Poroelastic analysis of amplitude-versus-frequency variations

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. N41-N48 ◽  
Author(s):  
Haitao Ren ◽  
Gennady Goloshubin ◽  
Fred J. Hilterman
Keyword(s):  
Low Frequency ◽  
Bright Spot ◽  
Frequency Dependent ◽  
Low Frequencies ◽  
Amplitude Variation With Offset ◽  
Seismic Amplitude ◽  
Nondispersive Medium ◽  
Seismic Anomalies ◽  
Frequency Variations ◽  
Lower Frequencies

Although significant advancement has occurred in the interpretation of seismic amplitude-variation-with-offset (AVO) anomalies, a theory is lacking to guide the interpretation of frequency-dependent seismic anomalies. Using analytic equations and numerical modeling, we have investigated characteristics of the normal-incident reflection coefficient (NI) as a function of frequency at an interface between a nondispersive medium and a patchy-saturated dispersive medium. Because of velocity dispersion, the variation of NI magnitude is divided into three general classes. These classes are (1) low-frequency dim-out reservoirs, in which NI magnitude decreases toward lower frequencies; (2) phase-shift reservoirs, in which NI is a small negative value at low frequencies but becomes positive at higher frequencies; and (3) low-frequency bright-spot reservoirs, in which NI magnitude increases toward lower frequencies. This classification could provide insight for frequency-dependent seismic interpre-tation.

Anisotropic correction of sonic logs in wells with large relative dip

Geophysics ◽  
2008 ◽  
Vol 73 (1) ◽  
pp. E1-E5 ◽  
Author(s):  
Lev Vernik
Keyword(s):  
Reservoir Characterization ◽  
Low Frequency ◽  
P Wave ◽  
Amplitude Variation With Offset ◽  
Seismic Amplitude ◽  
Avo Inversion ◽  
Pore Pressure Prediction ◽  
Siliciclastic Rocks ◽  
Dip Angle ◽  
Sonic Logs

Seismic reservoir characterization and pore-pressure prediction projects rely heavily on the accuracy and consistency of sonic logs. Sonic data acquisition in wells with large relative dip is known to suffer from anisotropic effects related to microanisotropy of shales and thin-bed laminations of sand, silt, and shale. Nonetheless, if anisotropy parameters can be related to shale content [Formula: see text] in siliciclastic rocks, then I show that it is straightforward to compute the anisotropy correction to both compressional and shear logs using [Formula: see text] and the formation relative dip angle. The resulting rotated P-wave sonic logs can be used to enhance time-depth ties, velocity to effective stress transforms, and low-frequency models necessary for prestack seismic amplitude variation with offset (AVO) inversion.

Are bright spots always hydrocarbons? A case study in the Taranaki Basin, New Zealand

2020 ◽  
Vol 8 (4) ◽  
pp. SR45-SR51
Author(s):  
Peter Reilly ◽  
Roberto Clairmont ◽  
Heather Bedle
Keyword(s):  
New Zealand ◽  
High Amplitude ◽  
Seismic Survey ◽  
Bright Spot ◽  
Temporal Region ◽  
Cross Sectional ◽  
Seismic Amplitude ◽  
Bright Spots ◽  
Seismic Amplitudes ◽  
Seismic Anomalies

