The travel time of seismic waves with frequencies up to 20 kHz is measured in borehole acoustics. Registration of the acoustic wavetrain (Fullwave Sonic-Log) allows the estimation of the dynamic elastic moduli of the formation in situ. But use of this method presupposes the traveling of longitudinal (P-) and transversal (S-) waves only as refracted waves along the borehole. In practice, the formation in the near borehole is altered or damaged by the drilling process. If the source to receiver offset (spacing) is short, as is usual in engineering, the measured results mainly reflect the velocity of this altered zone. In this case the measured arrival times will tend to be greater than those of a homogeneous formation and will depend on the penetration depth of acoustic waves through the formation (Fig. 1). Because of higher penetration depth sondes with longer spacings register the true formation velocity - by loss of structural-resolution - more exactly.


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