Tube‐wave suppression in single‐well seismic acquisition

Single‐well seismic imaging is significantly hampered by the presence of borehole tube waves. A tube‐wave suppressor has been tested using single‐well seismic equipment at the Lost Hills (California) oil field. The suppressor uses a gas‐filled bladder kept slightly above borehole fluid pressure. Field tests show a measurable reduction in tube‐wave energy as compared to body waves propagating in the surrounding reservoir rock. When using a high‐frequency (500–4000 Hz) piezoelectric source, the P‐wave–tube‐wave amplitude ratio was increased by 33 dB. When using a lower frequency (50–350 Hz) orbital vibrator source, the S‐wave–tube‐wave amplitude ratio was increased by 21 dB while the P‐wave–tube‐wave amplitude ratio was increased by 23 dB. These reductions in tube‐wave amplitudes significantly improve single‐well data quality.

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The 1998 Valhall microseismic data set: An integrated study of relocated sources, seismic multiplets, and S-wave splitting

Geophysics ◽

10.1190/1.3205028 ◽

2009 ◽

Vol 74 (5) ◽

pp. B183-B195 ◽

Cited By ~ 40

Author(s):

K. De Meersman ◽

J.-M. Kendall ◽

M. van der Baan

Keyword(s):

Seismic Anisotropy ◽

Amplitude Ratio ◽

Temporal Variations ◽

Oil Field ◽

P Wave ◽

Wave Form ◽

S Wave ◽

Data Set ◽

Wave Splitting ◽

Source Mechanisms

We relocate 303 microseismic events recorded in 1998 by sensors in a single borehole in the North Sea Valhall oil field. A semiautomated array analysis method repicks the P- and S-wave arrival times and P-wave polarizations, which are needed to locate these events. The relocated sources are confined predominantly to a [Formula: see text]-thick zone just above the reservoir, and location uncertainties are half those of previous efforts. Multiplet analysis identifies 40 multiplet groups, which include 208 of the 303 events. The largest group contains 24 events, and five groups contain 10 or more events. Within each multiplet group, we further improve arrival-time picking through crosscorrelation, which enhances the relative accuracy of the relocated events and reveals that more than 99% of the seismic activity lies spatially in three distinct clusters. The spatial distribution of events and wave-form similarities reveal two faultlike structures that match well with north-northwest–south-southeast-trending fault planes interpreted from 3D surface seismic data. Most waveform differences between multiplet groups located on these faults can be attributed to S-wave phase content and polarity or P-to-S amplitude ratio. The range in P-to-S amplitude ratios observed on the faults is explained best in terms of varying source mechanisms. We also find a correlation between multiplet groups and temporal variations in seismic anisotropy, as revealed by S-wave splitting analysis. We explain these findings in the context of a cyclic recharge and dissipation of cap-rock stresses in response to production-driven compaction of the underlying oil reservoir. The cyclic nature of this mechanism drives the short-term variations in seismic anisotropy and the reactivation of microseismic source mechanisms over time.

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Spectral analysis of body waves from the earthquake of February 18, 1956

Bulletin of the Seismological Society of America ◽

10.1785/bssa05406a2017 ◽

1964 ◽

Vol 54 (6A) ◽

pp. 2017-2035 ◽

Cited By ~ 1

Author(s):

Tomowo Hirasawa ◽

William Stauder

Keyword(s):

Amplitude Ratio ◽

Source Region ◽

Focal Depth ◽

Mean Amplitude ◽

Theoretical Models ◽

Body Waves ◽

P Wave ◽

Type I ◽

Type Ii ◽

S Wave

abstract The earthquake which occurred south of Honshu, Japan, on February 18, 1956 is studied by means of Fourier analysis. The focal depth of the shock is about 450 km and the magnitude is 714 to 712. Three theoretical models of the source mechanism, that is, Type Ia, Type Ib, and Type II, are examined by the observed amplitude spectra of S and ScS waves. It is found that the observed amplitude ratios of the Fourier components between two horizontal components of the S wave and of the ScS wave, respectively, agree well with the theoretical ratios for a Type II source. Under the assumption that spectral structures should be the same at all observing points, the scattering from the mean amplitude is calculated. The result shows that the Type II model is preferable to either of the Type I models. Assuming Honda's volume model, whose radiation pattern corresponds to that of a Type II point source, the radius of the source region is estimated by making use of the amplitude ratio of the Fourier component of the S wave to that of the P wave. The radius of the source is found to be 11 km ± 2 km.

