Shear Wave Energy Research Articles

High‐frequency wave propagation from mantle earthquakes in the Tyrrhenian Sea: New constraints for the geometry of the South Tyrrhenian Subduction Zone

Propagation of shear waves produced by 25 mantle earthquakes (80–600 km depth) in the subduction zone of the south Tyrrhenian Sea (southern Italy) has been investigated to infer the geometry and extent of the descending lithosphere. From all hypocentral depths high‐frequency, high‐amplitude shear waves are recorded at most of the stations in southern Italy and easternmost Sicily. This shear‐wave energy is interpreted to travel as a guided wave within the descending slab. In contrast, shear waves are either not recorded at all or they are recorded as low‐frequency, low‐amplitude signals at stations located in the peninsular part of Italy north of the Calabrian arc, in western Sicily and in Sardinia. This systematic S‐wave attenuation is interpreted in terms of an active and continuous slab correlated with and limited to the Calabrian arc.

Beamformed nearfield imaging of a simulated coronary artery containing a stenosis.

This paper is concerned with the potential for the detection and location of an artery containing a partial blockage by exploiting the space-time properties of the shear wave field in the surrounding elastic soft tissue. As a demonstration of feasibility, an array of piezoelectric film vibration sensors is placed on the free surface of a urethane mold that contains a surgical tube. Inside the surgical tube is a nylon constriction that inhibits the water flowing through the tube. A turbulent field develops in and downstream from the blockage, creating a randomly fluctuating pressure on the inner wall of the tube. This force produces shear and compressional wave energy in the urethane. After the array is used to sample the dominant shear wave space-time energy field at low frequencies, a nearfield (i.e., focused) beamforming process then images the energy distribution in the three-dimensional solid. Experiments and numerical simulations are included to demonstrate the potential of this noninvasive procedure for the early identification of vascular blockages-the typical precursor of serious arterial disease in the human heart.

P- and SV-wave separation by polarization-dependent velocity filtering: application to vertical seismic profiles from Kola superdeep borehole, Russia

In multicomponent seismic surveys, separation of interfering compressional (P) and shear-wave (S) energy provides valuable information for the interpretation of mode conversions and seismic anisotropy. We present a technique for such a separation using a polarization-dependent velocity filtering. At the first step of the procedure, a linear two-channel filter is designed in the frequency-wavenumber domain, with filter coefficients determined by the angles of incidence of respective seismic waves. Due to several physical reasons, the filter is unstable near the high end of the P-wave spectrum. We stabilize the filter with the use of S-wave spectrum balancing. This operation results in a robust decomposition into P- and SV-responses, while retaining the full energy of the vector wavefield. We implement this method in a module of our seismic processing system. The module employs a versatile parametrization scheme, as well as structured data input/output. It is able to process VSP or surface data, with velocities varying by either depth or offset. Application of the module to the processing of an offset VSP from the Kola Superdeep Borehole demonstrates that the program successfully separates P- and S-wave phases in crustal VSP data.

