Investigating the PS seismic imaging of faults using seismic modelling and data from the Snøhvit field, Barents Sea

PS seismic data from the Snøhvit field are compared with seismic modelling to understand the effect of azimuthal separation and incidence angle on the imaging of faults and associated horizon discontinuities. In addition, the frequency content of seismic waves backscattered from faults is analysed. The study area consists of a horst structure delimited by a northern fault dipping NW and oblique to the E-W survey orientation, and a southern fault dipping SSW and subparallel to the survey. Due to the raypath asymmetry of PS reflections, the northern fault is imaged better by azimuthally partitioned W data that include receivers downdip of the fault, relative to the sources, than by E data where the receivers are updip from the sources. Partial stack data show a systematic increase in the PS fault-reflected amplitude and therefore quality of fault imaging with increasing incidence angle. Fault images are dominated by internal low-medium frequency shadows surrounded by medium-high frequencies haloes. Synthetic experiments suggest that this is due to the interaction of specular waves and diffractions, and the spectral contribution from the fault signal, which increases with fault zone complexity. These results highlight the impact of survey geometry and processing workflows on fault imaging.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5727552

A simple seismic input model for estimating the seismic resistance of structures

The paper formulates the principles for shaping the design input, in particularly that the design input is not required to be similarto the real input. It is suggested that the seismic input should be set as a sinusoidal segment. This requires that the sinusoid be hazardous to the structure and causes it to reach the same limit state as a real earthquake. The amplitude of the sine wave is set equal to the average value of the peak boosts. The frequency of the exposure is set as dangerous for the structure to be designed and the duration is set according to the frequency of the exposure. The proposed seismic modelling approach makes it possible to assess the potential for progressive collapse and low-cycle fatigue of the designed structure. The model is based on statistical data on past earthquakes to estimate the average level of peak accelerations and the correlation between the prevailing period and the duration of the seismic event. The proposed input model greatly simplifies the computational assessment of seismic stability and the modeling of inputs on the seismic platform.

The out of plane behaviour of masonry infilled frames

The great interest about out of plane behavior of masonry infill walls has recently increased since it is a key point in the seismic modelling of framed structures. Their contribute to the whole seismic resistance of a framed building cannot be skipped. After a review of the literature on the subject, this paper presents a trilinear constitutive model for the out of plane behavior of masonry infills based on the tensile strength of the constituents. Comparisons with literature model are provided and the identification of the model is based on experimental tests.

Recognition and characterization of small-scale sand injectites in seismic data: Implications for reservoir development

Small-scale (< 20 m), non-resolvable sand injectites can constitute a large part of the net-to-gross volume and affect fluid flow in the reservoir. However, they may also cause challenges for well placement and reservoir development because they are too small to be reliably constrained by reflection seismic data. It is therefore important to better understand how small-scale injectites influence seismic images and may be recognized and characterized above reservoirs. The Grane Field (North Sea) hosts numerous small-scale sand injectites above the main reservoir unit, causing challenges for well placement, volume estimates and seismic interpretation. Here, we investigate how such small-scale sand injectites influence seismic images and may be characterized by (1) using well-, 3D seismic- and outcrop data to investigate geometries of small-scale sand injectites (0-15 m) and creating conceptual models of injectite geometries, (2) performing seismic convolution modelling to investigate how these would be imaged in seismic data, and (3) compare these synthetic seismic images to actual 3D seismic from the well-investigate Grane Field.Our results show that despite injectites being below seismic resolution, small-scale sand injectites can be detected in seismic data. They are more likely to be detected with high thickness (> 5 m), steep dip (> 30°), densely spaced sand injectites, and homogeneous background stratigraphy. Furthermore, as fraction of sand injectites increases the top reservoir amplitude will decrease. Moreover, comparison of the synthetic seismic images with real seismic data from the Grane Field indicates that the low-amplitude anomalies and irregularities observed above the reservoir may be a result of the overlying sand injectites. Additionally, the comparison strongly suggests that the Grane Field hosts sand injectites that are thicker and located further away from the top reservoir than what is indicated by well observations. These results may be used to improve well planning and develop reservoirs with overlying sand injectites.Supplementary material: A PDF file containing all the seismic modelling results allowing the reader to flip back and forth between the different models is available at https://www.doi.org/10.6084/m9.figshare.14333102 . Well logs from well 25/11-18 T2 are available at https://factpages.npd.no/pbl/wellbore_documents/2358_25_1_18_COMPLETION_REPORT_AND_LOG.pdf

Simplified Seismic Modelling of Fractured Rock: How Effective is the Localised Effective Medium Compared to Explicit Representation of Individual Fractures

Seismic waves can be an effective probe to retrieve fracture properties particularly when measurements are coupled with forward and inverse modelling. These seismic models then need an appropriate representation of the fracturing. The fractures can be modelled either explicitly, considering zero thickness frictional slip surfaces, or by considering an effective medium which incorporates the effect of the fractures into the properties of the medium, creating anisotropy in the wave velocities. In this work, we use a third approach which is a hybrid of the previous two. The area surrounding the predefined fracture is treated as an effective medium and the rest of the medium is made homogeneous and isotropic, creating a Localised Effective Medium (LEM). LEM can be as accurate as the explicit but more efficient in run-time. We have shown that the LEM model can closely match an explicit model in reproducing waveforms recorded in a laboratory experiment, for wave propagating parallel and perpendicular to the fractures. The LEM model performs close to the explicit model when the wavelength is much larger than the element size and larger than the fracture spacing. By the definition of the LEM model, we expect that as the LEM layer becomes coarser the model will start approaching the effective medium result. However, what are the limitations of the LEM and is there a balance between the stiffness, the frequency and the thickness, where the LEM performs close to an explicit model or approaches the effective medium model? To define the limits of the LEM we experiment varying fracture stiffness and source frequency. We then compare for each frequency and stiffness the explicit and effective medium with five models of LEM with different thickness. Finally, we conclude that the thick LEM layers with lower resolution perform the same as the thinner and finer resolution LEM layers for lower frequencies and higher fracture stiffness.

Investigation on Alamparai Fort by Utilization of Organic Materials for Improvement of Stability of Heritage Structure

Heritage structures are valuable monuments to describe the culture and traditions of the country. These heritage structures get deformed in today's scenario by natural or artificial disasters. Hence, to preserve these heritage structures, restoration was introduced to restore the ancient building with new binding agents. Rehabilitation can take place only by analysing the properties of existing structures. Based on the existing structure properties, the alternative binding agent selected; can regain the same strength and shape of the heritage structures. Based on these, the restoration of Alamparai fort was performed by analysing the fort materials using mortar strength analysis by core-drilling, double punch test, and small-scale masonry test. The arch properties are also analysed by performing seismic analysis based on the mortar strength properties. The stability analysis of the organic and existing materials shows that Gur and haritaki is the best agent for restoring the fort. Hence, the mortar strength and seismic analysis of these materials performed using diagonal shear test and seismic modelling of the fort. The proposed material strength tests results indicate that the Gur and Haritaki is the best agent to restore the fort. The fort was restored with these materials; it survived in Nivar cyclone crossed on 26th November 2020.