In the shallower regions of the 3D Nimitz seismic survey, there exist multiple interesting bright seismic amplitude anomalies. These anomalies, or funny looking things, occur in a confined spatial and temporal region of the seismic. They have a concave-up seismic appearance along the cross section. Bright seismic amplitudes can be a direct hydrocarbon indicator, or they can be representative of strong lithologic contrasts and/or acquisition artifacts. We have set out to investigate misinterpreted seismic anomalies along cross-sectional lines. Therefore, we apply seismic attributes to indicate that these bright spot features, which we interpret to be submarine gullies looking along time-slice intersections, can possibly be mistaken for hydrocarbon anomalies in a cross-sectional view. However, we cannot fully rule out the presence of hydrocarbons because it is common for gas sands to create similar anomalies. Previously drilled wells within the survey (Korimako-1 and Tarapunga-1) point to a lack of hydrocarbon potential in the subsurface. Although it is possible that these bright spots are due to hydrocarbon presence, we develop a more likely hypothesis: The lithology of the interfluve sediments is similar to the gully-margin drapes but differs from the gully sediment fill. Funny looking thing (FLT): Submarine gullies Seismic appearance: High-amplitude spotted features Alternative interpretations: Lithologic anomalies, gas seeps, bright spots Features with a similar appearance: Gas accumulation, sediment fills in limestone paleocaves Formation: Giant Foresets Formation Age: Pleistocene Location: Taranaki Basin, New Zealand Seismic data: Nimitz 3D (cropped volume) Analysis tools: Curvature, instantaneous frequency, and sweetness attributes; well reports

Permeability and borehole Stoneley waves: Comparison between experiment and theory

Geophysics ◽  
1989 ◽  
Vol 54 (1) ◽  
pp. 66-75 ◽  
Author(s):  
Kenneth W. Winkler ◽  
Hsui‐Lin Liu ◽  
David Linton Johnson
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Theoretical Models ◽  
Oil Field ◽  
Stoneley Wave ◽  
Biot Theory ◽  
Frequency Dependent ◽  
Low Frequencies ◽  
High Frequencies ◽  
Theoretical Predictions

We performed laboratory experiments to evaluate theoretical models of borehole. Stoneley wave propagation in permeable materials. A Berea sandstone and synthetic samples made of cemented glass beads were saturated with silicone oils. We measured both velocity and attenuation over a frequency band from 10 kHz to 90 kHz. Our theoretical modeling incorporated Biot theory and Deresiewicz‐Skalak boundary conditions into a cylindrical geometry and included frequency‐dependent permeability. By varying the viscosity of the saturating pore fluid, we were able to study both low‐frequency and high‐frequency regions of Biot theory, as well as the intermediate transition zone. In both low‐frequency and high‐frequency regions of the theory, we obtained excellent agreement between experimental observations and theoretical predictions. Velocity and attenuation (1/Q) are frequency‐dependent, especially at low frequencies. Also at low frequencies, velocity decreases and attenuation increases with increasing fluid mobility (permeability/viscosity). More complicated behavior is observed at high frequencies. These results support recent observations from the oil field suggesting that Stoneley wave velocity and attenuation may be indicative of formation permeability.

Dynamic Mechanical Properties of Cross-Linked Rubbers. II. Effects of Crosslink Spacing and Initial Molecular Weight in Polybutadiene

1966 ◽  
Vol 39 (4) ◽  
pp. 905-914
Author(s):  
Etsuji Maekawa ◽  
Ralph G. Mancke ◽  
John D. Ferry
Keyword(s):  
Molecular Weight ◽  
Crosslink Density ◽  
Low Frequency ◽  
Dynamic Mechanical Properties ◽  
Cross Linking ◽  
Loss Mechanism ◽  
Low Frequencies ◽  
Molecular Weights ◽  
Monomeric Friction Coefficient ◽  
Lower Frequencies

Abstract The complex shear compliances of eight samples of polybutadiene crosslinked by cumyl peroxide and four samples crosslinked by sulfur have been measured over a frequency range from 0.2 to 2 cps at temperatures from − 6 to 45° C by a torsion pendulum. On four of the samples, measurements were extended by the Fitzgerald transducer from 45 to 600 cps at temperatures from − 71 to 55°. The vulcanizates had been prepared from polymers of two different molecular weights (180,000 and 510,000) with sharp molecular weight distribution; the physical crosslink density ranged from 0.57 to 2.68×10−4 mole/cm3, and the chemical crosslink density calculated following Kraus ranged from 0.22 to 1.49×10−4 mole/cm3. The mechanical data were all reduced to T0=298° K by shift factors calculated from the equation log aT=−3.64(T−T0)/(186.5+T−T0). In the transition zone of frequencies, the viscoelastic functions of the cumyl peroxide vulcanizates were closely similar, except for a shift toward lower frequencies with increasing crosslinking, corresponding to a small but unexpected increase in the monomeric friction coefficient. Cross-linking by sulfur caused a somewhat larger shift toward lower frequencies at a comparable crosslink density. In the rubbery zone, the sample with least cross-linking exhibited a substantial secondary loss mechanism at very low frequencies. The low-frequency losses are evident in all the samples, but their magnitude falls rapidly with increasing crosslink density as previously found for natural rubber. It also falls somewhat with increasing initial molecular weight, indicating a contribution from network strands with loose ends. The possible relation of the low-frequency losses to trapped entanglements is discussed.

