Imaging beneath a high‐velocity layer using converted waves

Geophysics ◽  
1992 ◽  
Vol 57 (11) ◽  
pp. 1444-1452 ◽  
Author(s):  
Guy W. Purnell
Keyword(s):  
High Velocity ◽  
Energy Partitioning ◽  
P Wave ◽  
Converted Wave ◽  
P Waves ◽  
Lower Velocity ◽  
S Waves ◽  
High Velocity Layer ◽  
The Waves ◽  
Velocity Layer

High‐velocity layers (HVLs) often hinder seismic imaging of deeper reflectors using conventional techniques. A major factor is often the unusual energy partitioning of waves incident at an HVL boundary from lower‐velocity material. Using elastic physical modeling, I demonstrate that one effect of this factor is to limit the range of dips beneath an HVL that can be imaged using unconverted P‐wave arrivals. At the same time, however, partitioning may also result in P‐waves outside the HVL coupling efficiently with S‐waves inside. By exploiting some of the waves that convert upon transmission into and/or out of the physical‐model HVL, I am able to image a much broader range of underlying dips. This is accomplished by acoustic migration tailored (via the migration velocities used) for selected families of converted‐wave arrivals.

scholarly journals The Velocity Structure within the Subducted Slab Below the Lower North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Pauline Maria Galea
Keyword(s):  
New Zealand ◽  
Upper Mantle ◽  
High Velocity ◽  
P Wave ◽  
Low Velocity ◽  
The North ◽  
The Pacific ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

<p>A shallow aftershock sequence in the Hawkes Bay region of the North Island, New Zealand (May 1990) was recorded with high quality on an L-shaped, 7-station array of 3-component, short-period seismographs at Wellington, such that the seismic waves travelled almost along strike of the subducted Pacific plate in this region. The arrival times at the stations of the Pn wave pulse from a number of aftershocks could be picked sufficiently accurately for a least-squares inversion to be carried out for wavefront speed, c, and incident azimuth, [theta]. The results show a high apparent velocity, 8.7 [plus or minus] 0.2 km/s, and an azimuth which is shifted by 6.0 [plus or minus] 2.5 degrees east of the true epicentre - station azimuth. The azimuthal anomaly, [delta][theta], has been interpreted as due to lateral refraction of Pn off the subducted slab. The effect of different geometries of the slab on the Pn wavefront characteristics (c and [delta][theta]) at Wellington have been explored through both simple geometrical considerations (in the case of a plane or cylindrical slab) as well as through 3-dimensional ray tracing (in the case of irregular curvature of the slab). It has been shown that a plane or cylindrical slab would require P-wave velocities of about 9.0 km/s to exist within it in order to fit both c and [delta][theta], whereas a model of the slab which departs from a regular cylinder and has a small updip component along strike can fit the observations with P-wave velocities of 8.75 km/s in the high velocity medium. This model has been proposed by Ansell and Bannister (1991) after detailed consideration of the shallow seismicity that defines the slab surface in the lower North Island. Information about the nature of the high velocity medium has been obtained by modeling the waveforms through generation of synthetic seismograms by the reflectivity technique of Kennett (1983). The large number of aftershocks within a small source region, and the sampling of much the same wavepath, meant that a sufficient number of seismograms had very similar and characteristic features that could be modelled. The typical seismogram of the data set had a simple Pn wavepulse, followed immediately by a complex. high frequency (up to 15 Hz) phase (here referred to as Phf) and a high amplitude, lower frequency phase that dominated the P-wavetrain (here referred to as P). A velocity profile that contained a layer of 8.75 km/s material at least 4 km thick, underlying "normal" mantle material of P-velocity 8.2 [plus or minus] 0.2 km/s. and whose surface lies approximately 18 km below the slab surface reproduced the observed seismogram features well. The presence of velocity gradients above and below the layer is not excluded. A gradual decrease in velocity below the layer in fact gives a better fit of the Pn pulse shape. By breaking down the synthetic seismogram into simpler versions. using Kennett's wavefield approximation technique, it has been shown that the Pn wave propagates through the high velocity layer, the Phf phase through the overlying layers as a sequence of reflections and refractions, and the P group as a reverberatory phase in a crustal waveguide, with its energy mostly in the form of free surface reflections and S to P conversion. These results have also been confirmed by ray tracing. Waveform modelling has also clearly shown that a low velocity layer (representing subducted sediment) on the top of the subducted slab produces a highly characteristic imprint on the synthetic seismogram, in the form of an energetic, reverberatory, lower frequency signal late in the P-wavetrain. Wavefield approximations show that this is also a crustal waveguide effect, with a strong component of mode conversion at the free surface, but P - S conversion appears to be the dominant mechanism. Seismograms very similar to such synthetic ones have been observed for the Weber aftershocks recorded at stations along the northern East Coast. The presence of such a low velocity layer in the East Coast region is thus implied, consistently with previous proposals. The petrological implications of the high velocity layer in the subducted Pacific plate are discussed. The most likely explanation is that it represents the maximum P velocity of an anisotropic layer within the Pacific upper mantle. It is proposed that the conditions of stress orientation, pressure and temperature at approximately 36 - 50 km depth in this region induces a strong realignment of olivine crystals with their fast direction along strike of the slab, normal to the maximum compressive stress axis. The upper mantle of the segment of the Pacific ocean just east of the Tonga - Kermadec trench and the North Island has been shown in this study to possess P-wave anisotropy, with the P-velocity reaching a maximum of 8.37 km/s in a direction N60 degrees E. This result was obtained by analysing a large set of ISC travel times from earthquakes along the Tonga - Kermadec - New Zealand subduction zone recorded at stations Niue, Rarotonga and the Chatham Islands. It is suggested that an enhancement of this anisotropy, accompanied by some re-orientation, takes place as the upper mantle medium is subjected to the new stress conditions in the initial stages of subduction.</p>

