Local and global fluid effects on sonic wave modes

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. D369-D381 ◽  
Author(s):  
Elliot J. H. Dahl ◽  
Kyle T. Spikes
Keyword(s):  
Rayleigh Wave ◽  
Wave Velocity ◽  
Fluid Pressure ◽  
Wave Mode ◽  
P Wave ◽  
Stoneley Wave ◽  
S Wave ◽  
Local Pressure ◽  
Wave Velocities ◽  
Wave Modes

Most subsurface formations of value to exploration contain a heterogeneous fluid-filled pore space, where local fluid-pressure effects can significantly change the velocities of passing seismic waves. To better understand the effect of these local pressure gradients on borehole wave propagation, we combined Chapman’s squirt-flow model with Biot’s poroelastic theory. We applied the unified theory to a slow and fast formation with permeable borehole walls containing different quantities of compliant pores. These results are compared with those for a formation with no soft pores. The discrete wavenumber summation method with a monopole point source generates the wavefields consisting of the P-, S-, leaky-P, Stoneley, and pseudo-Rayleigh waves. The resulting synthetic wave modes are processed using a weighted spectral semblance (WSS) algorithm. We found that the resulting WSS dispersion curves closely matched the analytical expressions for the formation compressional velocity and solutions to the period equation for dispersion for the P-wave, Stoneley-wave, and pseudo-Rayleigh wave phase velocities in the slow and fast formations. The WSS applied to the S-wave part of the waveforms, however, did not correlate as well with its respective analytical expression for formation S-wave velocity, most likely due to interference of the pseudo-Rayleigh wave. To separate changes in formation P- and S-wave velocities versus fluid-flow effects on the Stoneley-wave mode, we computed the slow-P wave dispersion for the same formations. We found that fluid-saturated soft pores significantly affected the P- and S-wave effective formation velocities, whereas the slow-P wave velocity was rather insensitive to the compliant pores. Thus, the large phase-velocity effect on the Stoneley wave mode was mainly due to changes in effective formation P- and S-wave velocities and not to additional fluid mobility.

Numerical and Theoretical Investigation on the Origin of the Multiple Peaks of the H/V Ratio Curve

2021 ◽  
Author(s):  
Wanbo Xiao ◽  
Siqi Lu ◽  
Yanbin Wang
Keyword(s):  
Rayleigh Wave ◽  
Wave Velocity ◽  
Subsurface Layer ◽  
P Wave ◽  
Theoretical Formula ◽  
S Wave ◽  
Multiple Peaks ◽  
Ratio Curve ◽  
Multiple Layer ◽  
Wave Resonance

<p>Despite the popularity of the horizontal to vertical spectral ratio (HVSR) method in site effect studies, the origin of the H/V peaks has been controversial since this method was proposed. Many previous studies mainly focused on the explanation of the first or single peak of the H/V ratio, trying to distinguish between the two hypotheses — the S-wave resonance and ellipticity of Rayleigh wave. However, it is common both in numerical simulations and practical experiments that the H/V ratio exhibits multiple peaks, which is essential to explore the origin of the H/V peaks.</p><p>The cause for the multiple H/V peaks has not been clearly figured out, and once was simply explained as the result of multi subsurface layers. Therefore, we adopted numerical method to simulate the ambient noise in various layered half-space models and calculated the H/V ratio curves for further comparisons. The peak frequencies of the H/V curves accord well with the theoretical frequencies of S-wave resonance in two-layer models, whose frequencies only depend on the S wave velocity and the thickness of the subsurface layer. The same is true for models with varying model parameters. Besides, the theoretical formula of the S-wave resonance in multiple-layer models is proposed and then supported by numerical investigations as in the cases of two-layer models. We also extended the S-wave resonance to P-wave resonance and found that its theoretical frequencies fit well with the V/H peaks, which could be an evidence to support the S-wave resonance theory from a new perspective. By contrast, there are obvious differences between the higher orders of the H/V ratio peaks and the higher orders of Rayleigh wave ellipticity curves both in two-layer and multiple-layer models. The Rayleigh wave ellipticity curves are found to be sensitive to the Poisson’s ratio and the thickness of the subsurface layer, so the variation of the P wave velocity can affect the peak frequencies of the Rayleigh wave ellipticity curves while the H/V peaks show slight change. The Rayleigh wave ellipticity theory is thus proved to be inappropriate for the explanation of the multiple H/V peaks, while the possible effects of the Rayleigh wave on the fundamental H/V peak still cannot be excluded.</p><p>Based on the analyses above, we proposed a new evidence to support the claim that the peak frequencies of the H/V ratio curve, except the fundamental peaks, are caused by S-wave resonance. The relationship between the P-wave resonance and the V/H peaks may also find further application.</p>

