Seismic site survey investigations in urban environments: The case of the underground metro project in Copenhagen, Denmark.
Near surface geophysics applications are gaining more widespread use in geotechnical and engineering projects. The development of data acquisition, processing tools and interpretation methods have optimized survey time, reduced logistics costs and increase results reliability of seismic surveys during the last decades. However, the use of wide-scale geophysical methods under urban environments continues to face great challenges due to multiple noise sources and obstacles inherent to cities. A seismic pre-investigation was conducted to investigate the feasibility of using seismic methods to obtain information about the subsurface layer locations and media properties in Copenhagen. Such information is needed for hydrological, geotechnical and groundwater modeling related to the Cityringen underground metro project. The pre-investigation objectives were to validate methods in an urban environment and optimize field survey procedures, processing and interpretation methods in urban settings in the event of further seismic investigations. The geological setting at the survey site is characterized by several interlaced layers of clay, till and sand. These layers are found unevenly distributed throughout the city and present varying thickness, overlaying several different unit types of limestone at shallow depths. Specific results objectives were to map the bedrock surface, ascertain a structural geological framework and investigate bedrock media properties relevant to the construction design. The seismic test consisted of a combined seismic reflection and refraction analyses of a profile line conducted along an approximately 1400 m section in the northern part of Copenhagen, along the projected metro city line. The data acquisition was carried out using a 192 channels array, receiver groups with 5 m spacing and a Vibroseis as a source at 10 m spacing. Complementarily, six vertical seismic profiles (VSP) were performed at boreholes located along the line. The reflection data underwent standard interpretation and the refraction included wavepath Eikonal traveltime tomography. The reflection results indicate the presence of horizontal reflectors with discontinuities likely related to deep lying structural features in deeper lying chalk layers. The refraction interpretation allowed the identification of the upper limestone surface, relevant to map for tunneling design. The VSP provided additional information regarding limestone quality and provided correlation data for improved refraction interpretation. In general, the pre-investigation results demonstrated that it is possible to image the limestone surface using the seismic method. The satisfactory results lead to the implementation of a 15 km survey planned during the spring 2009. The survey will combine reflection, refraction, walkaway-VSP and electrical resistivity tomography (ERT). The authors wish to acknowledge Metroselskabet I/S for permission in presenting the preliminary results and the Cityringen Joint Venture partners Arup and Systra.
Application of Near-Surface Geophysical Techniques for Earthquake Microzonation Mapping in the Ottawa, Ontario Region
Earthquake Microzonation maps of the Ottawa area have been developed by a combined team of researchers from the Geological Survey of Canada (GSC) and Carleton University. The city of Ottawa has an areal extent of 2796 square kilometers, consisting of three basic geological/geotechnical units: bedrock outcrop (15%), a thin veneer of glacial deposits (30%) and thick post-glacial lacustrine and marine sediments (Champlain Sea) known locally as the Leda Clay (55%). Following the guidelines of the current National Building Code of Canada (NBCC), soil classification zones were established according to the National Earthquake Hazards Reduction Program (NEHRP) which are based on shear wave velocity measurements of the upper 30m of soil/rock and are related to ground-motion amplification. For adequate characterization of the soil properties, geophysical data were collected at 680 surface refraction-reflection sites, 35 MASW sites, 31 borehole geophysical sites, and 185 passive soil resonance sites. Twenty-five line-km of high resolution Minivib Landstreamer shear wave reflection were collected in areas of complex bedrock topography. In addition, a borehole database compiled from existing water-well and geotechnical drilling consisting of approximately 32000 borings from previous GSC work was converted into the three major geotechnical units and assigned shear wave velocity-depth functions to each borehole site based on an inverse distance weighting algorithm of adjacent shear wave measurement sites. We have found extremely low shear wave velocities associated with the post-glacial sediments (∼150m/s), and extremely high shear wave velocities associated with the competent Paleozoic or PreCambrian bedrock (∼2800 m/s), yielding very large seismic impedance ratios. Within the city limits we have shown that all six NEHRP zones occur, and that changes between solid rock (NERHP zone A) and very soft soil (NEHRP zone E or F) can occur within a lateral distance of a few hundred meters. Buried bedrock valleys >100m deep with infilling of soft soil have been delineated, some of which may result in 2- or 3- dimensional ground motion amplification effects. A map of the travel-time weighted average shear wave velocity to a depth of 30m (Vs30) has been made which shows the current NBCC zone boundaries and can be used to estimate amplification for the 'design' earthquake. An accompanying map shows the fundamental site period variation within the city, based on shear wave velocities measured to bedrock. We have shown that the fundamental site periods estimated from shear wave velocities assigned to an equivalent single layer post- glacial model deviate systematically from those measured by passive Horizontal-to-Vertical Spectral Ratios (HVSR) at longer periods (∼2.0 seconds). Although the hazards maps we have developed reflect the recommendations of the current NBCC, our data compilation offers shear wave velocity-depth structure into firm bedrock at depth. This information can provide the framework for future soft soil research and be of value in the development of modifications to future Canadian national building codes.