scholarly journals The Velocity Structure within the Subducted Slab Below the Lower North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Pauline Maria Galea
Keyword(s):  
New Zealand ◽  
Upper Mantle ◽  
High Velocity ◽  
P Wave ◽  
Low Velocity ◽  
The North ◽  
The Pacific ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

<p>A shallow aftershock sequence in the Hawkes Bay region of the North Island, New Zealand (May 1990) was recorded with high quality on an L-shaped, 7-station array of 3-component, short-period seismographs at Wellington, such that the seismic waves travelled almost along strike of the subducted Pacific plate in this region. The arrival times at the stations of the Pn wave pulse from a number of aftershocks could be picked sufficiently accurately for a least-squares inversion to be carried out for wavefront speed, c, and incident azimuth, [theta]. The results show a high apparent velocity, 8.7 [plus or minus] 0.2 km/s, and an azimuth which is shifted by 6.0 [plus or minus] 2.5 degrees east of the true epicentre - station azimuth. The azimuthal anomaly, [delta][theta], has been interpreted as due to lateral refraction of Pn off the subducted slab. The effect of different geometries of the slab on the Pn wavefront characteristics (c and [delta][theta]) at Wellington have been explored through both simple geometrical considerations (in the case of a plane or cylindrical slab) as well as through 3-dimensional ray tracing (in the case of irregular curvature of the slab). It has been shown that a plane or cylindrical slab would require P-wave velocities of about 9.0 km/s to exist within it in order to fit both c and [delta][theta], whereas a model of the slab which departs from a regular cylinder and has a small updip component along strike can fit the observations with P-wave velocities of 8.75 km/s in the high velocity medium. This model has been proposed by Ansell and Bannister (1991) after detailed consideration of the shallow seismicity that defines the slab surface in the lower North Island. Information about the nature of the high velocity medium has been obtained by modeling the waveforms through generation of synthetic seismograms by the reflectivity technique of Kennett (1983). The large number of aftershocks within a small source region, and the sampling of much the same wavepath, meant that a sufficient number of seismograms had very similar and characteristic features that could be modelled. The typical seismogram of the data set had a simple Pn wavepulse, followed immediately by a complex. high frequency (up to 15 Hz) phase (here referred to as Phf) and a high amplitude, lower frequency phase that dominated the P-wavetrain (here referred to as P). A velocity profile that contained a layer of 8.75 km/s material at least 4 km thick, underlying "normal" mantle material of P-velocity 8.2 [plus or minus] 0.2 km/s. and whose surface lies approximately 18 km below the slab surface reproduced the observed seismogram features well. The presence of velocity gradients above and below the layer is not excluded. A gradual decrease in velocity below the layer in fact gives a better fit of the Pn pulse shape. By breaking down the synthetic seismogram into simpler versions. using Kennett's wavefield approximation technique, it has been shown that the Pn wave propagates through the high velocity layer, the Phf phase through the overlying layers as a sequence of reflections and refractions, and