Velocity and amplitude of P-waves transmitted through fractured zones composed of multiple thin low-velocity layers

1995 ◽  
Vol 32 (4) ◽  
pp. 313-324 ◽  
Author(s):  
T. Watanabe ◽  
K. Sassa
Keyword(s):  
Low Velocity ◽  
P Waves ◽  
Fractured Zones

MODEL STUDY OF RADIATION FROM AN EXPLOSION IN A CIRCULAR DISK

Geophysics ◽  
1969 ◽  
Vol 34 (2) ◽  
pp. 213-221 ◽  
Author(s):  
Dhari S. Bahjat ◽  
Carl Kisslinger
Keyword(s):  
Solid Medium ◽  
Circular Disk ◽  
Peak Frequency ◽  
Approximate Theory ◽  
Low Velocity ◽  
P Waves ◽  
Compressional Waves ◽  
Model Study ◽  
Disk Material ◽  
Solid Circular Cylinder

The effect of the medium immediately surrounding the shot on the properties of the primary compressional waves has been studied by means of two‐dimensional models. Circular disks of Plexiglas and Styrofoam of various diameters were inserted in large sheets of aluminum and of Plexiglas, respectively. Charges were fired in the center of the disk. An approximate theory for radiation from a simple harmonic line source of P waves on the axis of a solid circular cylinder embedded in an infinite homogeneous solid medium has been developed. The transfer function of the disk has been calculated from the observed spectra in order to compare it with the frequency response calculated from theory. The results show that for these cases of a low‐velocity disk a peak frequency exists which decreases with increasing diameter. The maximum peak frequency is that for a shot in an infinite sheet of the high‐velocity material, and the minimum peak frequency is that for a shot in a sheet of the low‐velocity disk material. This minimum peak frequency is approached rapidly as the disk diameter increases. The geometric effect of radiation from the circular inhomogeneity is important only over a narrow range of cavity diameters for which the resonance associated with the cavity happens to fall near the peak of the spectrum of the explosion‐generated wave.

Rapid map and inversion of P-SV waves

Geophysics ◽  
1991 ◽  
Vol 56 (6) ◽  
pp. 859-862 ◽  
Author(s):  
Robert R. Stewart
Keyword(s):  
Spatial Resolution ◽  
Seismic Data ◽  
P Wave ◽  
Rock Properties ◽  
Low Velocity ◽  
P Waves ◽  
Near Surface ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
And Inversion

Multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional P‐wave sections and to find and use rock properties associated with shear waves (e.g. Dohr, 1985; Danbom and Dominico, 1986). Mode‐converted (P-SV) waves hold a special interest for several reasons: They are generated by conventional P‐wave sources and have only a one‐way travel path as a shear wave through the typically low velocity and attenuative near surface. For a given frequency, they will have a shorter wavelength than the original P wave, and thus offer higher spatial resolution; this has been observed in several vertical seismic profiling (VSP) cases (e.g., Geis et al., 1990). However, for surface seismic data, converted waves are often found to be of lower frequency than P-P waves (e.g., Eaton et al., 1991).

The horizontal propagation of P waves through scattering media: Analog model studies relevant to long-range Pn propagation

1983 ◽  
Vol 73 (1) ◽  
pp. 125-142
Author(s):  
William Menke ◽  
Paul G. Richards
Keyword(s):  
Long Range ◽  
Crustal Material ◽  
Scattering Media ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
P Waves ◽  
Model Studies ◽  
Homogeneous Models ◽  
Amplifying Effect ◽  
Lateral Heterogeneities

abstract We compare seismograms produced through the use of analog models of scattering and nonscattering Earth structure to access the effect of lateral heterogeneities on the horizontal propagation of P waves. Our results are applicable to propagation out to 16° and to periods as short as 5 sec. We find that layers of scatterers within the mantle near the turning points of P waves can cause these phases to have long coda similar in certain respects to the coda observed in long-range Pn. The overall appearance of this coda is similar to that produced in laterally homogeneous models that include low-velocity crustal material near their surfaces. Scatterers at shallower depths in the mantle or crust have a different effect. Those in very uppermost mantle seem to suppress the horizontal propagation of P waves while those in the crust have very little overall effect, at least at the perod range we studied. We also find that the scatterers have an amplifying effect on the horizontal propagation of the very longest periods (>20 sec), an effect that may be related to the scattering-controlled boundary waves discovered by Biot (1968).

