Changes In Seismic Properties Research Articles

Defining Continental Lithosphere as a Layer With Abundant Frozen‐In Structures That Scatter Seismic Waves

AbstractWe investigate the structure of the continental lithosphere by combining two approaches: a systematic survey of abrupt changes in seismic properties detected by P‐to‐S converted body waves and an integrated geophysical‐petrological inversion for temperature and density in the upper mantle. We refine the global thermo‐chemical model WINTERC‐G in eastern North America by including detailed regional information on the crust into petrological inversions and combine it with the upper mantle layering beneath eastern North America yielded by anisotropy‐aware receiver‐function analysis. Eastern North America's Archean, Proterozoic and Paleozoic lithospheres show an excellent agreement between the depth to the 1,300°C isotherm that bounds the lithosphere and the depth range where converted waves detect abrupt changes in seismic properties. Boundaries with these abrupt changes reside within the rigid mechanical lithosphere and are uncommon in the convecting mantle beneath it. The boundaries include both impedance increases and decreases with depth, as well as anisotropy changes, and must have developed over the course of the assembly and evolution of the lithosphere. In the asthenosphere below, such heterogeneities appear to have been largely mixed out by convection. The existence of abundant interfaces with diverse origin can account for the commonly observed scattered signals from within the continental lithosphere and presents an alternative to the end‐member concept of the mid‐lithospheric discontinuity as a ubiquitous feature with a uniform origin. Generally, we can define continental lithosphere as a region of conductive heat transport and steep geotherm that is characterized by pervasive internal layering of density, elastic moduli and texture.

Implications of Laterally Varying Scattering Properties for Subsurface Monitoring With Coda Wave Sensitivity Kernels: Application to Volcanic and Fault Zone Setting

AbstractMonitoring changes of seismic properties at depth can provide a first‐order insight into Earth’s dynamic evolution. Coda wave interferometry is the primary tool for this purpose. This technique exploits small changes of waveforms in the seismic coda and relates them to temporal variations of attenuation or velocity at depth. While most existing studies assume statistically homogeneous scattering strength in the lithosphere, geological observations suggest that this hypothesis may not be fulfilled in active tectonic or volcanic areas. In a numerical study we explore the impact of a non‐uniform distribution of scattering strength on the spatio‐temporal sensitivity of coda waves. Based on Monte Carlo simulation of the radiative transfer process, we calculate sensitivity kernels for three different observables, namely travel‐time, decorrelation, and intensity. Our results demonstrate that laterally varying scattering properties can have a profound impact on the sensitivities of coda waves. Furthermore, we demonstrate that the knowledge of the mean intensity, specific intensity, and energy flux, governed by spatial variation of scattering strength, is key to understanding the decorrelation, travel‐time, and scattering kernels, respectively. A number of previous works on coda wave sensitivity kernels neglect the directivity of energy fluxes by employing formulas extrapolated from the diffusion approximation. In this work, we demonstrate and visually illustrate the importance of the use of specific intensity for the travel‐time and scattering kernels, in the context of volcanic and fault zone setting models. Our results let us foresee new applications of coda wave monitoring in environments of high scattering contrast.

Noise-based passive ballistic wave seismic monitoring on an active volcano

SUMMARY Monitoring temporal changes of volcanic interiors is important to understand magma, fluid pressurization and transport leading to eruptions. Noise-based passive seismic monitoring using coda wave interferometry is a powerful tool to detect and monitor very slight changes in the mechanical properties of volcanic edifices. However, the complexity of coda waves limits our ability to properly image localized changes in seismic properties within volcanic edifices. In this work, we apply a novel passive ballistic wave seismic monitoring approach to examine the active Piton de la Fournaise volcano (La Réunion island). Using noise correlations between two distant dense seismic arrays, we find a 2.4 per cent velocity increase and −0.6 per cent velocity decrease of Rayleigh waves at frequency bands of 0.5–1 and 1–3 Hz, respectively. We also observe a −2.2 per cent velocity decrease of refracted P waves at 550 m depth at the 6–12 Hz band. We interpret the polarity differences of seismic velocity changes at different frequency bands and for different wave types as being due to strain change complexity at depth associated with subtle pressurization of the shallow magma reservoir. Our results show that velocity changes measured using ballistic waves provide complementary information to interpret temporal changes of the seismic properties within volcanic edifices.

