Nongeometrically converted shear waves in marine streamer data

Geophysics ◽  
2012 ◽  
Vol 77 (6) ◽  
pp. P45-P56 ◽  
Author(s):  
Guy Drijkoningen ◽  
Nihed el Allouche ◽  
Jan Thorbecke ◽  
Gábor Bada
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Body Wave ◽  
Shear Waves ◽  
Compressional Wave ◽  
Body Waves ◽  
P Wave ◽  
Streamer Data ◽  
Propagating Shear ◽  
Water Bottom

Under certain circumstances, marine streamer data contain nongeometrical shear body wave arrivals that can be used for imaging. These shear waves are generated via an evanescent compressional wave in the water and convert to propagating shear waves at the water bottom. They are called “nongeometrical” because the evanescent part in the water does not satisfy Snell’s law for real angles, but only for complex angles. The propagating shear waves then undergo reflection and refraction in the subsurface, and arrive at the receivers via an evanescent compressional wave. The required circumstances are that sources and receivers are near the water bottom, irrespective of the total water depth, and that the shear-wave velocity of the water bottom is smaller than the P-wave velocity in the water, most often the normal situation. This claim has been tested during a seismic experiment in the river Danube, south of Budapest, Hungary. To show that the shear-related arrivals are body rather than surface waves, a borehole was drilled and used for multicomponent recordings. The streamer data indeed show evidence of shear waves propagating as body waves, and the borehole data confirm that these arrivals are refracted shear waves. To illustrate the effect, finite-difference modeling has been performed and it confirmed the presence of such shear waves. The streamer data were subsequently processed to obtain a shear-wave refraction section; this was obtained by removing the Scholte wave arrival, separating the wavefield into different refracted arrivals, stacking and depth-converting each refracted arrival before adding the different depth sections together. The obtained section can be compared directly with the standard P-wave reflection section. The comparison shows that this approach can deliver refracted-shear-wave sections from streamer data in an efficient manner, because neither the source nor receivers need to be situated on the water bottom.

Sonic logging of compressional‐wave velocities in a very slow formation

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1627-1633 ◽  
Author(s):  
Bart W. Tichelaar ◽  
Klaas W. van Luik
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Compressional Wave ◽  
P Wave ◽  
Dipole Source ◽  
Frequency Spectra ◽  
Unconsolidated Sands ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Sonic Logging

Borehole sonic waveforms are commonly acquired to produce logs of subsurface compressional and shear wave velocities. To this purpose, modern borehole sonic tools are usually equipped with various types of acoustic sources, i.e., monopole and dipole sources. While the dipole source has been specifically developed for measuring shear wave velocities, we found that the dipole source has an advantage over the monopole source when determining compressional wave velocities in a very slow formation consisting of unconsolidated sands with a porosity of about 35% and a shear wave velocity of about 465 m/s. In this formation, the recorded compressional refracted waves suffer from interference with another wavefield component identified as a leaky P‐wave, which hampers the determination of compressional wave velocities in the sands. For the dipole source, separation of the compressional refracted wave from the recorded waveforms is accomplished through bandpass filtering since the wavefield components appear as two distinctly separate contributions to the frequency spectrum: a compressional refracted wave centered at a frequency of 6.5 kHz and a leaky P‐wave centered at 1.3 kHz. For the monopole source, the frequency spectra of the various waveform components have considerable overlap. It is therefore not obvious what passband to choose to separate the compressional refracted wave from the monopole waveforms. The compressional wave velocity obtained for the sands from the dipole compressional refracted wave is about 2150 m/s. Phase velocities obtained for the dispersive leaky P‐wave excited by the dipole source range from 1800 m/s at 1.0 kHz to 1630 m/s at 1.6 kHz. It appears that the dipole source has an advantage over the monopole source for the data recorded in this very slow formation when separating the compressional refracted wave from the recorded waveforms to determine formation compressional wave velocities.

Near-surface velocities and attenuation at two boreholes near Anza, California, from logging data

1990 ◽  
Vol 80 (4) ◽  
pp. 807-831 ◽  
Author(s):  
Jon B. Fletcher ◽  
Tom Fumal ◽  
Hsi-Ping Liu ◽  
Linda C. Carroll
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Spectral Ratio ◽  
Body Waves ◽  
P Wave ◽  
Wave Source ◽  
Near Surface ◽  
Wave Velocities ◽  
Rock Structure ◽  
Over The Top

