Geoscience Reference
In-Depth Information
concretion, at a particular level, which would give log readings quite different from
those typical of the formation in general. Also, in permeable formations the zone ad-
jacent to the borehole wall will be invaded by the drilling fluid, and this may alter the
log response from the original formation values. In some cases, seismic velocity may
vary significantly with frequency, a phenomenon usually called
dispersion
. In rocks of
low porosity and permeability, the sonic log measured at 20 kHz may not be a reliable
guide to velocities at seismic frequencies of 20 Hz.
Multiplying the sonic and density logs together will give us an acoustic impedance
log. A typical display from a well synthetic software package is shown in
fig. 3.1
. On
the left-hand side there is a scale marked in both depth and reflection (two-way) time
(TWT); how we find the correspondence between depth and TWT is explained below.
The values at the top of the scale show that a reflection time of zero corresponds to
Fig. 3.1
Synthetic seismogram generation. Tracks show time/depth scale, sonic and density log,
calculated impedance, and synthetic for two wavelets (5-10-40-50 Hz and 3-5-70-90 Hz, both
zero-phase).
