Seismic imaging of sandbox experiments – laboratory hardware setup and first reflection seismic sections

Abstract. With the study and technical development introduced here, we combine analogue sandbox simulation techniques with seismic physical modelling of sandbox models. For that purpose, we designed and developed a new mini-seismic facility for laboratory use, comprising a seismic tank, a PC-driven control unit, a positioning system, and piezo-electric transducers used here the first time in an array mode. To assess the possibilities and limits of seismic imaging of small-scale structures in sandbox models, different geometry setups were tested in the first experiments that also tested the proper functioning of the device and studied the seismo-elastic properties of the granular media used. Simple two-layer models of different materials and layer thicknesses as well as a more complex model comprising channels and shear zones were tested using different acquisition geometries and signal properties. We suggest using well sorted and well rounded grains with little surface roughness (glass beads). Source receiver-offsets less than 14 cm for imaging structures as small as 2.0–1.5 mm size have proven feasible. This is the best compromise between wide beam and high energy output, and being applicable with a consistent waveform. Resolution of the interfaces of layers of granular materials depends on the interface preparation rather than on the material itself. Flat grading of interfaces and powder coverage yields the clearest interface reflections. Finally, sandbox seismic sections provide images of very good quality showing constant thickness layers as well as predefined channel structures and fault traces from shear zones. Since these can be regarded in sandbox models as zones of decompaction, they behave as reflectors and can be imaged. The multiple-offset surveying introduced here improves the quality with respect to S/N-ratio and source signature even more; the maximum depth penetration in glass bead layers thereby amounts to 5 cm. Thus, the presented mini-seismic device is already able to resolve structures within simple models of saturated porous media, so that multiple-offset seismic imaging of shallow sandbox models, that are structurally evolving, is generally feasible.</jats:p

Detection of bearing damage by statistic vibration analysis

The condition of bearings, which are essential components in mechanisms, is crucial to safety. The analysis of the bearing vibration signal, which is always contaminated by certain types of noise, is a very important standard for mechanical condition diagnosis of the bearing and mechanical failure phenomenon. In this paper the method of rolling bearing fault detection by statistical analysis of vibration is proposed to filter out Gaussian noise contained in a raw vibration signal. The results of experiments show that the vibration signal can be significantly enhanced by application of the proposed method. Besides, the proposed method is used to analyse real acoustic signals of a bearing with inner race and outer race faults, respectively. The values of attributes are determined according to the degree of the fault. The results confirm that the periods between the transients, which represent bearing fault characteristics, can be successfully detected

Crustal structure of the Peruvian continental margin from wide-angle seismic studies

Active seismic investigations along the Pacific margin off Peru were carried out using ocean bottom hydrophones and seismometers. The structure and the P-wave velocities of the obliquely subducting oceanic Nazca Plate and overriding South American Plate from 8°S to 15°S were determined by modelling the wide-angle seismic data combined with the analysis of reflection seismic data. Three detailed cross-sections of the subduction zone of the Peruvian margin and one strike-line across the Lima Basin are presented here. The oceanic crust of the Nazca Plate, with a thin pelagic sediment cover, ranging from 0–200 m, has an average thickness of 6.4 km. At 8°S it thins to 4 km in the area of Trujillo Trough, a graben-like structure. Across the margin, the plate boundary can be traced to 25 km depth. As inferred from the velocity models, a frontal prism exists adjacent to the trench axis and is associated with the steep lower slope. Terrigeneous sediments are proposed to be transported downslope due to gravitational forces and comprise the frontal prism, characterized by low seismic P-wave velocities. The lower slope material accretes against a backstop structure, which is defined by higher seismic P-wave velocities, 3.5–6.0 km s−1. The large variations in surface slope along one transect may reflect basal removal of upper plate material, thus steepening the slope surface. Subduction processes along the Peruvian margin are dominated by tectonic erosion indicated by the large margin taper, the shape and bending of the subducting slab, laterally varying slope angles and the material properties of the overriding continental plate. The erosional mechanisms, frontal and basal erosion, result in the steepening of the slope and consequent slope failure

Analogue experiments of salt flow and pillow growth due to basement faulting and differential loading

