Compressional Seismic Waves Research Articles

Impacts of hydrofacies geometry designed from seismic refraction tomography on estimated hydrogeophysical variables

Abstract. Understanding the critical zone processes related to groundwater flows relies on subsurface structure knowledge and its associated parameters. We propose a methodology to draw the patterns of the subsurface critical zone at the catchment scale from seismic refraction data and show its interest for hydrological modelling. The designed patterns define the structure of a physically based distributed hydrological model applied to a mountainous catchment. With that goal, we acquired 10 seismic profiles covering the different geomorphology zones of the studied catchment. We develop a methodology to analyse the geostatistical characteristics of the seismic data and interpolate them over the whole catchment. The applied geostatistical model considers the scale variability of the subsurface structures observed from the seismic data analysis. We use compressional seismic wave velocity thresholds to identify the depth of the soil and saprolite bottom boundaries. Assuming that such porous compartments host the main part of the active aquifer, their patterns are embedded in a distributed hydrological model. We examine the sensitivity of classical hydrological data (piezometric heads) and geophysical data (magnetic resonance soundings) to the applied velocity thresholds used to define the soil and saprolite boundaries. Different sets of hydrogeological parameters are used in order to distinguish general trends or specificities related to the choice of parameter values. The application of the methodology to an actual catchment illustrates the interest of seismic refraction in constraining the structure of the critical zone subsurface compartments. The sensitivity tests highlight the complementarity of the analysed hydrogeophysical data sets.

Seismic Formation Fluid Pressure Observations Reveal High Anisotropy of Oceanic Crust

AbstractFractures and faults in igneous oceanic crust affect hydrothermal circulation and hydration of the oceanic plates. Seismic observations have provided information about lithospheric fabrics, but direct measurements of oceanic crust’s elastic properties were not made. We report determinations of compressibility of the upper igneous crust (of 3.6 Ma) of the Juan de Fuca plate, using observed formation fluid pressure oscillations in sealed boreholes associated with passing surface waves from distant earthquakes. We determine an azimuthal variation of formation‐matrix compressibility by a factor of ∼5, with the crust being most compressible in the plate‐spreading direction across the structural fabric inherited from crustal creation. This is equivalent to a seismic compressional wave speed anisotropy of ∼50%–60%, much greater than that of standard seismic measurements (typically <20%). This likely reflects a previously unresolved degree of fracturing of the uppermost oceanic crust, consistent with existing observations that suggest a high degree of hydraulic permeability anisotropy.

A seismometer maps Mars’s anatomy

NASA’s InSight is the first mission to explore seismic waves in a planetary body since Apollo 17 in 1972.

Constraining Crustal Properties With Bayesian Joint Inversion of Vertical and Radial Teleseismic P‐Wave Coda Autocorrelations

AbstractThe sensitivity of seismic compressional and shear waves and their velocity ratios to rock lithology, pore fluids, and high‐temperature materials makes these parameters very useful for constraining the physical state of the crust. In this study, we develop a joint inversion approach utilizing both radial and vertical components’ autocorrelations of teleseismic P‐wave coda for imaging the crust by simultaneously characterizing the crustal , , and ratio. Autocorrelations of the radial and vertical components contain P and S waves that are reflected from the subsurface. Therefore, joint inversion of them can account for the variations of both and , and consequently, the ratio. Synthetic inversions show significant improvement in the estimation of these parameters compared to those from the inversion of either, P receiver functions or the autocorrelation of the vertical component. The velocity models inferred from the application of the approach to teleseismic data recorded along a north‐south passive seismic profile (BILBY experiment) in central Australia reveal a distinct pattern of the Moho and the variations across the crustal blocks/domains. The general trend of the Moho structure corresponds well with the change of the reflectivity that can normally be seen at the base of the crust and also with the Moho estimated from previous studies including the deep seismic reflection profiling method. The structure at depths greater than 10 km shows dominant high values beneath locations where the crustal domains interact (e.g., at transition from one domain to another).

