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Full waveform acoustic well logging has become instrumental to hydrocarbon exploration because of its ability to determine in situ velocity information for P and S waves as well as the attenuation (or absorption) of seismic energy. It is therefore important that the factors infiuencing these logs be understood. In addition to formation properities, Poisson's ratio, the borehole environment, and tool centering can affect the full waveform acoustic logs. These latter factors are evaluated through modeling of elliptic boreholes and de-centralized tools in the borehole. INTRODUCTION A number of factors affect the observed full waveform acoustic log amplitudes. Formation properities play an important role. In addition the borehole environment can cause variations. For example, borehole rugosity impedes the propagation of head waves (Morris et aI., 1964). In areas of' cavitation and washouts, critical refraction may be prevented, resulting in low amplitude waveforms. Lebreton et al. (1978) report a strong reduction in waveform amplitude proportional to an increase in mud cake thickness. Centering of the sonde and a cylindrical borehole are crucial to waveform amplitudes and arrival times. In most full waveform logging systems the tool is held centralized in the borehole by several bow type springs which press against the formation. In other systems, rubber "fingers" keep the tool centralized. The reliability of these methods in keeping the tool centralized in holes with a true circular cross section as well as in irregular holes has not been well documented. The actual amount of "rattle" that the tool experiences is not generally known. Morris estimates that moving the source one quarter of an inch (0.64 cm) off-center decreases the P wave amplitude by one half (Morris et al., 1964). This is a significant effect for such a small displacement. In experiments with a tool placed in a trough to simulate a cased borehole it was determined that moving the centralized source completely against the casing not only decreases the amplitude of the recorded waveform but also increases its complexity and reverses the apparent polarity of the first motion (Riddle, 1962). Studies by Forristall and Ingram (1969) show that an off-centered condition for a fiuid cylinder yields two types of refiected arrivals: one associated with the source waveform, and the other with its derivative.

Approximate effects of off‐center acoustic sondes and elliptic boreholes upon full waveform logs