AVO METHODOLOGY Flashcards
What’s the input for AVO ?
NMO-corrected gathers
fully processed and migrated/ Pre-Stack Inv/
Basic Rock Properties - what does AVO response depend on ?
Properties of P-wave velocity (Vp) S-wave velocity (Vs) and density (Rho) in a porous reservoir rock.
Density formula variables
1 = Sw + Hhc Rho = density Phi= porosity Sw = water saturation subscripts: sat = saturated m = matrix hc = hydrocarbon w= water
what is seismic velocity ?
deformation of rock as a function of time
rock can be compressed (changes shape volume)
rock can be sheared (changes shape but NOT volume)
P wave - Compressional velocity
deformation
changes shape and changes volume
direction of particle motion is in the same direction of wave movement
S wave - Shear Velocity
deformation
changes shape but does NOT change volume
direction of particle motion is perpendicular to direction of wave movement
Velocity equations with Lambda and Mu
Vp = Pwave velocity Vs = Swave velocity Lambda - incompressibility Mu = shear modulus Rho = density
Poisson’s ratio from strains
applying compressional force
Longitudinal strain = delta L/L
Transverse strain = delta R/R
Poisson Ratio
Sigma = -(deltaR/R) / (deltaL/L)
Poisson’s ratio from velocity
small gamma = Vp/Vs
with Pwave and Swave velocities
calculate Poisson Ratio to know whether it’s HC or H20
if Vp/Vs = 1.5 then sigma = 0.1 (GAS)
if Vp/Vs = 2 then sigma = 0.33 (wet case)
3 AVO classes
Class1 – polarity reversal — bow tie – deep sand
Class2 – no polarity reversal – medium sand
Class3 – no polarity reversal – shallow sand
Conversion from offset to angle
In Reflectivity, AI calculations depends on angle of incidence`
How do you convert from offset to angle
straight ray assumption – simplest
ray parameter approx
ray-tracing
