AVO/AVA

AVO or AVA (amplitude versus angle) analysis utilizes seismic amplitude variations with offset/angle in a prestack seismic gather to infer Fluid and Lithology information.

Offset synthetic seismic data (generated for various reservoir properties) are compared to real seismic data to infer reservoir properties. For effective AVO analysis, real seismic data should be related to only reservoir properties and not related to seismic acquisition parameters, geology above the target, and seismic processing. Recorded seismic amplitudes (Figure 1) are function of (Zimmerman and Fahmy, 1990; Sheriff, 1977): 1)source directivity (arrays and ghosting), 2)transmission loss through the sediments above the target, 3) interference of interbed multiples, 4) wavefront spreading (spherical divergence), 5) attenuation (Q), 6) receiver directivity (arrays and directivity), and 7) instrument loss.



AVO Equations: modeling
There are various relations available to estimate offset (or angle) dependent PP-reflectivity at an interface between 2 layers (given the rock properties for both above and underlying layers). Figure 2 (from Gray, 2004) summarizes various AVO equations. A linearized there-term equation (Figure 3) is used in general, where "A" is AVO intercept, "B" is AVO gradient, and "C" is the higher order coefficient (Figure 3).





AVO Analysis: crossplotting
Prestack seismic amplitude plotting is a robust step in AVO analysis to understand fluid and lithology effects on real seismic and therefore characterize reservoir based on AVO response. Again there are various publications for this. A summary of AVO crossplots and analysis is shown in Figure 4. For example, you can identify AVO classes based on impedance and porosity of the sand. It also shows that if the top of the sand response is in the third quadrant (class III), then the base of sand and hydrocarbon contacts response will be in the first quadrant (Figure 4).





Rock properties relationships and tuning curve
When doing AVO analysis, we should understand first the rock properties relation (Figure 5), impedance variations, tuning effects (Figure 6), etc.