Does the Microtremor Array Method Provide Reliable Vs Profiles in Comparison with Commerical Techniques?
Earthquake engineering practice is based significantly on the shear-wave velocity of subsurface sediments and rock. Our research examines the relatively-new microtremor array method of estimating shear-wave velocity profiles at two urban centers in SW British Columbia that represent very different geological environments, and compare the results to traditional methods. The microtremor array method is based on recording background seismic noise using a spatial array of several seismographs to extract the associated surface wave dispersion that is used to invert for the shear-wave velocity profile of the site. The method has become increasingly popular worldwide because it makes use of continuously-available and wide-band (0.02- 50 Hz) seismic noise, requires little equipment, and is unobtrusive to the site, resulting in relatively fast, easy, and low-cost measurements. We performed microtremor array measurements in the cities of Victoria and Vancouver, which represent end member geological conditions. The local geology of Victoria generally exhibits a strong near-surface impedance contrast (< 30 m of soft marine silts over very stiff bedrock), in contrast to Vancouver where impedance contrasts are much deeper and weaker (up to 500 m of over-consolidated glacial material and up to 500 m of deltaic sands and silts from the Fraser river, resulting in a maximum of 1 km of material over bedrock). The wide frequency band of the noise wavefield (due primarily to ocean waves at < 0.5 Hz and anthropogenic sources at > 1 Hz) is found to adequately characterize the subsurface, even in Vancouver. The shear-wave velocity profiles obtained at each site using various array configurations and dispersion analysis procedures are compared with shear-wave measurements from three different commercial techniques; seismic cone penetration testing, surface refraction, and surface-to-downhole.
Prediction of Fracture Behavior in Rock and Rock-like Materials Using Discrete Element Models
The study of fracture initiation and propagation in heterogeneous materials such as rock and rock-like materials are of principal interest in the field of rock mechanics and rock engineering. It is crucial to study and investigate failure prediction and safety measures in civil and mining structures. Our work offers a practical approach to predict fracture behaviour using discrete element models. In this approach, the microstructures of materials are presented through the combination of clusters of bonded particles with different inter-cluster particle and bond properties, and intra-cluster bond properties. The geometry of clusters is transferred from information available from thin sections, computed tomography (CT) images and other visual presentation of the modeled material using customized AutoCAD built-in dialog- based Visual Basic Application. Exact microstructures of the tested sample, including fractures, faults, inclusions and void spaces can be duplicated in the discrete element models. Although the microstructural fabrics of rocks and rock-like structures may have different scale, fracture formation and propagation through these materials are alike and will follow similar mechanics. Synthetic material provides an excellent condition for validating the modelling approaches, as fracture behaviours are known with the well-defined composite's properties. Calibration of the macro-properties of matrix material and inclusions (aggregates), were followed with the overall mechanical material responses calibration by adjusting the interfacial properties. The discrete element model predicted similar fracture propagation features and path as that of the real sample material. The path of the fractures and matrix-inclusion interaction was compared using computed tomography images. Initiation and fracture formation in the model and real material were compared using Acoustic Emission data. Analysing the temporal and spatial evolution of AE events, collected during the sample testing, in relation to the CT images allows the precise reconstruction of the failure sequence. Our proposed modelling approach illustrates realistic fracture formation and growth predictions at different loading conditions.