Regional seismic discriminants using wave-train energy ratios

AbstractWe have examined broadband regional waveforms of recent (since 1988) Nevada Test Site (NTS) underground explosions and earthquakes throughout the southwestern United States and Baja, Mexico, recorded by TERRAscope and other IRIS stations in order to characterize seismic sources for the purposes of event identification. As expected, earthquakes tended to be richer in long-period surface-wave and short-period shear-wave energy relative to explosions of comparable P-wave strength. Also, explosions, in general, were found to be richer in 1- to 6-sec surface-wave (Rg) energy and other late-arriving coda energy than were earthquakes. Most earthquakes show relatively little long-period (T > 6 sec) Rg and surface-wave coda energy, which we attribute to their deeper source depths, whereas known shallow earthquakes do exhibit these phases. We have developed several seismic discriminants based on our observations. The most promising discriminant is the ratio of short-period (f ≧ 1.0 Hz), vertical component, Pnl wave-train energy (EspPz) to long-period (0.05 to 0.167 Hz), three-component, surface-wave energy (Elp−3). For this ratio, explosions tend to have a higher value than do earthquakes. This discriminant works on the same premise as the teleseismic mb:MS ratio, for which earthquakes are richer in long-period surface-wave energy relative to explosions with the same body-wave magnitude. The long-period passband was chosen to limit the effect of longer-period noise and to remove the effect of the coda surface waves. Another potential discriminant examined is the ratio of short-period (f ≧ 1.0 Hz), vertical-component, P-wave to S-wave energy (EspPz:EspSz). We find that this criterion only yields marginal separation of the source populations but becomes more effective at higher frequency bands (f ≧ 4.0 Hz) or when looking at single-station observations. It does, however, help to quantify significant short-period waveform differences between the three test subsites, with Pahute Mesa shots generating relatively little S-wave energy compared to those of Yucca Flat for which the S wave (or Lg) is often the largest phase, while Rainier's shots are intermediate in character with distinct but less prominent S waves. This S-wave generation is thought to be caused by near-source scattering to converted phases and appears to be highly dependent on the near-source geology. These two discriminants are useful in that they are simple and fast to calculate. Using regional stations for sources 200 to 1300 km away, the magnitude threshold for the EspPz:Elp−3 discriminant is roughly ML ≧ 4.0, the limiting factor being the signal level of the Airy phase, while that for the EspPz:EspSz discriminant is roughly ML ≧ 3.0 for the same distance ranges.

Scanning tomographic acoustic microscopy using shear waves

We propose to use shear waves instead of longitudinal waves in a novel scanning tomographic acoustic microscope (STAM) in which the specimens are solid. When a specimen with a shear modulus is immersed in the microscope's water bath, mode conversion takes place at the water-solid interface. The shear wave energy is detectable and can be used for image reconstruction. Although wave transmission in most solid specimens is limited to about 20 degrees for longitudinal waves, it is about twice that for shear waves. Also, velocities of shear waves are lower than those of longitudinal waves and hence the wavelengths at the same frequency are smaller. For these and other reasons we can expect that for many specimens the resolution of a shear-wave STAM to be substantially better than that of a longitudinal-wave STAM. We use computer simulation in order to compare the operation of a shear-wave STAM with that of the conventional longitudinal-wave STAM. We have simulated tomographic reconstruction for each. The corresponding critical angles of incidence are computed and tomographic reconstructions of a particular solid specimen is obtained by using the back-and-forth propagation algorithm (BFP). Our simulation results show that shear-wave STAM has better resolution than longitudinal-wave STAM.

Vertical seismic profile results from the Kola Superdeep Borehole, Russia

Multi-offset vertical seismic profiles (VSPs) from the Kola Superdeep Borehole (SG-3), as part of a larger seismic study of the Kola region conducted during the spring of 1992, sample the dipping Pechenga complex from 2175 m to 6000 m and contribute to the understanding of reflectivity in crystalline and Precambrian environments. From the surface to 6000 m, the SG-3 borehole penetrates interlayered Proterozoic metavolcanic and metasedimentary units and a mylonitic shear zone ranging from greenschist to amphibolite metamorphic grade, respectively. The Kola VSPs display a 6% velocity decrease which coincides to a mylonitic shear zone located between 4500 m and 5100 m within the SG-3 borehole. Seismic interfaces are identified by mode-converted energy (PS, and SP transmissions and reflections) in addition to primary seismic phases. The VSP shear wave energy is generated at or near the source by vertical vibrators. P-wave and S-wave reflections are generally detected from the same reflecting horizons, but increases in relative S-wave and SP reflection amplitudes originate at 1900 m, 3800 m, 4500 m, and 5100 m depths. These depths coincide with zones of elevated V p V s and may support the presence of free pore fluid which is reported from initial drilling. For the Proterozoic lithologies sampled by the VSP, reflection events result from five mylonitic shear zones and three lithologic contrasts.

