Seismic Wave Measurements Research Articles

Determination of the Small-Strain Stiffness of Hard Soils by Means of Bender Elements and Accelerometers

Direct determination of seismic wave velocities in the laboratory is becoming common practice worldwide, given its great potential in the definition of the stiffness at very small strains. One of the techniques for seismic wave measurement makes use of piezoelectric transducers, such as bender elements (BE). However, some limitations remain to the applicability of this technique, namely for stiff geomaterials, such as compacted soils, naturally or artificially cemented soils and soft or weak rocks. To overcome this issue, two accelerometers have been used in conjunction with BE. In the present paper, this combined test setup implemented on a stress-path triaxial chamber will be detailed. An application study will be presented for a hard soil, prepared by laboratory compaction and tested in triaxial compression at different isotropic stress levels. The equipments, procedures and interpretation analyses will also be described. The advantages of this setup are twofold: (1) the interpretation of the acceleration measurements is straightforward, since the signals are of the same nature; (2) these measurements can be used to verify the BE signals, and thus minimize the subjectivity of the interpretation of BE results. Additionally, the accelerometers can be used autonomously wherever the interpretation of BE becomes too complex. The results of this research enabled to validate the interpretation methods used for BE testing. Moreover, this combined setup of transducers provided a simple yet powerful tool for eliminating the subjectivity inherent to BE testing, enabling reliable measurements of small-strain stiffness for a wide range of materials.

Novel Fiber Bragg Grating Accelerometer Based on Diaphragm

We present the modeling, design, fabrication, optimization, and characterization of a novel fiber Bragg grating (FBG) accelerometer based on a diaphragm. The principle of the FBG accelerometer and the acceleration response versus FBG wavelength are analyzed theoretically. Experimental results indicate that the FBG accelerometer provides a linear response over a broad frequency range from 10 to 200 Hz, with a high sensitivity of 36.6 pm/G, which agrees well with the theoretical sensitivity; linear fitting is 99.8% and relative error is 3.68%. Dynamic resolution is 2 ${\rm mG}/\sqrt{\rm Hz}$ in flat range, cross-axis sensitivity less than 1.3% of the main-axis sensitivity, and it has a strong anti-interference capacity, so it is a good candidate for cross well seismic wave measurement $({>}{\rm 50}~{\rm Hz})$ .

A Robust and Compact Fiber Bragg Grating Vibration Sensor for Seismic Measurement

A compact fiber Bragg grating (FBG) vibration sensor consisting a flat diaphragm and two L-shaped rigid cantilever beams for seismic measurement has been proposed and experimentally demonstrated. The specially designed sensing configuration contributes many desirable features such as a wide frequency response range (10-120 Hz), an extremely high sensitivity coefficient (~100pm/g) together with a robust metal package for improving the mechanical strength and a decreased transverse sensitivity (<; 5%), making it a good candidate for in-field seismic wave measurement in oil and gas exploration.

Resolving the lithosphere-asthenosphere boundary with seismic Rayleigh waves

SUMMARY Seismic surface wave measurements offer tight constraints on shear wave speed values within the lithosphere and asthenosphere. It is still a matter of debate, however, how accurately and under what conditions surface waves can resolve the depth and thickness of the lithosphere–asthenosphere boundary (LAB). We investigate the sensitivity of Rayleigh waves to LAB properties and find that if the LAB is associated with a 2–3 per cent shear speed reduction at a depth less than 100 km, then 10–20 km variations in its depth translate into phase-velocity changes of up to 1 per cent. Variations in the thickness of the LAB from zero to a few tens of kilometres cause much smaller phase-velocity changes (∼0.1 per cent), although the signal of the LAB-thickness variations is enhanced by a distinct frequency-dependent pattern of phase-velocity increases and decreases. For LAB depths increasing beyond 100 km, larger absolute variations in the LAB depth and thickness are needed to generate response of the same amplitude in phase velocities. We introduce a grid-search inversion of phase-velocity curves for the LAB depth and thickness, defining them as the middle and the width, respectively, of a depth interval with a linear shear-speed decrease in it. The inversion comprises dense sampling of the LAB depthLAB thickness parameter plane and non-linear, gradient-search inversions at every point; it accountsforanytrade-offsoftheLABparameterswithshearspeedvariationsaboveandbelow the LAB. Inversions of phase-velocity curves for the LAB depth and thickness show that the two parameters have uncorrelated uncertainties, with the LAB depth constrained better than the LAB thickness. Random errors in phase-velocity measurements have a limited effect on LAB-depth measurements, causing errors of only a few kilometres. Measurements of the LAB thickness are less robust and are not possible in the presence of large (∼1 per cent) random errors. Substantial systematic biases in the measurements (errors that persist over broad period ranges of tens of seconds) can affect both LAB-depth and LAB-thickness estimates; phasevelocity curves with such biases are not suitable for inversions for LAB properties. Resolving the LAB, however, in particular the LAB depth, should already be possible with some of the accurate, broad-band, phase-velocity measurements available today. Applying our method to phase-velocity data measured in Phanerozoic west-central Germany, we find best-fitting LAB depths in the 60–80 km range (the LAB depth always understood to be the middle of the shear-speed reduction depth range), with minimum misfits achieved near the 70-km LAB depth. Beneath the Archean Guyana Shield, we determine a 140–175 km LAB-depth range, with a 160 km best-fitting (mid-)LAB depth.

