Pore pressure prediction from S‐wave, C‐wave, and 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.

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Delineation of geologic contacts by seismic refraction with application to the fault zone between the Thompson nickel belt and the Churchill Tectonic Province

Canadian Journal of Earth Sciences ◽

10.1139/e80-121 ◽

1980 ◽

Vol 17 (9) ◽

pp. 1141-1151 ◽

Cited By ~ 1

Author(s):

A. G. Green

Keyword(s):

Fault Zone ◽

Delay Time ◽

Wave Attenuation ◽

Seismic Refraction ◽

P Wave ◽

S Wave ◽

Wave Velocities ◽

P Wave Velocities ◽

A New Technique ◽

The Times

Refracted P-wave and S-wave arrivals are studied from a fourfold multicoverage seismic experiment that has been conducted across a region that spans the contact between the Thompson nickel belt and the Churchill Province in northern Manitoba. A new technique for the calculation of accurate delay times and basement velocities for unreversed multicoverage data is introduced. In this technique, the times of rays between selected shots and receivers are combined to give initial delay time corrections and a subsequent iterative least-squares analysis yields the final delay time corrections and estimates of the basement P-wave velocities. The P-wave velocities correlate well with the basement geology and have been used to refine the location of the contact between the Moak Lake gneisses of the Thompson nickel belt and the Kisseynew gneisses of the Churchill Province. From the P-wave velocities and S-wave attenuation it is concluded that this contact is a fault zone.

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Pore-pressure prediction challenges in chemical compaction regimes: An alternative VP/VS-based approach

Interpretation ◽

10.1190/int-2015-0106.1 ◽

2016 ◽

Vol 4 (4) ◽

pp. T443-T454 ◽

Cited By ~ 2

Author(s):

Ajesh John

Keyword(s):

High Temperature ◽

Pore Pressure ◽

Effective Stress ◽

Text Analysis ◽

Correction Coefficient ◽

S Wave ◽

Wave Velocities ◽

Temperature Regimes ◽

Pore Pressure Prediction ◽

High Degree

Understanding pressure mechanisms and their role in porosity-effective stress relationship is crucial in pore-pressure prediction estimation, particularly in complex geologic and high-temperature regimes. Overpressures are commonly associated with undercompaction and/or unloading mechanisms; those associated with undercompaction generally possess a direct relationship between effective stress and porosity, whereas those associated with unloading do not provide such direct indications from porosity trends. The type of associated unloading mechanism can be correlated when the effective stress and velocity become distorted with the onset of unloading. In the Ravva field, the pore-pressure distribution and overpressure mechanism in the Miocene and below it is a classic example of the unloading mechanism related to chemical compaction, thereby making it difficult to resolve the magnitude and trend of pore pressures. Here, the ratio of P- and S-wave velocities ([Formula: see text]) is analyzed from the drilled locations to understand the effects of lithology, pressure, and fluids on formation velocities and indicates a distinct decreasing trend across the overpressure formations, which I have corresponded to excess pressure resulting from chemical compaction. Across the high-pressured zones, [Formula: see text] ratios show low values compared with normally pressured zones possibly due to the presence of hydrocarbon and/or overpressures. A velocity correction coefficient ranging 0.83–0.71 is resolved for overpressure zones by normalizing the [Formula: see text] values across the normally pressured formations, and thereby assuring that a pore-pressure estimation using corrected velocity from [Formula: see text] analysis shows a high degree of accuracy on prediction trends. Pore-pressure predictions based on [Formula: see text] are a more effective and valid approach in high-temperature settings, in which numerous factors can contribute to pressure generation and a direct effective stress-porosity relationship deviates from the trend.

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Using P- to S- wave conversions from controlled sources to determine the shear-wave velocity structure along Hikurangi Margin Forearc, New Zealand

10.5194/egusphere-egu2020-11940 ◽

2020 ◽

Author(s):

Pasan Herath ◽

Tim Stern ◽

Martha Savage ◽

Dan Bassett ◽

Stuart Henrys ◽

...