Abstract To investigate near-surface site effects in granite rock, we drilled 300-m-deep boreholes at two sites which are collocated with stations from the digital array at Anza, California. The first borehole was sited at station KNW (Keenwild fire station), which is located along a ridge line about 8.7 km east of the San Jacinto Fault zone. Station PFO (Piñon Flat Observatory), chosen for the second site, is another 6 km further to the east of station KNW and is located on a gently sloping hillside. We logged each borehole for P- and S-wave velocities, as well as for crack density and orientation. P waves were generated by striking a plate with a hammer at the surface. A tool consisting of weighted anvils driven by compressed air against end plates along a 3.5-m beam was used to generate shear waves. Signals were recorded downhole with a three-component sensor package at 2.5-m intervals from the surface to 50 m depth, and at 5-m intervals from 50 m depth to the bottom of the hole. Velocities were determined by differencing the measured arrival times of first arrivals or peaks over each interval in depth. Travel times were computed for the first breaks at shallow depths, however, below about 100 m depth, times were computed for the first peaks rather than for first breaks since the first arrival was no longer clearly distinguishable. The KNW site yielded a shear velocity of 1.9 km/sec by only 30 m in depth and reached close to 2.6 km/sec at the bottom of the hole. P-wave velocities at KNW were also high at 5.4 km/sec starting at 120 m depth. The PFO site had similar but slightly higher shear-wave velocities. The bottom-hole shear-wave velocity reached 3.0 km/sec, and the P-wave velocity was 5.4 km/sec. Shear-wave attenuation was computed using both the pulse rise time and spectral ratio methods. At station KNW, attenuation was significant only in an interval between 17.5 and approximately 40 m in depth. Over the top 50 m, attenuation corresponding to a Q of about 8 was obtained. A total T* of 0.004 sec was measured for this interval. Pulse rise times also increased rapidly in this zone. The spectral ratio data for station PFO yields two peaks in attenuation above 50 m. Similar to the attenuation found for station KNW, the peak in attenuation corresponds to a Q of about 11, averaged over the top 50 m. Spectra of the seismic pulses produced by the hammer give good signal between 20 to 80 Hz. Significant motion perpendicular to the polarizations of the first shear-wave arrival was recorded within a few meters of the surface. Apparently, the rock structure is sufficiently complicated that body waves are being converted (SH to SV at oblique incidence) very close to the surface. The presence of these elliptical particle motions within a mere few m of the pure shear-wave source suggests that the detection of polarizations perpendicular to the main shear arrival at a single location at the surface is not, by itself, a good method for detecting shearwave splitting within the upper few tens of kilometers of the earth's crust. Crack densities and orientations were determined from televiewer records. These records showed cracks with a preferred direction at station KNW and of a greater density than at station PFO. At station PFO, crack densities were smaller and more diffuse in orientation.

Converted waves in a shallow marine environment: Experimental and modeling studies

Geophysics ◽  
2011 ◽  
Vol 76 (1) ◽  
pp. T1-T11 ◽  
Author(s):  
Nihed Allouche ◽  
Guy G. Drijkoningen ◽  
Willem Versteeg ◽  
Ranajit Ghose
Keyword(s):  
Shallow Water ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Shear Waves ◽  
Seismic Survey ◽  
Wave Source ◽  
Shallow Marine ◽  
Modeling Studies ◽  
Water Bottom

Seismic waves converted from compressional to shear mode in the shallow subsurface can be useful not only for obtaining shear-wave velocity information but also for improved processing of deeper reflection data. These waves generated at deep seas have been used successfully in hydrocarbon exploration; however, acquisition of good-quality converted-wave data in shallow marine environments remains challenging. We have looked into this problem through field experiments and synthetic modeling. A high-resolution seismic survey was conducted in a shallow-water canal using different types of seismic sources; data were recorded with a four-component water-bottom cable. Observed events in the field data were validated through modeling studies. Compressional waves converted to shear waves at the water bot-tom and at shallow reflectors were identified. The shear waves showed distinct linear polarization in the horizontal plane and low velocities in the marine sediments. Modeling results indicated the presence of a nongeometric shear-wave arrival excited only when the dominant wavelength exceeded the height of the source with respect to the water/sediment interface, as observed in air-gun data. This type of shear wave has a traveltime that corresponds to the raypath originating not at the source but at the interface directly below the source. Thus, these shear waves, excited by the source/water-bottom coupled system, kinematically behave as if they were generated by an S-wave source placed at the water bottom. In a shallow-water environment, the condition appears to be favorable for exciting such shear waves with nongeometric arrivals. These waves can provide useful information of shear-wave velocity in the sediments.

