scispace - formally typeset
Search or ask a question
Topic

Amplitude versus offset

About: Amplitude versus offset is a research topic. Over the lifetime, 1190 publications have been published within this topic receiving 21193 citations. The topic is also known as: AVO & amplitude variation with offset.


Papers
More filters
Journal ArticleDOI
TL;DR: In this paper, the compressional wave reflection coefficient R(θ) given by the Zoeppritz equations is simplified to the following: R0+[A0R0+Δσ(1-σ)2]sin2θ+1/2ΔVpVp(tan 2θ-sin2
Abstract: The compressional wave reflection coefficient R(θ) given by the Zoeppritz equations is simplified to the following: R(θ)=R0+[A0R0+Δσ(1-σ)2]sin2θ+1/2ΔVpVp(tan2θ-sin2θ). The first term gives the amplitude at normal incidence (θ = 0), the second term characterizes R(θ) at intermediate angles, and the third term describes the approach to critical angle. The coefficient of the second term is that combination of elastic properties which can be determined by analyzing the offset dependence of event amplitude in conventional multichannel reflection data. If the event amplitude is normalized to its value for normal incidence, then the quantity determined is A=A0+1(1-σ)2ΔσR0. A0 specifies the normal, gradual decrease of amplitude with offset; its value is constrained well enough that the main information conveyed is Δσ/R0, where Δσ is the contrast in Poisson’s ratio at the reflecting interface and R0 is the amplitude at normal incidence. This simplified formula for R(θ) accounts for all of the relations between R(θ...

1,115 citations

Journal ArticleDOI
TL;DR: The P-wave reflection coefficient at an interface separating two media is known to vary with angle of incidence and the manner in which it varies is strongly affected by the relative values of Poisson's ratio in the two media as mentioned in this paper.
Abstract: The P-wave reflection coefficient at an interface separating two media is known to vary with angle of incidence. The manner in which it varies is strongly affected by the relative values of Poisson’s ratio in the two media. For moderate angles of incidence, the relative change in reflection coefficient is particularly significant when Poisson’s ratio differs greatly between the two media. Theory and laboratory measurements indicate that high-porosity gas sands tend to exhibit abnormally low Poisson’s ratios. Embedding these low-velocity gas sands into sediments having “normal” Poisson’s ratios should result in an increase in reflected P-wave energy with angle of incidence. This phenomenon has been observed on conventional seismic data recorded over known gas sands.

666 citations

Journal ArticleDOI
TL;DR: In this paper, the amplitude-versus-offset (AVO) characteristics of a gas sand reflector were investigated and the two factors that most strongly determine the AVO behavior of gas sand reflections were the normal incidence reflection coefficient R0 and the contrast in Poisson's ratio at the reflector.
Abstract: Seismic reflections from gas sands exhibit a wide range of amplitude‐versus‐offset (AVO) characteristics. The two factors that most strongly determine the AVO behavior of a gas‐sand reflection are the normal incidence reflection coefficient R0 and the contrast in Poisson’s ratio at the reflector. Of these two factors, R0 is the least constrained. Based on their AVO characteristics, gas‐sand reflectors can be grouped into three classes defined in terms of R0 at the top of the gas sand. Class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for R0. Class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of R0 near zero. Class 3 sands have lower impedance than the encasing shale with negative, large magnitude values for R0. Each of these sand classes has a distinct AVO characteristic. An example of a gas sand from each of the three classes is presented in the paper. The Class 1 example involves a Hartshorn channel s...

635 citations

Journal ArticleDOI
TL;DR: In this article, the authors found that anomalously high amplitude fluid factormore reflections occurred at the top and base of the gas-reservoir sandstone and that the highest amplitude values were restricted mainly to the gas field area as determined by drilling.
Abstract: The Geostack technique is a method of analyzing seismic amplitude variation with offset (AVO) information. One of the outputs of the analysis is a set of direct hydrocarbon indicator traces called fluid factor traces. The fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the mudrock line. The mudrock line is the line on a crossplot of P-wave velocity against S-wave velocity on which water-saturated sandstones, shales, and siltstones lie. Some of the rock types that lie off the mudrock line are gas-saturated sandstones, carbonates, and igneous rocks. In the absence of carbonates and igneous rocks, high amplitude reflections on fluid factor traces would be expected to represent gas-saturated sandstones. Of course, this relationship does not apply exactly in nature, and the extent to which the mudrock line model applies varies from area to area. However, it is a useful model in many basins of the world, including the one studied here. Geostack processing has been done on a 3-D seismic data set over the Mossel Bay gas field on the southern continental shelf of South Africa. The authors found that anomalously high amplitude fluid factormore » reflections occurred at the top and base of the gas-reservoir sandstone. Maps were made of the amplitude of these fluid factor reflections, and it was found that the high amplitude values were restricted mainly to the gas field area as determined by drilling. The highest amplitudes were found to be located roughly in the areas of best reservoir quality (i.e., highest porosity) in areas where the reservoir is relatively thick.« less

597 citations

Journal ArticleDOI
TL;DR: In this article, the angle of incidence of a P-wave as a function of time and offset was computed for each sample in a normal moveout corrected CMP gather, which can then be fitted to the amplitudes of all traces at each time sample of the gather, and certain rock properties can be estimated.
Abstract: Amplitude versus offset concepts can be used to generate weighted stacking schemes (here called geo-stack) which can be used in an otherwise standard seismic data processing sequence to display information about rock properties. The Zoeppritz equations can be simplified and several different approximations appear in the literature. They describe the variation of P-wave reflection coefficients with the angle of incidence of a P-wave as a function of the P-wave velocities, the S-wave velocities and the densities above and below an interface. Using a smooth, representative interval velocity model (from boreholes or velocity analyses) and assuming no dip, the angle of incidence can be found as a function of time and offset by iterative ray tracing. In particular, the angle of incidence can be computed for each sample in a normal moveout corrected CMP gather. The approximated Zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather, and certain rock properties can be estimated. The estimation of the rock properties is achieved by the application of time- and offset-variant weights to the data samples before stacking. The properties which can be displayed by geo-stack are: P-wave reflectivity (or true zero-offset reflectivity), S-wave reflectivity, and the reflectivity of P-wave velocity divided by S-wave velocity (or ‘pseudo-Poisson's ratio reflectivity’). If assumptions are made about the relation between P-wave velocity and S-wave velocity for water-bearing clastic silicate rocks, then it is possible to create a display which highlights the presence of gas.

591 citations


Network Information
Related Topics (5)
Fault (geology)
26.7K papers, 744.5K citations
76% related
Sedimentary rock
30.3K papers, 746.5K citations
70% related
Fracture (geology)
41K papers, 677.6K citations
70% related
Lithosphere
14.5K papers, 723.8K citations
70% related
Crust
20.7K papers, 933.1K citations
68% related
Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202317
202262
202134
202046
201926
201840