Seismicity Sequences Driven by Pore Pressure Oscillations
It is widely acknowledged that processes such as surface water reservoir impoundment and subsurface oil and hydrocarbon extraction can influence pore pressure, and subsequently induce seismicity. However, the precise causes of protracted seismicity in these circumstances are a source of continuing debate. Field evidence suggests that protracted seismicity can be caused by cyclic water level variation; however, there is currently a severe lack of experimental support. We report new data in which cyclic pore pressure experiments were carried out using a laboratory triaxial deformation apparatus to investigate the how pore pressure oscillations induce seismicity. The seismic responses were recorded using a 3D array of acoustic emission sensors, allowing the temporal and spatial reconstruction of hypocentres. Experiments were conducted in two stages. The first stage simulated fluid- induced initial seismicity by simulating a main shock - aftershock sequence through fault reactivation, where an increase in pore pressure reduced the effective pressure and induced slipping along the existing fault. We quantitatively demonstrate that the amplitude of the applied pore pressure is a key factor for causing rapid seismic responses. However, the decline in the number of aftershocks occurred more rapidly for high porosity sandstone than for low porosity sandstone. The higher porosity sandstone allowed downstream pore pressures to equilibrate with the upstream pore pressures instantaneously, while low porosity sandstone experiment exhibited phase shifts and transient pore pressure cycles at the downstream. In the second stage, fluid-induced protracted seismicity was simulated by a foreshock - main shock sequence, in which pore fluid cycles were kept at a lower peak level than the previous stage. The increase in the number of cycles allowed damages to accumulate and develop into a main slip. This protracted seismicity stage is well defined in low porosity sandstone, but is not obvious in the high porosity sample. This protracted seismicity phenomenon can be observed in shallow crustal reservoir-induced seismicity where the reduction in water level or pore pressure did not reduce seismicity. We show that, by analysing AE data in terms of inverse rate over time, we can successfully forecast the time of slip in the laboratory within ~10%. Our results suggest that by controlling the upstream reservoir impoundment and monitoring downstream observation wells for phase shifts and pressure change, more effective mitigation measures for reservoir-induced seismicity can be implemented.
Seismic Waveform Parameters and the Engineering Properties of Unconsolidated Sediments: Laboratory Measurements and Models
The ability to locate and monitor weaker soil/rock units in the subsurface non-invasively using geophysical measurements would be very useful for geotechnical engineers involved in geo-hazard mitigation. Velocity and attenuation studies indicate that velocity and attenuation of transmitted P-waves are affected by the microstructure and mechanical state of the sediments. This investigative work explores the use of direct information from the spectra of waveforms propagating though the unconsolidated medium, hypothesized here to provide us with useful information about the engineering and petrophysical properties of the medium. Numerical investigations using a reformulation of Biot's theory by indicate that the spectral signature, shape and frequency content as well as the distribution of spectral energy are sensitive to the porosity, degree of saturation and the skeletal frame modulus of the medium, which are important in determining its mechanical stability. It will be shown from laboratory investigations that the spectral signature, spectral energy distribution and frequency content of seismic waveforms propagating through unconsolidated geomaterials provide valuable information that can be used to characterize their engineering and petrophysical properties. Such investigations are desirable and will be of great interest to geotechnical engineers involved in monitoring and assessment of the strength and stability conditions of subsurface geo-materials and a geo-hazard mitigation and assessment.