the P group as a reverberatory phase in a crustal waveguide, with its energy mostly in the form of free surface reflections and S to P conversion. These results have also been confirmed by ray tracing. Waveform modelling has also clearly shown that a low velocity layer (representing subducted sediment) on the top of the subducted slab produces a highly characteristic imprint on the synthetic seismogram, in the form of an energetic, reverberatory, lower frequency signal late in the P-wavetrain. Wavefield approximations show that this is also a crustal waveguide effect, with a strong component of mode conversion at the free surface, but P - S conversion appears to be the dominant mechanism. Seismograms very similar to such synthetic ones have been observed for the Weber aftershocks recorded at stations along the northern East Coast. The presence of such a low velocity layer in the East Coast region is thus implied, consistently with previous proposals. The petrological implications of the high velocity layer in the subducted Pacific plate are discussed. The most likely explanation is that it represents the maximum P velocity of an anisotropic layer within the Pacific upper mantle. It is proposed that the conditions of stress orientation, pressure and temperature at approximately 36 - 50 km depth in this region induces a strong realignment of olivine crystals with their fast direction along strike of the slab, normal to the maximum compressive stress axis. The upper mantle of the segment of the Pacific ocean just east of the Tonga - Kermadec trench and the North Island has been shown in this study to possess P-wave anisotropy, with the P-velocity reaching a maximum of 8.37 km/s in a direction N60 degrees E. This result was obtained by analysing a large set of ISC travel times from earthquakes along the Tonga - Kermadec - New Zealand subduction zone recorded at stations Niue, Rarotonga and the Chatham Islands. It is suggested that an enhancement of this anisotropy, accompanied by some re-orientation, takes place as the upper mantle medium is subjected to the new stress conditions in the initial stages of subduction.</p>

A simple method for resolving large converted‐wave (P-SV) statics

Geophysics ◽  
1993 ◽  
Vol 58 (3) ◽  
pp. 429-433 ◽  
Author(s):  
Peter W. Cary ◽  
David W. S. Eaton
Keyword(s):  
P Wave ◽  
S Wave ◽  
Converted Wave ◽  
P Waves ◽  
Simple Method ◽  
Near Surface ◽  
S Waves ◽  
Static Solutions ◽  
Surface Fluctuations

The processing of converted‐wave (P-SV) seismic data requires certain special considerations, such as commonconversion‐point (CCP) binning techniques (Tessmer and Behle, 1988) and a modified normal moveout formula (Slotboom, 1990), that makes it different for processing conventional P-P data. However, from the processor’s perspective, the most problematic step is often the determination of residual S‐wave statics, which are commonly two to ten times greater than the P‐wave statics for the same location (Tatham and McCormack, 1991). Conventional residualstatics algorithms often produce numerous cycle skips when attempting to resolve very large statics. Unlike P‐waves, the velocity of S‐waves is virtually unaffected by near‐surface fluctuations in the water table (Figure 1). Hence, the P‐wave and S‐wave static solutions are largely unrelated to each other, so it is generally not feasible to approximate the S‐wave statics by simply scaling the known P‐wave static values (Anno, 1986).