Modeling of ground motion from a 1994 Northridge aftershock using a tomographic velocity model of the Los Angeles Basin

Abstract The 1994 Northridge mainshock and its aftershocks show a complex pattern of peak accelerations at stations located in the Los Angeles Basin. The waveforms contain multiples of body-wave phases and extensive surface waves at frequencies mostly below 1 Hz. In particular, for stations at distances greater than 18 km, secondary arrivals show larger accelerations than the direct S-wave arrivals. The mainshock waveforms are further complicated by irregularities of the source rupture. We use 2D finite difference to evaluate the effect of lateral variations in seismic velocity on the amplitude of shear-wave energy and to distinguish the effects of source and propagation path. We model waveforms from one aftershock recorded at nine stations deployed along a 60-km-long profile extending into the Los Angeles Basin. We use a two-dimensional slice through the 3D tomography model of the Los Angeles Basin in the 2D finite-difference calculations. These synthetic waveforms fit the aftershock waveforms significantly better than corresponding waveforms determined from simple 1D velocity models. With the addition of a thin low-velocity surface layer above the tomography model, the finite-difference synthetics reproduce most of the important features of the recorded data, in particular, the large-amplitude arrivals 7 to 10 sec following the direct S arrival. These arrivals correspond to the SS arrival, which is sharply refracted at the basin edge, and the S-wave with multiple legs trapped by the dipping near surface gradient. For large earthquakes located either inside or outside the basin, these phases can be the cause of the largest and hence potentially most hazardous shaking in the Los Angeles Basin.

Causes of low-frequency ground motion amplification in the Salt Lake Basin: the case of the vertically incident P wave

We use simulations of 1-D, 2-D and 3-D wave propagation to identify the major causes of low-frequency (0.2–1.2 Hz) seismic amplification in the Salt Lake Basin. For a simple two-layer basin model and a vertically incident P wave, we examine how amplification is influenced by mode conversion, surface-wave generation, impedance effects at the sediment-bedrock boundary, resonance, and 2-D and 3-D focusing and scattering. Results show the following. (1) Approximately 30 per cent of the total cumulative kinetic energy at the Salt Lake Valley floor consists of shear-wave energy generated by P-to-S converted waves and surface waves. The surface waves appear to be generated primarily along the edges of the basin, and the instantaneous S/P energy ratio in the sedimentary layer is as large as 3. (2) The largest peak particle velocity at the free surface is due to the direct P wave. The value is roughly predicted by the transmission coefficient of 1.46 at the sediment-bedrock interface, i.e. a normally incident P wave in the stiff bedrock will be magnified in amplitude by 1.46 times as it enters the softer sediments. (3) The low-frequency elastic response of the two-layer Salt Lake Basin model is characterized by surface-wave propagation and resonance from vertically interfering waves. (4) The peak particle velocities, cumulative kinetic energies, and mean spectral magnitudes computed from the 2-D (1-D) synthetics underestimate the values computed from the 3-D synthetics by up to 40 per cent (48 per cent) along a profile above the deepest part of the basin model. The 2-D and 1-D signal duration times underestimate the 3-D values by up to 59 and 94 per cent, respectively. Our results suggest that 2-D basin modelling may yield good approximations to the 3-D ground motion amplification above the deepest part of the Salt Lake Basin. Our results show that several mechanisms contribute significantly to low-frequency seismic amplification in the semi-consolidated sediments of the Salt Lake Basin—P-to-S wave conversion, surface-wave generation, impedance effects at the sediment-bedrock boundary, and resonance. Future attempts to estimate ground motion amplification in the Salt Lake Basin should therefore account for the amplification effects of all these mechanisms.

Use of mode-converted waves in marine seismic data to investigate the lithology of the sub-bottom sediments in Isfjorden, Svalbard

The compression wavefield is efficiently converted to shear-wave energy at post-critical angles in areas of high impedance contrast at the sea floor. We have analysed mode-converted shear waves in a data set acquired with a hybrid marine/land geometry in Isfjorden, Svalbard. Through a kinematic 2D ray-tracing modellingVp/Vs ratios for part of the uppermost 5km of the crust are obtained. Low values (Vp/Vs=1.65) are tentatively associated with the section of Devonian sandstones which appears to attain a minimum thickness of 1.5km below 3 km depth about 10km west of Kapp Thorden.

