Arch. Min. Sci., Vol. 54 (2009), No 2, p. 189–201

Arch. Min. Sci., Vol. 54 (2009), No 2, p. 189–201

Arch. Min. Sci., Vol. 54 (2009), No 2, p. 189–201
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Electronic version (in color) of this article is available: http://mining.archives.pl
MARIUSZ MŁYNARCZUK*, MIROSŁAW WIERZBICKI*
STEREOLOGICAL AND PROFILOMETRY METHODS IN DETECTION OF STRUCTURAL
DEFORMATIONS IN COAL SAMPLES COLLECTED FROM THE ROCK AND OUTBURST ZONE
IN THE “ZOFIÓWKA” COLLIERY
METODY STEREOLOGICZNA I PROFILOMETRYCZNA WYKRYWANIA ZABURZEŃ
STRUKTURALNYCH WĘGLI KAMIENNYCH NA PRZYKŁADZIE PRÓB POBRANYCH
Z REJONU WYRZUTU SKALNO-GAZOWEGO W KWK “ZOFIÓWKA”
Deformations of coal structure affect the major parameters of the gas-coal system, such as gas bearing
capacity, desorption kinetics or cohesion. Those factors, in turn, determine the methane hazard and rock
and gas outburst conditions in collieries. To get a better insight into these phenomena, coal samples were
collected from the side walls in the coal and methane outburst zone in a loading gate D-6 on the coal
seam 409/4 in the “Zofiówka” Colliery (Poland). The samples were analysed to identify and describe
structurally deformed coals. The stereological analysis revealed the presence of sheared and fissured coal
in the region adjacent to the fault in an end gate and to a cavern produced during an outburst. Such coal
structure encourages gas accumulation and prompts quick gas release in the condition of sudden pressure
changes. This study briefly outlines the new measurement method enabling us to detect coals with this
structure. The method makes use of laser profilometry to analyse the morphology of the coal sample
surfaces. It appears that the method is adequate to detect the zones of structurally deformed coal. Unlike
stereological methods, the new method does not require extensive preparations or lengthy calculation
procedures, which appears to be its major advantage.
Keywords: mylonitic coal, methane hazard, rock and gas outburst hazard, stereology, profilometry
Analiza literaturowa publikacji dotyczących wyrzutów gazów i skał wskazuje, że jednym z istotnych
czynników wpływających na wzrost zagrożenia metanowego i wyrzutowego jest występowanie węgla
o strukturze odmienionej (przetartej, zmylonityzowanej). Węgle te charakteryzują się gęstą sieci spękań
wewnętrznych, rozbudowaną strukturą, małą zwięzłością i wysoką pojemnością gazową. Aktualnie brak
jest efektywnych metod wykrywania tego typu struktur. Występowanie węgla odmienionego strukturalnie
było jedną z przyczyn wystąpienia wyrzutu metanu i skał w KWK „Zofiówka” w roku 2005 (Jakubów i in.
2006). W pracy przedstawiono dwie metody wykrycia i opisu węgla o strukturze przetartej a przedstawione
*
STRATA MECHANICS RESEARCH INSTITUTE OF THE POLISH ACADEMY OF SCIENCES, REYMONTA 27 STR. 30-059
KRAKOW, POLAND; E-MAIL: [email protected], [email protected]
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wyniki dotyczą badań wykonanych na próbkach węglowych pobranych w rejonie uskoku, w bezpośrednim
sąsiedztwie kawerny wyrzutowej w KWK ”Zofiówka”- rys. 1.
Dla każdej próbki przeprowadzono analizę punktową w trakcie której identyfikowano ślady następujących obiektów: kleju, witrynitu (V), inertynitu oraz liptynitu, (I+L), mylonitu (M), spękań na
witrynicie (Fr(V)), spękań na inertynicie oraz liptynicie (Fr(I+L)), spękań na mylonicie (Fr(M)) oraz
substancji nieorganicznych (N).
Przykłady węgla nieodmienionego strukturalnie oraz węgla zmylonityzowanego pokazano na rys. 2.
Wyniki analizy stereologicznej przeprowadzonej dla wszystkich pobranych prób węgla prezentuje tabela
1. Próby pobrane w bezpośrednim sąsiedztwie uskoku i kawerny powyrzutowej charakteryzuje duży
udział objętościowy mylonitu. W pozostałych próbkach węgla, mylonit występuje jedynie w ilościach
śladowych. Udział mylonitu w całości substancji węglowej widocznej na poszczególnych zgładach
pokazano na wykresie z rys. 3. Obliczono również gęstość spękań na węglu odmienionym (mylonicie)
oraz na całej strukturze węgla. Wyniki obliczone ze wzorów (1-2) przedstawione w tabeli 2 pokazują, że
węgiel pobrany w rejonie szczeliny uskokowej posiada o rząd większą ilość spękań niż węgiel pobrany
w pewnej odległości od niej.