Keyword(s):

New Zealand ◽

Pore Pressure ◽

Shear Wave ◽

P Wave ◽

Unconsolidated Sediments ◽

Eigenvalue Decomposition ◽

Slow Slip ◽

S Wave ◽

Wave Velocities ◽

Basement Rocks

The Hikurangi subduction margin offshore of the east coast of New Zealand displays along-strike variations in subduction-thrust slip behavior. Geodetic observations show that the subduction-thrust of the southern segment of the margin is locked on the 30-100 year scale and the northern segment displays periodic slow-slip on the 1-2 year scale. It is hypothesised that spatial variations in pore-pressure may play a role in this contrasting phenomenon. Higher pore-pressures would result in lower effective stresses, which promote slow-slip of the subduction-thrust. In addition, the presence of a sedimentary wedge with very low shear wave-speeds in the northern Hikurangi margin has been proposed to fit the ultra-long duration of ground motions observed following the 2016 Kaikoura earthquake. Compressional (P-) wave velocities (Vp) of the subsurface provide useful information about the lithological composition. Combined with shear (S-) wave velocities (Vs), the Vp/Vs ratio which is directly related to Poisson&#8217;s ratio can be obtained. This is a diagnostic property of a rock&#8217;s consolidation and porosity. Typical Vp/Vs ratio of consolidated and crystalline rocks range from 1.6 to 1.9 and that of unconsolidated sediments can range from 2.0 to 4.0.We use the controlled sources of R/V Marcus G Langseth recorded by a profile of 49 multi-component ocean bottom seismometers (OBS) along the Hikurangi margin forearc for the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) to derive the Vs structure and estimate the Vp/Vs ratio. The orientations of the horizontal components of each OBS are found by a hodogram analysis and by an eigenvalue-decomposition of the covariance matrix. Using the orientations, the horizontal components of each OBS are rotated into radial and transverse components. P to S converted phases are identified on the radial and transverse components considering their linear moveout, polarisation angle, and ellipticity. We confirm incoming S-waves to OBSs by comparing them with their hydrophone components. We identify both PPS (up-going P-wave after reflection or refraction converts to an S-wave at an interface) and PSS (down-going P-wave from the controlled source converts to an S-wave at an interface) type conversions. The identified conversion interfaces are the sediment-basement interface and the top of the subducting crust. The travel-time delay of a PPS type conversion relative to its P-wave arrival is indicative of Vs above the converting interface. The linear-moveout of PSS type conversions are indicative of Vs along the raypath after the conversion. Preliminary results from the southern Hikurangi margin suggest Vp/Vs ratios of ~1.70 for the basement rocks above the subducting crust and ~1.90 for the sediments overlying the basement rocks. These values indicate that the basement rocks are consolidated and less porous than the overlying sediments.We expect to estimate the Vp/Vs ratios in the northern Hikurangi margin to assess the role played by pore-pressure in the along-strike variation in subduction-thrust slip behavior. We also expect to ascertain the presence and estimate the thickness of the low-velocity sediment wedge in the northern Hikurangi margin.

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S-wave velocity measurements in deep soil deposit and bedrock by means of an elaborated down-hole method

Bulletin of the Seismological Society of America ◽

10.1785/bssa0700010363 ◽

1980 ◽

Vol 70 (1) ◽

pp. 363-377

Author(s):

Y. Ohta ◽

N. Goto ◽

F. Yamamizu ◽

H. Takahashi

Keyword(s):

Wave Velocity ◽

Earthquake Engineering ◽

Underground Structure ◽

P Wave ◽

Velocity Measurements ◽

S Wave ◽

Deep Soil ◽

Tokyo Metropolitan Area ◽

Wave Velocities ◽

P Wave Velocities

abstract Deep S-wave velocity measurements were planned at two separate sites in the Tokyo area from the earthquake engineering point of view, and actually carried out down to 2 to 3 km in depth using geophysical observation wells. S-waves were produced by means of ordinary small explosions and a specially designed SH-wave generator. A set of three component seismometers was installed in a capsule having a device that is clamped to the borehole wall. Measurements to the bottom of the wells were conducted at about 15 different depths at intervals of 100 to 500 m. The S-wave velocities are around 0.8 km/sec in Pleistocene soils, 1.2 to 1.6 km/sec in Miocene soils, and 2.5 to 2.7 km/sec in Cambrian rocks. The corresponding P-wave velocities are 2.0 to 2.3 km/sec, 2.6 to 3.0 km/sec, and 4.7 to 4.9 km/sec, respectively. These data show both S- and P-wave velocities in deep soil deposit increasing with depth. The greatest velocity difference is at the boundary above the pre-Tertiary rocks. The velocity structures completely agree with the known data such as sonic logs, density distributions, and geological sections. A comparison with velocity profiles at two separate sites was also made as the first step to visualize the three-dimensional underground structure in the Tokyo metropolitan area. The seismological and earthquake engineering importance of shear-wave velocity measurements for thick soil deposits was demonstrated by approximate calculations of the amplification of seismic waves between ground surface and bedrock.

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