Weathering and hydrological processes in Earth's shallow subsurface are influenced by inherited bedrock structures, such as bedding planes, faults, joints, and fractures. However, these structures are difficult to observe in soil‐mantled landscapes. Steeply dipping structures with a dominant orientation are detectable by seismic anisotropy, with fast wave speeds along the strike of structures. We measured shallow (~2–4 m) seismic anisotropy using “circle shots,” geophones deployed in a circle around a central shot point, in a weathered granite terrain in the Laramie Range of Wyoming. The inferred remnant fracture orientations agree with brittle fracture orientations measured at tens of meters depth in boreholes, demonstrating that bedrock fractures persist through the weathering process into the shallow critical zone. Seismic anisotropy positively correlates with saprolite thickness, suggesting that inherited bedrock fractures may control saprolite thickness by providing preferential pathways for corrosive meteoric waters to access the deep critical zone.

International audienceThe 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

The simultaneous estimation of 2D pressure (P-) and S-wave velocities ([Formula: see text] and [Formula: see text], respectively) is a promising approach for imaging subsurface mechanical properties. It can be performed with a single acquisition setup by combining P-wave refraction and surface-wave (SW) analysis. Although SW methods are commonly applied for the 1D estimation of [Formula: see text], 2D profiling requires the implementation of specific processing and inversion tools not yet widely available in the community. We have developed an open-source MATLAB-based package that performs SW inversion and profiling (SWIP) so as to retrieve 1D to 2D variations of [Formula: see text] from any kind of linear active-source near-surface seismic data. Each step of the workflow involves up-to-date processing and inversion techniques and provides ready-to-use outputs with quality control tools. First, windowing and stacking techniques are implemented to enhance the signal-to-noise ratio and extract local dispersion images along the line. Then, dispersion curves are picked for each window with an uncertainty range in the phase velocity including higher uncertainties at low frequency. These curves are next inverted using a Monte Carlo approach with various parameterizations (e.g., user defined, refraction based). The best models are finally selected according to their fit to the data to build an average final model with a suggested investigation depth. As an example, we used SWIP to process data collected at a Yellowstone hydrothermal system. Our results show the benefits of estimating [Formula: see text] and [Formula: see text] from a single seismic setup to highlight subsurface gas pathways.