Shape Of Bedrock Research Articles

New method to estimate bedrock shape of small-scale basin using modal properties of sediment

2-D resonances are known to occur in sedimentary basins, and it has been reported that the fundamental mode shapes appear to reflect the bedrock shape. This suggests that the bedrock shape of a valley can be estimated by evaluating the relationship between the mode shapes and the bedrock shape, and by identifying the resonance modes from the observation records of the ground motions. In the present study, the fundamental mode shapes of a small-scale valley were identified by applying the frequency domain decomposition (FDD) technique to microtremor observation records. 2-D finite difference models were used to investigate the relationship between the mode shapes and the bedrock shape. A new method was proposed for estimating the bedrock shape by comparing the mode shapes obtained from observation records and a numerical analysis. Furthermore, the applicability of the new method was confirmed using synthetic and observed data.

Geophysical methods for mapping Quaternary sediment thickness: Application to the Saint-Lary basin (French Pyrenees)

This study aims at mapping the sediment infill thickness in the Saint-Lary basin (Aure valley, French Pyrenees). For this purpose, we combine passive seismic and gravity surveys. The resonance frequencies of the sediment body are retrieved from seismic ambient measurements, while the gravimetric survey shows negative residual anomaly of about −3 mGal in the basin. Both methods reveal unexpected but consistent bedrock shape. The southern Saint-Lary basin appears deeper than its northern part, with maximal infill thickness of about 300 m and 150 m, respectively. Valley cross sections show regular and smooth “U”-shape in the southern Saint-Lary basin, in contrast to an irregular and asymmetric pattern in the northern basin. This basin shape may be related to Quaternary fluvio-glacial carving processes especially controlled by a regional fault (the Soulan fault), variations in bedrock hardness, and preferential ice flow paths.

A novel technique to monitor subsurface movements of landslides

Slope deformation sensors (SDSs) were developed to monitor profiles of soil deformation at a high frequency during slope-monitoring and landslide-triggering experiments. It was hypothesized that the surface and subsurface movements could be combined to integrate the temporal development of the movements and help monitor the initiation and propagation of the shear bands indirectly, as well as predict the volume of the eventual landslide. Four SDSs were installed in a 38° slope in Northern Switzerland and slope movements due to two artificial rainfall sprinkling experiments in October 2008 and March 2009 were monitored. This paper describes the design, numerical validation, installation details, and performance of the SDSs during the first rainfall event. The data acquired from SDSs in terms of bending strain, deformation profiles, and an indication of the mechanical energy transmitted from the surrounding soil are analysed and compared with the patterns of surface movements of the slope and changes in the horizontal earth pressure. The findings are interpreted based on the applied rainfall, hydrological properties of the slope, bedrock shape, and specifications of the observed failure surface in the subsequent landslide triggering experiment. Details of the data acquired from SDSs during the second experiment in March 2009 are reported and analysed in a forthcoming paper.

A simplified geographical information systems (GIS)-based methodology for modeling the topography of bedrock: illustration using the Canadian Shield

Many geology, mining, and geotechnical applications require or depend upon some form of modeling of bedrock topography. Optimizing the manner with which bedrock topography is modeled poses a significant challenge because of the unpredictable or erratic presentation of the surface shape of bedrock. Unlike surface topography, bedrock topography is more difficult to determine because direct observation points are often not readily or directly accessible, unless the bedrock outcrops at the surface and is exposed, a relatively rare occurrence. When bedrock is covered by granular deposits, the only methods that allow practitioners to objectively establish the location of the top of the bedrock are to drill boreholes or conduct geophysical surveys. This makes the determination of bedrock topography not only difficult but also expensive. This study proposes a new approach for optimizing the modeling of complex bedrock topography, whose originality is based on the addition of “virtual” data points derived from cross-sections located between known boreholes. The proposed methodology is thus composed of four steps: gathering the maximum amount of relevant surface and subsurface data (from observation points), selecting the most appropriate technique for interpolating the observed bedrock elevations that will be entered into the dataset to be modeled, enriching the quantity of modeling data by adding “virtual” data elements based on geological interpretations of cross-sections (inserted into the model alongside the original objective data), and finally the modeling itself. The proposed approach is illustrated using data from a study area located in the Canadian Shield. Thousands of borehole records and surficial geological data as well as geological cross-section records were integrated to construct a three-dimensional bedrock topography model. The new proposed methodology can be applied to other regions worldwide.

