NUllERICAL STUDIES OF BODY WAVE AMPLITUDES IN FULL WAVEFORM ACOUSTIC LOGS

The amplitudes of P and S head waves in a full waveform acoustic log microseismogram are studied numerically as a function of borehole and formation parameters. The technique used is contour integration around the respective branch cuts in the complex wavenumber plane (Tsang and Rader. 1979). The results showed that the P wave amplitUde depends on the Poisson's ratio, but the S wave amplitude does not. The well accepted geometric spreading factor for P and S waves in the "far field" is only valid for a limited range of source-receiver spacings, and the onset of "far field" depends on the Poisson's ratio as well as the wavelength. The wave shape factor Ie for the P wave as defined by Lebreton st at. (1978) has a direct relationship to in situ attenuation. INTRODUCTION Synthetic microseismograms are extremely useful tools for studying the behavior of full waveform acoustic logs in different borehole environments. It allows us to investigate the arrivals and amplitUdes of different body waves and gUided waves, and how they are affected by formation velocities, attenuation, borehole radius and tl.uid (mud) properties. However, most of the studies so far have concentrated on the generation of the total waveform using different forms of the real axis integration technique. This technique is rather time consuming and does not allow for the separation of different arrivals. In order to isolate the formation and borehole effects on the different body and gUided waves, it is necessary to generate the different waves separately. Tsang and Rader (1979) used contour integrals to study the effects of formation velocity on the P and S wave amplitudes. Kurkjian (1983) discussed the separation of the S wave from the pseudo-Rayleigh wave in the complex wavenumber domain. In both papers, only the elastic case (no attenuation) was considered. OI'ennanent addresll: lGan Institute of Petroleum, lGan, People's Repuhlie of ChiDa.