Wavefield simulation and data-acquisition-scheme analysis for LWD acoustic tools in very slow formations

Geophysics ◽  
2011 ◽  
Vol 76 (3) ◽  
pp. E59-E68 ◽  
Author(s):  
Hua Wang ◽  
Guo Tao
Keyword(s):  
Wave Velocity ◽  
Cutoff Frequency ◽  
P Wave ◽  
Flexural Wave ◽  
S Wave ◽  
Low Frequencies ◽  
Wave Velocities ◽  
Dipole System ◽  
P Wave Velocity ◽  
Wavefield Simulation

Propagating wavefields from monopole, dipole, and quadrupole acoustic logging-while-drilling (LWD) tools in very slow formations have been studied using the discrete wavenumber integration method. These studies examine the responses of monopole and dipole systems at different source frequencies in a very slow surrounding formation, and the responses of a quadrupole system operating at a low source frequency in a slow formation with different S-wave velocities. Analyses are conducted of coherence-velocity/slowness relationships (semblance spectra) in the time domain and of the dispersion characteristics of these waveform signals from acoustic LWD array receivers. These analyses demonstrate that, if the acoustic LWD tool is centralized properly and is operating at low frequencies (below 3 kHz), a monopole system can measure P-wave velocity by means of a “leaky” P-wave for very slow formations. Also, for very slow formations a dipole system can measure the P-wave velocity via a leaky P-wave and can measure the S-wave velocity from a formation flexural wave. With a quadrupole system, however, the lower frequency limit (cutoff frequency) of the drill-collar interference wave would decrease to 5 kHz and might no longer be neglected if the surrounding formation becomes a very slow formation, with S-wave velocities at approximately 500 m/s.

Dispersion patterns of the ground roll in eastern Saudi Arabia

Geophysics ◽  
1981 ◽  
Vol 46 (2) ◽  
pp. 121-137 ◽  
Author(s):  
Moujahed I. Al‐Husseini ◽  
Jon B. Glover ◽  
Brian J. Barley
Keyword(s):  
Saudi Arabia ◽  
Rayleigh Wave ◽  
Wavelength Range ◽  
Noise Analysis ◽  
Regional Scale ◽  
P Wave ◽  
S Wave ◽  
Wave Velocities ◽  
Ground Roll ◽  
P Wave Velocities

Seismic surveys on land must be designed so that the source‐generated noise, such as ground roll, is preferentially attenuated before P‐wave signal amplification and recording. The correct specification of spatial and frequency filters requires prior knowledge of the noise properties in the area. We show that the strong Rayleigh wave component of source‐generated noise has a wavelength range which is predictable on a regional scale, using widespread P‐wave velocity measurements in shallow upholes. This predictive capability decreases the number of noise analyses required to map the boundaries between areas with different Rayleigh wave properties. The case history presented is for northeastern Saudi Arabia, an area of roughly [Formula: see text]. The data comprise 80 noise analyses and a data base of over 10,000 up‐hole measurements of P‐wave velocities, supplemented by maps of topography and geologic outcrops. Examples show that the frequency‐wavenumber transforms of time‐offset records can be interpreted in detail in terms of Rayleigh wave dispersion and air wave coupling, dictated by the elastic properties of the very shallow layers. P‐wave velocities, measured in shallow upholes at noise analysis sites, are used to form initial estimates of the corresponding shear‐wave velocities and subsequently refined by matching the observed and predicted dispersion curves. Even without this refinement process, the initial S‐wave velocities can be used to estimate Rayleigh wave velocities at frequencies which typify the top and bottom of current vibrator sweeps (10 and 80 Hz). These velocities are mapped for the area and used to determine the wavelength range of Rayleigh waves. An effort is also made to map regions where Rayleigh wave scattering from surface topography is likely to occur.

Acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection

Geophysics ◽  
2002 ◽  
Vol 67 (2) ◽  
pp. 405-412 ◽  
Author(s):  
Manika Prasad
Keyword(s):  
Wave Velocity ◽  
P Wave ◽  
Acoustic Measurements ◽  
S Wave ◽  
Deepwater Drilling ◽  
Unconsolidated Sands ◽  
Wave Velocities ◽  
P Wave Velocity ◽  
Shallow Water Flows ◽  
Low Pressures

Shallow water flows and over‐pressured zones are a major hazard in deepwater drilling projects. Their detection prior to drilling would save millions of dollars in lost drilling costs. I have investigated the sensitivity of seismic methods for this purpose. Using P‐wave information alone can be ambiguous, because a drop in P‐wave velocity (Vp) can be caused both by overpressure and by presence of gas. The ratio of P‐wave velocity to S‐wave velocity (Vp/Vs), which increases with overpressure and decreases with gas saturation, can help differentiate between the two cases. Since P‐wave velocity in a suspension is slightly below that of the suspending fluid and Vs=0, Vp/Vs and Poisson's ratio must increase exponentially as a load‐bearing sediment approaches a state of suspension. On the other hand, presence of gas will also decrease Vp but Vs will remain unaffected and Vp/Vs will decrease. Analyses of ultrasonic P‐ and S‐wave velocities in sands show that the Vp/Vs ratio, especially at low effective pressures, decreases rapidly with pressure. At very low pressures, Vp/Vs values can be as large as 100 and higher. Above pressures greater than 2 MPa, it plateaus and does not change much with pressure. There is significant change in signal amplitudes and frequency of shear waves below 1 MPa. The current ultrasonic data shows that Vp/Vs values can be invaluable indicators of low differential pressures.

scholarly journals Evidence for deep icequakes in an Alpine glacier

2000 ◽  
Vol 31 ◽  
pp. 85-90 ◽  
Author(s):  
N. Deichmann ◽  
J. Ansorge ◽  
F. Scherbaum ◽  
A. Aschwanden ◽  
F. Bernard ◽  
...  
Keyword(s):  
Rayleigh Wave ◽  
Three Dimensional ◽  
Focal Depth ◽  
Vertical Component ◽  
Swiss Alps ◽  
P Wave ◽  
S Wave ◽  
Glacier Surface ◽  
Simultaneous Inversion ◽  
Wave Velocities

AbstractTo obtain more reliable information about the focal-depth distribution of icequakes, in April 1997 we operated an array of seven portable digital seismographs on Unteraargletscher, central Swiss Alps. Over 5000 events were detected by at least two instruments during the 9 day recording period. P-wave velocities (3770 m f) were determined from several calibration shots detonated at the glacier surface as well as in a 49 m deep borehole, whereas S-wave velocities (1860 ms–1) were derived from a simultaneous inversion for Vp/Vs6 applied to 169 icequakes. So far, hypocentral locations have been calculated for over 300 icequakes. Besides confirming the occurrence of shallow events associated with the opening of crevasses, our results show that a small but significant fraction of the hypocenters are located at or near the glacier bed. One event was found at an intermediate depth of about 120 m. Three-dimensional particle-motion diagrams of both explosions and icequakes clearly demonstrate that all vertical component seismograms from shallow sources are dominated by the Rayleigh wave. On the other hand, for events occurring at depths greater than about 40 m, the Rayleigh wave disappears almost entirely. Therefore, a qualitative analysis of the signal character provides direct information on the focal depth of an event and was used as an independent check of the locations obtained from traditional arrival-time inversions. Thus, our results demonstrate that deep icequakes do occur and that simple rheological models, according to which brittle deformation is restricted to the uppermost part of a glacier, may need revision.

EXPERIMENTAL EVALUATION OF ULTRASONIC VELOCITIES AND ANISOTROPY IN THE TUSCALOOSA MARINE SHALE FORMATION

2020 ◽  
pp. 1-62 ◽  
Author(s):  
Jamal Ahmadov ◽  
Mehdi Mokhtari
Keyword(s):  
Wave Velocity ◽  
Mineralogical Composition ◽  
Organic Content ◽  
P Wave ◽  
S Wave ◽  
Velocity Anisotropy ◽  
Wave Velocities ◽  
Marine Shale ◽  
P Wave Velocity ◽  
Degree Of Anisotropy