基于L型刚性悬臂梁的结构小巧型光纤布拉格光栅膜片加速度计

A compact fiber Bragg grating (FBG) diaphragm accelerometer based on L-shaped rigid cantilever beam is proposed and experimentally demonstrated. The sensing system is based on the integration of a flat diaphragm and an L-shaped rigid cantilever beam. The FBG is pre-tensioned and the two side points are fixed, efficiently avoiding the unwanted chirp effect of grating. Dynamic vibration measurement shows that the proposed FBG diaphragm accelerometer provides a wide frequency response range (0–110 Hz) and an extremely high sensitivity (106.5 pm/g), indentifying it as a good candidate for embedding structural health monitoring and seismic wave measurement.

Effective antiplane properties in the presence of frictional shear cracks

Cracked solids are viewed as equivalent homogenous media that sustain the propagation of coherent shear horizontal waves in the presence of multiple scattering. Strip‐like cracks are filled with viscous fluid and are randomly distributed. Their faces undergo viscous friction. The effective wave number and shear stiffness are calculated for parallel cracks in terms of the far‐field response of a single crack. Attention is focused on the derivation of this single‐scattering property in the general case of oblique incidence. Effect of the crack density and fluid viscosity on the effective properties is investigated. Simple low‐ and high‐frequency analytical approximations allow a direct assessment of the crack density and fluid viscosity from seismic wave measurements. Knowledge of these parameters facilitates more precise estimation of the crack size in the Earth's crust.

Development of a Time-Domain, Variable-Period Surface-Wave Magnitude Measurement Procedure for Application at Regional and Teleseismic Distances, Part I: Theory

A major problem with time-domain measurements of seismic surface waves is the significant effect of nondispersed Rayleigh waves and Airy phases, which can occur at both regional and teleseismic distances. This article derives a time-domain method for measuring surface waves with minimum digital processing by using zero-phase Butterworth filters. The method can effectively measure surface- wave magnitudes at both regional and teleseismic distances, at variable periods between 8 and 25 sec, while ensuring that the magnitudes are corrected to accepted formulae at 20-sec reference periods, thus providing historical continuity. For applications over typical continental crusts, the proposed magnitude equation is, for zero- to-peak measurements in millimicrons: M s(b) = log( a b ) + 1/2 log(sin(Δ)) + 0.0031(20/ T ) 1.8 Δ − 0.66 log (20/ T ) − log( f c ) − 0.43, where: f c ≤ 0.6/ T √Δ. To calculate M s(b) , the following steps should be taken: Determine the epicentral distance in degrees to the event Δ and the period T . Calculate the corner filter frequency f c using the preceding inequality. Filter the time series using a zero-phase, third-order Butterworth bandpass filter with corner frequencies 1/ T − f c , 1/ T + f c . Calculate the maximum amplitude a b of the filtered signal and calculate M s(b) . At the reference period of 20 sec, the equation is equivalent to von Seggern’s formula (1977) scaled to Vanĕk (1962) at 50 degrees. For periods 8 ≤ T ≤ 25, the equation is corrected to T = 20 sec, accounting for source effects, attenuation, and dispersion. Online material: Design and realization of Butterworth filters.

Ionospheric remote sensing of the Denali Earthquake Rayleigh surface waves

Using the Global Positioning System, we have detected ionospheric disturbances associated with the long‐period Rayleigh waves from the 2002 Denali earthquake (Ms = 7.9). The dense California GPS networks allowed us to map the ionospheric perturbations and to compute the group velocity with a high spatial resolution above the Pacific coasts. Due to a low sampling rate, a large error in the velocity determination remains. Nonetheless, it demonstrates that bi‐static remote sensing measurements of seismic waves with GPS networks can be performed. Monostatic measurements with a dedicated satellite could possibly be used to record in the ionosphere surface waves originating from large earthquakes. Such a space‐based remote sensing of the local group velocity of Rayleigh surface waves would effectively complement the seismic networks for high‐resolution global tomography of the Earth's lithosphere.