95/01951 1993 carbon dioxide fact sheet (notebook)

In multicomponent seismic surveys, separation of interfering compressional (P) and shear-wave (S) energy provides valuable information for the interpretation of mode conversions and seismic anisotropy. We present a technique for such a separation using a polarization-dependent velocity filtering. At the first step of the procedure, a linear two-channel filter is designed in the frequency-wavenumber domain, with filter coefficients determined by the angles of incidence of respective seismic waves. Due to several physical reasons, the filter is unstable near the high end of the P-wave spectrum. We stabilize the filter with the use of S-wave spectrum balancing. This operation results in a robust decomposition into P- and SV-responses, while retaining the full energy of the vector wavefield. We implement this method in a module of our seismic processing system. The module employs a versatile parametrization scheme, as well as structured data input/output. It is able to process VSP or surface data, with velocities varying by either depth or offset. Application of the module to the processing of an offset VSP from the Kola Superdeep Borehole demonstrates that the program successfully separates P- and S-wave phases in crustal VSP data.

95/01955 Climate alteration. A global issue for the electric power industry in the 21st century

In multicomponent seismic surveys, separation of interfering compressional (P) and shear-wave (S) energy provides valuable information for the interpretation of mode conversions and seismic anisotropy. We present a technique for such a separation using a polarization-dependent velocity filtering. At the first step of the procedure, a linear two-channel filter is designed in the frequency-wavenumber domain, with filter coefficients determined by the angles of incidence of respective seismic waves. Due to several physical reasons, the filter is unstable near the high end of the P-wave spectrum. We stabilize the filter with the use of S-wave spectrum balancing. This operation results in a robust decomposition into P- and SV-responses, while retaining the full energy of the vector wavefield. We implement this method in a module of our seismic processing system. The module employs a versatile parametrization scheme, as well as structured data input/output. It is able to process VSP or surface data, with velocities varying by either depth or offset. Application of the module to the processing of an offset VSP from the Kola Superdeep Borehole demonstrates that the program successfully separates P- and S-wave phases in crustal VSP data.

95/01950 Canadian tribes consider waste store - Canada

In multicomponent seismic surveys, separation of interfering compressional (P) and shear-wave (S) energy provides valuable information for the interpretation of mode conversions and seismic anisotropy. We present a technique for such a separation using a polarization-dependent velocity filtering. At the first step of the procedure, a linear two-channel filter is designed in the frequency-wavenumber domain, with filter coefficients determined by the angles of incidence of respective seismic waves. Due to several physical reasons, the filter is unstable near the high end of the P-wave spectrum. We stabilize the filter with the use of S-wave spectrum balancing. This operation results in a robust decomposition into P- and SV-responses, while retaining the full energy of the vector wavefield. We implement this method in a module of our seismic processing system. The module employs a versatile parametrization scheme, as well as structured data input/output. It is able to process VSP or surface data, with velocities varying by either depth or offset. Application of the module to the processing of an offset VSP from the Kola Superdeep Borehole demonstrates that the program successfully separates P- and S-wave phases in crustal VSP data.

The effects of free-methane bubbles on the propagation and scattering of compressional and shear wave energy in muddy sediments

Free-methane bubbles cause significant scattering of acoustic energy in the soft sediments of Eckernfoerde Bay, Baltic Sea. Insitu and laboratory measurement of sediment geoacoustic and physical properties were made in an attempt to understand the physical mechanisms responsible for this scattering. Insitu shear wave velocities (at 100–500 Hz) increased from 5–7 near the surface to 15–20 m s−1 at 2 m into the seafloor, whereas insitu compressional wave velocities (at 38 and 58 kHz) varied little (∼1425 m s−1) with depth. Methane bubbles apparently caused significant attenuation of compressional waves at depths below 1 m, whereas shear wave attenuation was unaffected by gas and decreased with depth. Compressional waves (at 400 kHz) in cores (1%–5% free gas) maintained at insitu pressures were highly attenuated but show little evidence of velocity dispersion. Comparison of geoacoustic data with theory suggest that primary control of the interaction of acoustic profilers with this gassy seafloor is by bubbles larger than about 1 mm diameter and that the observed high-frequency scattering can be described by models of the seafloor bubbles as individual scatters of acoustic energy. Response of the seafloor as a medium with modified bulk propagation properties would occur at lower frequencies than those used in the Eckernfoerde experiments.