Travel-time curves for a low-velocity channel in the upper mantle

1964 ◽  
Vol 54 (6A) ◽  
pp. 1981-1996 ◽  
Author(s):  
John Dowling ◽  
Otto Nuttli
Keyword(s):  
Velocity Gradient ◽  
Upper Mantle ◽  
Body Wave ◽  
Shadow Zone ◽  
Low Velocity ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Time Distance ◽  
Velocity Channel

abstract Velocities within the earth can be determined from body wave time-distance (T-D) data by the Herglotz-Wiechert method provided the velocity does not decrease too rapidly with depth. Until the present time, the properties of T-D curves for rapid decreases of velocity with depth have been considered only qualitatively. This paper presents a technique for calculating a T-D curve for any velocity distribution, including continuous and discontinuous increases and decreases of velocity with depth. Some properties of T-D curves are quantitatively studied by systematically varying the characteristics of a single model and noting the corresponding variations in the calculated T-D curves. From this it is concluded that a significant low-velocity channel may not be evidenced by a shadow zone but rather by an overlapping of two distinct branches of the T-D curve. It is further concluded that the presence of a shadow zone implies a very gentle velocity gradient below the low-velocity channel. By fitting a calculated T-D curve to observed data one can determine velocity as a function of depth even when the velocity decreases rapidly with depth, when a low-velocity channel exists. Observed T-D data for two underground nuclear explosions (gnome and bilby) measured in four different azimuths were fitted with T-D curves calculated for assumed velocity distributions. It is concluded that these data can be satisfied by a low-velocity channel for P waves in the upper mantle. The character of this channel (depth, thickness and velocity) was determined in each azimuth. The depth to its top was shallow (70 ± km) in the western U.S. and deep (125 ± km) in the eastern U.S. The velocity gradient below the channel is sharp enough to produce no prominent shadow zones. There are significant lateral changes in upper mantle velocities in the western U. S.

VARIATIONS IN SPECTRA OF P WAVES RECORDED AT CANADIAN ARCTIC SEISMOGRAPH STATIONS

1966 ◽  
Vol 3 (5) ◽  
pp. 597-621 ◽  
Author(s):  
Tokuji Utsu
Keyword(s):  
Surface Layer ◽  
Epicentral Distance ◽  
Canadian Arctic ◽  
The Arctic ◽  
Spectral Amplitude ◽  
Layered System ◽  
Low Velocity ◽  
P Waves ◽  
Low Frequencies ◽  
Spectral Curves

To investigate the effects of location of a seismograph station on the records obtained, P waves from seismograms of Alaskan earthquakes recorded at four Canadian Arctic stations, Coppermine, Mould Bay, Resolute, and Alert have been analyzed spectrally. Differences in spectral amplitude at very low frequencies among the stations and between earthquakes can reasonably be explained by a consideration of earthquake magnitude, epicentral distance, and mechanism. Differences in the shape of the spectral curves between stations result mainly from local crustal structures beneath the station. Increased absorption of waves is observed beneath Resolute. Use of the vertical to radial-horizontal spectral amplitude ratios gives an approximate estimate of the thickness of the crust and surface layer beneath these stations, although some phenomena noted cannot be explained by the theory of signal reverberation in a horizontally layered system with perfect elasticity. This analysis suggests that the crust thins toward the Arctic Ocean: the best estimates of crustal thickness are about 45 km at Coppermine, about 33 km at Resolute, about 23 at Alert, and about 18 km at Mould Bay. The latter depth is the most uncertain. There is a low velocity surface layer with a thickness of several hundred meters at Mould Bay, Resolute, and Alert.

A Strong Seismic Reflector within the Mantle Wedge above the Ryukyu Subduction of Northern Taiwan

2019 ◽  
Vol 91 (1) ◽  
pp. 310-316
Author(s):  
Cheng‐Horng Lin ◽  
Min‐Hung Shih ◽  
Ya‐Chuan Lai
Keyword(s):  
Opposite Direction ◽  
Mantle Wedge ◽  
Seismic Velocity ◽  
Low Velocity ◽  
P Waves ◽  
Deep Earthquake ◽  
Subduction System ◽  
Subducting Slab ◽  
Seismic Reflector ◽  
Northern Taiwan