Validating compositional fluid flow simulations using 4D seismic interpretation and vice versa in the SECARB Early Test—A critical review

This paper discusses strengths and weaknesses of 4D seismic interpretation as a technique for monitoring carbon dioxide that was injected as part of a large scale test associated with commercial enhanced oil recovery (EOR) at Cranfield Field, Mississippi, USA. The goals of the monitoring effort are 1) to make measurements to verify that the CO2 is contained in the reservoir according to operational designs and model predictions, and 2) that if there are deviations, to provide data which can be used to update the earth models and determine if any mitigation is needed. The current work uses a compositional numerical simulation to model CO2 flow in the reservoir and compares the results with estimates of changes in seismic properties between a pre-injection and a survey after more than 2 million metric tons injected. The complicated physics of the problem in Cranfield field present challenges to seismic interpretations. Our results show partial agreement between the results of the numerical simulation and the seismic interpretations. Possible causes of discrepancies among the fluid flow model and multiple interpretations of the same seismic data sets performed with different work flows illuminates the types of uncertainties that should be considered to achieve the goals of monitoring.

Monitoring temporal changes of seismic properties

Temporal changes of seismic properties, such as velocity, attenuation, anisotropy, and scattering properties, have been inferred by active methods for decades and more recently by passive methods. In particular, passive methods are capable of monitoring seismic properties because they do not require earthquakes but rely on continuously excited signals in the ocean, for example. A collection of continuous monitoring of seismic velocities has revealed that the susceptibility of velocity changes to stress perturbations are highly variable. These variations can be translated to variability of third-order elastic moduli, elastic moduli arising by considering finite deformation. The third-order elastic moduli are shown by theoretical studies to be a good indicator of granular properties of rocks and, in general, as to how fluids interact with solid rocks. Advancement of theoretical and observational studies will gain more insights into the nature of third-order elastic moduli, which will eventually become yet another parameters to characterize the properties of rocks.

Properties and Processes of Crustal Fault Zones: Volume II

Active faults and earthquake rupture zones are largely buried under miles of rocks. Due to the effects of attenuation, typical seismological and other geophysical data lack detailed information on faulting processes. For these reasons as well as the inherent complexity of faulting, many fundamental questions on the structure and processes of fault zones in relation to earthquake properties and generated ground motion remain unanswered. To discuss the state of the art in the field, the 40th Workshop of the International School of Geophysics, titled ‘‘Properties and Processes of Crustal Fault Zones’’, was held May 18‐24, 2013, in the Ettore Majorana Foundation and Centre for Scientific Culture of Erice, Sicily. The papers in a previous volume I and the present volume II discuss observational and theoretical results based on presentations at the workshop and additional contributions that advance the understanding of earthquakes and faults. Topics covered in this volume include imaging of fault zones and the crust with seismic and other signals, microstructural analyses of fault zone rocks, long paleoseismic records of large earthquakes, inferences on stress, stress drops and fault geometries from seismological and geological data, properties of dynamic ruptures, generation and healing of rock damage, temporal changes of seismic properties, postseismic deformation, and scaling of earthquake rupture areas with moment. In the first paper of the volume, Zigone et al. use Rayleigh and Love waves constructed from the ambient seismic noise to obtain a 3D model of S wave velocities in the southern California plate-boundary region. The results show clear damage zones in the top few kilometres around the San Andreas, San Jacinto, and Elsinore faults, and velocity contrasts across various fault sections, which augment earthquake tomography images at greater depth. Additional findings include 2-h azimuthal anisotropy with fast directions parallel to geometrically simple fault sections and mixed directions in complex areas. Bennington et al. develop a method for joint inversion of P wave velocity and electrical resistivity using a normalized cross-gradient constraint, and apply it to obtain a structural profile across the San Andreas Fault Observatory at Depth (SAFOD). The results provide strong constraints on several key geological units and are used to infer possible fluid-rich regions in the fault zone structure. Bradbury et al. describe the microstructure and chemical composition of faultrelated rocks from the SAFOD borehole and surface outcrops that may be useful analogs for San Andreas Fault rocks. The observed rock textures are consistent with largely aseismic deformation punctuated by seismic slip. The mineralogy and whole-rock geochemistry indicate that the fault zone experienced transient fluid‐rock interactions. Ferrarini et al. constrain the fault segmentation pattern and regional stress tensor in the intra-Apennine area of central Italy using geological and seismological fault-slip data associated with the Colfiorito 1997 Mw 6.0 and L’Aquila 2009 Mw 6.1 seismic sequences. The results indicate a prevailing tensional regime involving a subhorizontal NE‐SW minimum stress axis and only minor strike-slip regimes. The findings are at odds with results of other studies indicating wide areas characterized by strike