Druga, nowo zaproponowana metoda, oparta jest na wykorzystaniu profilometrii laserowej w celu
analizy ukształtowania powierzchni próbek węgla. Do odwzorowania powierzchni z opisanych uprzednio
próbek węglowych użyto profilomierz laserowy zaprojektowany i zestawiony w Instytucie Mechaniki
Górotworu Polskiej Akademii Nauk (Młynarczuk, 2004). Zmierzono pola o wielkości 256x256 punktów
oddalonych od siebie o 20μm. W celu wyróżnienia składowej chropowatości zmierzonych powierzchni,
otrzymane dane filtrowano filtrami sekwencyjnymi przemiennym o rozmiarze 10 punktów (200 μm).
W prezentowanych badaniach użyto odmiany wariogramu zwanej madogramem (Emery, 2005), zdefiniowanej równaniami (7) i (8) . Na rysunku 4 pokazano wykresy madogramów dla badanych węgli. Progi
madogramów dla węgli o strukturze odmienionej plasują się poniżej madogramów dla węgli nieodmienionych. Nieco odmienny jest madogram próbki L4 związany jest prawdopodobnie z występowaniem dużych
ilości wtrąceń substancji nieorganicznych (patrz tabela 1). Jeżeli wyeliminujemy tą próbkę z madogramu
(rys. 5), różnice pomiędzy węglami odmienionymi strukturalnie a nienaruszonymi stają się dużo bardziej
widoczne. Wykazano, że metoda ta doprowadzić może do wykrycia stref węgla o strukturze przetartej.
Jej zaletą jest fakt, że nie wymaga ona długiego etapu przygotowawczego i żmudnych zliczeń jak ma
to miejsce we wspomnianej wcześniej analizie stereologicznej. Tego typu badania, przy zastosowaniu
odpowiedniego (iskrobezpiecznego) sprzętu można wykonać bezpośrednio na ociosie, lub np. w otworach
wyprzedzających.
Autorzy uważają, że proponowane badania strukturalne węgla mogą być przydatne w prognozowaniu
i ocenie stanu zagrożenia metanowego oraz wyrzutami metanu i skał dlatego też wymagają one dalszego
rozwinięcia.
Słowa kluczowe: węgiel zmylonityzowany, zagrożenie metanowe, zagrożenie wyrzutami węgla i gazu,
stereologia, profilometria
1. Introduction
Hazardous conditions are inherent in mining. Natural hazards encountered by Polish
miners include the presence of gas in coalbeds. After the closure of several collieries in
the Lower Silesia Coal Basin in the 1990s, the major hazard becomes the presence of
coalbed methane, which might also lead to rock and methane outbursts. As the mining
depth increases (at the average rate of 5-8 m/year), the methane bearing capacity of
coal beds being mined increases too, further enhancing the hazardous conditions. Key
issues associated with methane emissions in the mining sector world-wide are outlined
by Bibler et al. (1998).
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Natural hazards associated with the presence of methane are continuous and, to
a large extent, predictable (methane hazard category, methane forecasts for the developed or currently operated longwall face or mine working). In such situations adequate
preventive measures can be put in place to control the methane risk. The organisational
and technical aspects include: effective ventilation preventing too high methane concentrations, methane flow measurements, degassification strategies, sealing of rock strata
around the mine working. These issues are addressed in more detail elsewhere (Branny
and Filipek 2008; Su et al., 2008).
There are also regions in the rock strata that are categorised as “high –risk’ zones
where the methane emission and rock and gas outburst hazard levels are elevated. These
spots, commonly referred to as “gas traps”, “gas nests” or “gas pockets” are characterised by an enhanced gas-bearing capacity and higher gas pressure whilst coal in that
region has physical, mechanical and structural properties different from the neighbouring strata. Such spots, typically encountered in the faulted zones might be the cause of
mining disasters, such as gas and rock outburst in the “Zofiówka” Colliery (Jakubów
et al. 2006) and might impair the drifting activities (Tor et al., 2007) or lead to abrupt
changes of methane bearing capacity of mine workings.
