Difference between revisions of "Dictionary:Amplitude variation with angle/offset (AVA/AVO)"

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{{#category_index:A|amplitude variation with angle/offset (AVA/AVO)}}
  
The variation in the amplitude of a seismic reflection with angle of incidence or source-geophone distance. Depends on changes in velocity, density, and Poisson's ratio. Often used as a hydrocarbon gas indicator because gas generally decreases Poisson&#x2019;s ratio and often increases amplitude with incident angle/offset. Other conditions can produce similar effects. The amplitude of an event is often plotted (Figure [[Dictionary:Fig_A-12|A-12]]) against sin<sup>2</sup>''&theta;'' (or sin<sup>2</sup>''x''), where ''&theta;'' is incidence angle (and ''x'' is offset), and the slope (gradient) of a best-fit line is measured as the indicator:  
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The variation in the amplitude of a seismic reflection with angle of incidence or source-geophone distance. Depends on changes in velocity, density, and Poisson's ratio. Often used as a hydrocarbon gas indicator because gas generally decreases Poisson&#x2019;s ratio and often increases amplitude with incident angle/offset. Other conditions can produce similar effects. The amplitude of an event is often plotted (Figure [[Special:MyLanguage/Dictionary:Fig_A-12|A-12]]) against sin<sup>2</sup>''&theta;'' (or sin<sup>2</sup>''x''), where ''&theta;'' is incidence angle (and ''x'' is offset), and the slope (gradient) of a best-fit line is measured as the indicator:  
  
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<center><math>A(\theta) = A + B \sin^2\theta \ </math></center>
 
<center><math>A(\theta) = A + B \sin^2\theta \ </math></center>
  
Because measurements have to be made with prestack data, the noise level is usually large. The gradient is often determined by the ratios of amplitudes of large-offset to short-offset stacks. Also <b>amplitude versus angle/offset. Class 1</b> reservoirs have higher impedance than the surrounding rocks, <b>class 2</b> are those with very small, either positive or negative, impedance contrast, and <b>class 3</b> are low-impedance reservoirs. In Tertiary clastic sections, class 1 reservoirs often yield dim spots, class 3 bright spots, and class 2 reservoirs are difficult to see unless they have appreciable increase of amplitude with offset. <b>Class 4</b> are low-impedance reservoirs where the magnitude decreases with offset. See also Figure [[Dictionary:Fig_A-12|A-12]], Castagna and Backus (1993)<ref>Castagna, J. P. and Backus, M. M., Eds., 1993, Offset-dependent reflectivity–Theory and practice of AVO analysis: Soc. Expl. Geophys.</ref>, Allen and Peddy (1993)<ref>Allen, J. L. and Peddy, C. P., 1993, Amplitude variation with offset Gulf Coast case studies: Soc. Expl. Geophys.</ref>, Rutherford and Williams (1989)<ref>Rutherford, S. R. and R. H. Williams, 1989, Amplitude-versus-offset variations in gas sands: Geophysics, 54, 680–688.</ref>, and Hilterman (2001)<ref>Hilterman, F. J., 2001, Seismic amplitude interpretation. SEG-EAEG Distinguished Instructor Series #4: Soc. Expl. Geophys.</ref>.
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Because measurements have to be made with prestack data, the noise level is usually large. The gradient is often determined by the ratios of amplitudes of large-offset to short-offset stacks.  
  