Abstract Major structures within the mantle wedge are often revealed from seismic velocity anomalies, such as low‐velocity zones at magma reservoirs, partial melting regions, or the upwelling asthenosphere. However, no significant seismic boundaries have been reported in the shallow mantle wedge beneath volcanic arcs. Here, we present evidence for a strong seismic reflector dipping in the opposite direction of the subducting slab in the mantle wedge beneath northern Taiwan in the western end of the Ryukyu subduction system. We find that two unambiguous P waves generated by a deep earthquake (ML 5.1) at a depth of 132.5 km were clearly recorded by the dense seismic array (Formosa Array), composed of 140 broadband seismic stations with a station spacing of approximately 5 km in northern Taiwan. Forward modeling using both raytracing and travel times shows that a seismic reflector exists beneath the Tatun volcano group (TVG) around depths of 80–110 km. The reflector dips in the opposite direction of the subducting slab and is unlikely to be associated with mantle wedge corner flow. Instead, it probably belonged to parts of possible structures such as the asthenospheric flow, the mantle diapir, or other undiscovered structures above the subducting slab. No matter what the seismic boundary is exactly, it might be associated with the active volcanism in the TVG. The detailed geometry and mechanism of the seismic boundary in the mantle wedge will be obtained as the Formosa Array collects more seismic data in the near future.

Investigation of multiple reflections and wave conversion by means of a vertical wave test (vertical seismic profiling) in southern Mississippi

Geophysics ◽  
1982 ◽  
Vol 47 (7) ◽  
pp. 977-1000 ◽  
Author(s):  
C. C. Lash
Keyword(s):  
Wave Train ◽  
Low Frequency ◽  
P Wave ◽  
Multiple Reflections ◽  
Low Velocity ◽  
P Waves ◽  
S Waves ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
Sonic Log

A vertical wave test employing the vertical seismic profiling (VSP) technique in southern Mississippi confirmed suspicions that apparent multiple reflections might include converted waves as well as multiply reflected compressional waves. Both compressional (P) and shear (S) waves generated near the source were observed to travel to great depths, and P‐to‐S conversions were apparent in deep zones as well as shallow. P‐wave reflections were observed in agreement with predictions from synthetic records based on the sonic log. Up‐traveling P‐waves were reflected a short distance below the surface, at the base of the low‐velocity layer, and were followed as down‐traveling P‐waves to 200 ft depth by means of a vertical spread. Below 2000 ft and following the first P wave train, the predominate energy appeared to be down‐traveling P‐waves which could not be traced back to the reflection of up‐traveling P‐waves. The continuity of wavelets indicated instead that the strong down‐traveling S‐waves generated near the source produced P‐waves by S‐to‐P conversion somewhere in the zone between 800 and 1400 ft. The interference on the recordings made with an individual seismometer, or a small group of seismometers, using dynamite shots as the source was generally of a low‐frequency nature, so that the signal‐to‐noise (S/N) ratio was improved by the use of a high passband filter. The interference was greatly reduced without the need for a filter on recordings in which the source was a distributed charge of 100 ft length. The distributed charge produced much less shear‐wave energy in the P reflection band, demonstrating that the interference encountered when using a concentrated charge source was the consequence of the generation of S‐waves near the source. The distributed charges were previously chosen as a means for effectively eliminating secondary (ghost) reflections, an unwanted form of multiple reflections.

Physical modeling of anisotropic domains: Ultrasonic imaging of laser-etched fractures in glass

Geophysics ◽  
2013 ◽  
Vol 78 (1) ◽  
pp. D11-D19 ◽  
Author(s):  
Robert R. Stewart ◽  
Nikolay Dyaur ◽  
Bode Omoboya ◽  
J. J. S. de Figueiredo ◽  
Mark Willis ◽  
...  
Keyword(s):  
Physical Modeling ◽  
Laser Etching ◽  
Etching Technique ◽  
P Waves ◽  
S Waves ◽  
Fractured Zones ◽  
Scattered Energy ◽  
Zero Offset ◽  
Very High ◽  
Velocity Decrease

Many regions of subsurface interest are, or will be, fractured. Seismically characterizing these zones is a complicated but essential task for resource development. Physical modeling, using ultrasonic sources and receivers over scaled exploration targets, can play a useful role as an analog for reservoir imaging and assessment. We explored the anisotropic response of glass blocks containing internal fractures created by a novel laser-etching technique. We compared transmitted and reflected signals for P- and S-waves from fractured and unfractured zones in a suite of ultrasonic (1–5 MHz) experiments. The unaltered glass velocities have averages of [Formula: see text] and [Formula: see text] for P- and S-waves, respectively (giving [Formula: see text]). The unfractured glass has a very high quality (Q) factor of over 500 for P-waves and S-waves. The fractured zones have a small (up to 1.5%) velocity decrease. Signals propagating through the fractured zone have diminished amplitudes and increased coda signatures. Reflection surveys (zero-offset and with variable polarizations) record significant scatter from the fractured zones. The fracture-scattered energy can be migrated to provide a sharper image. The glass specimens with laser-etched fractures display a rich anisotropic response, which can help inform field-scale imaging.

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