Amplitude Variation with Offset
Abstract
Subsurface variation in the ratio compressional of P-wave velocity to shear (S-) wave velocity, Vp/Vs, yields an opportunity to observe amplitude variation with offset (AVO) effects. Ideally, such variations can be inverted to indicate lithology, porosity, constituent fluids, and/or presence of free gas. In practice, AVO techniques are somewhat limited by factors such as noisy data and actual subsurface materials.
One geological situation to which AVO techniques are ideally suited is the delineation of free gas-bearing sandstones. The dramatic decrease of Vp, relative to sandstone without gas, and the negligible change in Vs results in very large changes in Vp/Vs, i.e., Poisson's ratio. The AVO response is heavily dependent on that one parameter.
Data-quality control is critical with AVO measurements because many of the analyses are done with single trace common offset or CMP gathers, and measured amplitude response is very sensitive to noise. Data acquisition must include a large range of offsets, i.e., angles of incidence, ideally from 0 to 40°. A general rule of thumb is that far offsets need to be about 1.6 times the depth of the zone of interest. Recording of at least two components (vertical and radial) is desirable to rigorously derive true amplitude response of a P-wave reflection; however, the advantage of using a second component may not be truly cost-effective. Measured amplitude can be extremely accurate when corrected from offset information to an estimated true amplitude. Processing must maintain relative amplitude response of a reflection. This consideration affects and limits the use of AGC gain, deconvolution, and other space (trace) or time-adaptive processes. The display of AVO can be in the form of unstacked common offset gathers or CMP (CDP) gathers of instantaneous amplitude. Instantaneous amplitude compensates for phase-response differences on the peak amplitude of the reflection wavelet.