[[File:Sega12.jpg|thumb|center|Amplitude variation with angle. The ''[[Dictionary:Rutherford-Williams_AVO_classification|Rutherford-Williams classification]]'' (q.v.) based predominantly on normal reflectivity is also indicated. (From Castagna et al., 1998.)<ref>Castagna, J. P., Swan, H. W., and Foster, D. J., 1998, Framework for AVO gradient and intercept interpretation: Geophysics, 63, 948–956.</ref>]]
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Also <b>amplitude versus angle/offset. Class 1</b> reservoirs have higher impedance than the surrounding rocks, <b>class 2</b> are those with very small, either positive or negative, impedance contrast, and <b>class 3</b> are low-impedance reservoirs. In Tertiary clastic sections, class 1 reservoirs often yield dim spots, class 3 bright spots, and class 2 reservoirs are difficult to see unless they have appreciable increase of amplitude with offset. <b>Class 4</b> are low-impedance reservoirs where the magnitude decreases with offset. See also Figure [[Special:MyLanguage/Dictionary:Fig_A-12|A-12]], Castagna and Backus (1993)<ref>Castagna, J. P. and Backus, M. M., Eds., 1993, Offset-dependent reflectivity–Theory and practice of AVO analysis: Soc. Expl. Geophys.</ref>, Allen and Peddy (1993)<ref>Allen, J. L. and Peddy, C. P., 1993, Amplitude variation with offset Gulf Coast case studies: Soc. Expl. Geophys.</ref>, Rutherford and Williams (1989)<ref>Rutherford, S. R. and R. H. Williams, 1989, Amplitude-versus-offset variations in gas sands: Geophysics, 54, 680–688.</ref>, and Hilterman (2001)<ref>Hilterman, F. J., 2001, Seismic amplitude interpretation. SEG-EAEG Distinguished Instructor Series #4: Soc. Expl. Geophys.</ref>.
  
==References==
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[[File:Sega12.jpg|thumb|center|Amplitude variation with angle. The ''[[Special:MyLanguage/Dictionary:Rutherford-Williams_AVO_classification|Rutherford-Williams classification]]'' (q.v.) based predominantly on normal reflectivity is also indicated. (From Castagna et al., 1998.)<ref>Castagna, J. P., Swan, H. W., and Foster, D. J., 1998, Framework for AVO gradient and intercept interpretation: Geophysics, 63, 948–956.</ref>]]
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==See also==
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Main page: [[AVO_equations|AVO equations]]
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Latest revision as of 15:24, 8 September 2020

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The variation in the amplitude of a seismic reflection with angle of incidence or source-geophone distance. Depends on changes in velocity, density, and Poisson's ratio. Often used as a hydrocarbon gas indicator because gas generally decreases Poisson’s ratio and often increases amplitude with incident angle/offset. Other conditions can produce similar effects. The amplitude of an event is often plotted (Figure A-12) against sin2θ (or sin2x), where θ is incidence angle (and x is offset), and the slope (gradient) of a best-fit line is measured as the indicator:

Because measurements have to be made with prestack data, the noise level is usually large. The gradient is often determined by the ratios of amplitudes of large-offset to short-offset stacks.

Also amplitude versus angle/offset. Class 1 reservoirs have higher impedance than the surrounding rocks, class 2 are those with very small, either positive or negative, impedance contrast, and class 3 are low-impedance reservoirs. In Tertiary clastic sections, class 1 reservoirs often yield dim spots, class 3 bright spots, and class 2 reservoirs are difficult to see unless they have appreciable increase of amplitude with offset. Class 4 are low-impedance reservoirs where the magnitude decreases with offset. See also Figure A-12, Castagna and Backus (1993)[1], Allen and Peddy (1993)[2], Rutherford and Williams (1989)[3], and Hilterman (2001)[4].

Amplitude variation with angle. The Rutherford-Williams classification (q.v.) based predominantly on normal reflectivity is also indicated. (From Castagna et al., 1998.)[5]

See also

Main page: AVO equations

References

  1. Castagna, J. P. and Backus, M. M., Eds., 1993, Offset-dependent reflectivity–Theory and practice of AVO analysis: Soc. Expl. Geophys.
  2. Allen, J. L. and Peddy, C. P., 1993, Amplitude variation with offset Gulf Coast case studies: Soc. Expl. Geophys.
  3. Rutherford, S. R. and R. H. Williams, 1989, Amplitude-versus-offset variations in gas sands: Geophysics, 54, 680–688.
  4. Hilterman, F. J., 2001, Seismic amplitude interpretation. SEG-EAEG Distinguished Instructor Series #4: Soc. Expl. Geophys.
  5. Castagna, J. P., Swan, H. W., and Foster, D. J., 1998, Framework for AVO gradient and intercept interpretation: Geophysics, 63, 948–956.