RECENT DEVELOPMENTS IN BOREHOLE SEISMIC SURVEYS, OFFSHORE NORTHERN AUSTRALIA

Offshore northern Australia has long been recognised as a region where the quality of seismic data is frequently adversely affected by several factors which include the following.The presence of a hard water bottom.The presence of carbonate units in the Eocene and Paleocene which are highly reflective and reduce the amount of seismic energy which penetrates to the deeper target horizons. These also produce interbedded multiples which are difficult to remove from the seismic data.Complex shallow faulting which causes ray path distortion and disperses the seismic energy.The confidence with which well data can be tied to the poor quality seismic data through well velocity surveys is further reduced by the complicated deeper faulting associated with the structures drilled and the low acoustic impedance contrast at the target horizon.Borehole seismic surveys provide the most reliable link between the subsurface intersected by a well and seismic data. The recent introduction of a new borehole seismic tool has improved the quality of Vertical Seismic Profiles (VSPs) acquired. This tool has three component geophones mounted in a sensor module which is isolated from the main body of the tool by springs which minimise the effect of source-generated noise.This reduction in sensitivity to the source-generated noise has allowed the introduction of more powerful source arrays to improve the signal to noise ratio. The use of source arrays has increased the bandwidth of the seismic impulse and decreased the effect of the bubble pulse.The quality of the horizontal component VSP data recorded using the new tool has also improved significantly and this has increased the possibility of detecting mode converted reflected and transmitted shear wave energy and the more accurate measurement of shear wave velocities.

Attenuation of high‐frequency shear waves in the crust: Measurements from New York State, South Africa, and southern California

We compare the attenuation of high‐frequency (3–30 Hz) shear waves for crustal paths in New York State, South Africa, and southern California over source‐receiver distances of about 10–400 km. The data consist of digital recordings of S waves (Δ = 5–100 km) and Lg waves (Δ = 100–400 km) produced by earthquakes. We use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the S and Lg waves. Over the entire distance range studied (10–400 km), the amplitude decay of 3‐Hz shear wave energy is considerably less for the tectonicaily stable areas of New York and South Africa than for the tectonicaily active region of southern California. High‐frequency (30 Hz) S wave attenuation is significantly less for New York and South Africa than for southern California, for distances between 15 and 90 km. We parameterize the decay with distance (R) of coda‐normalized shear wave amplitudes with a frequency‐independent Q and geometrical spreading exponent γ, where geometrical spreading is proportional to R−γ. For New York State the S wave amplitude decay (3–30 Hz) is well described by a frequency‐independent Q of 2100−330+490 and γ of 1.3±0.1. The decay of Lg wave amplitudes from 3 to 15 Hz in the New York State region is fit with a frequency‐independent Q of 1600−280+330 γ of 0.70±0.2. The S wave amplitudes (3–30Hz) in South Africa yield a Q of 1500−190+380 and γ of 1.3±0.1. Fixing the geometrical spreading at R−0.5 produces an Lg wave Q estimate at 3 Hz in South Africa of 360−50+80. This Lg wave Q is low considering that South Africa is a cratonic, tectonicaily stable area. The S wave amplitudes from southern California are described with a frequency‐independent Q of 800−150+240 and a large geometrical spreading exponent of γ of 1.9±0.2. We find an Lg wave Q at 3 Hz of 260±30 for southern California, after constraining the geometrical spreading at R−0.5.

