Sylvain Pasquet

Geophysical imaging of shallow degassing in a Yellowstone hydrothermal system

The Yellowstone Plateau Volcanic Field, which hosts over 10,000 thermal features, is the worlds largest active continental hydrothermal system, yet very little is known about the shallow “plumbing” system connecting hydrothermal reservoirs to surface features. Here we present the results of geophysical investigations of shallow hydrothermal degassing in Yellowstone. We measured electrical resistivity, compressional-wave velocity from refraction data, and shear wave velocity from surface-wave analysis to image shallow hydrothermal degassing to depths of 15–30 m. We find that resistivity helps identify fluid pathways and that Poissons ratio shows good sensitivity to saturation variations, highlighting gas-saturated areas and the local water table. Porosity and saturation predicted from rock physics modeling provide critical insight to estimate the fluid phase separation depth and understand the structure of hydrothermal systems. Finally, our results show that Poissons ratio can effectively discriminate gas- from water-saturated zones in hydrothermal systems.

2D characterization of near-surface V P/V S: surface-wave dispersion inversion versus refraction tomography

The joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling.

Surface-wave Dispersion Inversion versus SH-wave Refraction Tomography in Saturated and Poorly Dispersive Quick Clays

A seismic survey involving two distinct acquisition setups, with vertical and horizontal component geophones, has been carried out along the same line on a site presenting a simple vertical structure (peat, quick clays and bedrock) and no strong lateral variations. SH-wave refraction tomography and Rayleigh-wave dispersion inversion provided the same shear-wave velocity gradient in the quick clays. SH-wave tomography failed to correctly depict the peat layer and to reach the bedrock. A collection of Rayleigh-wave dispersion images were extracted along the line using windowing and stacking techniques. A thorough analysis of these images made it possible to give a complete description of the site velocity structure, illustrating the complementarity of both methods.

Temporal Variations of Near-surface Seismic Data at the Ploemeur (France) Hydrogeological Observatory

Near-surface seismic methods are mainly used to determine the geometrical characteristics of hydrosystems (and to provide elements that are interesting for hydrogeologists such as separating aquifer layers, setting up systems boundaries, highlighting fractures etc.). Recent methodological advances suggest the high potential of seismic methods to investigate the mechanical properties of the Critical Zone (CZ), by exploiting the full wealth of seismic records. Indeed, the behavior of Shear (S) and Pressure (P) waves in the presence of water is partially decoupled, so that the ratio of their propagation velocities VP/VS is strongly linked to water saturation. We propose here a time-lapse application of this approach. Two seismic acquisitions were carried out under distinct hydrogeological conditions along the same line at the Ploemeur hydrogeological observatory (South Brittany, France). Vertical component seismic data were recorded to extract: (i) P-wave first arrival times and (ii) Rayleigh-wave phase velocities. The significant variations with time and space, of both datasets, indicate marked changes in mechanical properties of the CZ that have to be compared to soil moisture variations in the unsaturated zone and groundwater level variations.

Contribution of Seismic Methods to Hydrogeophysics

The characterisation and monitoring of aquifer systems mainly rely on piezometric and log data. Delineating spatial variations of lithology between piezometers is a delicate task, which inevitably generates errors possibly propagating into hydrogeological models. Seismic methods have been proposed to: (i) improve the low spatial resolution of borehole data, (ii) provide a characterisation of the subsurface geometry, and (iii) estimate the physical parameters of the medium influenced by the presence of water and the associated flow and transport processes. The joint study of pressure (P-) and shear (S-) wave seismic velocities (VP and VS, respectively), whose evolution is strongly decoupled in the presence of fluid, has been proposed through the estimation of the VP/VS ratio and Poissons ratio. A specific methodology has been developed for the combined exploitation of P- and surface waves present on single seismic records. The use of this methodology in several geological and hydrogeological contexts allowed for estimating VP/VS ratio lateral and temporal variations in good agreement with a priori geological information and existing geophysical and piezometric data. Laser-based ultrasonic techniques were also proposed to put these processing techniques in practice on perfectly controlled physical models and study elastic wave propagation in partially saturated porous media.

Integrated Workflow for Surface-wave Dispersion Inversion and Profiling

While surface-wave prospecting methods are classically applied for the one-dimensional (1D) estimation of shear (S-) wave velocities (Vs), two-dimensional (2D) profiling still requires implementing specific processing and inversion tools that are not yet widely available in the community. We present here a free and open-source tool performing surface-wave inversion and profiling (SWIP) in order to retrieve 2D lateral variations of Vs from typical seismic shot records. Windowing and stacking techniques are implemented to compute dispersion images with smooth lateral variations and enhanced signal-to-noise ratio. Dispersion curves are extracted for each window with an error in phase velocity taking into account the higher uncertainties at low frequency. These curves are then inverted for each window position using a Monte Carlo approach and a refraction tomography-based parameterization. Models matching the observed data within the error bars are selected to build a misfit-weighted final model and estimate the investigation depth. Finally, 1D models obtained for each window position are merged into a 2D Vs section.

Seismic Surface-wave Analysis for Railway Platform Auscultation

The renewal of existent railways requires the characterisation of the mechanical properties of railway platforms (RP), thus raising the need to select appropriate maintenance actions. Conventional techniques (geotechnical soundings, coring) remain local, destructive, expensive and with low yields. Using non-destructive investigation techniques for local diagnosis and monitoring thus appears of great interest for enhancing RP control. Seismic surface-wave methods have been proposed to estimate in situ mechanical parameters of the superficial layers below railways. In this context, a joint geotechnical and seismic survey was carried out along the Northern Europe high-speed line (LGV) in order to precisely determine the origins of a phenomenon affecting the geometry of the track. Strong a priori knowledge of the RP structure allowed for inverting dispersion measurements for 1D VS models along the track. The results showed a contrast of VS in the loess lying below the RP, between areas where the phenomenon was observed and those it was not. This contrast was confirmed by Bender Elements measurements of VS performed on core drilling samples, and corresponded to the lateral variations observed along the track. These results encourage considering dispersion measurements as an appropriate tool of RP monitoring.

Surface-wave Analyses in Unconsolidated Granular Models with Increasing Degrees of Complexity

Using micrometric glass beads, we build small scale physical models with increasing degrees of complexity in order to address theoretical and methodological issues of seismic methods (velocity gradients, lateral variations, pore overpressure, etc.). We simulate seismic records at the surface of the laboratory models thanks to a mechanical source and a laser-Doppler vibrometer. From recorded seismograms, we are able to invert surface-wave dispersion for one or two-dimensional velocity structures. These experiments are for instance used as benchmarks for processing and inversion techniques, enable the validation of numerical methods, or make it possible to study issues related to pore fluids.

Surface-wave Dispersion Stacking on a Granite- micaschists Contact at Ploemeur Hydrological Observatory, France

In the context of a geophysical survey at the Ploemeur hydrological observatory (France), we performed surface-wave profiling for the characterisation of shallow subsurface Shear-wave velocities. Since we anticipated lateral variations but needed great investigation depth, we deployed multifold acquisition geometries and used roll-along dispersion stacking to enable efficient measurements of multi-modal dispersion data. Several offset moving windows have been tested. Represented as pseudo-sections, the phase velocities extracted using a 12-trace window clearly showed three areas coherent with field observation and interestingly consistent with electrical conductivities and P-wave first arrival times. This cross-quality control has been of great help in the choice of the moving window size and revealed itself to be a rewarding step prior to the inversion process.