White Paper

Drill Hole Spacing

Analysis
Geostatistical Assessment of
Borehole Spacing Relative to
Resource Confidence
Drill Hole Spacing Analysis
Geostatistical Assessment of Borehole Spacing
Relative to Resource Confidence

Use your data to inform and support your resource

classification and drilling decisions.
Not all coal seams and quality variables have the same continuity or variability, so investigating
drill hole spacing for your data makes sense.

Drill Hole Spacing Analysis (DHSA) lets you understand variations in estimation precision across
your deposit for different seams, domains and variables.

Many deposits, variables, and most importantly, a range of geological settings have been analysed
using DHSA to obtain the optimal drill spacing information to support resource classification
decisions.

DHSA has been implemented internationally by Geovariances for over 12 years. In Australia, DHSA
has been applied to coal operations and projects in the Bowen, Surat and Gunedah Basins.
 Variability in coal quality
Continuity and variability are unique for each coal seam and
deposit. So why apply the same drill hole spacings for resource
classification at different mine sites or even across a deposit?

Does your resource The Coal Guidelines for coal resource classification are based on
classification process the perceived precision of estimates. The recommended spacings
reflect the variability apply to all seams, variables, coal measures and geological
settings. These spacings do not consider the continuity and
you see at your
variability of the individual coal seams or plies, nor the specific
operation?
geology.

If you have sufficient drilling data to understand the continuity

and variability of coal properties for your site, why not use it to
assist with operational decisions, like resource classification,
product specifications or drilling optimisation?

Fig. 1 - The spatial continuity of a deposit is captured in a DHSA study

thanks to variography.

DHSA - an objective measure

DHSA provides a geostatistical means of assessing the estimate
Objectively inform precision for coal resource estimates based on the sampling data
your resource and its geostatistical properties. The DHSA results can be used to
classification and objectively inform your resource classification and drilling
drilling decisions. decisions.

DHSA results can be used to:

References Support changes to resource classification.

Coalfields Geology Council of Prioritise drilling targets by identifying areas requiring

New South Wales and the
Queensland Mining Council,
increased or reduced sampling density.
2003 Australian guidelines for
estimating and reporting of Indicate further drilling required to attain a given resource
Inventory Coal, Coal Reserves classification. This can be critical in budget planning for
project development.
O. Bertoli (Geovariances), C.
Mawdesley (Geovariances), S.
Martinez (BMA), A. Paul Suggest grade control drill densities required to capture the
(formerly BMA), D. Dunn (BMA), short-term variability to meet mill or product specifications.
2012 - "Measure for Measured -
Geostatistical coal resource Compare different domains and seams or plies to identify
classification in the Bowen
areas of greater variability (or insufficient data) that require
Basin", 34th Int. Geol. Congress,
Brisbane 5-10 August 2012

C
o
a
 further work to define or mitigate the potential impact of high
variability on planned mine production.

The use of a geostatistical method, where the classification of a

Through DHSA, use resource is driven by the actual in situ variability of the given
your data to resource, is strongly recommended as best practice for the
understand, inform industry (Bertoli et al, 2012).
and support your
resource classification
decisions.
Benefits
The principal benefit of DHSA is gained from using your data and
its characteristics to develop site-specific resource classification
recommendations.

DHSA is repeatable and objective, which can be crucial as projects

progress and drilling is updated.

Experience shows that DHSA spacing recommendations for

resource classifications can differ significantly from the Coal
Guidelines.

DHSA methodology
DHSA is a geostatistical technique that uses the global estimation
variance to calculate the precision levels for various sample
densities for a specific deposit.

Firstly, exploratory data analysis and variography are completed

for the available sampling data (and where appropriate, domaining
may also be applied to achieve stationarity).