Tuscaloosa Marine Shale (TMS) formation is a clay- and organic-rich emerging shale play with a considerable amount of hydrocarbon resources. Despite the substantial potential, there have been only a few wells drilled and produced in the formation over the recent years. The analyzed TMS samples contain an average of 50 wt% total clay, 27 wt% quartz and 14 wt% calcite and the mineralogy varies considerably over the small intervals. The high amount of clay leads to pronounced anisotropy and the frequent changes in mineralogy result in the heterogeneity of the formation. We studied the compressional (VP) and shear-wave (VS) velocities to evaluate the degree of anisotropy and heterogeneity, which impact hydraulic fracture growth, borehole instabilities, and subsurface imaging. The ultrasonic measurements of P- and S-wave velocities from five TMS wells are the best fit to the linear relationship with R2 = 0.84 in the least-squares criteria. We observed that TMS S-wave velocities are relatively lower when compared to the established velocity relationships. Most of the velocity data in bedding-normal direction lie outside constant VP/VS lines of 1.6–1.8, a region typical of most organic-rich shale plays. For all of the studied TMS samples, the S-wave velocity anisotropy exhibits higher values than P-wave velocity anisotropy. In the samples in which the composition is dominated by either calcite or quartz minerals, mineralogy controls the velocities and VP/VS ratios to a great extent. Additionally, the organic content and maturity account for the velocity behavior in the samples in which the mineralogical composition fails to do so. The results provide further insights into TMS Formation evaluation and contribute to a better understanding of the heterogeneity and anisotropy of the play.

Seismic imaging using microearthquakes induced by hydraulic fracturing

Geophysics ◽  
1994 ◽  
Vol 59 (1) ◽  
pp. 102-112 ◽  
Author(s):  
Lisa V. Block ◽  
C. H. Cheng ◽  
Michael C. Fehler ◽  
W. Scott Phillips
Keyword(s):  
Hydraulic Fracturing ◽  
Wave Velocity ◽  
Seismic Imaging ◽  
P Wave ◽  
Basement Rock ◽  
S Wave ◽  
Fractured Zone ◽  
Wave Velocities ◽  
The Absolute ◽  
Wave Arrival

Seismic imaging using microearthquakes induced by hydraulic fracturing produces a three-dimensional (3-D), S-wave velocity model of the fractured zone, improves the calculated locations of the microearthquakes, and may lead to better estimates of fractureplane orientations, fracture density, and water flow paths. Such information is important for predicting the amount of heat energy that may be extracted from geothermal reservoir. A fractured zone was created at the Los Alamos Hot Dry Rock Reservoir in north-central New Mexico within otherwise impermeable basement rock by injecting [Formula: see text] of water into a borehole under high pressure at a depth of 3.5 km. Induced microearthquakes were observed using four borehole seismometers. The P-wave and S-wave arrival times have been inverted to find the 3-D velocity structures and the microearthquake locations and origin times. The inversion was implemented using the separation of parameters technique, and constraints were incorporated to require smooth velocity structures and to restrict the velocities within the fractured region to be less than or equal to the velocities of the unfractured basement rock. The rms amval time residuals decrease by 11–15 percent during the joint hypocenter-velocity inversion. The average change in the microearthquake locations is 20–27 m, depending on the smoothing parameter used. Tests with synthetic data imply that the absolute locations may improve by as much as 35 percent, while the relative locations may improve by 40 percent. The general S-wave velocity patterns are reliable, but the absolute velocity values are not uniquely determined. However, studies of inversions using various degrees of smoothing suggest that the S-wave velocities decrease by at least 13 percent in the most intensely fractured regions of the reservoir. The P-wave velocities are poorly constrained because the P-wave traveltime perturbations caused by the fluid-filled fractures are small compared to the amval time noise level. The significant difference in the relative signal-to-noise levels of the P-wave and S-wave arrival time data, coupled with the limited ray coverage, can produce a bias in the computed [Formula: see text] ratios, and corresponding systematic rotation of the microearthquake cluster. These adverse effects were greatly reduced by applying a [Formula: see text] lower bound based on the [Formula: see text] ratio of the unfractured basement rock.

