Maria-Daphne Mangriotis

Effects of the near-field on shallow seismic studies

Seismic studies in the near surface typically involve path lengths that are on the order of a wavelength. However, most investigators extrapolate seismic imaging and parameter extraction techniques to the near surface without incorporating the near-field in their processing approach. For such short path lengths, we find that the near-field is strongly present in the recorded waveforms, which consequently biases seismic parameter estimation, such as Q measurement. To suppress the near-field effects, we examined the challenges of near-field removal and the issues involving the accurate modeling of the near-field. Using the mathematical expression for a source monopole, which describes an elastic impact source typically used in shallow seismic studies, we modeled the near- and far-field terms of the waveform with distance in a homogeneous elastic medium. For thiscase, we found that the near-field can be suppressed through frequen-cy-domain or polarization filtering. However, in heterogeneous media, the near-...

Geo-Characterization at selected accelerometric stations in Crete (Greece) and comparison of earthquake data recordings with EC8 elastic spectra

To estimate the seismic response according to Eurocode (EC8) and almost all other national codes, site conditions have to be properly characterized so that soil amplification and the corresponding peak ground motion can be calculated.In this work, different geophysical and geotechnical methods are combined in order to define the detailed ground conditions in selected sites of the Hellenic Accelerometric Network (HAN) in Crete. For this purpose, the geological information of the sites and shear wave velocity, calculated from surface wave measurements, is used. Additionally, ground acceleration data recorded through HAN have been utilized from intermediate depth earthquakes in the broader area of South Aegean Sea.Using the recorded ground motion data and the procedure defined in EC8, the corresponding elastic response spectrum is calculated for the selected sites. The resulting information is compared to the values defined in the corresponding EC8 spectrum for the seismic zone that includes the island of Crete.The comparison shows that accurate definition of ground type through geological, geotechnical and geophysical investigations is important. However, our current comparison focuses on the distribution of values rather than the absolute values of EC8-prescribed spectra, and the results should be considered in this context.

A review of post-stack 4D seismic time-shifts, part 1: values and interpretation

A review and analysis of post-stack time-lapse time-shifts has been carried out that covers published literature supplemented by in-house datasets available to the authors. Time-shift data are classified into those originating from geomechanical effects and those due to fluid saturation changes. From these data, conclusions are drawn regarding the effectiveness of post-stack time-shifts for overburden and reservoir monitoring purposes. A variety of field examples are shown that display the range and magnitude of variation for each class of application. The underlying physical mechanisms creating these time-shifts are then described, and linked to a series of generic and field-specific rock physics calculations that predict their magnitudes. These calculations serve as a guide for practitioners wishing to utilize this information on their own datasets. Conclusions are drawn regarding the reliability of this attribute for monitoring purposes, and the extent to which further development is required and how it should be reported by authors.

Different Processing and Inversion Methods for Resolving Vs Profiles in Engineering Geophysics Using Surface Wave Data

Recent advances in Seismic Codes increased the necessity in Engineering Geophysics to use surface wave analysis as a tool. The shear wave velocity profile had to be resolved to calculate the site amplification in case of an earthquake event, necessary also for seismic hazard studies. In our approach we present the results from Surface Wave data acquired in different site in Crete Island (Greece) at strong motion sites. Passive and Active data at linear and circular geometries have been analyzed and the corresponding processing and inversion results are compared. Although one would comment that there are discrepancies on Dispersion Curve results the resulted ground profiles up to a shallow depth show considerable agreement. However, this is site dependent and in order to increase the accuracy and reliability of results one should be careful with the assignment of different modes in surface wave analysis and also combine the results provided.

Vehicle Traffic as a Source for Near‐Surface Passive Seismic Imaging

In this paper, we present preliminary results from a field experiment in which we explored the use of cars and traffic as a source for passive seismic imaging. We set out a line of geophones at a 45 degree angle between two intersecting roads at the University of California, Richmond Field Station. We collected data sets including background noise, an idling car at the intersection of the roads, one car driving on one road and two cars driving on both roads. A freight train contributed an appreciable signal. We found that the signal propagated best in the 3-20 Hz range. Average power spectra at each of the geophones showed a constructive interference structure which we had seen in simple synthetic models, computed by modeling the road as a line source. We were able to use the data from the idling car to estimate a dispersion curve.