Quantitative interpretation using facies-based seismic inversion

2017 ◽  
Vol 5 (3) ◽  
pp. SL1-SL8 ◽  
Author(s):  
Ehsan Zabihi Naeini ◽  
Russell Exley
Keyword(s):  
Seismic Data ◽  
Low Frequency ◽  
Seismic Inversion ◽  
Quantitative Interpretation ◽  
Frequency Model ◽  
Amplitude Variation With Offset ◽  
Limited Bandwidth ◽  
Seismic Amplitude ◽  
Primary Signal

Quantitative interpretation (QI) is an important part of successful exploration, appraisal, and development activities. Seismic amplitude variation with offset (AVO) provides the primary signal for the vast majority of QI studies allowing the determination of elastic properties from which facies can be determined. Unfortunately, many established AVO-based seismic inversion algorithms are hindered by not fully accounting for inherent subsurface facies variations and also by requiring the addition of a preconceived low-frequency model to supplement the limited bandwidth of the input seismic. We apply a novel joint impedance and facies inversion applied to a North Sea prospect using broadband seismic data. The focus was to demonstrate the significant advantages of inverting for each facies individually and iteratively determine an optimized low-frequency model from facies-derived depth trends. The results generated several scenarios for potential facies distributions thereby providing guidance to future appraisal and development decisions.

scholarly journals The Low Frequency Variability of Extragalactic Radio Sources: A Relativistic Effect or Galactic Scintillation?

1988 ◽  
Vol 129 ◽  
pp. 297-298 ◽  
Author(s):  
L. Padrielli ◽  
R. Fanti ◽  
A. Ficarra ◽  
L. Gregorini ◽  
F. Mantovani ◽  
...  
Keyword(s):  
Structural Changes ◽  
Low Frequency ◽  
Relativistic Electrons ◽  
List Type ◽  
Low Frequencies ◽  
Vlbi Observations ◽  
Low Frequency Variability ◽  
5 Ghz ◽  
Frequency Variations ◽  
3C 120

Results obtained on the Low Frequency Variability (LFV) phenomenon, by means of combined multifrequency observations of 50 sources, on a period of more than ten years on a frequency grid of 0.4, 2.3, 4.8, 8.0, and 14.4 GHz and two epoch VLBI observations at 18 cm can be summarized as follows: 1.15–20% of variables appear to have variations consisting either of quasi-simultaneous outbursts at all frequencies or of bursts which drift to lower frequencies with time and decreasing amplitude. In our sample, we find five good cases: 3C 120, 0605-085, 1510-089, 3C 345, BL Lac. Three of these are famous superluminals; the other two show significant structural changes between our 18 cm VLBI measurements. The corresponding expansion rate for these five sources is in agreement with the γ's derived from LFV with the usual causality arguments. For the sources of this class, the observations are therefore in agreement with models that explain the phenomenon of the variability as synchrotron emission of relativistic electrons beamed in a direction close to the line of sight.2.35% of variables show only low frequency (<1 GHz) variability and little or no intermediate high frequency variations. In DA 406, prototype of the category, no superluminal motions have been observed, even if the resolution of our VLBI observations should allow the detection of the structural change expected on the basis of intrinsic LFV. In this case we do not find direct evidence of relativistic motions associated with the LFV and the process is most easily explained if the variations are extrinsic (propagation effects through the interstellar medium as the slow refractive scintillation).3.The remaining 40–45% of variables show uncorrelated high (<5 GHz) and low frequency variability with a minimum of activity at the intermediate frequencies. The explanation of the phenomenon is less clear. It could be attributed to intrinsic (superluminal) variations at high frequencies, coexisting with unrelated processes at low frequencies.

