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13: SOMA LIGNITE BASIN, TURKEY 297

500-m centers to establish the resource poten- is lost during drilling there is no way of deter-
tial of the basin. mining the quality of the lost core and often the
The second exploration program was super- better quality, more brittle, bright sections are
vised by Otto Gold GmbH. Open-hole drilling lost on these occasions. It is usual to have a
was used with predetermined intervals cored drilling contract that requires the drillers to
to include lignite seams. Lignite cores were recover at least 95% of the core in the seam.
analyzed for ash and moisture content, and Recoveries less than this (within reason) usu-
calorific value. The sulfur and volatile content ally require redrilling. Techniques that help
were determined on a few samples. The pro- to improve core recoveries include the use of
gram was designed to identify lateral and large diameter wireline (see section 10.3.2)
down-dip lignite limits, as well as to undertake or air flush core barrels (e.g. HQ series with
infill drilling at 250 m centers within the basin. a nominal hole diameter of 96.1 mm) and the
TKI initiated the third drilling program as use of triple tube core barrels (Cummings &
part of their preliminary mine feasibility study. Wickland 1985, Berkman 2001).
They also used rotary drilling techniques with
spot coring. Full proximate analyses and calo-
rific value determinations were done on these 13.2.3 Geophysical logging
cores, and occasionally sulfur analyses. These Down-the-hole (DTH) geophysical logging (see
holes were drilled to obtain additional informa- section 7.13) was first used in the oil industry
tion in areas of structural complexity or areas but soon found its way into coal exploration
with a paucity of data. TKI produced isopach, (Ellis 1988). It is now used routinely in the
structure contour, isoquality, and polygonal evaluation of coal deposits because geophysical
reserve maps, as well as reserve tables, surface logs can help reduce drilling costs by enabling
and underground mine plans, manpower sched- the use of cheaper, rotary, open-hole drilling
ules, etc. of, say, 80% of the holes. Good comparison
A fourth drilling program was carried out of open hole and cored holes is achieved with
during the feasibility study. Five of the holes the use of DTH logs. They can also help
were fully cored for geotechnical studies, in identifying the top and bottom of the
while the remainder were rotary drilled and seams, partings within the seams, lithological
spot cored. Lignite cores were analyzed for changes, in checking on core recovery and the
proximates, calorific value, and specific grav- depth of each hole, and ensuring that drillers do
ity. Hardgrove grindability tests (section 13.6) not claim for more meters than they actually
and size analyses were also carried out on drilled. Seams often have characteristic geo-
samples from the existing open pit. Although physical signatures which, in structurally com-
122 boreholes were drilled, only 83 of them plex areas, can help with seam correlation.
intersected the lowest lignite seam, or the hori- Typical DTH geophysical logs used on coal
zon at which the equivalent of the lignite seam and lignite exploration programs are natural
occurred. This was due to common drilling gamma, density, neutron, caliper, and resistiv-
problems such as loss or sticking of rods in the ity, as well as sonic and slim line dip meter logs
hole, or burning the bit in at zones of serious (Ellis 1988). Shale, mudstone, and marl usually
and sudden water loss. This resulted in an over- have a high natural gamma response while coal
−2
all density of 11 holes km , equivalent to a rec- has a low response, with sharp contacts often
tangular grid roughly 330 m × 250 m, although being observed. Coarsening-upward or fining-
some holes are closer and some farther apart upward sequences in the clastic sections of the
than this. This is considered to have given suf- logs can also be inferred from the gamma logs.
ficient density to classify the lignite in terms The density log, as its name implies, reflects
of measured (= proved) reserves (USGS 1976). the change in density of the rocks, with coal
and lignite having low densities and shale and
sandstone having higher densities. The neu-
13.2.2 Core recovery
tron tool is used in estimating porosity, and the
One of the major problems of assessing lignite resistivity tool may be used to indicate bed
or coal deposits is that of core recovery. If core boundaries. The caliper tool defines the size of