected based on the corresponding granulometric characteristics, as well as with the available evidence form laboratory measurements. These findings are also consistent with evidence from ocean acoustics, which indicate that, over a frequency range of several orders-of-magnitude, the classical theory of poro-elasticity is generally capable of explaining the observed P-wave velocity dispersion in medium- to fine-grained seabed sediments.

Neutrally buoyant tracers in hydrogeophysics: Field demonstration in fractured rock

Electrical and electromagnetic methods are extensively used to map electrically conductive tracers within hydrogeologic systems. Often, the tracers used consist of dissolved salt in water, leading to a denser mixture than the ambient formation water. Density effects are often ignored and rarely modeled but can dramatically affect transport behavior and introduce dynamics that are unrepresentative of the response obtained with classical tracers (e.g., uranine). We introduce a neutrally buoyant tracer consisting of a mixture of salt, water, and ethanol and monitor its movement during push-pull experiments in a fractured rock aquifer using ground-penetrating radar. Our results indicate a largely reversible transport process and agree with uranine-based push-pull experiments at the site, which is in contrast to results obtained using dense saline tracers. We argue that a shift toward neutrally buoyant tracers in both porous and fractured media would advance hydrogeophysical research and enhance its utility in hydrogeology.

Analysis of the Velocity Dispersion and Attenuation Behavior of Multi‐Frequency Sonic Logs

Modern slim-hole sonic-logging tools designed for surficial environmental and engineering applications allow for measurements of the phase velocity and the attenuation of P-waves at multiple emitter frequencies over a bandwidth covering five to 10 octaves. One can explore the possibility of estimating the permeability of saturated surficial alluvial deposits based on the poroelastic interpretation of the velocity dispersion and frequency-dependent attenuation of such broadband sonic-log data. Methodological considerations indicate that for saturated, unconsolidated sediments in the fine silt to coarse sand range and typical nominal emitter frequencies ranging fromapproximately1 to30 kHz, the observable P-wave velocity dispersion should be sufficiently pronounced to allow for reliable first-order estimations of the underlying permeability structure based on the theoretical foundation of poroelastic seismic-wave propagation. Theoretical predictions also suggest that the frequency-dependent attenuation behavior should show a distinct peak and detectable variations for the entire range of unconsolidated lithologies. With regard to the P-wave velocity dispersion, results indicate that the classical framework of poroelasticity allows for obtaining first-order estimates of the permeability of unconsolidated clastic sediments with granulometric characteristics ranging between fine silts and coarse sands. The results of attenuation measurements are more difficult to interpret because the inferred attenuation values are systematically higher than the theoretically predicted ones, and the form of their dependence on frequency is variable and is only partially consistent with theoretical expectations. Introduction Permeability arguably is the most important but most elusive hydraulic parameter in native earthen materials, and it commonly can bemeasured only throughdedicated hydrologic laboratory and field experiments (e.g., Butler, 2005). Knowledge of the permeability distribution within an aquifer is a key prerequisite for reliable predictions of fluid flow and contaminant transport. This information is critical for the effective protection, remediation, and sustainable management of increasingly scarce and fragile groundwater resources in densely populated and/or highly industrialized regions. Geophysical constraints with regard to aquifer structure in general and to the distribution of hydraulic parameters in particular are considered to be especially valuable. The underlying methods are comparatively cheap and noninvasive. In addition, in terms of spatial resolution and coverage, they have the potential to bridge the gap between traditional hydrogeologic methods, such as core analyses and tracer or pumping tests (e.g., Hubbard and Rubin, 2005). Although standard traditional geophysical techniques cannot provide any direct information on the permeability of the integrated medium,more specialized approaches exhibit a more or less direct sensitivity to this important parameter. Along with nuclear magnetic resonance and spectrally induced polarization measurements, the interpretation of seismic data in a so-called poroelastic context arguably represents the most promising avenue to this end (e.g., Holliger, 2008). The methodological foundations of seismic-wave propagation in saturated porous media generally are credited to Biot (1956a, 1956b). The corresponding theoretical Institute of Geophysics, University of Lausanne, Lausanne, Switzerland.

