Appendix A 181
is with several mapping overlays that might be used for an air photograph). As with
any map, the use of colour will maximize the information content. The lithological
data on the core map are recorded according to a legend that is drawn up for each
drilling project. A copy of this legend should accompany each drill hole log. A num-
ber of standard symbols that can be used are shown at the head of the form. Where
the geology is not complex, some of these columns (e.g. lithology and structure) can
be combined.
The pictorial log does not aim to give a detailed, near-photographic representa-
tion of everything that can be seen in the core. As with any geological map it presents
a selection of what are considered to be the key geological elements. The down-hole
depths of important contacts are accurately plotted, but the detail shown is to some
extent symbolic. The aim is to preserve in a visual way the characteristic style and
relationships seen in the core. Where complex or important relationships need to be
shown in a more precise and accurate way, they should be sketched separately in the
Geology notes (Column 8) of the log.
At a scale of 1:100 (or at less detailed, smaller scales) the width of a drill core
would be less than 1 mm. In order to provide space to show observations it is there-
fore necessary to project the structures/lithologies seen in the core for several core
widths on either side of the drill line, thus enabling a core map several millimetres
wide to be made. By doing this, the horizontal and vertical scales of the core map
remain the same and there is no distortion. A surface drawn at 45
◦
on the mapping
column represents a surface at 45
◦
to the core axis (alpha angle).
Planar structural elements (bedding, lithology contacts, faults, veins, etc.) will
usually be shown on the pictorial log in the view that gives the maximum core axis
to surface angle. However, a special case exists where the hole is not drilled at right
angles to the strike of the major planar structural element (usually the lithological
contacts or bedding surfaces) within it. In this case, if the orientation of the core
itself is known, it is a good idea to represent the attitude of this dominant structure
on the pictorial log as the apparent dip that it would make on the drill section. The
apparent dip will always be less than the true dip and can be quickly calculated as
logging proceeds either by looking up a table of correction factors or by the use
of a stereonet (see Sect. B.3.6 for how to do this). Plotting surfaces in this way on
the pictorial log and drill section will facilitate correlation of major lithological units
and structures between holes on the same section. The true orientation of the surface
can then be recorded as a measurement in the Geology notes column.
The centre line of each of the mapping columns is the point at which the down-
hole depth of a particular feature is recorded on the form, just as its intersection with
the long core axis is the point at which the feature would be measured in the core.
A.7 Column 7 (Histogram Logs)
The histogram enables the distribution of quantitative data to be shown as a function
of hole depth. The type of measurements shown and the appropriate horizontal scale
are chosen for each project. Percentage sulphide or percentage quartz are common
variables which could be recorded. The use of colour will allow more than one