Biot elastic moduli of sea ice

Ice moduli may be determined from acoustic velocities measured via techniques such as resonance vibration of ice rods, seismic and flexural wave measurement, and propagation of high-frequency pulses in core samples. However, due to the porous nature of sea ice, handling procedures can alter the brine volume of, and introduce air into, the saline ice. This in turn changes velocities from their in situ values. This is demonstrated with velocities measured during temperature cycling of an ice core sample. These results indicate the advantages of a pulse-type experiment designed to allow rapid measurement of velocities and related ice properties in the field. One such design is presented, including the equipment used, and then experimental results obtained are related to the elastic moduli of a sealed-pore Biot porous media model of sea ice. The expressions for the longitudinal and shear velocities in ice, derived assuming a sealed-pore structure, most generally apply to sea ice away from the growing ice/water interface. The results from the type of analysis presented here ultimately lead to prediction of the acoustic impedance layers in sea ice, a crucial factor in understanding and predicting acoustic propagation processes involving sea ice. [Work supported by ONT with technical management by NOARL.]

Directional spectra observations of seafloor microseisms from an ocean-bottom seismometer array

Observations of the directional spectra of seabed motion in shallow water were conducted off the New Jersey coast during the summer of 1987. Using a six-point ocean-bottom seismometer array, each instrument supporting a pressure transducer, and two horizontal and vertical accelerometers, measurements of gravity and seismic waves across the ULF/VLF band were collected in 12.5 m of water. Array dimensions were tuned particularly for directional spectra observations of short-period seafloor microseisms. Directional spectra analysis indicates that in the short-period microseismic band, 1.5–2.5 s, motion of the seafloor is primarily a result of slow seismic waves traveling at apparent velocities near 200 m/s. These propagation velocities for the microseismic band in shallow water are an order of magnitude less than microseismic velocities from similar studies on land. Contemporaneous measurements of the directional spectra of long-period ocean gravity waves, 15–85 s, show an eastern direction of origin; short-period ocean gravity waves, 5–9 s, measured using particle motion analysis, are from the south. The direction of propagation of the microseisms, found from the maximum response of the array, is shown to be approximately N150E—a direction midway between long- and short-period ocean-wave propagation directions. Correlation of particle motion and directional spectra analysis indicates that microseisms have retrograde motion. These results suggest that the microseisms are most likely Scholte interface waves.

New global map of seismic crust

An up‐to‐date global compilation of the thickness of the earth's crust and a compilation of upper mantle seismic velocity measurements has been published recently by Soller, Ray, and Brown (Tectonics, 1, 125–149, 1982). The data base indicate 2508 crustal thicknesses and 1806 Pn (uppermost mantle) velocity values. The data are from seismic refraction and surface wave measurements. The compilation represents a significant expansion of available crustal thickness maps, with more uniform coverage. Thus, in agreement with current geophysical interpretations, numerous tectonic features are recognizable as local variations in crustal thickness. For example, mountain chains in isostatic equilibrium, such as the Alps, the Andes, and the Pamir and Tien Shan ranges, display a thickening of the crust and a deeper Mohorovicic Boundary. Beneath the Tibetan plateau, the crust is 80 km thick—the maximum observed. The area of Japan and the Kurile Islands are represented on the map as a thickened crustal ridge separating the thin oceanic crust of the Pacific plate from the thicker crust underlying the marginal basins of the Seas of Japan and Okhotsk. Contours define the Pacific‐Eurasion plate boundary, located southeast of the Kurile island arc. Very thin crust, often less than 4 km, occasionally outlines oceanic spreading centers, such as the Midocean Ridge in the North Atlantic.

The constitution of the core: seismological evidence

The present best estimates of seismic velocities in the core are compared with the 1939 solution of Jeffreys, with emphasis on the remaining uncertainties and present resolution capability. The relative contributions of measurements of seismic body waves and terrestrial eigenspectra to the inverse problem of the determination of elastic parameters, density and damping in the core are compared. Linear perturbation algorithms and smoothing functions used with the spectral data reduce their capacity for fine structural definition. Between radii of 1400 and 3300 km (shell E), the outer core appears to be substantially homogeneous and non-stratified, with small or zero rigidity and a dimensionless seismic quality factor, Q ,of order 10 4 . It is a sufficient but still not a necessary condition that density p follows precisely the Adams-Williamson equation in E; for an averaging interval of 400 km, estimates of have a standard error there of about 0.2 g cm -3 . There is as yet no unequivocal seismological evidence for or against a boundary shell (thickness less than about 200 km) at the top of the liquid outer core. At the bottom of the outer core, the evidence is becoming stronger that any reduction in the rate of increase with depth of P wave velocity is confined to a minor transition layer little more than 100 km thick. The inner core has a sharp outer boundary at about 1216 km radius, but below it only average physical properties are estimated with any confidence. The average seismic compressions and shear velocities are about — 11.2 and /? — 3.5 km s -1 and 12.5 &lt; p &lt; 13.6 g cm-3, yielding a peculiar mean Poisson ratio of 0.44 or greater. At the inner core boundary, jumps in parameters are: A « 0.65, A/? — 2.0- 3.0 km s-1 and A p» 1.0 g cm -3 . Recent travel-time and waveform synthetics suggest a strong increase of P (and perhaps S) velocity in the upper 300 km of the inner core, which could be interpreted as a mixing or melting effect. Damping properties in the inner core may have an unusual dependence on wave frequency with an order of magnitude increase in Q from 1 Hz to 4 mHz vibrations.

