A theoretical study of acoustic <i>S</i>-wave and <i>P</i>-wave velocity logging with conventional and dipole sources in soft formations

Winbow, Graham A.

doi:10.1190/1.1442411

Cited by 19 publications

(11 citation statements)

References 4 publications

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“…that there is no cutoff frequency for the flexural mode, which is different from the traditional point of view [7,18] . P However, the calculation shows that the contributions of the mode to the wave field are weak in the low frequency range [19] .…”

Section: The Distribution Of Complex Poles On Various Riemann Sheetsmentioning

confidence: 86%

“…Initially, White [5] brought forward the virtue of a dipole source for generating S-waves. PIn the following years, the physics of dipole source excitation in the borehole has been studied in many papers, either by numerical simulation [6,7] or by laboratory experiments [8,9] . It was realized that the full waveform was a combination of several arrivals of modes and body waves, which are dependent on the parameters of the borehole and the surrounding formation [10] .…”

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confidence: 99%

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Acoustic mode waves and individual arrivals excited by a dipole source in fluid-filled boreholes

Zhang

Wang

2009

Sci. China Ser. G-Phys. Mech. Astron.

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An approach of separating individual wave arrivals for a dipole logging is presented. The branch points are treated as a type of logarithm and the Riemann surface structure of the multivalued function is studied, that is, the displacement potential within the borehole. Based on the theoretical analysis, the complex poles contributing to the wave field on various Riemann sheets are investigated in detail for the case of a fast formation. It is shown that poles on Riemann sheet (0,0) are real and form branches of modes with dispersion. Mathematically, it is demonstrated that the flexural mode has no cutoff frequency, which is different from the traditional point of view. Poles on other relevant Riemann sheets are complex and form many branches on the complex frequency-wavenumber plane. Further investigation on the pole and branch cut contributions indicates that the vertical branch cut integration method has limitations in separating wave arrivals. By properly taking into account the complex poles on various Riemann sheets together with branch cut integrations, wave arrivals are separated from the full waveforms effectively for both the fast and slow formation models. Specially, there are complex poles on Riemann sheet (0,−1) in the vicinity of the compressional branch cut for a slow formation with a large Poisson's ratio, which have small imaginary parts and contribute a lot to the p-wave arrival.dipole sources, Riemann sheet, cutoff frequency, individual arrivals Acoustic wave propagation within a borehole excited by multipole sources has attracted intensive attention, and multipole acoustic logging has been widely used in oilfields for measuring the shear wave velocities and anisotropy caused by fractures or residual stress in formation surrounding boreholes [1 -4] . Initially, White [5] brought forward the virtue of a dipole source for generating S-waves. PIn the following years, the physics of dipole source excitation in the borehole has been studied in many papers, either by numerical simulation [6,7] or by laboratory experiments [8,9] . It was realized that the full waveform was a combination of several arrivals of modes and body waves, which are dependent on the parameters of the borehole and the surrounding formation [10] . Therefore, the studies of all these arrivals separately are required although difficult. For example, the popular numerical simulation method, real axis integration (RAI), gives the full waveform only, and it does not show the individuals separately except for the compressional head arrivals with a long source-receiver offset.The individual arrival separation, such as compressional head arrival and shear wave arrival separation from full borehole acoustic waveforms, is interesting to many research scientists [10,11] . Usually it is believed that the full wave field is represented as a sum of the contributions of poles and branch cuts on the complex wavenumber (k) plane [10][11][12] .P Tsang and RaderP [11] presented a numerical method, called the vertical branch cut integration (VBCI),...

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Section: The Distribution Of Complex Poles On Various Riemann Sheetsmentioning

confidence: 86%

mentioning

confidence: 99%

Acoustic mode waves and individual arrivals excited by a dipole source in fluid-filled boreholes

Zhang

Wang

2009

Sci. China Ser. G-Phys. Mech. Astron.