Research on joint inversion method of seismic compressional and shear waves from ocean-bottom nodes

Research on joint inversion method of seismic compressional and shear waves from ocean-bottom nodes

Continuous evolution of oceanic crustal structure following an eruption at Axial Seamount, Juan de Fuca Ridge

Abstract We present the first continuous observations of the temporal evolution of oceanic crustal shear velocity beneath Axial Seamount, a submarine volcano on the Juan de Fuca Ridge (offshore northwestern North America). Weekly values of seafloor compliance, the periodic deformation of the seafloor under ocean waves, were estimated over the time period between December 2014 and May 2018 using data from two cabled broadband ocean-bottom seismometers with collocated absolute pressure sensors. We inverted these measurements for shear-wave velocity within the volcano beneath the two stations as a function of depth and time. Our results, combined with estimates of seismic compressional wave velocity, suggest that the shallow melt reservoir and the lower crust beneath the central caldera contain melt fractions of 14% and at least 4%, respectively. The eruption of April 2015 induced a dramatic drop in shear velocities beneath the central station, primarily in the lower crust, which could have been caused by an increase in melt fraction, a change in small-scale melt geometry, or both. The absence of such a change beneath the eastern flank of the caldera indicates that there is a lower-crustal conduit beneath the caldera center, which is much narrower in cross section (<1 km2) than the overlying melt reservoir (≥42 km2). Our study demonstrates the promise of using continuous data to understand submarine volcanism and crustal accretionary processes.

Using Shallow Seismic Method for Evaluation of the Foundation Beds of New Cairo Extension (Andalus Area), Egypt.

Shallow Seismic Refraction Method (SSR) in engineering investigation is growing interest after the October 1992 earthquake in Egypt. In the present study, eight shallow seismic refraction (SSR) profiles were acquired along the Al-Andalus area, which located at the southeastern part of new Cairo extension, Egypt, in order to identify the shallow subsurface geological features of the near-surface section. Both Primary (VP)-wave and Secondary (VS)-wave velocities that were picking and computed during processing and interpretation. The seismic compressional wave velocity (Vp) distribution indicates that there are three geoseismic layers ranging between (420 and 680 m/s), (1040 and 1680 m/s) and (2170 and 3100 m/s), for first, second and third layers, respectively. The first shear wave velocity (VS) is ranging between (240 – 390 m/s), the second VS layer ranging between (570 - 980 m/s), and the third VS layer is ranging between (1240 - 1800 m /s). The thickness of the first layer varies from 1.8 to 2.6 m and the second layer ranges between 7.6 and 10.5 m. The geoseismic cross sections are interpreted geologically as the first low velocity values which indicate weathered sediments fine sands change to sandy gravels sometimes. However, the second geoseismic layer is represented by consolidated sands with clay intercalations and the third layer reflected sandstones. The geotechnical parameters (elastic modulus, material competence and bearing capacities) are estimated. The distribution of these parameters noticed that the areas can be carried heavy loads at the central, east central and north central parts of the study area.

Azimuthal investigation of compressional seismic-wave attenuation in a fractured reservoir

Three-dimensional vertical seismic profiling data acquired from an oilfield located in Abu Dhabi, United Arab Emirates, were used to obtain a high-resolution multioffset azimuthal estimate of compressional seismic wave attenuation. On the basis of the assumption that the fracture strike corresponds to the azimuthal direction [Formula: see text] at which the attenuation is minimized, fracture orientations were obtained in three reservoir units. Two approaches were used to determine [Formula: see text]: first from the variation of the absolute attenuation [Formula: see text] with the azimuth and second from the variation of the relative attenuation [Formula: see text] with the azimuth. The rose diagrams of [Formula: see text] estimated from the [Formula: see text] variation indicated better agreement with those showing the strikes of open and cemented fractures obtained from core interpretation than with either of those showing the two types of fractures separately. However, the rose diagrams of [Formula: see text] estimated from the variation of [Formula: see text] were more similar to those showing the strikes of open fractures obtained from core and Fullbore Formation Microimager data. This observation can be explained by the fact that in the first approach, all types of fractures contribute to the scattering and fluid-related mechanisms of [Formula: see text]. However, in the second approach, [Formula: see text] is obtained from a least-squares fitting of the variation of [Formula: see text] with the azimuth, which is based on the squirt flow mechanism that is caused by the movement of fluid between grain pores and fractures. Therefore, a comparison of the orientations obtained using these two approaches can be an efficient way to separately determine the orientations of open and cemented fractures.