The continuity and variability of a specific area and variable are

Benchmarking DHSA characterised by the variogram model. DHSA uses the variogram
model to determine the estimation variance for a single block.
The precision of the DHSA
results have been The annual area mined (or uncovered) is required as an input into
benchmarked against a the DHSA process. This gives the size of the area for the global
suite of conditional co- estimate.
simulations (CCS). The
DHSA spacings were in very
close agreement with CCS
results over periods
equating to five years of
production.

Geovariances recommend
using DHSA instead of
conditional simulation
approaches for the
significant time savings and
relative simplicity.
Fig. 2 - Calculating global estimation variance
 Many different drill spacings (e.g. block sizes) are tested, with the
global estimation variance calculated based on the total area and
the number of drill holes that will cover that area at a specified
drill spacing. This analysis is repeated for many different sample
spacings and the global estimation variances are used to
approximate the precision.

Current coal industry Global Estimation Relative Precisions

practice for coal Ash over nominal 1, 2, 5 and 10-year periods
resource classification
25.00%

is to derive global
Global Estimation Precision

estimation precisions 20.00%

for the variables

1 year
thickness and ash over 15.00%

a five year period and 2 years

apply the following 10.00%
5 years
resource categories:
10 years
5.00%

Measured <10%

Indicated 10-20%
0.00%
100 250 400 550 700 850 1000 1150

Drill Hole Spacing (m)

Inferred >20%

Fig. 3 - DHSA utilises the site variability (captured by the variogram model)
to recommend drill spacings for resource classification

Drill hole spacings are determined for a defined global estimation

precision.

In the example above, for Raw Ash, if a measured coal resource is

assigned as having a global estimation precision lower than 10%
over a 5 year period, a DHSA study advises a drill hole spacing of
900 metres.

DHSA is typically carried out for a range of variables, including

thickness, ash, volatiles, sulphur and phosphorous. DHSA will
provide a separate drill hole spacing recommendation for each
variable investigated. The DHSA results for all variables will not
have equal weighting or consideration for resource classification.
Current coal industry practice for coal resource classification is to
Case Story for the
consider global estimation precisions obtained from DHSA for the
Bowen basin variables thickness and ash.

DHSA has been used at 14

of BHP Billiton Mitsubishi MEASURED INDICATED INFERRED
THICKNESS
Alliance (BMA) coal (±10%) (±20%) (±50%)
operations and projects in Drill hole Spacing Required – Rounded down
the Bowen Basin. The DHSA Seam/Domain the closer 50 meters (area equivalent to 5
results suggest that in the years production)
Rangal Coal Measures, the THICKNESS 2,000 3,650 7,650
Coal Guidelines are RAW ASH 900 1,700 3,100
probably too strict for WASHED ASH 1,200 2,250 4,700
Indicted Resources (Bertoli
et al, 2012). With DHSA results like these, would you reconsider your resource
classification decisions and drilling decisions? You should.
 Our expertise
Who is Geovariances?
Geovariances has 12 years of experience in DHSA projects. Most
Geovariances is a specialist
geostatistical consulting and
of the work has been performed for the Australian coal industry.
software company. We have
over 45 staff, including We can provide a unique expertise through both our French and
specialist mining consultants Australian offices.
and statisticians.
Our software, Isatis, is the Geovariances is dedicated to applied geostatistics and has set the
accomplishment of 25 years of standards in geosciences, providing the mining industry with the
dedicated experience in Isatis software for more than 25 years.
geostatistics. It is the global
software solution for all
geostatistical questions.
Other technical specialties For more information
Geovariances are world leaders
in developing and applying new Let us help you understand the variability and continuity of your
and practical geostatistical coal deposit and use the global precisions and drill spacings to
solutions to mining operations. refine your resource classification and drilling decisions.
We have strong experience in
all commodities, and have
Contact our consultants:
gained trust from the biggest
international companies.
In Australia, Clare Mawdesley:
Our expertise is in applying
mawdesley@geovariances.com
geostatistics to resource
evaluation. Our services are In France, Olivier Bertoli:
through consulting, training,
and software. bertoli@geovariances.com

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