Geology Reference
In-Depth Information
Fig. 1.45.
Representative segment of a
dipmeter log. The depth scale
could be in feet or meters.
Solid points
indicate the higher
quality correlations,
open
points
lower quality correla-
tions
responses are correlated around the borehole, and the dip of the unit is determined by
a version of the 3-point method (Sect. 2.4.2). Dips may be calculated for depth inter-
vals as small as 8-16 cm. A typical record (Fig. 1.45) shows the dips as “tadpoles”, the
heads of which mark the amount of dip and the tails of which point in the direction
of dip. The dips presented on the log are corrected for well deviation.
The correlations required to determine the dip for a dipmeter log are not always
possible and may not always be correct. On the printed log, solid points (Fig. 1.45)
indicate the highest quality correlations and open points indicate lower quality corre-
lations. Sparse data or gaps on the dipmeter record indicate that no correlations were
possible, a likely occurrence in a very homogeneous lithology (including fault gouge).
Closely spaced dips that are scattered in amount and direction, such as between the
depths of 2 715 and 2 725 in Fig. 1.45, suggest miscorrelations or perhaps small-scale
bedding features, and are probably not reliable dips for structural purposes. A log may
use a special symbol to show dips that are consistent over vertical intervals five or
more times that of the minimum correlation interval. These large-interval dips are
more likely to represent the structural dip. The correlations in a dipmeter log are made
by scanning some distance (the scan angle) up and down the individual tracks to look
for correlations. If the angle between the well bore and bedding is small (equivalent to
a steep dip in a vertical well), the correlative units may lie outside the search interval.
Thus dipmeter logs rarely show dips that are at angles of less than 30-40° from the well
bore (50-60° dip in a vertical well) unless they were specifically programmed to look
for them. If the dipmeter interpretation program is unable to make good correlations
across the well it will probably show either no dips in the interval or may have made
false correlations and so show low-quality dips at a high angle to the well bore.