Cross‐borehole simulation of wave propagation

Geophysics ◽  
1991 ◽  
Vol 56 (7) ◽  
pp. 1103-1113 ◽  
Author(s):  
Philippe Jean ◽  
Michel Bouchon
Keyword(s):  
Wave Propagation ◽  
High Velocity ◽  
Guided Waves ◽  
Seismic Energy ◽  
Synthetic Seismograms ◽  
P Wave ◽  
Receiver Arrays ◽  
High Velocity Layer ◽  
Discrete Wavenumber Method ◽  
Velocity Layer

Synthetic seismograms are used to study elastic wave propagation in multi‐layered media for cross‐borehole geometries. The calculations are done using the discrete wavenumber method. We compute the wavefield for a series of receiver arrays located at various offsets to follow the evolution of the wavefronts and the distribution of the seismic energy in space. The results show the complexity of the wavefield at large offsets. Trapped waves and conical waves perturb the identification of direct and primary reflected phases. The display of polarization diagrams of the guided waves shows elliptical prograde motions. The source position plays an important role in the energy distribution within the medium. In order to study this dependency, we compare the cases of sources located in relatively low and high‐velocity layers. In the former case, most of the energy is trapped within the source layer and in the neighboring region and propagates horizontally. The S‐converted waves generated at the interfaces bounding the source layer have larger amplitude than the direct P‐wave. When the source is placed in a relatively high‐velocity layer the energy of the P‐wavefront spreads rapidly throughout the medium as the offset increases.

scholarly journals Three-component amplitude versus offset analysis

1989 ◽  
Vol 20 (2) ◽  
pp. 257
Author(s):  
D.R. Miles ◽  
G. Gassaway ◽  
L. Bennett ◽  
R. Brown
Keyword(s):  
Seismic Data ◽  
P Wave ◽  
S Wave ◽  
Converted Wave ◽  
P Waves ◽  
S Waves ◽  
Avo Analysis ◽  
Avo Inversion ◽  
Amplitude Versus Offset ◽  
Converted Waves

Three-component (3-C) amplitude versus offset (AVO) inversion is the AVO analysis of the three major energies in the seismic data, P-waves, S-waves and converted waves. For each type of energy the reflection coefficients at the boundary are a function of the contrast across the boundary in velocity, density and Poisson's ratio, and of the angle of incidence of the incoming wave. 3-C AVO analysis exploits these relationships to analyse the AVO changes in the P, S, and converted waves. 3-C AVO analysis is generally done on P, S, and converted wave data collected from a single source on 3-C geophones. Since most seismic sources generate both P and S-waves, it follows that most 3-C seismic data may be used in 3-C AVO inversion. Processing of the P-wave, S-wave and converted wave gathers is nearly the same as for single-component P-wave gathers. In split-spread shooting, the P-wave and S-wave energy on the radial component is one polarity on the forward shot and the opposite polarity on the back shot. Therefore to use both sides of the shot, the back shot must be rotated 180 degrees before it can be stacked with the forward shot. The amplitude of the returning energy is a function of all three components, not just the vertical or radial, so all three components must be stacked for P-waves, then for S-waves, and finally for converted waves. After the gathers are processed, reflectors are picked and the amplitudes are corrected for free-surface effects, spherical divergence and the shot and geophone array geometries. Next the P and S-wave interval velocities are calculated from the P and S-wave moveouts. Then the amplitude response of the P and S-wave reflections are analysed to give Poisson's ratio. The two solutions are then compared and adjusted until they match each other and the data. Three-component AVO inversion not only yields information about the lithologies and pore-fluids at a specific location; it also provides the interpreter with good correlations between the P-waves and the S-waves, and between the P and converted waves, thus greatly expanding the value of 3-C seismic data.