A through examination of publications on the subject on rock and gas outbursts
reveals that the occurrence of sheared coal zones is to be treated as major factors responsible for the enhanced methane and outburst hazard (Li et al. 2003). Cao et al. (2003)
classify coals in terms of the destruction level of its internal structure, distinguishing the
following coal categories: normal, cataclastic, granular and mylonitic. Investigations of
rock and gas outbursts in the Pindingshan Coalfield (China) revealed that outbursts occur
only in deformed coal layers with granulated or mylonitic microstructure coals. These
granular or mylonitic coals are now referred to as outburst-prone coals.
Hazards associated with the occurrence of coals with deformed structure are
discussed by Beamish and Crosdale (1998) and Shepherd et al. (1981). Basing on the
research data from China, Cao et al. (2001) assert that nearly all outbursts occur in
structurally deformed regions because mylonitic coals there are unstable, mechanically
non-resistant and their gas bearing capacity is enhanced. These researchers suggest
that most perilous outburst hazard arises when the thickness of a tectonically deformed
stratum should exceed 0.8 m. Williams and Weissman (1995) showed the schematic
diagram of gas pressure changes and stress variations in the strata surrounding the
mine work “approaching’ the mylonitic coal zone. They hold the view that when the
distance is reduced between the end gate and the mylonitic coal zone, the stress levels
tend to increase and an artificial pressure gradient is produced in the proximity of the
end gate. According to them, breaking the protective barrier becomes a direct cause of
an outburst. Wang and Yang (1980) suggest that high gas bearing capacity displayed by
outburst-prone coals might be attributable to large adsorption surface area, associated
with structural deformations and tectonic stresses.
Cao et al. (2000) found out that shearing zones, present mostly in the faulted regions, are largely responsible for outbursts and other natural phenomena; however, no
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adequate detection methods were available yet. The very name “gas trap” suggests it
to be a dangerous and hard- to- locate spot. The lack of reliable methods for detecting
shear zones is also commented on by Hargraves (1983), Zhang (1992) and Beamish
and Crosdale (1998).
The occurrence of sheared and deformed coals caused the outburst in the loading
gate D-6 on the coal seam 409/4 in the colliery “Zofiówka” (Sprawozdanie Komisji,
2006). Stereological analysis of burst rock reported by Wierzbicki and Młynarczuk (2006)
reveal the presence of mylonitic coal. The presence of such substances there suggest
they must come from the vicinity of the cavern. To measure how far this coal type can
be found, coal samples were analysed collected from the fault zone. Two methods were
used to detect and describe this microstructure: the first method utilises the conventional
stereological approach, the latter method makes use of laser profilometry to examine
the surface of coal samples.
2. Sample collection and preparation of microsections
Coal for tests was collected in the form of lumps from the loading gate D-6 on the
coal seam 409/4 in the Zofiówka Colliery, where a methane and gas outburst occurred in
2005. There were two faults in the proximity of the cavern. The fault fissure were about
30 cm in width. The plane of the first fault was inclined at 30-40° in the NW direction,
the inclination angle of the other plane was 75-85° (NW). The rough sketch is shown
in Fig. 1.
Fig. 1. Sketch of a loading gate D-6 on the coal seam 409/4 with indicated sample collection points
193
Samples (Fig. 1) were collected for the purpose of the research program at the front
end (sample L11), from the fault in the side wall on the right (samples L3, L4, L6, L9)
and from the side wall on the right at the distance of 5.6, 9.8 and 13 m from the front
end (samples: L12, L13, L14). Because of the risk of roof roughing-out, no samples
were collected in the direct vicinity of the outburst cavern.
3. Sterelogical analysis of coal samples
To permit the point-by-point stereological analysis, the collected samples were
crushed, the grains 0.5-1 mm in size were selected and covered with glue. Microsection
samples were then prepared and used in measurements. The program uses an AXIOPLAN polarisation microscope (Zeiss) and a computer-controlled table XYZ. Samples
for observations were magnified 200 times, in accordance with the recommendations
set forth in the standard PN-ISO 7404-3. The point-by-point analysis was carried out for
each microsection, using a square 50 × 50 grid, the mesh distance being 0.3 mm, yielding
2500 measurement points per one microsection. The analysed area was 14.7 × 14.7 [mm].