Nonbright‐spot AVO: Two examples

Geophysics ◽  
1995 ◽  
Vol 60 (5) ◽  
pp. 1398-1408 ◽  
Author(s):  
Christopher P. Ross ◽  
Daniel L. Kinman
Keyword(s):  
Seismic Data ◽  
Acoustic Impedance ◽  
Normal Incidence ◽  
Bright Spot ◽  
Seismic Section ◽  
Amplitude Variation With Offset ◽  
Avo Analysis ◽  
Impedance Contrast ◽  
Seismic Anomalies ◽  
Low Amplitude

The use of amplitude variation with offset (AVO) attribute sections such as the product of the normal incidence trace (A) and the gradient trace (B) have been used extensively in bright spot AVO analysis and interpretation. However, while these sections have often worked well with low acoustic impedance bright spot responses, they are not reliable indicators of nonbright‐spot seismic anomalies. Analyzing nonbright‐spot seismic data with common AVO attribute sections will: (1) not detect the gas‐charged reservoir because of near‐zero acoustic impedance contrast between the sands and encasing shales, or (2) yield an incorrect (negative) AVO product if the normal incidence and gradient values are opposite in sign. We divide nonbright‐spot AVO offset responses into two subcategories: those with phase reversals and those without. An AVO analysis procedure for these anomalies is presented through two examples. The procedure exploits the nature of the prestack response, yielding a more definitive AVO attribute section, and this technique is adaptive to both subcategories of nonbright‐spot AVO responses. This technique identifies the presence of gas‐charged pore fluids within the reservoir when compared to a conventionally processed, relative amplitude seismic section with characteristically low amplitude responses for near‐zero acoustic impedance contrast sands.

Hydrocarbon indicators on seismic data: Insights from poroviscoelastic modeling, amplitude, and frequency variation with offsets from the Drake Point gas field, Western Arctic Islands, Canada

2014 ◽  
Vol 2 (4) ◽  
pp. SP45-SP59
Author(s):  
Mathieu J. Duchesne ◽  
Bernard Giroux ◽  
Kezhen Hu
Keyword(s):  
Seismic Data ◽  
Low Frequency ◽  
Frequency Variation ◽  
Class Iii ◽  
Gas Reservoirs ◽  
Gas Field ◽  
Amplitude Variation With Offset ◽  
Seismic Amplitude ◽  
Impedance Modeling ◽  
Western Arctic

The evaluation of drilling prospects is frequently based on seismic amplitude anomalies. To decipher “true” seismic prospects from “false” ones, we used poroviscoelastic (PVE) models, as opposed to other formalisms such as acoustic, elastic, viscoelastic, and poroelastic models, that provided a solution that takes into account solid and fluid attenuation mechanisms separately to model the earth’s response to the propagation of a seismic wavefield. Here, a PVE impedance modeling scheme was tested using seismic and well-log data collected on a conventional gas reservoir in the Canadian Arctic. Comparisons between seismic-to-well ties achieved using acoustic and PVE media indicated that the latter provided more realistic synthetic seismograms. Although prestack analysis revealed that the present lithological context was of class I amplitude variation with offset (AVO), the seismic signature observed was of class III AVO. Consequently, the increase in amplitude with offset was interpreted to be induced not by a lithological change (i.e., shale to sand) combined with a gas-charged interval, but rather by an increase in porosity within the sandstone reservoir itself where the gas has accumulated. Frequency variation with offset analysis using spectral decomposition, image low-frequency shadows on the far offsets attributed to the gas accumulation that were correlative with the AVO anomaly. This highlighted the importance of far offsets in anomalous amplitude and frequency events attributed to the occurrence of gas reservoirs observed on stacked data and that these events can be missed if seismic hydrocarbon indicators were solely investigated on stacked data. Finally, the method of analysis emphasized the importance of combining indirect arguments coming from the observation of prestack and stacked seismic data in the time and frequency domains for reducing risk to an acceptable level before a prospect can be drilled.