Stand-Alone GNSS Sensors as Velocity Seismometers: Real-Time Monitoring and Earthquake Detection.

By means of the time derivatives of Global Navigation Satellite System (GNSS) carrier-phase measurements, the instantaneous velocity of a stand-alone, single GNSS receiver can be estimated with a high precision of a few mm/s; it is feasible to even obtain the level of tenths of mm/s. Therefore, only data from the satellite navigation message are needed, thus discarding any data from a reference network. Combining this method with an efficient movement-detection algorithm opens some interesting applications for geohazard monitoring; an example is the detection of strong earthquakes. This capability is demonstrated for a case study of the 6.5 Mw earthquake of October 30, 2016, near the city of Norcia in Italy; in that region, there are densely deployed GNSS stations. It is shown that GNSS sensors can detect seismic compressional (P) waves, which are the first to arrive at a measurement station. These findings are substantiated by a comparison with data of strong-motion (SM) seismometers. Furthermore, it is shown that the GNSS-only hypocenter localization comes close (less than a kilometer) to the solutions provided by official seismic services. Finally, we conclude that this method can provide important contributions to a real-time geohazard early-warning system.

Elastic Softening of (Mg0.8Fe0.2)O Ferropericlase Across the Iron Spin Crossover Measured at Seismic Frequencies

AbstractWe experimentally determined the bulk modulus of (Mg0.8Fe0.2)O ferropericlase across the iron spin transition and in the low‐spin phase by employing a new experimental approach. In our measurements, we simulate the propagation of a compressional seismic wave (P wave) through our sample by employing a piezo‐driven dynamic diamond anvil cell that allows to oscillate pressure at seismic frequencies. During pressure oscillations, X‐ray diffraction images were continuously collected every 5–50 ms. The bulk modulus is directly calculated from these data at different pressures. Our experiments show a pronounced softening of the bulk modulus throughout the spin crossover, supporting previous single‐crystal measurements at very high frequencies and computations. Comparison of our results to previous data collected on (Mg,Fe)O with lower iron contents shows that the magnitude of softening strongly depends on iron content. Our experiments at seismic frequencies confirm that the iron spin crossover markedly affects the ratio of seismic compressional to shear wave velocities in Earth's lower mantle.

Seismological evidence for a localized mushy zone at the Earth\u2019s inner core boundary

Although existence of a mushy zone in the Earth’s inner core has been hypothesized several decades ago, no seismic evidence has ever been reported. Based on waveform modeling of seismic compressional waves that are reflected off the Earth’s inner core boundary, here we present seismic evidence for a localized 4–8 km thick zone across the inner core boundary beneath southwest Okhotsk Sea with seismic properties intermediate between those of the inner and outer core and of a mushy zone. Such a localized mushy zone is found to be surrounded by a sharp inner core boundary nearby. These seismic results suggest that, in the current thermo-compositional state of the Earth’s core, the outer core composition is close to eutectic in most regions resulting in a sharp inner core boundary, but deviation from the eutectic composition exists in some localized regions resulting in a mushy zone with a thickness of 4–8 km.

Frequency-dependent attenuation of compressional wave and seismic effects in porous reservoirs saturated with multi-phase fluids

