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.

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.

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.

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.

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.

Mode Conversion Technique Employed in Shear Wave Velocity Studies of Rock Samples Under Axial and Uniform Compression

1967 ◽  
Vol 7 (02) ◽  
pp. 136-148 ◽  
Author(s):  
A.R. Gregory
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Wave Energy ◽  
Pressure Range ◽  
Mode Conversion ◽  
P Wave ◽  
S Wave ◽  
Rock Samples ◽  
Mode Converters

Abstract A shear wave velocity laboratory apparatus and techniques for testing rock samples under simulated subsurface conditions have been developed. In the apparatus, two electromechanical transducers operating in the frequency range 0.5 to 5.0 megahertz (MHz: megacycles per second) are mounted in contact with each end of the sample. Liquid-solid interfaces of Drakeol-aluminum are used as mode converters. In the generator transducer, there is total mode conversion from P-wave energy to plain S-wave energy, S-wave energy is converted back to P-wave energy in the motor transducer. Similar transducers without mode converters are used to measure P-wave velocities. The apparatus is designed for testing rock samples under axial or uniform loading in the pressure range 0 to 12,000 psi. The transducers have certain advantages over those used by King,1 and the measurement techniques are influenced less by subjective elements than other methods previously reported. An electronic counter-timer having a resolution of 10 nanoseconds measures the transit time of ultrasonic pulses through the sample; elastic wave velocities of most homogeneous materials can be measured with errors of less than 1 percent. S- and P-wave velocity measurements on Bandera sandstone and Solenhofen limestone are reported for the axial pressure range 0 to 6,000 psi and for the uniform pressure range 0 to 10,000 psi. The influence of liquid pore saturants on P- and S-wave velocity is investigated and found to be in broad agreement with Biot's theory. In specific areas, the measurements do not conform to theory. Velocities of samples measured under axial and uniform loading are compared and, in general, velocities measured under uniform stress are higher than those measured under axial stress. Liquid pore fluids cause increases in Poisson's ratio and the bulk modulus but reduce the rigidity modulus, Young's modulus and the bulk compressibility. INTRODUCTION Ultrasonic pulse methods for measuring the shear wave velocity of rock samples in the laboratory have been gradually improved during the last few years. Early experimental pulse techniques reported by Hughes et al.2, and by Gregory3 were beset by uncertainties in determining the first arrival of the shear wave (S-wave) energy. Much of this ambiguity was caused by the multiple modes propagated by piezoelectric crystals and by boundary conversions in the rock specimens. Shear wave velocity data obtained from the critical angle method, described by Schneider and Burton4 and used later by King and Fatt5 and by Gregory,3,6 are of limited accuracy, and interpreting results is too complicated for routine laboratory work. The mode conversion method described by Jamieson and Hoskins7 was recently used by King1 for measuring the S-wave velocities of dry and liquid-saturated rock samples. Glass-air interfaces acted as mode converters in the apparatus, and much of the compressional (P-wave) energy apparently was eliminated from the desired pure shear mode. A more detailed discussion of the current status of laboratory pulse methods applied to geological specimens is given in a review by Simmons.8

Changes in dynamic shear moduli of carbonate rocks with fluid substitution

Geophysics ◽  
2009 ◽  
Vol 74 (3) ◽  
pp. E135-E147 ◽  
Author(s):  
Gregor T. Baechle ◽  
Gregor P. Eberli ◽  
Ralf J. Weger ◽  
Jose Luis Massaferro
Keyword(s):  
Shear Modulus ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Water Saturation ◽  
Acoustic Properties ◽  
P Wave ◽  
Shear Moduli ◽  
Water Saturated ◽  
Fluid Interaction