Salt flow in sedimentary basins is mainly driven by differential loading and can be described by the concept of hydraulic head. A hydraulic head in the salt layer can be imposed by vertically displacing the salt layer (elevation head) or the weight of overburden sediments (pressure head). Basement faulting in salt-bearing extensional basins is widely acknowledged as a potential trigger for hydraulic heads and the growth of salt structures. In this study, scaled analogue experiments were designed to examine the kinematics of salt flow during the early evolution of a salt structure triggered by basement extension. In order to distinguish flow patterns driven by elevation head or by pressure head, we applied a short pulse of basement extension, which was followed by a long-lasting phase of sedimentation. During the experiments viscous silicone putty simulated ductile rock salt, and a PVC-beads/quartz-sand mixture was used to simulate a brittle supra-salt layer. In order to derive 2-D incremental displacement and strain patterns, the analogue experiments were monitored using an optical image correlation system (particle imaging velocimetry). By varying layer thicknesses and extension rates, the influence of these parameters on the kinematics of salt flow were tested. Model results reveal that significant flow can be triggered in the viscous layer by small-offset basement faulting. During basement extension downward flow occurs in the viscous layer above the basement fault tip. In contrast, upward flow takes place during post-extensional sediment accumulation. Flow patterns in the viscous material are characterized by channelized Poiseuille-type flow, which is associated with subsidence in regions of "salt" expulsion and surface uplift in regions of inflation of the viscous material. Inflation of the viscous material eventually leads to the formation of pillow structures adjacent to the basement faults (primary pillows). The subsidence of peripheral sinks adjacent to the primary pillow causes the formation of additional pillow structures at large distance from the basement fault (secondary pillows). The experimentally obtained structures resemble those of some natural extensional basins, e.g. the North German Basin or the Mid-Polish Trough, and can aid understanding of the kinematics and structural evolution of sedimentary basins characterized by the presence of salt structures

Crustal structure along the Peruvian Margin from wide angle seismic data

Within the GEOPECO project (Geophysical Experiments at the Peruvian Continental Margin - investigations of tectonics, mechanics, gas hydrates and fluid transport) seismic refraction and reflection data were acquired during RV 'Sonne' cruise SO 146 along with bathymetric and gravimetric mapping, sea-floor sampling, observation of the ocean floor and heat flow measurements. The objectives were a quantitative characterization of the structures and geodynamics of the Peruvian section of the Andean subduction zone and the associated gas hydrate systems in regions with differing tectonic development. The oceanic Nazca Plate, which is approximately 28 to 38 million years new at the Peruvian trench, is subducting under the South American Plate. The Peruvian Continental Margin has been influenced over the last 8 million years by collision with the Nazca Ridge, a 400 km long and 50 km wide basement high. Collision migrated progressively from north to south, is presently in the area of 15°S and has influenced the area to the north in several ways. Six wide angle seismic profiles, each approximately 100nm long, were shot with three 32 liter Bolt-airguns over 9 to 14 OBH/S instruments at the Peruvian Margin. During the cruise a total amount of 127 OBH/S were successfully deployed showing high quality data. Forward modeling was performed to characterize the structure and the velocities of the different stages of the evolution of the margin after collision with the Nazca Ridge. The coincident reflection seismic profiles were used to constrain the structure and thickness of the upper layers. The resulting crustal cross sections reveal a rough surface and a thin sediment layer of the subducting oceanic Nazca Plate. The crust thickens beneath the Nazca Ridge. Its thickness also varies north and south of Mendana Fracture Zone (MFZ), which separates younger (~25 Ma old) from older (~35 Ma old) oceanic crust at about 11°S. There is no accretionary wedge where Nazca Ridge currently subducts. 3 Ma after the ridge has passed, a new accretionary prism is already set up with a width of 20 to 30 km and 4 to 5 km thickness which does not further increase in size as revealed by the profiles recorded further north of Nazca Ridge. This indicates that current subduction along the Peruvian Margin is non-accreting. The slope angle of the accretionary prism increases south of MFZ, whereas the profile north of MFZ shows a smaller slope angle. As the subducting Nazca Plate dips at about 6° on all profiles north of Nazca Ridge, the resulting taper is 12° to 17°, indicative of high basal friction and non-accretionary subduction. The horst and graben like structure and rough topography of the oceanic plate also substantiates non-accretionary even erosional subduction for the graben structures are filled with sediment before subduction. Two cross profiles from Lima Basin reveal the crustal structure of the continental slope. Lima Basin is some 80 km wide (along dip) and its thickness varies from 1 to 3 km below sea floor. Furthermore it shows an asymmetric shape and is divided into two parts by a basement high at the landward termination