Shear‐wave logging with dipole sources

Geophysics ◽  
1988 ◽  
Vol 53 (5) ◽  
pp. 659-667 ◽  
Author(s):  
S. T. Chen
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Wave Train ◽  
P Wave ◽  
Dipole Source ◽  
Stoneley Wave ◽  
S Wave ◽  
S Waves ◽  
On Line ◽  
First Arrival

Laboratory measurements have verified a novel technique for direct shear‐wave logging in hard and soft formations with a dipole source, as recently suggested in theoretical studies. Conventional monopole logging tools are not capable of measuring shear waves directly. In particular, no S waves are recorded in a soft formation with a conventional monopole sonic tool because there are no critically refracted S rays when the S-wave velocity of the rock is less than the acoustic velocity of the borehole fluid. The present studies were conducted in the laboratory with scale models representative of sonic logging conditions in the field. We have used a concrete model to represent hard formations and a plastic model to simulate a soft formation. The dipole source, operating at frequencies lower than those conventionally used in logging, substantially suppressed the P wave and excited a wave train whose first arrival traveled at the S-wave velocity. As a result, one can use a dipole source to log S-wave velocity directly on‐line by picking the first arrival of the full wave train, in a process similar to that used in conventional P-wave logging. Laboratory experiments with a conventional monopole source in a soft formation did not produce S waves. However, the S-wave velocity was accurately estimated by using Biot’s theory, which required measuring the Stoneley‐wave velocity and knowing other borehole parameters.

Subsurface Shear Wave Velocity Characterization Using P-Wave Seismograms in Central and Eastern North America

2016 ◽  
Vol 32 (1) ◽  
pp. 143-169 ◽  
Author(s):  
Byungmin Kim ◽  
Youssef M. A. Hashash ◽  
Ellen M. Rathje ◽  
Jonathan P. Stewart ◽  
Sidao Ni ◽  
...  
Keyword(s):  
North America ◽  
Wave Velocity ◽  
P Wave ◽  
Motion Prediction ◽  
Eastern North America ◽  
Ground Motion Prediction ◽  
Profile Data ◽  
S Wave ◽  
Wave Velocities ◽  
Linear Relationships

The time-averaged shear ( S) wave velocity in the upper 30 meters of sediment ( V S30) is a widely used site parameter for ground motion prediction. When unavailable from measurements, as is often the case at accelerograph stations in Central and Eastern North America (CENA), V S30 is typically estimated from proxies. We propose an alternative for CENA based on a theoretical relationship between S-wave velocity and the ratio of radial to vertical components of the compressional ( P)-wave–dominated portion of the velocity time series. This method is applied to 31 CENA accelerograph sites having measured S-wave velocity profiles. Time-averaged S-wave velocities to depth z ( V SZ) from the proposed method agree well with those from measurements. We develop linear relationships between V SZ and V S30 using CENA S-wave velocity profile data. Values of V S30 established from the proposed method (including depth extrapolation) have lower dispersion relative to data ( σln V = 0.43) than do estimates from available CENA proxies.

Unexpected Consequences of Transverse Isotropy

2020 ◽  
Author(s):  
Hitoshi Kawakatsu
Keyword(s):  
Rayleigh Wave ◽  
Wave Velocity ◽  
Seismic Analysis ◽  
Receiver Function ◽  
Function Analysis ◽  
Incidence Angle ◽  
Body Wave ◽  
Transverse Isotropy ◽  
P Wave ◽  
S Wave

ABSTRACT In a series of articles, Kawakatsu et al. (2015) and Kawakatsu (2016a,b, 2018) introduced and discussed a new parameter, ηκ, that characterizes the incidence angle dependence (relative to the symmetry axis) of seismic body-wave velocities in a transverse isotropy (TI) system. During the course of these exercises, several nontrivial consequences of TI were realized and summarized as follows: (1) P-wave velocity (anisotropy) strongly influences the conversion efficiency of P-to-S and S-to-P, as much as S-wave velocity perturbation does; (2) Rayleigh-wave phase velocity has substantial sensitivity to P-wave anisotropy near the surface; (3) a trade-off exists between ηκ and the VP/VS ratio if the latter is sought under an assumption of isotropy or the elliptic condition. Among these findings, the first two deserve careful attention in interpretation of results of popular seismic analysis methods, such as receiver function analysis and ambient-noise Rayleigh-wave dispersion analysis. We present simple example cases for such problems to delineate the effect in actual situations, as well as scalings among TI parameters of the crust and mantle materials or models that might help understanding to what extent the effect becomes important.

Sign in / Sign up

Export Citation Format

Share Document