Sound absorptive light comprising nanofibrous resonant membrane applicable in room acoustics

2017 ◽  
Vol 39 (3) ◽  
pp. 362-370 ◽  
Author(s):  
Klara Kalinova
Keyword(s):  
Broad Band ◽  
Low Frequency ◽  
Acoustic Power ◽  
Acoustic Properties ◽  
Room Acoustics ◽  
Sound Waves ◽  
Final Thickness ◽  
Low Frequencies ◽  
Perforated Sheet ◽  
Lower Frequencies

Room acoustic solutions are based on measurements of the acoustic power of the room and acoustic elements with different functions (absorption tiles, absorption ceilings, absorption bodies, diffusers, barriers). This work is focused only on absorption elements with an emphasis on addressing lower-middle frequencies. The design of the material is based on broad band noise. Damping of lower frequencies is restricted to a certain extent by the final thickness of the acoustic material. Nanofibrous resonant membranes will be used in the design to achieve higher sound absorption at lower frequencies in comparison with commercially available materials. The principle of the acoustic system is to use combination of a perforated sheet covered by a nanofibrous resonant membrane, which is brought into forced vibration upon impact of sound waves of low frequency. Practical application:To absorb sounds of high frequencies, porous materials are used. To absorb sounds of low frequencies, resonant membranes are employed. However, these structures absorb only sounds of certain frequency. Nanofibrous layers have unique acoustic properties due to the large specific surface area of the nanofibres, where viscous losses may occur, and also the ability to resonate at its own frequency. The advantage of this technology is the space between the acoustic element with a thickness of 1–2 mm and the wall/ceiling, which can be used for the installation of lighting/audio speakers, etc. The acoustic light prototype has been made.

Observations and suggested mechanisms for generation of low-frequency seismic anomalies: Examples from the Johan Sverdrup field, central North Sea Norwegian sector

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. B1-B14 ◽  
Author(s):  
Suhail Sayyid Ahmad ◽  
Wiktor W. Weibull ◽  
R. James Brown ◽  
Alejandro Escalona
Keyword(s):  
North Sea ◽  
Low Frequency ◽  
Dominant Frequency ◽  
Oil Field ◽  
Clear Correlation ◽  
Fracture Density ◽  
Seismic Amplitude ◽  
Porosity And Permeability ◽  
Central North Sea ◽  
Seismic Anomalies

Using 3D broadband seismic data, we have investigated low-frequency seismic amplitude anomalies associated with and below various geologic formations in the Johan Sverdrup oil field situated in the central North Sea. Low-frequency anomalies are observed below the Intra-Draupne and the Heather and Hugin reservoirs, at the Svarte and Tor Fms, and below shallow channels. The Intra-Draupne Fm is the main oil reservoir of interest in the field, and it is relatively homogeneous, with observed low [Formula: see text] values of approximately 25–30. The Heather and Hugin reservoirs, which underlie the Intra-Draupne, are heterogeneous and oil bearing. We performed three-layer elastic modeling on a simple reservoir model based on the properties of the Intra-Draupne Fm, and the results suggest that as the thickness of the middle oil-bearing layer increases the dominant frequency decreases due to the tuning effect. The Svarte and Tor Formations are shallower in the section and are fractured. Low-frequency anomalies associated with these formations seem to indicate a clear correlation with zones of increased fracture density interpreted from high-resolution most-negative-curvature attribute maps. Low-frequency anomalies are also observed below shallow gas channels consisting of stratified sandy and shaly intervals with vertical variations in porosity and permeability. In addition, stacking tests using coarse and fine velocity analysis indicate no noticeable difference in the characteristics of the low-frequency anomalies, in general, at all levels. We conclude, therefore, that the observed low-frequency anomalies are unrelated to stacking issues.

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