Vp/Vs—A POTENTIAL HYDROCARBON INDICATOR

Geophysics ◽  
1976 ◽  
Vol 41 (5) ◽  
pp. 837-849 ◽  
Author(s):  
Robert H. Tatham ◽  
Paul L. Stoffa
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Mode Conversion ◽  
High Water ◽  
Compressional Wave ◽  
P Wave ◽  
Angle Of Incidence ◽  
Seismic Velocities ◽  
Compressional Wave Velocity ◽  
Reflection Seismic

Theoretically and experimentally, the shear‐wave velocity of a porous rock has been shown to be less sensitive to fluid saturants than the compressional wave velocity. Thus, observation of the ratio of the seismic velocities for waves which traverse a changing or laterally varying zone of undersaturation or gas saturation could produce an observable anomaly which is independent of the regional variation in compressional wave velocity. One source of shear‐wave data in reflection seismic prospecting is mode conversion of P waves to shear waves in marine areas of high water bottom P-wave velocity. A relatively simple interpretative technique, based on amplitude variation as a function of the angle of incidence, is a possible discriminant between shear and multiple compressional arrivals, and data for a real case are shown. A normal moveout velocity analysis, carefully coupled with this offset discriminant, leads to the construction of a shear‐wave reflection section which can then be correlated with the usual compressional wave section. One such a section has been constructed, the variation in the ratio of the seismic velocities can be mapped, and potentially anomalous subsurface regions observed.

SHEAR‐WAVE RECORDING USING CONTINUOUS SIGNAL METHODS PART I—EARLY DEVELOPMENT

Geophysics ◽  
1968 ◽  
Vol 33 (2) ◽  
pp. 229-239 ◽  
Author(s):  
J. T. Cherry ◽  
K. H. Waters
Keyword(s):  
Shear Wave ◽  
Early Development ◽  
Wave Velocity ◽  
Field Work ◽  
Compressional Wave ◽  
P Wave ◽  
Wave Reflections ◽  
Continuous Signal ◽  
Equipment Development ◽  
Wave Recording

In recent years, it has been found possible to record shear‐wave reflections and horizontally traveling shear waves using continuous signal methods. Thus paper traces the equipment development and field work performed during this research. The earliest work with a version of a swinging‐weight vibrator showed that shear‐wave reflections could be recorded. This fact provided the impetus to make modifications to equipment to meet difficulties caused by lack of energy and lack of frequency bandwidth. Examples are given which show the flexibility of the system in providing comparison between the horizontally traveling surface waves induced and recorded by the various combinations of vibrator sources and geophone types and their relative orientations. Frequency selection by the different modes is well illustrated. For most of the reflection examples, the average ratio of shear‐wave velocity to compressional‐wave velocity in the first few thousands of feet is near 0.5. Finally, to complete the early development, the version of the shear‐wave vibrator and recording system which was used for most of the additional work is described. In order to make comparison of P‐wave and SH‐wave reflection records easier, this system provided for a 2:1 compression of the shear‐wave time scale as well as a 2:1 ratio of frequency output between the P‐ and SH‐vibrator systems. A few examples of SH reflection profiles achieved with this system are presented.

High-frequency approximation to the nongeometrical S* arrival

1983 ◽  
Vol 73 (1) ◽  
pp. 109-123
Author(s):  
P. F. Daley ◽  
F. Hron
Keyword(s):  
Free Surface ◽  
Point Source ◽  
Shear Wave ◽  
High Frequency ◽  
Energy Content ◽  
Body Wave ◽  
Body Waves ◽  
P Wave ◽  
Saddle Point Method ◽  
Frequency Approximation

abstract When 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.

A teleseismic body-wave analysis of the May 1980 Mammoth Lakes, California, earthquakes

1983 ◽  
Vol 73 (2) ◽  
pp. 419-434
Author(s):  
Jeffery S. Barker ◽  
Charles A. Langston
Keyword(s):  
Body Wave ◽  
Moment Tensor ◽  
Normal Fault ◽  
Strike Slip ◽  
Body Waves ◽  
P Wave ◽  
Moment Tensors ◽  
Double Couple ◽  
The Moment ◽  
Mammoth Lakes