Rock physics modelling of 4D time‐shifts and time‐shift derivatives using well log data – a North Sea demonstration

ABSTRACTRock physics models for fluid and stress dependency in reservoir rocks are essential for quantification and interpretation of 4D seismic signatures during reservoir depletion and injection. For siliciclastic sandstone reservoirs, the Gassmann theory successfully predicts changes in seismic properties associated with fluid changes. However, our ability to predict the sensitivity to pressure from first principles is poor, especially for cemented sandstones. In this study, we demonstrate how we can use a patchy cement rock physics model to quantify the combined effect of stress and fluid changes in terms of seismic time‐shifts and time‐shift derivatives during depletion or injection. The time‐shifts are estimated directly from well log data without core calibration of stress sensitivity. By assuming non‐uniform grain contacts where some grain contacts are cemented and others are loose, we can combine the contact theory for cemented sandstones with the contact theory for loose sands in order to predict stress sensitivity in a patchy cemented sandstone reservoir. Time‐shift derivatives are also useful estimates, as this parameter reveals which part of the reservoir is most stress sensitive and contributes most to the cumulative time‐shift.We test out our new approach on well log data from Troll East, North Sea and compare the predicted time‐shifts with observed 4D seismic time‐shifts. We find that there are good agreements between predicted time‐shifts and observed time‐shifts. Furthermore, we confirm that there are local geological trends controlling the fluid and stress sensitivity of the reservoir sands on Troll East. In particular, we observe a lateral stiffening of the reservoir from west to east, probably associated with the tectonic and burial history of the area. The combined effect of a thinning gas cap and stiffening reservoir sands amplifies the eastward decrease in time‐shifts associated with reservoir depletion. We manage to disentangle these two effects using rock physics analysis. It is essential to identify and map the static rock stiffness spatial trends before interpreting time‐shifts and time‐shift derivatives in terms of dynamic (i.e., 4D) pressure and fluid changes.

Lava dome collapse detected using passive seismic interferometry

[1] The collapse of the lava dome at the Soufriere Hills Volcano on Montserrat in July 2003 is the largest recorded in historical times. I use noise correlation Green's functions to measure the changes in seismic properties that resulted from this collapse. Continuous three component seismic data recorded at two pairs of stations were cross-correlated to retrieve three-component Green's functions along two paths that intersect the volcanic edifice before and after the dome collapse. Particle motion analysis shows that the Green's functions are dominated by Rayleigh waves and are consistent with the expected Green's tensor for a vertical point force source at one station recorded by a three-component receiver at the other. Following the collapse, there is a clear decorrelation and phase shift in the Green's functions corresponding to a change in velocity of approximately 0.5% that can be interpreted in terms of the unloading of the lava dome.

Temperature dependence of seismic properties in geothermal rocks at reservoir conditions

Petrophysical experiments on two Icelandic geothermal rock samples at simulated in situ reservoir conditions are analysed to delineate the effect of temperature on seismic velocity and attenuation. A goal of the present work is to predict the effect of the saturating pore fluid on seismic velocity using the Gassman equation, which has been modified for this purpose. To include the temperature effect in the equation, two assumptions are made: (1) the grain/mineral and dry bulk moduli are independent of temperature; and (2) the temperature dependence follows solely from the thermophysical characteristics of the saturating fluid through the fluid bulk modulus and fluid density. Laboratory measurements show that P-wave velocities decrease with increasing temperature. This change is related to the thermophysical characteristics of the saturating fluid; at higher temperatures bubbles and thermal microfractures are formed affecting seismic velocities. The measurements also show that at low temperatures seismic attenuation decreases with temperature due to the rapid decrease in the fluid viscosity. On the other hand, at higher temperatures the attenuation increases because of the generation of bubbles and thermal microfractures. Although having data from only two samples and that no measurements on dry samples were done, thus limiting the generality of the claims that can be made, the study presents a plausible approach to relate changes in seismic properties to geothermal system temperatures.

