2-D and 3-D Trend Analysis - Geomathematics: Theoretical Foundations, Applications and Future Developments - page 258

the noise. The function has elliptical contours which are elongated in approximately
the NW-SE direction. This directional anisotropy would indicate that the actual
topography features (channels?) which constitute the signal are elongated in the
NW-SE direction.
The signal was extracted from the residuals by using the theoretical autocorrelation
function. The kriging method was used to estimate the values of the signal at the
intersection points of a grid (UTM grid) with 1-km spacing. Every kriging value was
estimated from all observations falling within the ellipse described by the 0.01 contour
of the theoretical Gaussian autocorrelation function. This ellipse is approximately 13 km
long and about 7 km wide. The estimated kriging values on the 1-km grid defined a
relatively smooth pattern which was contoured yielding the pattern of Fig. 7.15 .
The pattern of Fig. 7.15 shows a number of minima and maxima. The amplitudes
of these fluctuations are very small in comparison with the variations described by the
trend (Fig. 7.16 ) as can be seen in Fig. 7.17 representing the sum of trend and signal.
Ideally, the signal would correspond to small depressions and uplifts in the Archean
basement, as it seems to do in most of the southern part of the area. However, other
features of the estimated signal can be interpreted as belonging to the later structural
deformation pattern. The lack of a clearly developed negative at Quirke Lake in the
northern part of the area where significant uranium mineralization occurs may be
caused by the fact that the channel direction is approximately parallel to the structural
trend in this part of the area. This would make it hard and perhaps impossible to
discriminate between the two patterns by means of the present statistical model.
7.2.3 Sulphur in Coal: Lingan Mine Example
The Lingan Mine was located in the Sydney coalfield on Cape Breton Island in Nova
Scotia. All seams in this coalfield belong to the Morien Group that is about 2 km thick
and Pennsylvanian in age. The strata of the Morien Group dip eastward, usually
between 4 and 20 . When production of coal from the Lingan Mine commenced in
1972, there was great interest in prediction of the sulphur content of the 1.5-2 m thick
Harbour and Phalen seams from which about two million tons of coal was to be
extracted annually. From mined-out areas close to the shoreline, these two seams
were known to contain less sulphur than other seams. Figure 7.19 shows the study
area. Lingan Mine reserves consisted of (1) a 4-km wide zone between two worked
areas (No. 12 and No. 26 Collieries on the Harbour seam); and (2) an east-west zone,
more than 16 km wide, extending from the initial area and adjacent worked areas
toward the 4,000 ft. depth (below sea level) of the seams. The area that was to be
mined could not be sampled beforehand because it was too far offshore.
In general, Caper Breton seams have relatively high sulphur content with pyrite
as the main sulphide-bearer. However, a large part of the Harbour seam in the
No. 26 Colliery averages about 1 % sulphur, yielding a high-grade metallurgical
coal. It was planned that much of the future output of the Lingan Mine would be
sold by the Cape Breton Development Corporation (DEVCO) as a metallurgical
coal. Coal with a sulphur content of up to 2 % could be desulphurized at the Lingan