Fracture Detection with Marine P-S Mode Conversion
Authors: Li Xiang-Yang , MacBeth Colin 
Published: October 2008

Over recent years, the use of mode converted shear-waves has become increasingly common. Apart from applications in imaging through gas clouds, lithology and fluid prediction, analysis of P-S waves may elso prove to be beneficial to fracture detection. The azimuthal variations in P-S amplitudes appear to be more sensitive to fracturing, although the P-S data from seabed recordings may suffer systematic phase changes. In particular, for near vertical propagating P-S waves, the polarization and time delay of the shear-wave provide a direct measurement of the fracture orientation and intensity. Two methods may be used to recover the polarization azimuth of the fast shear wave. In the first, a 2D acquisition is assumed where the survey line is along the receiver cable. The method use rotation analysis and assumes that the fast and slow shear-wawes have similar waveforms. In the second, a 3D cross geometry is assumed where the survey line is perpendicular to the receiver cable. The fracture strike (polarization azimuth) is diagnosed by a polarity change and amplitude dimming in the azimuthal gathers of the transverse-geophone component.

Introduction

Recently, the use of P-waves for fracture detection has attracted considerable interest because of the relatively low cost in acquisition. However, the most significant P-wave effects are apparent at only sufficiently large offsets and multi-azimuths, and are often complicated by other factors. This limits the application of P-wave analysis to some extent. With the advent of multicomponent seabed seismic technology, the study of mode converted shearwaves has risen in popularity in the industry. It appears that the mode-converted P-to-S wave retains the benefit of both P- and S-wave surveysl, and thus offers the potential for more cost-effective reservoir characterization and monitoring.

The effects of azimuthal anisotropy on P-waves are relatively well known, and there are many publications in the literature. For example, Refs. 2 through 7 addressed the amplitude effects, and Refs. 8 through 13 studied the velocity effects. The effect of azimuthal anieotropy on P-S wave amplitudes is not generally well understood. Ref. 1 first presented the P-S AVO equations for anisotropic media, and they conclude that P-S AVO and its azimuthal variations may in fact be more sensitive to fractures. In contrast, the variations of P-S wave polarizations and time delays are better understood arising from S-S studies. For near-vertical propagation, the polarization of the fast shear-wave is parallel to the fracture strike, and the tie delay between the two split shear-waves is proportional to the fracture density. Ref. 14 reviewed these effects in more detail.

Here, assuming fracture-induced transverse isotropy with a horizontal axis of symmetry (TIH), which is the simplest form of azimuthal anisotropy, we review some of the fundamental effects on wave propagation, and present processing methods for anisotropy analysis. These include the amplitude behaviour, seabed waveform distortions, and shear-wave birefringence. Two new methods are presented to recover the polarization azimuth of the fast split shear-waves.