Varying fluid types and saturations in porous media cause attenuation of compressional seismic waves and thus contribute to frequency-dependent reflections. We analyze the frequency-dependent attenuation and the corresponding seismic reflection behavior by taking into account the effects of the multi-phase fluid on the wave-induced fluid flow in porous reservoir. Fluid viscosity, the property that largely controls pore-fluid mobility, is proportional to the relaxation time, which in turn influences the frequency-dependent velocities. Here, we describe the variation of effective viscosity varies with saturation for a sandstone reservoir saturated for different multi-phase fluid mixtures using the Refutas equation. Then we explore the frequency-dependent velocity and inverse quality factors under different fluid saturation cases by employing equivalent-medium theory of Chapman's model. Next, we present a frequency-dependent phase-shift wavefield extrapolation method to simulate frequency-dependent seismic wavefield on 4-layer models. Comparative analyses indicate that the frequency-dependent attenuation and the seismic behaviors are functions of fluid viscosity associated with hydrocarbon saturations. Hydrocarbon saturations in fluid mixture strongly affect attenuation and characteristic frequencies. The frequency-dependent attenuation within the seismic frequency range can be increased due to the presence of hydrocarbons in multi-phase fluids. Synthetic seismic records indicate that the frequency-dependent attenuation of compressional wave and seismic reflection signatures are significantly dependent on hydrocarbon saturation, in terms of amplitude, waveform and traveltime of the reflection located at the base of the saturated reservoir and the underlying shale layer.

Two decades of temporal change of Earth's inner core boundary

AbstractWe report two decades of changing behavior of the Earth's inner core boundary (ICB), which provides the simplest explanation for the observed temporal change of the compressional seismic waves that are reflected from the ICB (PKiKP) and refracted in the inner core (PKIKP), from earthquake doublets occurring in South Sandwich Islands between 1993 and 2013. In the early period (before 2003), the ICB is enlarged beneath the western coast of Gabon, Republic of Congo, and southwest Tanzania in the reflected points of the PKiKP observed at seismic stations OBN, AAK, and ARU, while it experiences little change beneath Zimbabwe or/and Kenya, and beneath west Angola or/and north Central African Republic, in the PKIKP entry or/and exit points of AAK and ARU observations, respectively. In the later period (after 1998), the ICB regions beneath the western coast of Gabon, Republic of Congo, and southwest Tanzania either shrink or remain unchanged, and the temporal change migrates to beneath Zimbabwe or/and Kenya, and beneath west Angola or/and north Central African Republic, with a decrease of inner core surface by 5.59 km between 1998 and 2009 beneath Zimbabwe or Kenya and by 1.73 km beneath west Angola or north Central African Republic between 1998 and 2013. These results indicate that ICB temporal change occurs in localized regions and is episodic, rapidly migrating, and alternately enlarged and shrunk.

Compressional seismic waves in permeable media: an asymptotic representation

Compressional seismic waves in permeable media: an asymptotic representation

Estimation of compressional seismic wave attenuation of carbonate rocks in Abu Dhabi, United Arab Emirates

The subsurface geology of Abu Dhabi in the United Arab Emirates is primarily composed of carbonate rocks. Such media are known to be highly heterogeneous. Very few studies have attempted to estimate attenuation in carbonate rocks. In Abu Dhabi no attenuation profile has been published. This study provides the first seismic wave attenuation profiles in Abu Dhabi using dense array of VSP data. We estimated three attenuation profiles: the apparent, the scattering, and the intrinsic attenuations. The apparent attenuation profile was computed using amplitude decay and spectral-ratio methods. The scattering attenuation profile was estimated using a generalized reflection–transmission matrix forward model. It is usually estimated from the sonic log, but to be more consistent with the apparent attenuation, we succeeded in this paper to estimate it from the VSP data. We subtracted the scattering attenuation from the apparent attenuation to deduce the intrinsic attenuation. The results of the study indicate that the scattering attenuation is significant compared to the published studies that are mainly based on clastic rocks. The high scattering attenuation can reach up to 0.02. It can be explained by the strong heterogeneity of the carbonate rocks. This study demonstrates that the Simsima and Rus Formations have considerable scattering and intrinsic attenuations. These formations are considered aquifers in Abu Dhabi; we therefore interpreted this high intrinsic attenuation zones to be due to the heterogeneity and to the fluids contained in these formations. The Umm-Er-Radhuma Formation is a more homogenous formation with limited aquifer potential. Hence, scattering and intrinsic attenuations of the Umm-Er-Radhuma Formation are low.