High-resolution investigations of seismoacoustic propagation in shallow-water areas

Data from a newly developed 30-element accelerometer/hydrophone array are used to study seismoacoustic propagation in shallow-water locations off New Jersey and Martha's Vineyard. The elements of the array have 1-m spacing; each consisting of three orthogonal accelerometers (flat from 2 to 500 Hz), a hydrophone, and a vertical direction sensor. The 120 accelerometer/hydrophone signals and 30 vertical direction signals are digitized and transmitted via 1 km of fiber optic cable to a PC-type recording system. The A/D converters digitize all hydrophones at 2048 samples/s and all accelerometers at 512 samples/s. Good coupling to the bottom is obtained since sensor symmetry has been maximized; coupling to the water has been minimized; and density is matched to the sediments. There are 10-m extensions between the 30-m sensor section and the A/D converter housing at either end of the array. Units containing port-starboard paired, shotgun-shell sources were mounted within the array in these extensions and fired through the array circuitry, providing precise time and distance control and reverse profile capability. Differenced shot pairs emphasize transverse horizontal shear (SH) and Love wave signals; summed pairs emphasize P, SV, and Rayleigh/Stoneley/Scholte waves. Shear boundary wave energy is observed with frequencies greater than 60 Hz and velocities and wavelengths as low as 20 m/s and 1 m, respectively. Considerable energy appears to result from lateral heterogeneity and/or anisotropy. [Research supported by ONR.]

Local magnitude and sediment amplification observations from earthquakes in the northern Baja California-southern California region

Abstract The data set that is analyzed in this study consists of strong motion and digital velocity records obtained during the last 6 yr for earthquakes along faults in Baja California North and southern California. From this set, local magnitudes for 60 earthquakes recorded at stations on hard rock and sediments are calculated following the technique proposed by Kanamori and Jennings (1978). The calculated near-source motion magnitudes ( M LSM ) are then compared with reported local magnitude values ( M L ) which are in the range from 3 to 6.6. From these comparisons, it is found that for earthquakes with magnitude M L between 3 and 5.5, almost all the M LSM values estimated from data registered on sediments exceed the M L values by amounts which at the shorter distances (less than about 10 km) may be as high as an order of magnitude, or even higher for a few cases. The strong motion records associated with these high M LSM values have highly coherent pulse-like S -wave signals of very short period and large accelerations. Similar estimations of local magnitude from more limited data obtained on hard rock were comparable or lower than the standard M L values. We conclude that the high M LSM values are the result of a combination of high-frequency source spectra and a strong shear wave energy amplification due to the presence of sediments. A sediment amplification factor of 3.2 is inferred from the data and is in good agreement with a theoretical factor of 3.4 estimated on the basis of the McMechan and Mooney (1980) velocity model for the Imperial Valley. It is also found from the analyzed data that near-source local magnitude decreases between 0 and 10 km, indicating that for smaller earthquakes the attenuation function, -log A 0 (richter, 1958), used in local magnitude determinations, needs to be modified to take into account the characteristics of the seismic energy radiated at short distances. Near-source strong motion data for the 1979 Imperial Valley ( M L = 6.6) and 1980 Victoria ( M L = 6.1) events do not show the strong discrepancy between M LSM and M L observed for the smaller magnitude events. Factors that might be responsible for this include nonlinear sediment attenuation for larger amplitude waves, as well as complex near-source spreading out of the waveform resulting from larger source size (near-source magnitude saturation). We give a numerical example, using small earthquakes as Green9s functions, to show that for a complex source function represented as a superposition of smaller earthquakes, M LSM and M L for the 1980 Victoria earthquake can be explained by the magnitude saturation phenomenon.