The 3-D structure of the Somma-Vesuvius volcanic complex (Italy) inferred from new and historic gravimetric data

Existing 3-D density models of the Somma-Vesuvius volcanic complex (SVVC), Italy, largely disagree. Despite the scientific and socioeconomic importance of Vesuvius, there is no reliable 3-D density model of the SVVC. A considerable uncertainty prevails concerning the presence (or absence) of a dense body underlying the Vesuvius crater (1944 eruption) that is implied from extensive seismic investigations. We have acquired relative gravity measurements at 297 stations, including measurements in difficult-to-access areas (e.g., the first-ever measurements in the crater). In agreement with seismic investigations, the simultaneous inversion of these and historic data resolves a high-density body that extends from the surface of the Vesuvius crater down to depths that exceed 2 km. A 1.5-km radius horseshoe-shaped dense feature (open in the southwestern sector) enforces the existing model of groundwater circulation within the SVVC. Based on its volcano-tectonic evolution, we interpret volcanic structures that have never been imaged before.

Characterisation and Imaging of a Near-vertical Fault Zone in Crystaline Rock from Hydrophone VSP Data

Summary A shallow near-vertical hydrothermally active fault zone embedded in fractured crystalline rocks of the central Alps has been drilled and geophysically explored in view of its potential analogies to planned deep petrothermal reservoirs in the Alpine foreland. Hydrophone-based zero-offset vertical seismic profiling (VSP) data were found to be highly effective for detecting open fractures and determining seismic velocity changes due to deformation. Walk-away hydrophone VSP data were acquired with a 45-degree crooked-line survey geometry with respect to the borehole plane, which requires 3D processing methods for seismic imaging. For imaging purposes, a laterally changing velocity cube was generated from the ZVSP velocities and projected along the strike of the fault. The subsequent, pre-stack-depth-migration imaging has been successful in delineating vertical structures, the target fault core, and an as of yet unknown, and correspondingly enigmatic, sub-horizontal structural feature.

Hydrogeophysical characterization of transport processes in fractured rock by combining push-pull and single-hole ground penetrating radar experiments: TRANSPORT PROCESSES IN FRACTURED ROCK

The in situ characterization of transport processes in fractured media is particularly challenging due to the considerable spatial uncertainty on tracer pathways and dominant controlling processes, such as dispersion, channeling, trapping, matrix diffusion, ambient and density driven flows. We attempted to reduce this uncertainty by coupling push-pull tracer experiments with single-hole ground penetrating radar (GPR) time-lapse imaging. The experiments involved different injection fractures, chaser volumes and resting times, and were performed at the fractured rock research site of Ploemeur in France (H+ network, hplus.ore.fr/en). For the GPR acquisitions, we used both fixed and moving antenna setups in a borehole that was isolated with a flexible liner. During the fixed-antenna experiment, time-varying GPR reflections allowed us to track the spatial and temporal dynamics of the tracer during the push-pull experiment. During the moving antenna experiments, we clearly imaged the dominant fractures in which tracer transport took place, fractures in which the tracer was trapped for longer time periods, and the spatial extent of the tracer distribution (up to 8 m) at different times. This demonstrated the existence of strongly channelized flow in the first few meters and radial flow at greater distances. By varying the resting time of a given experiment, we identified regions affected by density-driven and ambient flow. These experiments open up new perspectives for coupled hydrogeophysical inversion aimed at understanding transport phenomena in fractured rock formations.

Constraints on the permeability structure of alluvial aquifers from P-wave sonic logs

Summary Modern sonic logging tools designed for shallow environmental and engineering applications allow for Pwave phase velocity measurements over a wide frequency band. Methodological considerations indicate that, for saturated unconsolidated sediments in the silt to sand range and source frequencies ranging from approximately 1 to 30 kHz, the observable poro-elastic P-wave velocity dispersion is sufficiently pronounced to allow for reliable first-order estimations of the underlying permeability structure. These predictions have been tested on and verified for a surficial alluvial aquifer. Our results indicate that, even without any further calibration, the thus obtained permeability estimates as well as their variabilities within the pertinent lithological units are remarkably close to those expected based on the corresponding granulometric characteristics.