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Amplitude balancing in separating P- and S-waves in 2D and 3D elastic seismic data

Geophysics ◽

10.1190/1.3555529 ◽

2011 ◽

Vol 76 (3) ◽

pp. S103-S113 ◽

Cited By ~ 45

Author(s):

Robert Sun ◽

George A. McMechan ◽

Han-Hsiang Chuang

Keyword(s):

Wave Amplitude ◽

Amplitude Ratio ◽

Velocity Ratio ◽

Displacement Component ◽

P Wave ◽

Elastic Displacement ◽

Reverse Time ◽

S Wave ◽

Near Surface ◽

S Waves

The reflected P- and S-waves in elastic displacement component data recorded at the earth’s surface are separated by reverse-time (downward) extrapolation of the data in an elastic computational model, followed by calculations to give divergence (dilatation) and curl (rotation) at a selected reference depth. The surface data are then reconstructed by separate forward-time (upward) scalar extrapolations, from the reference depth, of the magnitude of the divergence and curl wavefields, and extraction of the separated P- and S-waves, respectively, at the top of the models. A P-wave amplitude will change by a factor that is inversely proportional to the P-velocity when it is transformed from displacement to divergence, and an S-wave amplitude will change by a factor that is inversely proportional to the S-velocity when it is transformed from displacement to curl. Consequently, the ratio of the P- to the S-wave amplitude (the P-S amplitude ratio) in the form of divergence and curl (postseparation) is different from that in the (preseparation) displacement form. This distortion can be eliminated by multiplying the separated S-wave (curl) by a relative balancing factor (which is the S- to P-velocity ratio); thus, the postseparation P-S amplitude ratio can be returned to that in the preseparation data. The absolute P- and S-wave amplitudes are also recoverable by multiplying them by a factor that depends on frequency, on the P-velocity α, and on the unit of α and is location-dependent if the near-surface P-velocity is not constant.

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A technique to eliminate the azimuth ambiguity in single-well imaging

Geophysics ◽

10.1190/geo2013-0310.1 ◽

2014 ◽

Vol 79 (6) ◽

pp. D409-D416 ◽

Cited By ~ 8

Author(s):

Yi-de Zhang ◽

Hengshan Hu

Keyword(s):

Wave Reflection ◽

Incident Angle ◽

Imaging Techniques ◽

Fluid Pressure ◽

Displacement Component ◽

P Wave ◽

Reflected Waves ◽

Single Well ◽

The Media ◽

Two Sides

Single-well imaging has been a technique increasingly used in the detection of near-borehole geologic structures. The azimuth of a geologic structure, however, cannot be uniquely determined with acoustic signals recorded in the borehole alone, due to the azimuth ambiguity existing in current imaging techniques. We eliminated such ambiguity by revealing the relevant acoustic principle underlying the P-wave reflection behavior. When a P-wave excited by a transducer in the logging tool impinges upon a planar interface, the P-wave reflection coefficient (RC) of the displacement is opposite in sign to that of the normal stress or fluid pressure, regardless of the incident angle and the parameters of the media on the two sides. The derived relation about signs of RCs was validated by finite-difference solutions for reflected waves from a near-borehole plane fault. With this newly discovered reflection principle, one can eliminate the azimuth ambiguity of any interface outside a borehole by checking if the waveforms of pressure and the displacement component are both changed in polarity after reflection. Furthermore, because the pressure and displacement are observable quantities and the waveform of the acoustic source is known in acoustic logging, it is convenient to implement the data acquisition for this technique, which is a major advantage over other techniques. We expounded and exemplified our new technique by numerical simulation.

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Biot slow‐wave effects in stratified rock

Geophysics ◽

10.1190/1.1451799 ◽

2002 ◽

Vol 67 (1) ◽

pp. 271-281 ◽

Cited By ~ 44

Author(s):

Steven R. Pride ◽

Eric Tromeur ◽

James G. Berryman

Keyword(s):

Slow Wave ◽

Wave Scattering ◽

Sedimentary Basin ◽

Fluid Pressure ◽

Low Frequency ◽

P Wave ◽

Reservoir Rock ◽

Biot Theory ◽

Angle Of Incidence ◽

Wave Effects

The transmission of P‐waves through the stratified layers of a sedimentary basin is modeled numerically using Biot theory. The effects on the transmissivity of frequency, angle of incidence, layer thickness, permeability and elastic compliance of the rocks are all considered. Consistent with previous analytical work, it is found that the equilibration of fluid pressure between the fine layers of a sedimentary sequence can produce significant P‐wave attenuation at low frequencies. For this attenuation mechanism to act within the surface‐seismic band (say, 3–300 Hz), we find that there must be layering present at the scale of centimeters to tens of centimeters. If the layering is restricted to layers of roughly 1 m thickness or greater, then for typical sandstone formations, the attenuation caused by the interlayer flow occurs below the seismic band of interest. Such low‐frequency interlayer flow is called Biot slow‐wave diffusion in the context of Biot theory and is likely to be the dominant source of low‐frequency attenuation in a sedimentary basin, even for relatively tight and stiff reservoir rock; however, the effect is enhanced in more compliant materials. At higher frequencies, the generation of slow‐waves at interfaces is also shown to significantly affect the P‐wave scattering so long as the layers are sufficiently thin and sufficiently compliant. This effect on the P‐wave scattering is shown to increase with increasing angle of incidence. Our work is limited to performing numerical experiments, with care given to making realistic estimates of all the material properties required. No attempt is made here to define an equivalent viscoelastic solid that allows for such slow‐wave effects.