Non-linearity and temporal changes of fault zone site response associated with strong ground motion

SUMMARY We systematically analyse temporal changes of fault zone (FZ) site response along the Karadere-Dbranch of the North Anatolian fault that ruptured during the 1999 u and Dearthquake sequences. The study is based primarily on spectral ratios of strong motion seismic data recorded by a FZ station and a station ∼400 m away from the fault and augmented by analysis of weak motion records. The observations are used to track non-linear behaviour and temporal changes of the FZ site response. The peak spectral ratio increases 80-150 per cent and the peak frequency drops 20-40 per cent at the time of the D¨ uzce main shock. These co-main shock changes are followed by a logarithmic recovery over an apparent timescale of ∼1 d. However, analysis of temporal changes at each individual station using weak motion waveforms generated by repeating earthquakes show lower-amplitude longer-duration logarithmic recoveries that are not detected by the spectral ratio analysis. The results are con- sistent with a reduction of S-wave velocities in the top 100-300 m during the Dmain shock of 20-50 per cent or more and logarithmic post-main shock recovery on a timescale of 3 months or more. The observations support previous suggestions that non-linear wave propagation effects and temporal changes of seismic properties are generated in the shallow material by strong ground motion of nearby major earthquakes.

Classification of lithology from seismic tomography: A case study from the Messum igneous complex, Namibia

Combined analysis of seismic P and S velocity information provides a reasonable and efficient basis for the petrologic interpretation of seismic cross sections. In this paper, a methodology is presented which allows extraction of prominent features related to well‐defined P velocities and Poisson's ratios from a tomographic velocity model using a classification approach. We used first‐arrival travel time data from a near‐vertical seismic experiment and independently determined P and S velocities by forward and inverse modeling. Resolution and uncertainties were estimated from inverting synthetic data. The classification procedure was carried out in two subsequent steps. First, prominent classes were identified in the parameter space spanned by Poisson's ratios and P wave velocities. For this purpose, a probability density function was calculated from the tomograms. A function measuring the topography of the probability density was then determined, and a histogram analysis was carried out to detect significant classes. In the second step, the results from principal component analysis for the identified classes were used to map their distribution along the seismic profile. We applied the method to the Messum intrusive complex of Namibia and identified three prominent classes. On the basis of the integration of petrophysical data and comparison with surface geology, we conclude that quartz‐syenite composition dominates the upper 800 m of the crust under the complex. Outside of the intrusion the upper crust has properties corresponding to felsic metasediments and granites which are abundant in the local basement. This material shows strong depth‐dependent changes of seismic properties which are ascribed to decreasing porosity and fluid saturation with depth.

Seismic and electrical properties of sandstones at low saturations

Laboratory measurements of elastic wave velocities, attenuation, and electrical resistivity of sandstones all show distinct changes in the measured properties at very low levels of water saturation. Velocity decreases, attenuation reaches a maximum, and electrical resistivity decreases over a limited range in saturation. We assume that all the water at low saturations is present in the rock as a layer of uniform thickness covering the entire internal surface area of the sample. With this model, we calculate the thickness of surface water that corresponds to the distinct low saturation region. In all cases, for both the seismic and the electrical data, the thickness is equal to one to four monolayers of water. This is interpreted as marking the transition in the behavior of water in a sandstone from that of a surface adsorbed phase to a bulk water phase. The changes in seismic properties can be modeled by considering the effect of adding surface and bulk water to grain contacts. In a similar treatment, the role of contacts is shown to be a dominant factor in the electrical resistivity of sandstones at low saturations, such that the addition of water to the contact area causes a dramatic decrease in the electrical resistivity.

Changes in seismic properties of loess soils under engineering site preparation : Ilichev, V A; Golubtsova, M N; Musaelyan, A A; Lavrusevich, L V; Gelman, T V Soil Mech Found EngngV26, N3, May–June 1989, P98–102

Changes in seismic properties of loess soils under engineering site preparation : Ilichev, V A; Golubtsova, M N; Musaelyan, A A; Lavrusevich, L V; Gelman, T V Soil Mech Found EngngV26, N3, May–June 1989, P98–102

Changes in seismic properties of loess soils under engineering site preparation

Changes in seismic properties of loess soils under engineering site preparation

Predicting changes in seismic properties of permafrost during utilization of construction sites : 4F, 1T, 5R. SOIL MECH. FOUNDATION ENG. V10, N6, 1973, P390–393

Predicting changes in seismic properties of permafrost during utilization of construction sites : 4F, 1T, 5R. SOIL MECH. FOUNDATION ENG. V10, N6, 1973, P390–393

Predicting changes in seismic properties of permafrost during utilization of construction sites

Predicting changes in seismic properties of permafrost during utilization of construction sites