Microscopic observations distinctly reveal the presence of two entirely different coal
structures: normal microstructure structure (non-deformed), Fig 2a, and that displaying
a dense network of internal fissures and extended structure (Fig 2b). This coal type is
referred to as structurally deformed or mylonitic coal.
a)
b)
Fig. 2. Analysed coal samples: a) undisturbed coal; b) structurally deformed coal
Further analysis identified the following substances and features: glue (neglected
in further considerations), vitrinite (V), inertinite and liptinite (I+L), mylonite (M),
vitrinite fissures (Fr(V)), fissures in inertinite and liptinite structures Fr(I+L)), fissures
in mylonite (Fr(M)) and inorganic substances (N).
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Data obtained from stereological analysis are compiled in Table 1.
TABLE 1
Results obtained from stereological point analysis of coal samples – compilation
No.
Structure
1
2
3
4
5
6
7
V
Fr(V)
I+L
Fr(I+L)
M
Fr(M)
N
L3
L4
L6
25,00
4,50
6,00
0,00
41,25
23,25
0,00
29,85
10,97
9,44
0,26
27,55
5,10
16,84
13,03
2,73
5,46
0,00
49,79
28,99
0,00
Volume fraction
[%]
L9
L 11
29,30
6,97
6,35
0
40,98
16,39
0
82,36
2,52
13,37
0,00
0,97
0,58
0,19
L 12
L13
L14
82,41
0,98
14,41
0,00
1,47
0,65
0,08
79,84
3,33
12,13
0,2
2,15
1,96
0,39
80,90
3,49
13,55
0
0,82
0,41
0,82
The analysis revealed the presence of trace amounts of inorganic substances in all
examined samples except L4, which was collected in the direct proximity of the fault
fissure. It displays the structure of a tectonic break, where coal bits adhere to crushed
rock material.
Analysed coal samples fall in two distinct categories. Samples from the side wall,
collected in the direct vicinity of the fault and an outburst cavern (samples L3, L4, L6,
L9) contain a large volume fraction of mylonitic coal. The remaining coal samples:
those collected at the front end near the left wall (L11) and behind the front gate (L12,
L13, L14) featured only trace amounts of structurally deformed coal. The graph in Fig. 3
shows the volume fraction of mylonitic coal in the entire coal substance for the analysed
samples. This parameter is derived from the equation:
PM (C ) =
M + Fr ( M )
V + ( I + L ) + M + Fr (V ) + Fr ( I + L ) + Fr (M )
× 100 %
(1)
The presence of fissures is a major determinant of gas-bearing capacity and gas
desorption kinetics of coal so the volumetric fraction of fissures on mylonite PFr(M) and
on the whole coal structure were computed accordingly. Calculation results obtained
from equations (1) and (2) are compiled in Table 2.
PFr ( M ) =
PFr (C ) =
Fr ( M )
× 100 %
M + Fr ( M )
Fr (V ) + Fr ( I + L ) + Fr (M )
V + ( I + L ) + M + Fr (V ) + Fr ( I + L ) + Fr ( M )
(2)
× 100 %
(3)
Volume fraction of structurally deformed coal [%]
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80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
L11
L4
L3
L6
L9
L12
L13
L14
Sample
Fig. 3. Volume fraction of deformed substance in the analysed coal samples
TABLE 2
The volume fraction of fissures in mylonitic coal and in the entire coal structure
No.
1 PFr(M) [%]
2
PFr(C) [%]
L3
L4
L6
L9
L11
L12
L13
L14
36.05
27.75
15.63
19.63
36.80
31.72
28.57
23.36
37.5
3.11
30.77
1.63
47.62
5.50
33.33
3.93
Table 2 shows that the proportion of fissures in coal collected in the vicinity of the
fault fissure is by an order of magnitude higher than in coal collected at some distance
from the fault. The average volume fraction of fissures PFr(C) for these two coal types
are 25.62% and 3.54%, respectively. The average volume fraction of fissures in the rock
mass after an outburst would range from 7.3% to 13.2% (Wierzbicki & Młynarczuk,
2006). It appears, therefore, that burst mass contains a mixture of structurally deformed
and non-deformed coals.
4. Analysis of surface morphology of coal samples
The results indicate that coal structure becomes deformed when approaching the
tectonic disturbance region. The applied research method, though leading to interesting
results, is not widely employed in the outburst control schemes as it requires time-consuming and cumbersome microscopic measurements (Bodziony et al., 1990).
Within the framework of the research program a new method was sought that would
facilitate the procedure and permit us to detect differences in coal structure in the vicinity of the faults. It appears that coal roughness analysis could be used as such method.
196
The analysis of coal surface morphology suggests that mylonitic coal should display
slightly different surface morphology than undisturbed coals. The research program was
undertaken to verify this hypothesis.
