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

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

a review and some new issues on the theory of the h v technique for ambient vibrations

In spite of the Horizontal-to-Vertical Spectral Ratio (HVSR or H/V) technique obtained by the ambient vibrations is a very popular tool, a full theoretical explanation of it has been not reached yet. A short excursus is here presented on the theoretical models explaining the H/V spectral ratio that have been development in last decades. It leads to the present two main research lines: one aims at describing the H/V curve by taking in account the whole ambient-vibration wavefield, and another just studies the Rayleigh ellipticity. For the first theoretical branch, a comparison between the most recent two models of the ambient-vibration wavefield is presented, which are the Distributed Surface Sources (DSS) one and the Diffuse Field Approach (DFA). A mention is done of the current developments of these models and of the use of the DSS for comparing the H/V spectral ratio definitions present in literature. For the second research branch, some insights about the connection between the so-called osculation points of the Rayleigh dispersion curves and the behaviour of the H/V curve are discussed

Terrestrial laser scanning and electrical resistivity tomography as combined tools for the geoarchaeological study of the Kritsa-Latô dolines (Mirambello, Crete, Greece)

International audienceUn complexe de dépressions karstiques de la chaîne de montagnes du Dikté (secteur de Kritsa-Latô, Crète orientale) a fait l'objet d'études visant à caractériser leur topographie et leur sous-sol, là où des vestiges archéologiques ont pu être identifiés sous une épaisse couverture de sédiments, composée d'argiles de décalcification, de colluvions et de dépôts éoliens, et cela grâce à une nouvelle approche méthodologique. L'utilisation combinée des méthodes du scanner laser terrestre (SLT), de tomographie résistivité électrique (TRE) et l'intégration des résultats géophysiques et morphométriques dans un Système d'Information Géographique (SIG) favorisent une interprétation des données sédimentologiques provenant de l'échantillonnage de carottages légers. Des Modèles Numériques de Terrain (MNT) à haute résolution (2 m, 0,25 m) ont révélé dans les moindres détails la microtopographie ainsi que les traces d'occupation humaine dans ces dépressions karstiques au cours des derniers siècles. Une vision tridimensionnelle (3D) du secteur d'études a ainsi pu être restituée. Les données acquises permettent de mesurer précisément pour la première fois les dimensions ainsi que la géométrie des archives sédimentaires stockées dans le fond des dépressions karstiques. Les résultats mettent ainsi en évidence que l'action conjuguée de la dissolution des calcaires et de la tectonique locale ont créé ce complexe de pièges sédimentaires, à l'échelle du Mirambello. Les principaux résultats révèlent que dans la partie supérieure du substratum rocheux, un épikarst intensément fracturé, forme typique de l'activité karstique obtenue en contexte de recouvrement par une couverture sédimentaire, peut être identifié en même temps que des points d'infiltration et des pinacles. Ce relief accidenté en profondeur contraste avec la partie supérieure du fond de la doline, essentiellement composé de matériaux très fins (argiles limoneuses). L'épaisseur du manteau d'argiles rouges de colluvionnement atteint par endroit une épaisseur de 30 m environ, et aucune stratification n'a pu être identifiée par l'étude géophysique dans ces dépôts d'âge quaternaire. Les traces de l'occupation humaine au cours de la période historique sont mises en évidence par de nombreuses anomalies géophysiques: on identifie notamment l'existence d'un réseau d'irrigation d'âge minoen à hellénistique. Dans la mesure où l'approvisionnement en eau était une préoccupation majeure dans ce relief karstique des montagnes crétoises, le rôle joué par les dépressions karstiques dans les stratégies d'implantation des populations et des activités agricoles a été fondamental. L'association des données géomorphologiques et archéologiques du complexe de dépressions karstiques de la région de Kritsa nous fait mieux comprendre l'évolution des paysages dans ce secteur, ainsi que l'adaptation des sociétés à ce milieu, à priori inhospitalier, de Crète orientale. Des cartographies précises de SLT et de tomographie électrique ont permis de mieux appréhender le contexte karstique et en particulier la zone recouverte par les couches épaisses d'argiles et la surface du sol, autorisant ainsi l'ouverture vers de nouvelles approches sur des sites à potentiel archéologique

Holocene environmental changes in the highlands of the southern Peruvian Andes (14° S) and their impact on pre-Columbian cultures