High-frequency approximation to the nongeometricalS* arrival

abstractWhen an explosive point source is buried less than one wavelength from the free surface in a perfectly elastic, homogeneous, isotropic half-space, a very strong non-geometric arrival, tentatively denoted as S* by Hron and Mikhailenko (1981), exists in the reflected shear wave field. It has all basic characteristics of an ordinary shear body wave (linear polarization, transverse particle motion, and shear wave velocity) and, as such, it can be also reflected or transmitted upon incidence at an interface according to the rules governing ordinary body waves. Because of its very strong amplitude which at most epicentral distances exceeds even the amplitude of a direct P wave, the S* arrival is undoubtedly responsible for a large part of the shear wave energy content seen in the seismic records in oil exploration, which deals almost exclusively with shallow explosive sources. In our paper, we develop a high-frequency approximation to the S* arrival applying a saddle-point method to the integral representing a shear wave reflected from the free surface due to the incidence of a spherical longitudinal wave radiated by the shallow point source. Numerical results presented in the paper show that our high-frequency approximation preserves all basic features of the S* arrival, thereby making it suitable for easy incorporation into any synthetic seismogram computation based on the ray approach.

Instrumentation Corrections to Wave Velocity Data

The detection of the arrival of seismic stress waves for a field measurement of wave velocity requires that the incoming signal be recorded and displayed by an instrument such as a seismograph. Filters, introduced into recording circuits to limit noise or to prevent aliasing in digital recording, can have significant effects on the observed arrival times and on the computation of wave velocity. Such filtration is often not taken into account in processing the data, but the example that follows indicates how the effects can be evaluated and the data corrected by simple techniques. The properties of electronic filters are, of course, well known to electrical engineers, and the following comments are concerned with an application in which their importance has not always been appreciated by those trained in other disciplines. Two basic conclusions can be drawn from this work: 1. The characteristics of instrumentation can have important effects on the results of field geophysical tests. In particular, the effects of filtration should not be ignored. 2. Regression analysis of travel time versus distance is effective in determining timing error and correcting for it. There is a widespread belief that, if one utilizes a hammer with reversible polarity, shear wave arrivals in cross-hole seismic tests can be identified unambiguously because the direction of first motion in the shear wave reverses when the direction of the impact is reversed. The direction of first motion in the P-wave is assumed not to reverse. Unfortunately, shear wave sources such as the Bison hammer also produce P-waves of opposite polarity when the direction of impact changes. The reason that P-waves reverse polarity is that a downward excitation of the hammer causes initial compression below the level of the hammer and initial tension above it. An upward excitation reverses this pattern so the P-waves must reverse polarity. A theoretical argument is that in a linearly elastic material, the superposition of equal and opposite upward and downward excitations gives a case of no net excitation. If the P-waves do not reverse polarity, they will not cancel, and there will be energy propagated with no input. Non-reversing P-waves may be caused by several factors, including late arrivals of reflected or refracted waves and secondary dilatational effects near the source. However, this misconception probably arises because in the down-hole test, shear wave energy is often created by striking a horizontal beam or plate on the surface in opposite horizontal direction. This unreversed vertical load causes an unreversing P-wave, which can be observed in field data. (Author)

Some laboratory experiments on shear wave propagation in unconsolidated sands

Abstract With the recent development of a method to transmit shear wave energy through laboratory unconsolidated sediments, an important new era of research on the dynamic properties of sediments has just begun. This, in time, will complement the vast amount of compressional wave data existent in the literature and provide the researcher with a very important tool to aid in the classification of sediments by correlating geophysical with geo‐technical parameters. A description is given of laboratory experiments in a specially designed sedimentation chamber to propagate shear waves in saturated sand sediments, using a method which employs a pair of novel design piezoelectric ceramic transducers. For comparison, similar measurements of wave velocity and amplitude were made on the sands using a pair of compressional wave transducers. Results are presented of compressional wave and shear wave velocity and amplitude versus porosity for these sands. A complete laboratory electronics testing facility necessary for acoustic measurements using both types of transducers is detailed. The performance of the shear wave transducers is also demonstrated in preliminary experiments using a plexiglass rod and then a container full of dry sand. Finally, the behavior of a transiently‐stressed saturated sand in the sedimentation cell is monitored geophysically by the continuous propagation of shear waves. It is shown that the duration of the liquefaction event in the sediment can be recognized by the temporary disappearance of shear waves from the record.