abstract Teleseismic P-wave first motions for the M ≧ 6 earthquakes near Mammoth Lakes, California, are inconsistent with the vertical strike-slip mechanisms determined from local and regional P-wave first motions. Combining these data sets allows three possible mechanisms: a north-striking, east-dipping strike-slip fault; a NE-striking oblique fault; and a NNW-striking normal fault. Inversion of long-period teleseismic P and SH waves for the events of 25 May 1980 (1633 UTC) and 27 May 1980 (1450 UTC) yields moment tensors with large non-double-couple components. The moment tensor for the first event may be decomposed into a major double couple with strike = 18°, dip = 61°, and rake = −15°, and a minor double couple with strike = 303°, dip = 43°, and rake = 224°. A similar decomposition for the last event yields strike = 25°, dip = 65°, rake = −6°, and strike = 312°, dip = 37°, and rake = 232°. Although the inversions were performed on only a few teleseismic body waves, the radiation patterns of the moment tensors are consistent with most of the P-wave first motion polarities at local, regional, and teleseismic distances. The stress axes inferred from the moment tensors are consistent with N65°E extension determined by geodetic measurements by Savage et al. (1981). Seismic moments computed from the moment tensors are 1.87 × 1025 dyne-cm for the 25 May 1980 (1633 UTC) event and 1.03 × 1025 dyne-cm for the 27 May 1980 (1450 UTC) event. The non-double-couple aspect of the moment tensors and the inability to obtain a convergent solution for the 25 May 1980 (1944 UTC) event may indicate that the assumptions of a point source and plane-layered structure implicit in the moment tensor inversion are not entirely valid for the Mammoth Lakes earthquakes.

scholarly journals Triplicated P-wave measurements for waveform tomography of the mantle transition zone

Solid Earth ◽  
2012 ◽  
Vol 3 (2) ◽  
pp. 339-354 ◽  
Author(s):  
S. C. Stähler ◽  
K. Sigloch ◽  
T. Nissen-Meyer
Keyword(s):  
Transition Zone ◽  
Epicentral Distance ◽  
Body Wave ◽  
Source Parameters ◽  
Body Waves ◽  
P Wave ◽  
Theoretical Modelling ◽  
Finite Frequency ◽  
Mantle Transition Zone ◽  
Band Limited

Abstract. Triplicated body waves sample the mantle transition zone more extensively than any other wave type, and interact strongly with the discontinuities at 410 km and 660 km. Since the seismograms bear a strong imprint of these geodynamically interesting features, it is highly desirable to invert them for structure of the transition zone. This has rarely been attempted, due to a mismatch between the complex and band-limited data and the (ray-theoretical) modelling methods. Here we present a data processing and modelling strategy to harness such broadband seismograms for finite-frequency tomography. We include triplicated P-waves (epicentral distance range between 14 and 30°) across their entire broadband frequency range, for both deep and shallow sources. We show that is it possible to predict the complex sequence of arrivals in these seismograms, but only after a careful effort to estimate source time functions and other source parameters from data, variables that strongly influence the waveforms. Modelled and observed waveforms then yield decent cross-correlation fits, from which we measure finite-frequency traveltime anomalies. We discuss two such data sets, for North America and Europe, and conclude that their signal quality and azimuthal coverage should be adequate for tomographic inversion. In order to compute sensitivity kernels at the pertinent high body wave frequencies, we use fully numerical forward modelling of the seismic wavefield through a spherically symmetric Earth.

P-wave velocity profile at very shallow depths in sand dunes

Geophysics ◽  
2020 ◽  
Vol 85 (5) ◽  
pp. U129-U137
Author(s):  
Sherif M. Hanafy ◽  
Ammar El-Husseiny ◽  
Mohammed Benaafi ◽  
Abdullatif Al-Shuhail ◽  
Jack Dvorkin
Keyword(s):  
Velocity Profile ◽  
Wave Velocity ◽  
Sand Dunes ◽  
Seismic Signal ◽  
Compressional Wave ◽  
Dominant Frequency ◽  
P Wave ◽  
Beach Sand ◽  
Low Velocity ◽  
Measured Velocity

We have addressed the problem of measuring the compressional wave velocity at a very shallow depth in unconsolidated dune sand. Because the overburden stress is very small at shallow depths, the respective velocity is small and the seismic signal is weak. This is why such data are scarce, in the lab and in the field. Our approach is to stage a high-resolution seismic experiment with a dense geophone line with spacing varying between 10 and 25 cm, allowing us to produce a velocity-depth relation in the upper 1 m interval. These results are combined with another survey in which the geophone spacing is 2 m and the dominant frequency is an order of magnitude lower than in the first survey. The latter results give us the velocity profile in the deeper interval between 1 and 7 m, down to the base of the dune. The velocity rapidly increases from about 48 m/s in the first few centimeters to 231 m/s at 1 m depth and then gradually increases to 425 m/s at 7 m depth. This is the first time when such a low velocity has been recorded at extremely shallow depths in sand in situ. The velocity profile thus generated is statistically fitted with a simple analytical equation. Our velocity values are higher than those published previously for beach sand. We find that using replacement or tomogram velocities instead of an accurately measured velocity profile may result in 23%–44% error in the static correction.

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