The body of this report is devoted to studying the effect of the invaded zone, mudcake, or steel casing on the components of the full wavetrain (P-wave, S-wave, reflected modes, and Stoneley mode). I conclude that a sonic logging tool has a very shallow depth of investigation. This depth, for P- and S-waves, depends upon the spacing between the source and receiver. An approximate rule of thumb is that if the source-to-receiver separation is n feet, the logging tool sees n inches into the formation. Thus, a conventional logging tool sees less than 6 inches into the surrounding formation. Since attenuation of the P- and S-wave arrivals places a practical contraint on realistic source-to-receiver separations, the depth of investigation is limited to a maximum of about 2 ft. Therefore, an acoustic logging tool is usually capable of measuring properties of the invaded zone only.--Modified journal abstract.