Scattering versus intrinsic attenuation: Measurements from permanent downhole geophones

Attenuation (Q) profiles were determined from a VSP study of a well in the Lawrence Livermore National Lab facility using permanent down-hole geophones and a vertical elastic generator source. We used the zero-offset data to determine the interval velocity profile, and combined gamma ray, and other log information to model the density and Pand Swave velocity profile. From these profiles we generated synthetic zero offset data using a 2-dimensional elastic finite difference model assuming laterally homogeneous layers and compared the attenuation measured from the synthetic data with the attenuation measured from the real data. The attenuation of the synthetic data is slightly larger than the attenuation predicted by the O’Doherty and Anstey formula for 1D scattering, whereas the attenuation of the real data is substantially greater than the attenuation of the synthetic data, and the amplitudes predicted by the ODA approach. We adjusted the amplitudes of the real data based on the attenuation of the synthetic data so as to remove the effect of scattering and estimated intrinsic attenuation. We computed Q factors for three different depth intervals which were equal to 3, 5, and 7 for the depth intervals of 0-60 ft, 60-110 ft and 110140 ft respectively, and concluded that the primary source of attenuation in our location is intrinsic. For comparison, we estimated Q using the standard technique of rise time.

Scale effects on modelling the seismic signature of gas: results from an outcrop analogue

ABSTRACT Numerous examples of reservoir fields from continental and marine environments involve thin‐bedded geology, yet, the inter‐relationship between thin‐bedded geology, fluid flow and seismic wave propagation is poorly understood. In this paper, we explore the 4D seismic signature due to saturation changes of gas within thin layers, and address the challenge of identifying the relevant scales and properties, which correctly define the geology, fluid flow and seismic wave propagation in the field. Based on the study of an outcrop analogue for a thin‐bedded turbidite, we model the time‐lapse seismic response to fluid saturation changes for different levels of model scale, and explore discrepancies in quantitative seismic attributes caused by upscaling. Our model reflects the geological complexity associated with thin‐bedded turbidites, and its coupling to fluid flow, which in turn affects the gas saturation distribution in space, and its time‐lapse seismic imprint. Rock matrix and fluid properties are modelled after selected fields to reproduce representative field models with realistic impedance contrasts. In addition, seismic modelling includes multiples, in order to assess their contribution in seismic propagation through thin gas layers. Our results show that multiples could contribute significantly to the measured amplitudes in the case of thin‐bedded geology. This suggests that forward/inverse modelling involving the flow simulation and seismic domains used in time‐lapse seismic interpretation should account for thin layers, when these are present in the geological setting.

A Practical Tool for Simultaneous Analysis of 4D Seismic Data, PEM and Simulation Model

Summary One of the main focuses of the geoscience industry is to constrain reservoir models to 3D and 4D seismic data using quantitative workflows that are suitable for model updating and history matching. Seismic history matching (SHM) closes the loop and minimizes the misfit between the observed 4D seismic and that predicted by the reservoir model. A key problem in formulating this misfit function is a lack of understanding as to how uncertainties in the seismic data, petroelastic model and simulation model interact. This can lead to lengthy and time consuming workflows. This study presents a simple and interactive way of visualizing these uncertainties whilst optimizing the SHM. The approach is applied initially to a synthetic example, and then to two different field datasets from the UKCS and Norwegian Sea. The results demonstrate that qualitative updates can be successfully applied to the simulation model in the presence of uncertainty in the PEM and noise in the 4D seismic data.

Estimation of time-lapse velocity changes using Gaussian reconstruction

A key task in 4D seismic analysis is to resolve changes in subsurface velocity ΔV/V that affect both imaging in the monitor data and our interpretation of time-lapse amplitudes. This study introduces a new approach to recover ΔV/V using a Gaussian mixture model. The Gaussians are found to be better representative of the property fields than other choices such as B-splines. This approach is tested by application to a North Sea field, where geomechanical effects are active. Recovery of ΔV/V from three different time-shift estimates, using three approaches is firstly compared with Gaussian reconstruction. A second comparison estimates ΔV/V directly from the trace data. In these tests, the new approach compares favourably in the presence of noise, and is relatively simple to implement.

The Effect of Shale Activation on 4D Seismic Interpretation of a UKCS Field

Shale typically has a low but non-negligible permeability of the order of nanodarcys that could affect magnitude and pattern of pressure diffusion over the lifetime of a producing field. The implications of this phenomenon for reservoir monitoring by 4D seismic can be significant, but depend on the geology of the field, the time-lines for production and recovery, and the timing of the seismic surveys. This study assesses pressure diffusion effects for an offshore Paleocene turbidite reservoir in the UKCS. First, we evaluate the petrophysical characteristics of the overburden, intra-reservoir and underburden shales. Next, we adjust the simulation model to activate the shale-related contributions, and then perform 3D and 4D seismic modelling. In our reservoir of interest, fluid flow simulation results indicate that gas dominates the 4D seismic signature. It is found that activation of the shale improves the overall reservoir connectivity, which in turn impacts strongly on the breakout and distribution of gas liberated from solution and improves the fit to the observed seismic data.