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Fig. 14-2. Stratigraphic diagram of relationship of abnormally pressured mudstones to
stratigraphy. (Time lines horizontal.)

normal pressures, and estimating the actual pressures, from their effects on
electrical and acoustic borehole logs (Hottman and Johnson, 1965). It was
found (Fig. 14-3) that both resistivity and sonic transit time showed depar-
tures from the normal trends of increasing resistivity arid decreasing sonic
transit time below the top of abnormal pressures. But these did not provide a
predictive tool, merely a method of locating and quantifying the abnormality.
The drilling break remains the only reliable indicator of impending abnormal
pressures, although the great improvements in seismic technology enable
qualitative prediction of abnormal pressures, and the depths with an accuracy
of 200 or 300 m, from interval velocity analyses. The drilling break was also
quantified and, through automation, provided a continuous computed pore
pressure. This was called the D-exponent, or d-exponent (see Fertl, 1976, pp.
122-130). Computers can, however, be dangerous because the printed figures
tend to be accepted uncritically*.
The development of indirect, geophysical methods (including borehole
logging) of quantifying abnormal pressures, coupled with the ever-widening
areas of the world in which these phenomena were found, led to increasing
awareness of the habitat of abnormal pressures - but this interest, naturally,
was confined to those in or connected with the petroleum industry. Where-
as Dickinson had worked with measured pressures and with pressures estimated
from the mudweight needed to contain a pressure, the borehole log responses
indicated that the mudstones themselves were abnormally pressured (as had
indeed been inferred earlier) although no pressure measurements could be

* I have been on an offshore Louisiana rig when the toolpusher ordered a reduction of
the mudweight on the grounds that the computer showed a reduced pore pressure. The
mud weight had been raised earlier for a part of the section that was still in open hole,
and he was surprised when the well kicked.