To assess saturation effects on acoustic properties in carbonates, we measure ultrasonic velocity on 38 limestone samples whose porosity ranges from 5% to 30% under dry and water-saturated conditions. Complete saturation of the pore space with water causes an increase and decrease in compressional- and shear-wave velocity as well as significant changes in the shear moduli. Compressional velocities of most water-saturated samples are up to [Formula: see text] higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity [Formula: see text] generally decreases, but nine samples show an increase of up to [Formula: see text]. Water saturation decreases the shear modulus by up to [Formula: see text] in some samples and increases it by up to [Formula: see text] in others. The average increase in the shear modulus with water saturation is [Formula: see text]; the average decrease is [Formula: see text]. The [Formula: see text] ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have distinctly higher [Formula: see text] ratios. Grainstone samples with high amounts of microporosity and interparticle macro-pores preferentially show shear weakening, whereas recrystallized limestones are prone to increase shear strengths with water saturation. The observed shear weakening indicates that a rock-fluid interaction occurs with water saturation, which violates one of the assumptions in Gassmann’s theory. We find a positive correlation between changes in shear modulus and the inability of Gassmann’s theory to predict velocities of water-saturated samples at high frequencies. Velocities of water-saturated samples predicted by Gassmann’s equation often exceed measured values by as much as [Formula: see text] for samples exhibiting shear weakening. In samples showing shear strengthening, Gassmann-predicted velocity values are as much as [Formula: see text] lower than measured values. In 66% of samples, Gassmann-predicted velocities show a misfit to measured water-saturated P-wave velocities. This discrepancy between measured and Gassmann-predicted velocity is not caused solely by velocity dispersion but also by rock-fluid interaction related to the pore structure of carbonates. Thus, a pore analysis should be conducted to assess shear-moduli changes and the resultant uncertainty for amplitude variation with offset analyses and velocity prediction using Gassmann’s theory.

Shear Wave Velocities Determined From Long-and Short-Spaced Borehole Acoustic Devices

1980 ◽  
Vol 20 (05) ◽  
pp. 317-326 ◽  
Author(s):  
E.A. Koerperich
Keyword(s):  
Shear Wave ◽  
Material Balance ◽  
Shear Velocity ◽  
High Amplitude ◽  
Compressional Wave ◽  
P Wave ◽  
Considerable Uncertainty ◽  
Seismic Amplitude ◽  
Wave Interference ◽  
Sand Control

Abstract Acoustic waveforms from long- and short-spacedsonic logs were investigated to determine ifshort-spaced tools give accurate measurements of shear wave velocity. Compressional wave interference canaffect shear velocities from both tools adversely.However, the short-spaced tool was useful over awider range of conditions. Introduction The areas where shear velocity data can be appliedtheoretically or empirically are diverse. Most of theseinvolve use of the dynamic elastic rock constants, which can be computed from shear (S) velocity[along with compressional (P) velocity and bulkdensity, which are obtained readily from existingwireline logging devices]. Some of these applicationareas are (1) seismic amplitude calibration andinterpretation, (2) sand control,(3) formationfracturing, reservoir material balance and subsidencestudies(through relationships between rock andpore-volume changes with stress),(4) lithologyand porosity, 14 and (5) geopressure prediction. While rich in possible application areas, shearvelocity is difficult to measure automatically withconventional acoustic devices and detection schemes.Except in limited lithology-logging conditions, manual examination of waveforms commonly isrequired to extract shear velocity. Even then there has been considerable uncertainty in shear arrivals onshort-spaced tools due to P-wave interference. Insofter rocks, conventional tools simply do nottransmit distinct shear arrivals. Current axial transmitter-receiver (T-R) toolsare designed primarily for detection of P waves.Downhole amplifiers adjusted to accentuate the firstP-wave arrival normally saturate through the shearand late P regions of the waveform. When downholegain is reduced to eliminate amplifier saturation, initial shear arrivals generally are superimposed onlate P arrivals. This interference makes automaticdetection difficult and leads to a concern about theconsistency and dependability of this arrival fordetermining shear velocity. The interference effect iscompounded in that the initial shear energycommonly is not extremely high relative to P-waveenergy. Rather shear amplitudes are generally lowinitially and increase with succeeding arrivals. Theshear breaking point, therefore, almost always isobscured by P-wave interference. In somelithologies, such as low-porosity carbonates, an earlyshear arrival (probably the second or third shearhalf-cycle)sometimes has relatively high amplitudecompared with superimposed P arrivals. This"high-amplitude" event is commonly used to determineshear velocity. SPEJ P. 317＾

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