Reinterpretation of the effective elastic thickness in terms of Young\u2019s modulus variation applying the analytical solution for an Elastic Plate (ASEP) to the Barents Sea

We apply the analytical solution for an elastic plate (ASEP), which solves the 4th order differential equation for the flexure of a thin plate to the Barents Sea in order to calculate the flexural rigidity. To constrain our analysis we make use of a 3D density model based on the Barents50 model [Ritzmann et al. 2006]. The density model provides information about the crustal configuration, e.g. the Moho and the loading in the crust including all internal density variation. The loading in combination with the ASEP allows us to calculate the flexure Mohos, and by comparison with the reference Moho, the flexural rigidity distribution. The resulting flexural rigidity distributions will be used to validate tectonic concepts, e.g. the location of the proposed Caledonian suture. In the past the effective elastic thickness (EET) has been used synonymously for the flexural rigidity, since it was defined by the material parameters of Young's modulus and Poisson ratio, which were assumed to be constant. The application of the ASEP shows, that it is sufficient to operate with a constant value for the Poisson's ratio, as the variation does not lead to a significant change in the result. However, concerning the vertical and horizontal variation of crustal composition, which corresponds to a change of Young modulus by orders of magnitude - the use of a constant standard value in the calculation process is doubtful. For that reason the EET distribution was recalculated including the Young's modulus variation, which could be estimated by using the p- wave velocities of the Barents50 model. From the viewpoint of solid-state physics the elastic thickness concept should be reconceived. The EET corresponds theoretically to a thickness of a flexed plate, which consists of a material describable by a constant Young's modulus. Therefore the obtained EET distribution could be related to a Young's modulus variation, if the calculation was done with a constant assumed standard value. If the crust and the upper mantle have a non-uniform Young's modulus, the calculated flexural rigidity distribution is only valid for the crust but not for the lithosphere. These investigations ought to demonstrate the importance of the consideration of the Young's modulus variation in the EET calculation

Seismic imaging of sandbox experiments – laboratory hardware setup and first reflection seismic sections

With the study and technical development introduced here, we combine analogue sandbox simulation techniques with seismic physical modelling of sandbox models. For that purpose, we designed and developed a new mini-seismic facility for laboratory use, comprising a seismic tank, a PC-driven control unit, a positioning system, and piezoelectric transducers used here for the first time in an array mode. To assess the possibilities and limits of seismic imaging of small-scale structures in sandbox models, different geometry setups were tested in the first 2-D experiments that also tested the proper functioning of the device and studied the seismo-elastic properties of the granular media used. Simple two-layer models of different materials and layer thicknesses as well as a more complex model comprising channels and shear zones were tested using different acquisition geometries and signal properties. We suggest using well sorted and well rounded grains with little surface roughness (glass beads). Source receiver-offsets less than 14 cm for imaging structures as small as 2.0–1.5 mm size have proven feasible. This is the best compromise between wide beam and high energy output, and is applicable with a consistent waveform. Resolution of the interfaces of layers of granular materials depends on the interface preparation rather than on the material itself. Flat grading of interfaces and powder coverage yields the clearest interface reflections. Finally, sandbox seismic sections provide images of high quality showing constant thickness layers as well as predefined channel structures and indications of the fault traces from shear zones. Since these were artificially introduced in our test models, they can be regarded as zones of disturbance rather than tectonic shear zones characterized by decompaction. The multiple-offset surveying introduced here, improves the quality with respect to S / N ratio and source signature even more; the maximum depth penetration in glass-bead layers thereby amounts to 5 cm. Thus, the presented mini-seismic device is already able to resolve structures within simple models of saturated porous media, so that multiple-offset seismic imaging of shallow sandbox models, that are structurally evolving, is generally feasible