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is held to the wall of the borehole, ploughing through the mud cake, lose
energy by collision with electrons in the formation. Thus, the greater the
absorption in the formation, the lower the reading from the detectors. Be-
cause the absorption rate is approximately proportional to the rock density
(the proportion of electrons is proportional to density) the bulk density can
be determined. This log is affected by the thickness of mud cake that may
intervene between the sonde and the wall of the borehole, so there is a cor-
rection to be applied to the raw log, and considerable care in its use is required.
Porosity can be estimated from eq. 3:

into which can be inserted the appropriate values of pg according to the
lithology and, in this context, pw is the density of the mud filtrate because
the depth of penetration of the device is very small.

Sonic log
The velocity of sound through sedimentary rocks is a parameter of con-
siderable geophysical and geological interest, and this log provides a profile
of the transit or travel time (ps/ft or ps/m, the inverse of velocity) - usually
on the right hand side of the log, with the SP or Gamma Ray on the left
from which lithologies may be distinguished. The old device (Fig. 6-12),
with one signal generator and three detectors, illustrates the principles. The

Signal
generator

Detectors

Fig. 6-12. Sonic logging device. (Courtesy of Schlumberger Seaco Inc., Sydney.)