Numerical simulation of the flow velocity and change in the future of the SG4

Based on glacier dynamic theory, a two-dimensional velocity field model is developed to apply for the Qilian Shuiguan River glacier no. 4 (SG4). Our analysis shows that the glacier bedrock slope and shape affect the ice flow velocity together. The hydrological parameter k was adjusted in the model to fit the measured average flow velocities in summer and winter. The simulated results show that parameter k has a considerable impact on the glacier surface velocity, indicating that the water pressure under the glacier’s base is a major factor leading to basal sliding and ice velocity. The simulation shows that the glacier thickness, bedrock slope, bedrock shape, and basal slide jointly effect the glacier movement. The annual average mass balance was used to forecast the changes over the next 50 years based on the continuity equation. The results forecast the shrinkage of nearly 400 m in the terminus and glacier area over the next 50 years; the longitudinal section will diminish by nearly 29.2 %, and the average thickness will decrease by 47 %. If the effective glacier hydrology and geothermal temperature changes are taken into account, the actual retreat will be more substantial than that shown in our model.

Delineating ShallowS‐Wave Velocity Structure Using Multiple Ambient‐Noise Surface‐Wave Methods: An Example from Western Junggar, China

Abstract Aiming for a geophysical survey to around 1.5 km depth, we conduct a passive‐source surface‐wave survey along a 70‐km‐long line on the western side of the Junggar basin in northwest China. We use frequency–time analysis and spatial autocorrelation methods to extract surface‐wave dispersion curves from ambient‐noise data of station pairs, as well as ambient‐noise multichannel analysis of surface waves from multistations. Dispersion curves corresponding to the same positions obtained by these three methods independently are comparable, which shows the validity of using these methods. After inverting more than 400 dispersion curves and aligning their results together, we construct a 2D shear S ‐wave velocity image around 1.5 km deep and 60 km long. A less than 6% difference between the inverted S ‐wave velocity profile and the borehole data in situ shows the relatively high accuracy of our method. The 2D S ‐wave velocity image clearly delineates the main geologic structures, including transition of landform, intrusion of rock mass, and shape of bedrock beneath the survey line.

Influence of Lateral Heterogeneities on Strong‐Motion Shear Strains: Simulations in the Historical Center of Rome (Italy)

Abstract The influence of lateral heterogeneities on alluvial deposits is a topic of particular interest in the field of urban planning and engineering design of structures and infrastructures. This work focuses on the effects of such heterogeneities on the shear strains produced within the recent alluvial deposits of the Tiber River in the historical center of Rome in case of the worst expected earthquake scenario. To this aim, a 3D engineering‐geology model of the subsoil is used to derive four geological sections across the Tiber River valley as well as 48 soil columns to perform numerical simulations. Various models are considered: a viscoelastic equivalent linear rheology in a 1D finite‐difference model for one‐component horizontal input, a nonlinear elastoplastic model in a 1D finite‐element scheme for three‐component input, and a nonlinear viscoelasto‐plastic rheology in a 2D finite‐difference model under one‐component horizontal input. After comparing these different simulations, results have shown that lateral heterogeneities play a key role with respect to the expected shear strains within multilayered soils. To this aim, some specific indexes are introduced to estimate the maximum shear strain (MSS) concentration within the soil layers as well as to highlight their effect due to the stratigraphic position of the layers, within the soil column, independently from its depth. A final differential index leads to the evaluation of the lateral heterogeneity effect on the estimated MSS, demonstrating their prevalent role with respect to the bedrock shape (i.e., the angle of inclination of the buried valley slopes). From these results, an MSS zoning map is obtained for the historical center of Rome, showing that the local seismic response should be modeled by assuming 1D or 2D conditions depending on the location considered.

Effects of Bedrock Shape on a Landslide under the Seismic Force

Effects of Bedrock Shape on a Landslide under the Seismic Force