Long-Period Surface Wave Seismology: Love Wave Phase Velocity and Polar Phase Shift

Summary Due to continuing improvement in instrumentation and data processing techniques, increasingly longer periods are being obtained in seismic surface wave measurements. Three aspects of surface wave propagation on a sphere, which become important at long periods, are treated in some detail: (1) the dependence of the spherical phase velocity on the relative positions of epicentre and station, (2) the importance of the polar component of Love waves, relative to the azimuthal component, and (3) the accuracy of the common practice of treating surface wave data as though they represent a continuous-frequency phenomenon. The original work on polar phase shift theory (Brune et al.) predated derivations of the response of a sphere to realistic models of the earthquake mechanism. We update the original theory by basing the development on these later derivations.

Measurements of seismic waves caused from underground extraction of coal seams : Isobe, T Hokkaido Univ. Sapporo, J Conference. 4F, 16R. Proc. Third Congress, Int. Soc. Rock Mech. Denver, 1974, V2, Part B, 1974, P1278–1282

Measurements of seismic waves caused from underground extraction of coal seams : Isobe, T Hokkaido Univ. Sapporo, J Conference. 4F, 16R. Proc. Third Congress, Int. Soc. Rock Mech. Denver, 1974, V2, Part B, 1974, P1278–1282

Caisson Construction Problems and Correction in Chicago

The possible causes for defective caissons encountered during construction have been examined. These causes included excess water, soil inclusions, voids, change in caisson dimensions, inadequate bearing and poor concrete. Methods of exploration outlined are coring, caliper logging, inclinometer reading, side test pits, seismic wave and velocity measurement, 3-D logging, Gamma ray logging and probing for bells. Methods of correction and repair suggested include grouting, addition of steel in core holes, partial removal and replacement, complete removal and replacement and additional caissons with transfer girder. The importance of adequate inspection and safe construction procedures has been emphasized.

Primary ground displacements and seismic energy near the gnome explosion

Abstract Near-source measurements of seismic waves from gnome have been compared with a theoretical seismic source model derived by Blake (1952). It is estimated that 75 percent of the seismic energy in the primary waves is contained in the first half cycle of the ground displacement as shown on the seismograms from instruments located between 0.3 and 10 km from the explosion. The geometrical attenuation of the radiation field of the displacement wave is probably closely approximated by spherical divergence at ranges near the explosion. There is some evidence that a long-period displacement field may exist near the explosion as predicted by the theoretical model. However, there are not sufficient empirical data from the gnome explosion to make a detailed comparison between theory and observation.

Flip returns from mid‐Pacific research cruise

The research platform Flip and the research vessel Horizon returned to San Diego on October 7, 1963, after completing 46 days at sea, the longest consecutive time spent between ports for either vessel. The ships spent 27 days on station near the western end of the Mendocino escarpment at 39°30′N, 148°30′W. Measurements of surface waves, internal waves, and seismic waves were made from Flip during the time; hydrographic casts, BT observations, PDR soundings, and gravity cores were made from Horizon. Flip was maintained in a vertical position the entire time on station and was resupplied with fresh water, fuel, and perishables by high line from Horizon. The duration of the cruise was such that Horizon moored next to Flip for four days in order to conserve fuel.

Short Period Spectral Measurements of Seismic Waves in the Northeastern U.S.A.

Short Period Spectral Measurements of Seismic Waves in the Northeastern U.S.A.

Air Blast and Ground Shock Waves Generated at Long Distances from Demolitions of High Explosives

Abstract A summary of air blast wave and seismic wave measurements at long distances from chemical detonations obtained during a survey of demolition activities at installations of the Field Service Division, Ordnance Ammunition Command, is presented. Disturbances to surrounding residents as a result of demolition activities were found to be traceable solely to air blast waves, seismic disturbances generated directly by the demolition being far too small at the distances in question to be of consequence. The intensities of the air blast waves were found to depend primarily on weather conditions at the time of detonation, and less than one might expect on the quantity of explosive detonated. Under ideal conditions for blast wave propagation as much as five-fold enhancement above “normal” intensity of the air blast wave pressure was noted, while under conditions least favorable for propagating the blast wave as much as a twenty-fold reduction from “normal” was measured. Comparison of results with informatio...