New algorithm to improve earthquake early warning systems

When a fault line ruptures, seismic waves race out from the earthquake epicenter. Compressional seismic waves, known as primary (P) waves, travel fastest. Shear waves—secondary (S) waves—travel more slowly but are the source of the bulk of earthquake‐induced damage. Using the opportunity afforded by the difference in travel times between these two types of waves, researchers have begun to design and implement earthquake early warning systems.

Poisson’s Ratio of Surface Soils and Shallow Sediments Determined from Seismic Compressional and Shear Wave Velocities

This paper presents the results of seismic compressional, P- and shear, S-wave measurements carried out on the unconsolidated top-soil at the different locations of the study area to determine Poisson’s ratio. In this work, seismic refraction data is used to determine Poisson’s ratio as an aid to engineering foundation. A 12-channel seismograph with signal stacking ability was used together with high frequency (100 Hz) geophones on the top-soil. The geophone intervals were set to 5 m at the all locations. In all the locations, Vp/Vs ratio ranged from 1.0289 to 1.4185 for the top layer. Vp/Vs ratio in the second layer ranged from 1.0512 to 1.5834. The Poisson’s ratio for the first layer ranged from -8.0324 to 0.2060. For the second layer, the Poisson’s ratio ranged from -0.7567 to 0.1683. The values of Vp/Vs ratio less than in the first layer and in some locations in the second layer resulted in negative Poisson’s ratio. The low and negative values of Poisson’s ratio are symptomatic of occurrence of ripable anisotropic materials in the locations where they occur, which suggests that indicated average depth should be removed and refilled with geomaterials that may be resilient to carry engineering loads.

Irregular topography at the Earth’s inner core boundary

Compressional seismic wave reflected off the Earth's inner core boundary (ICB) from earthquakes occurring in the Banda Sea and recorded at the Hi-net stations in Japan exhibits significant variations in travel time (from -2 to 2.5s) and amplitude (with a factor of more than 4) across the seismic array. Such variations indicate that Earth's ICB is irregular, with a combination of at least two scales of topography: a height variation of 14km changing within a lateral distance of no more than 6km, and a height variation of 4-8km with a lateral length scale of 2-4km. The characteristics of the ICB topography indicate that small-scale variations of temperature and/or core composition exist near the ICB, and/or the ICB topographic surface is being deformed by small-scale forces out of its thermocompositional equilibrium position and is metastable.

Velocity and Attenuation Characterization of the LIGO Site near Livingston, Louisiana

In April of 2009 a seismic survey utilizing explosive charges took place in Livingston parish, Louisiana. The area of the survey encompassed the location of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Livingston interferometer. In this paper, we present an analysis of seismic data recorded with three of the LIGO seismometers and a geophone array that was deployed during the time of the survey. In particular, the geophone-array data are used to study the propagation of surface waves, whereas first-arrival measurements with the LIGO seismometers provide estimates of the speed of compressional seismic waves as a function of depth. We find that fundamental Rayleigh waves have a speed close to 205 m/s consistent with results from previous borehole tests and that speed of compressional waves is 1650 m/s at 25 m depth, increasing to 2300 m/s at 1 km depth. Blast spectra are further investigated to determine the Q value of the ground medium experienced by Rayleigh waves (f > 1 Hz) and body waves deeper underground. The estimated Q value is approximately 50 for the surface waves and exceeds a value of 190 for body waves propagating at depths below 100 m.

Seismic ray tomography using L1 integral norm

Seismic ray tomography methods are usually associated with substantial computer processing time. The reason for this is that at each step of the iterative inversion process defined by the tomographic method the two-point ray tracing problem must be solved for each source-receiver pair. In order to resolve this, an Euclidean norm (L2 vector norm), commonly used in error functions which are to be minimized in inversion procedures, is substituted by an L1 integral norm, which enables the estimation of model parameters by minimizing the area between observed and calculated traveltime curves that are interpolated (or adjusted) to the data points. Relatively simple mathematical developments and numerical experiments with two-dimensional compressional seismic wave velocity field models showthat L1 integral norm saves an enormous amount of processing time with no significant loss of accuracy. Occasionally, parameters of the model can be better estimated using L1 integral norm than the L2 vector norm that is traditionally utilized in seismic inversion tomography.