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Bias in surface-wave magnitude Ms due to inadequate distance corrections

Bulletin of the Seismological Society of America ◽

10.1785/bssa0880010043 ◽

1998 ◽

Vol 88 (1) ◽

pp. 43-61

Author(s):

Mehdi Rezapour ◽

Robert G. Pearce

Keyword(s):

Surface Wave ◽

Wave Amplitude ◽

Body Wave ◽

The Body ◽

Body Waves ◽

P Wave ◽

Systematic Bias ◽

Surface Wave Magnitude ◽

Distance Curve ◽

Distance Correction

Abstract We investigate bias in surface-wave magnitude using the complete ISC and NEIC datasets from 1978 to 1993. We conclude that although there are some small differences between the ISC and NEIC magnitudes, there is no major difference between these agencies for this presentation of the global dataset. The frequency-distance plot for reported surface-wave amplitude observations exhibits detailed structure of the body-wave amplitude-distance curve at all distances; the influence of the surface-wave amplitude decay with distance is much less apparent. This censoring via the body waves represents a large deficit in the number of potentially usable surface-wave amplitude observations, particularly in the P-wave shadow zone between Δ = 100° and 120°. We have obtained two new modified Ms formulas based upon analysis of all ISC data between 1978 and 1993. In the first, the conventional logarithmic dependence of the distance correction is retained, and we obtain M s e = log ( A / T ) max + 1.155 log ( Δ ) + 4.269 . In the second, we make allowance for the theoretically known contribution of dispersion and geometrical spreading, to obtain M s t = log ( A / T ) max + 1 3 log ( Δ ) + 1 2 log ( sin Δ ) + 0.0046 Δ + 5.370. Comparison of these formulas with other work confirms the inadequacy of the distance-dependence term in the Gutenberg and Prague formulas, and we show that our first formula, as well as that of Herak and Herak, gives less bias at all epicentral distances to within the scatter of the observed dataset. Our second formula provides an improved overall distance correction, especially beyond Δ = 145°. We show evidence that Airy-phase distance decay predominates at shorter distances (Δ≦30°), but for greater distances, we are unable to resolve whether this or non-Airy-phase decay predominates. Assuming 20-sec surface waves with U = 3.6 km/sec, we obtain a globally averaged apparent Q−1 of 0.00192 ± 0.00026 (Q ≈ 500). We argue that our second formula not only improves the distance correction for surface-wave magnitudes but also promotes the analysis of unexplained amplitude anomalies by formally allowing for those contributions that are theoretically predictable. We conclude that there remains systematic bias in station magnitudes and that this includes the effects of source depth, different path contributions, and differences in seismometer response. For intermediate magnitudes, Mts shows less scatter against log M0 than does Ms calculated using the Prague formula.

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Fracture compliance estimation using borehole tube waves

Geophysics ◽

10.1190/geo2012-0521.1 ◽

2013 ◽

Vol 78 (4) ◽

pp. D249-D260 ◽

Cited By ~ 25

Author(s):

Sudhish Kumar Bakku ◽

Michael Fehler ◽

Daniel Burns

Keyword(s):

Amplitude Ratio ◽

Wave Generation ◽

Transition Frequency ◽

P Wave ◽

Fracture Aperture ◽

Generation Model ◽

Data Set ◽

Fracture Properties ◽

Lateral Extent ◽

Tube Wave

We tested two models, one for tube-wave generation and the other for tube-wave attenuation at a fracture intersecting a borehole that can be used to estimate fracture compliance, fracture aperture, and lateral extent. In the tube-wave generation model, we consider tube-wave excitation in the borehole when a P-wave is incident on the fracture. The amplitude ratio of the pressure due to the tube wave to that of the incident P-wave is a function of fracture compliance, aperture, and length. Similarly, the attenuation of a tube wave in the borehole as it crosses a fracture intersecting the borehole is also a function of fracture properties. Numerically solving the dispersion relation in the fracture, we study tube-wave generation and the attenuation coefficient as a function of frequency. We observed that measuring amplitude ratios or attenuation near a transition frequency can help constrain the fracture properties. The transition frequency corresponds to the regime in which the viscous skin depth in the fracture is comparable to its aperture. Measurements in the high-frequency limit can place a lower bound on fracture compliance and lateral extent. We evaluated the applicability of the tube-wave generation model to a previously published VSP data set and found that compliance values of the order [Formula: see text]–[Formula: see text] are likely in the field. These observations support scaling of fracture compliance with fracture size.

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