Abstract

The presence of unequal horizontal stresses and/or vertical aligned fractures affect seismic signal depending on their relative azimuths. The effect of the fractures and / or unequal horizontal stresses on the seismic signal depends on the relative scale length of the seismic waves and the aligned fractures. The azimuthal variations of seismic reflection signal are manifested in variations in travel time, amplitude, amplitude gradient, frequency and attenuation (Q). The increased use of wide-azimuth 3-D P-wave surveys has dramatically increased the awareness and use of azimuthal variations to infer pertinent features of the subsurface. S-waves are also sensitive to aligned fractures and / or unequal horizontal stresses. When an S-wave of arbitrary polarization (skew to the natural coordinate system) encounters fractures, it splits into two orthogonal modes which travel at different velocities and polarizations. These split S-waves (generally recorded as mode-converted PS reflections, or C-waves) provide additional information to the azimuthal signatures of P-wave data.

Acquiring seismic data at all azimuths and offsets is a necessary prerequisite to utilize seismic azimuthal anisotropy. 3D wide-azimuth PP field data acquisition requires offset equal to target depths, for all azimuths, with fold at least 2 times that required to form a migrated image (that is, to pick velocities). Processing can be accomplished in a variety of methods, but the all-azimuth scanning techniques have shown themselves to work well. Interpretation of azimuthal data is challenging and requires the co-rendering of, at least, 5 to 10 values for each map location. These values include the azimuthal seismic attributes as well as structure, faults, horizon curvature, and seismic inversion attributes.

This paper provides a basic understanding of the nature of P-wave and S-wave azimuthal anisotropy and the requirements for acquisition, processing and interpretation of full azimuth data. We will use recent field data publications from onshore North America to illustrate the practicality of the PP azimuthal measurements.