High-altitude peatlands of the Andes still remain relatively unexploited although they offer an excellent opportunity for well-dated palaeoenvironmental records. <br><br> To improve knowledge about climatic and environmental changes in the western Andes of southern Peru, we present a high-resolution record of the Cerro Llamoca peatland for the last 8600 years. The 10.5 m long core consists of peat and intercalated sediment layers and was examined for all kinds of microfossils. We chose homogeneous peat sections for pollen analysis at decadal to centennial resolution. The inorganic geochemistry was analysed in 2 mm resolution (corresponding >2 years) using an ITRAX X-ray fluorescence core scanner. <br><br> We interpret phases of relatively high abundances of Poaceae pollen in our record as an expansion of Andean grasslands during humid phases. Drier conditions are indicated by a significant decrease of Poaceae pollen and higher abundances of Asteraceae pollen. The results are substantiated by changes in arsenic contents and manganese/iron ratios, which turned out to be applicable proxies for in situ palaeoredox conditions. <br><br> The mid-Holocene period of 8.6–5.6 ka is characterised by a series of episodic dry spells alternating with spells that are more humid. After a pronounced dry period at 4.6–4.2 ka, conditions generally shifted towards a more humid climate. We stress a humid/relatively stable interval between 1.8 and 1.2 ka, which coincides with the florescence of the Nasca culture in the Andean foothills. An abrupt turn to a sustained dry period occurs at 1.2 ka, which is contemporaneous with the demise of the Nasca/Wari society in the Palpa lowlands. Markedly drier conditions prevail until 0.75 ka, providing evidence of the presence of a Medieval Climate Anomaly. Moister but hydrologically highly variable conditions prevailed again after 0.75 ka, which allowed re-expansion of tussock grasses in the highlands, increased discharge into the Andean foreland and resettling of the lowlands during this so-called late Intermediate Period (LIP). <br><br> On a supraregional scale, our findings can ideally be linked to and proved by the archaeological chronology of the Nasca–Palpa region as well as other high-resolution marine and terrestrial palaeoenvironmental records. Our findings show that hydrological fluctuations, triggered by the changing intensity of the monsoonal tropical summer rains emerging from the Amazon Basin in the north-east, have controlled the climate in the study area

Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments

Two types of dispersive seismic waves have been acquired in different geological settings to investigate the potential to reveal the elastic parameters of the shallow marine subsurface. Scholte waves as well as acoustic guided waves are excited by a near-surface towed airgun, and recorded using two acquisition methods: (1) the towed-acquisition system using a hydrophone streamer towed close to the sea floor, and (2) the stationary-receiver method using Ocean-Bottom Seismometers and/or Hydrophones (OBS/OBH). Our diverse data sets reveal that the spatial sampling of the wavefield required to avoid aliasing may vary significantly for different geological settings. Scholte waves are characterised by a few distinct modes observed at low frequencies and low phase velocities. Their dispersion is mainly controlled by the depth profile of the shear-wave velocity. Acoustic guided waves show profound amplitude variations of numerous higher modes over a broad frequency range. These are sensitive to shear-wave velocity, but more sensitive to compressional-wave velocity than Scholte waves are. To avoid the identification of distinct modes we infer 1-D models of elastic parameters of the subsurface from the inversion of the full wavefield spectra of acoustic guided waves. In the Siberian Laptev Sea we infer the presence of a soft sediment layer (8-10 m) with a well resolved strong S-velocity gradient (150-450 m/s). In the Baltic Sea a low P-velocity layer with a strong vertical gradient (1250-1440 m/s) corresponding to a post-glacial gassy mud layer could be resolved, which agrees well with the sediment stratigraphy derived from a gravity core

Interpretation of microtremor 2D array data using Rayleigh and Love waves: the case study of Bevagna (central Italy)

In the last decades, geophysicists and seismologists have focused their attention on the inversion of empirical surface-waves’ dispersion curves from microtremor measurements for estimating the Swaves velocity structure at a site. This procedure allows a fast and convenient investigation without strong active sources, which are difficult to deploy especially in urban areas. In this study we report on a 2D seismic noise array experiment carried out at Bevagna (Central Italy) near the station BVG of the Italian Accelerometric Network (RAN). The site was investigated within the DPC-INGV S4 Project (2007-2009). The Rayleigh- and Love- waves dispersion characteristics were estimated using different methods. The inversion of the dispersion curves was then performed independently, obtaining two estimations for the S-waves velocity profiles. The results of cross-hole logging near the seismic station are used for a comparison. The shear waves velocity profiles estimated by microtremor analyses range up to 150m depth. The two independent procedures provide consistent shear waves velocity profiles for the shallow part of the model (20-30 m in depth) in agreement with the results of the cross-hole logging. Some problems arise between 30 and 40 m in depth in the profile estimated by surface waves. In this range cross-hole logging evidences an inversion of S-waves velocity. Although the cross-hole logging stops at 40 m of depth, we are confident about the results provided by the Rayleigh-waves analysis below 40-50 m. This case study suggests that greater efforts should be devoted to exploit the potential of a coupled analysis of Rayleigh and Love waves from microtremor array measurements.Published529-5404.1. Metodologie sismologiche per l'ingegneria sismicaJCR Journalope