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“…In soft formations, with velocities ranging from 1500 to 2000 m/s, the different waves can be more difficult to identify (Winbow, 1988) and the refracted waves can be missed. Cheng and Toksöz (1981) provide a more detailed description of the effect of rock type on sonic waveforms.…”

Section: Wave Propagation In Boreholesmentioning

confidence: 99%

Analysis of Noise on Digital Sonic Log Data at Leg 120 Sites, Derived Synthetic Seismogram and Correlation with MCS Data at Site 747

Fritsch¹,

Munschy²,

Fezga³

1992

Proceedings of the Ocean Drilling Program

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During Leg 120, weather conditions and time constraints restricted logging operations to Sites 747 and 750; only the seismic-stratigraphic combination was run at these sites. For some intervals, the sonic digital tool produced extremely noisy velocity logs that made interpretation of the data very difficult, even impossible. Three types of noise were observed: phase skipping, oscillations caused by noncompensated heave, and discrete aberrant velocity values. The heave-related oscillations recorded on the velocity logs were eliminated by deriving a simple model simulating the tool movement. The aberrant velocity values, essentially caused by failures in recording the signal from one of the two transmitters, were eliminated by inspection of the individual waveforms using an interactive software package. This software program was also used to pick the compressional arrival time. A composite velocity log accounting for all these corrections and complemented for the unlogged part of the hole with the core-measured velocities was used to derive a synthetic seismogram at Site 747. The synthetic seismogram was correlated with a multichannel seismic reflection profile and the lithologic units, which allowed us to define four seismic sequences that could be identified with the K2-K3, P1-P2, PN1, and NQ1 sequences of the Raggatt Basin in the Southern Kerguelen Plateau.

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“…Dipole sources produce flexural waves in boreholes, which can be used to determine shear wave velocities in various formations, and evaluate the formation anisotropy or stresses [1][2][3][4][5][6] . In such cases, dipole logging has become an important logging technique since 1990s [6] .…”

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confidence: 99%

Leaky modes and their contributions to the compressional head wave in a borehole excited by a dipole source

Zhang

Wang

2009

Sci. China Ser. G-Phys. Mech. Astron.

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A method of studying the contributions of leaky modes to the wave field is presented based on the analysis of the Riemann surface structure of the characteristic function, and the sensitivities of contributions to various factors of interest are examimed. Numerical results show that their contributions to the compressional head wave are related to the distributions of complex poles on (−1, −1) and (0, −1) Riemann sheets on the frequency-wavenumber (ω − k) plane. For fast formations, their contributions are small, while for slow formations with large Poisson's ratio, their contributions are large because of those complex poles with small imaginary parts near the compressional vertical branch cut. The decaying factor of the contributions of leaky modes is approximately proportional to 1/distance 2 . dipole source, Riemann surface, leaky modes, compressional head wave Dipole sources produce flexural waves in boreholes, which can be used to determine shear wave velocities in various formations, and evaluate the formation anisotropy or stresses [1][2][3][4][5][6] . In such cases, dipole logging has become an important logging technique since 1990s [6] . The acoustical wavefield in the borehole excited by dipole sources includes compressional head waves (P-wave) propagating at near the compressional velocity except for flexural waves, and it influences the evaluation of formation parameters [3,6,7] . Meanwhile, the character of P-waves reflects parameters of the formation attenuation, clay content, etc. [8 -10] . Compared with monopoles cases, however, P-waves excited by dipole sources have not been widely used in evaluating those parameters for lack of analysis of the excitation mechanism and corresponding data processing techniques. The results got from some methods (e.g., spectra ratio, centroid frequency, and filter-correlation) [11][12][13] for monopole cases are not satisfactory either. Therefore, it is necessary to investigate further the acoustical borehole wavefield excited by dipole sources. Some researchers [7,14] pointed out that leaky modes influenced the P-waves and produced bias to attenuation estimate. Therefore, it is very useful to analyze the contributions of leaky modes to P-waves. Tsang et al. [15] believed that leaky modes would influence the head waves. Cheng et al. [16] illustrated with numerical results that the contributions of leaky modes to the wavefield increase with Poisson's ratio, and then Paillet et al. [17] divided leaky modes into two parts, the leaky compressional modes and leaky shear modes, and pointed out that contributions of the leaky compressional modes were apparent for slow formations (where the velocity of the fluid in the borehole is greater than the shear wave velocity in the formation outside), provided the Poisson'a ratio is greater than 0.3. The two authors of the paper Pstudied leaky modes and their contributions to the wavefield generated by a monopole source, they proposed a wave component isolation method, and

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A theoretical study of acoustic S-wave and P-wave velocity logging with conventional and dipole sources in soft formations

Cited by 19 publications

References 4 publications

Acoustic mode waves and individual arrivals excited by a dipole source in fluid-filled boreholes

Acoustic mode waves and individual arrivals excited by a dipole source in fluid-filled boreholes

Analysis of Noise on Digital Sonic Log Data at Leg 120 Sites, Derived Synthetic Seismogram and Correlation with MCS Data at Site 747

Leaky modes and their contributions to the compressional head wave in a borehole excited by a dipole source

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