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.

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.

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.

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.

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