Geoscience Reference
In-Depth Information
11.2.1
Field Procedures
The drilling methods used should be documented, record-
ing core or hole diameters, the presence or not of groundwa-
ter, and the drilling rate in meters per hour. Comparisons and
statistical evaluations of data obtained from different meth-
ods and sometimes hole diameters should be part of the data
validation work. If significant discrepancies are observed, a
detailed study attempting to resolve the issue should be com-
pleted. Core or chip recoveries (as percentage or recovered
weights, respectively) from the drill holes should be incorpo-
rated into the database and statistically evaluated. Relation-
ships between grades and recoveries or sample weights, if
any, should be understood and described.
Sample collection, preparation and splitting procedures
should be well documented, whether completed in the field
next to the drill rig or in the preparation laboratory. The sample
chain of custody should also be well documented, and checks
performed while the drilling is on-going, if at all possible.
It is important to observe and document if loss of fines
occurs, if there is excessive water being used at the time of
drilling, and all other issues that may impact sample quality.
It is often necessary to recover some of the fines produced at
the time of drilling and analyze them to verify whether it has
significant mineralization or not.
The checks and validation should begin in the field,
and should include checks related to sampling, collar loca-
tions, topographic and down-the-hole surveys, drilling meth-
ods, sample collection and preparation, assaying, and sample
quality control and quality assurance programs.
The topographic surface should be checked against the
drill hole collars to ensure that there is a reasonable match.
As a general guideline for open pit mines, while a differ-
ence greater than half the proposed or actual bench height
is considered a serious error, many practitioners would only
accept vertical differences of 2 m or less. It is best to relate
the acceptable error to the level of project development and
the detail required by the design engineering. At a feasibil-
ity-level stage, accurate estimates of ground or soil move-
ment and infrastructure location would require smaller than
2 m differences; projects at an earlier stage of development
can tolerate larger errors. In underground mines, topographic
precision is much more critical.
Sometimes topography is derived from widely-spaced
survey points or a smoothed version of an aerial photograph
with few ground control points. If there is sufficient confi-
dence on the elevation of the drill hole collar as provided by
an independent survey, it may be advisable to re-interpolate
the topographic surface including the drill hole collar eleva-
tions to better reflect local variations in topography.
Another important aspect is the definition of the coor-
dinate system (projection system) used, and the tie point to
the local grids. Sometimes the “official” coordinates of the
critical tie-in points as provided by government agencies
change, as re-surveys using modern techniques are used.
The history of the base points used in the survey should
be well understood. If there are changes in the coordi-
nates of the base tie-in points, a general project coordinate
transformation (translation and/or rotation) may be suffi-
cient to fix the problem.
Drill hole collar locations should be checked for accuracy.
Down-the-hole surveys, which measure hole deviation, can
also be a source of significant errors. The expected deviation
of a hole will depend on the drilling method and the driller's
experience, the type of rocks traversed, and whether they are
inclined or vertical holes. All drill holes should be surveyed
for deviations, and all measurements should be checked for
consistency and inaccuracies. Issues to be considered include
the measuring device used; whether it is affected by mag-
netic minerals present in the rock; whether the measurements
have been corrected for declination, if it is significant for the
project's latitude and time period; whether the azimuth and
dip measurements have been taken at sufficiently close spac-
ing; and whether the information has been properly inter-
preted, analyzed, and incorporated into the database. These
issues were discussed in Chap. 5.
11.2.2
Data Handling and Processing
Any transformation of the coordinate system used should be
checked, also considering the different levels and types of
surveying that may have occurred along the life of the proj-
ect. Additionally, coordinate transformations performed to
facilitate or improve the modeling, such as unfolding, should
also be checked.
The computerized sampling database is sometimes taken
for granted and not thoroughly checked. At best, both the
geologic codes and the assayed grade values are electroni-
cally recorded at the time of capturing the data. Digital entry
reduces the risk of introducing mistakes. In practice, manual
entry is still often encountered, and if so it should always in-
clude a double-entry procedure to minimize data entry errors.
Minimum checks to be automatically performed within
the database should include from-to checks (the “to” value
is always greater or equal than the “from” value); out-of-
boundary checks for collar coordinates (ensures that no digits
are lost or added to coordinate values); stoichiometric checks
if applicable (whereby the addition of grades are no greater
than a pre-specified value); values-within-range checks (such
as all Cu assays between 0 and 100 %); presence of duplicate
sample coordinates (to avoid batches of data being entered
twice); and others as deemed appropriate. It is important to
consider, however, that these checks are a first line of defense
against potential errors, but are not sufficient by themselves
to ensure database integrity, and thus do not preclude the
need for periodic reviews and additional checks.