Analysis of conventional and converted mode reflections at Putah sink, California using three‐component data

Geophysics ◽  
1990 ◽  
Vol 55 (6) ◽  
pp. 646-659 ◽  
Author(s):  
C. Frasier ◽  
D. Winterstein
Keyword(s):  
Time Window ◽  
Structural Features ◽  
Low Noise ◽  
P Wave ◽  
Bright Spot ◽  
High Signal ◽  
Converted Wave ◽  
Gas Field ◽  
Seismic Line ◽  
S Waves

In 1980 Chevron recorded a three‐component seismic line using vertical (V) and transverse (T) motion vibrators over the Putah sink gas field near Davis, California. The purpose was to record the total vector motion of the various reflection types excited by the two sources, with emphasis on converted P‐S reflections. Analysis of the conventional reflection data agreed with results from the Conoco Shear Wave Group Shoot of 1977–1978. For example, the P‐P wave section had gas‐sand bright spots which were absent in the S‐S wave section. Shot profiles from the V vibrators showed strong P‐S converted wave events on the horizontal radial component (R) as expected. To our surprise, shot records from the T vibrators showed S‐P converted wave events on the V component, with low amplitudes but high signal‐to‐noise (S/N) ratios. These S‐P events were likely products of split S‐waves generated in anisotropic subsurface media. Components of these downgoing waves in the plane of incidence were converted to P‐waves on reflection and arrived at receivers in a low‐noise time window ahead of the S‐S waves. The two types of converted waves (P‐S and S‐P) were first stacked by common midpoint (CMP). The unexpected S‐P section was lower in true amplitude but much higher in S/N ratio than the P‐S section. The Winters gas‐sand bright spot was missing on the converted wave sections, mimicking the S‐S reflectivity as expected. CRP gathers were formed by rebinning data by a simple ray‐tracing formula based on the asymmetry of raypaths. CRP stacking improved P‐S and S‐P event resolution relative to CMP stacking and laterally aligned structural features with their counterparts on P and S sections. Thus, the unexpected S‐P data provided us with an extra check for our converted wave data processing.

Fractionation trends in the Bay of Islands ophiolite of Newfoundland: polycyclic cumulate sequences in ophiolites and their classification

1977 ◽  
Vol 14 (5) ◽  
pp. 1156-1165 ◽  
Author(s):  
W. R. Church ◽  
L. Riccio
Keyword(s):  
Basaltic Magma ◽  
General Rule ◽  
Olivine Gabbro ◽  
Lower Velocity ◽  
Crystallization Sequence ◽  
Basaltic Rocks ◽  
Lower Temperature ◽  
High Velocity Layer ◽  
Velocity Layer ◽  
Temperature Crystallization

The fractionation range of the cumulate sequence of the allochthonous Bay of Islands ophiolite of the Western Platform of Newfoundland, measured in terms of the FeO(tolal)/MgO ratios of the liquids from which they were derived, encompasses entirely the range of known values exhibited by the overlying dikes and pillow lavas. Cryptic variations within the cumulate sequences are irregular, often inverse, and the crystallization sequences found in the cumulates suggest that they were formed from at least three different basaltic magma types, one of which is unusual in having given rise to co-existing highly aluminous clinopyroxenes and spinels. These features suggest that crystallization of the Bay of Islands plutonic rocks took place in an 'open system' magma chamber that was tapped repeatedly during fractionation to form dike rocks and lavas. Most of the cumulate rocks of the Bay of Islands ophiolite formed according to the crystallization sequence ol–cpx–(opx) or the sequence ol–plag–cpx–(opx). In contrast, the cumulate rocks of the Betts Cove ophiolite, located within the Fleur de Lys orthotectonic zone of the Newfoundland Appalachians, crystallized according to the sequences ol–opx–cpx and ol–cpx–plag. This difference in the nature of the cumulate sequences within the Bay of Islands and Betts Cove ophiolites is also reflected in the Ti characteristics of the basaltic rocks of the ophiolites, and in the morphology of the gabbroic units. Comparison with Mesozoic ophiolites suggests, as a general rule, that within ophiolite cumulate successions there is a tendency for ol–opx sequences to be followed by ol–cpx sequences, and for ol–cpx sequences to be followed by ol–plag sequences. Such a relationship may be related to processes involving remelting of lower-temperature crystallization products in a system open to either continuous or periodic additions of high temperature basaltic liquid. In terms of oceanic structures the Bay of Islands ophiolite corresponds to sonobouy model 2 of Christensen and Salisbury: the basal high velocity layer corresponding to the olivine-gabbro cumulate rocks, and the lower velocity 'gabbroic' layer to the upper part of the olivine-free cumulate sequence and overlying massive uralitized roof gabbro and dike rocks.

Numerical simulation of azimuthal acoustic logging in a borehole penetrating a rock formation boundary

Geophysics ◽  
2016 ◽  
Vol 81 (3) ◽  
pp. D283-D291 ◽  
Author(s):  
Peng Liu ◽  
Wenxiao Qiao ◽  
Xiaohua Che ◽  
Xiaodong Ju ◽  
Junqiang Lu ◽  
...  
Keyword(s):  
Domain Model ◽  
P Wave ◽  
S Wave ◽  
Angle Variation ◽  
P Waves ◽  
Acoustic Logging ◽  
S Waves ◽  
Rock Formation ◽  
Logging Tool ◽  
Difference Time

We have developed a new 3D acoustic logging tool (3DAC). To examine the azimuthal resolution of 3DAC, we have evaluated a 3D finite-difference time-domain model to simulate a case in which the borehole penetrated a rock formation boundary when the tool worked at the azimuthal-transmitting-azimuthal-receiving mode. The results indicated that there were two types of P-waves with different slowness in waveforms: the P-wave of the harder rock (P1) and the P-wave of the softer rock (P2). The P1-wave can be observed in each azimuthal receiver, but the P2-wave appears only in the azimuthal receivers toward the softer rock. When these two types of rock are both fast formations, two types of S-waves also exist, and they have better azimuthal sensitivity compared with P-waves. The S-wave of the harder rock (S1) appears only in receivers toward the harder rock, and the S-wave of the softer rock (S2) appears only in receivers toward the softer rock. A model was simulated in which the boundary between shale and sand penetrated the borehole but not the borehole axis. The P-wave of shale and the S-wave of sand are azimuthally sensitive to the azimuth angle variation of two formations. In addition, waveforms obtained from 3DAC working at the monopole-transmitting-azimuthal-receiving mode indicate that the corresponding P-waves and S-waves are azimuthally sensitive, too. Finally, we have developed a field example of 3DAC to support our simulation results: The azimuthal variation of the P-wave slowness was observed and can thus be used to reflect the azimuthal heterogeneity of formations.

Shallow near-surface effects

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. T221-T231 ◽  
Author(s):  
Christine E. Krohn ◽  
Thomas J. Murray
Keyword(s):  
Time Delay ◽  
Velocity Gradient ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
P Waves ◽  
Large Computer ◽  
Near Surface ◽  
S Waves ◽  
High Attenuation ◽  
Pulse Broadening

The top 6 m of the near surface has a surprisingly large effect on the behavior of P- and S-waves. For unconsolidated sediments, the P-wave velocity gradient and attenuation can be quite large. Computer modeling should include these properties to accurately reproduce seismic effects of the near surface. We have used reverse VSP data and computer simulations to demonstrate the following effects for upgoing P-waves. Near the surface, we have observed a large time delay, indicating low velocity ([Formula: see text]), and considerable pulse broadening, indicating high attenuation ([Formula: see text]). Consequently, shallowly buried geophones have greater high-frequency bandwidth compared with surface geophones. In addition, there is a large velocity gradient in the shallow near surface (factor of 10 in 5 m), resulting in the rotation of P-waves to the vertical with progressively smaller amplitudes recorded on horizontal phones. Finally, we have found little indication of a reflection or ghost from the surface, although downgoing reflections have been observed from interfaces within the near surface. In comparison, the following have been observed for upgoing S-waves: There is a small increase in the time delay or pulse broadening near the surface, indicating a smaller velocity gradient and less change in attenuation. In addition, the surface reflection coefficient is nearly one with a prominent surface ghost.

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