4.1. Waviness and roughness
Many researchers dealing with fracture surface distinguish the relevant components
of waviness and roughness of analysed surfaces and profiles. This approach is also
recommended by the standard PN-EN ISO 4287. One of the major drawback of this
approach, however, is the definition of a filter defining the boundary between roughness
and waviness. The influence of the filter magnitude on variations of basic parameters of
linear roughness specified in ISO is explored by Gurau et al. (2006). Problems associated with profile filtering using standard filters are addressed in the works by Trumpold
and Heldt (1998), the main emphasis being on 2D filtering. Filtration using the Fourier
transform is investigated by Hocheng and Hsieh (2004) and the applications of the wavelet
method are explored by Chen et al. (1999). Recently new data filtering methods have
appeared. Chen et al. (1998) propose their own model of 3D filtering. Others resort to
find the roughness component using morphological filtering with the use of sequential
alternating filters (Serra 1988). The rationale behind this procedure is that it yields the
wavy component. Furthermore, it is assumed that when thus obtained wavy component is
subtracted from the analysed fracture surface, another component is obtained that might
be regarded as the roughness component. The major step involves the selection of the
filter size to separate the wavy and roughness components. Though the selection process determines the final result of this analysis, it often remains subjective. Its adequacy
might can be verified to some extent by the quality of final results.
4.2. Variogram and madogram
Roughness analyses widely use the variogram function (Herzfeld & Overbeck, 1999;
Kwaśniewski & Wang 1997; Młynarczuk 2004). In practical applications the variogram
is obtained from the formula:
2g (h , a ) =
1
N
N
å (Z (xi ) - Z ( xi + h ))
2
(4)
i =1
Three characteristic variogram parameters are considered (see Fig. 4): when the
variogram increases not from zero, but from a certain level, this level is referred to as
the nugget value; the value at which the function ceases to increase is called the sill and
the stretch from zero to the sill is referred to as the range of a variogram.
In the case of semi-variogram analysis (i.e. the function γ(h,α)) the mathematical
interpretation of major parameters is as follows: the nugget effect expresses the variability
197
of the physical quantity under consideration with the scale smaller than the sampling
interval; the sill is approximately equal to the sample variance whilst the range expresses
the largest value of the parameter h at which the sampled values are mutually correlated
(Usowicz & Usowicz, 2004).
g(h, a)
sill
range
nugget effect
h
Fig. 4. Characteristic parameters of a variogram
The research program uses a version of a variogram known as a madogram (Emery
2005). The madogram function is defined similar to the variagram but the squared difference (Z(xi)-(xi+h)) is replaced by the absolute value expression. Furthermore, in the
research program reported here madograms were obtained for surfaces that are mapped
through measurements of M parallel lines with the use of laser profilometer:
2g1( h , a ) =
1
M ×N
M
N
åå
j =1 i =1
Z ( xi , j ) - Z ( xi , j + h )
(5)
4.3. Results obtained from the analysis of the coal samples
roughness
The samples were prepared from the material collected at each point specified in
previous sections, thus prepared samples were measured with the use of laser profilometer. Sample surfaces were mapped with the use of a laser profilometer designed and
assembled in the Strata Mechanics Research Institute of the Polish Academy of Sciences
(Młynarczuk, 2004). Measurements were taken of the fields 256 x 255 points with 20 μm
198
spacing, which would yield 65536 control points distributed over the field 5 × 5 mm.
In order to distinguish the roughness components of the surfaces being measured, thus
obtained data were filtered using sequential alternating filters 10 points in size (200 μm).
Data filtering was supported by the dedicated software for image analysis and
mathematical morphology. The profile measurement data are represented in the form
of a bitmap. This representation is possible since the fracture surface was mapped over
the square area and the applied step remained invariant throughout the measurement.
Thus obtained results might be therefore represented as a bitmap whereby each datum
is represented using one pixel whose position is associated with the numbering of measurements in the X and Y axis and the grey level is equal to the measured value at that
point (in micrometers).
Fig. 5 shows madograms obtained for the analysed coal samples and the relationship
between the coal structure and obtained results now becomes apparent. The madogram
sills for mylonitic coal samples are below those obtained for structurally undisturbed
coals. Madograms of structurally undisturbed coals are slightly similar to that obtained
for the sample L4, which contains large amounts of inorganic substances (see Table 1).
It is reasonable to suppose, therefore, that the grains of these inorganic substances cause
the sill to be elevated with respect to other mylonitic coal samples. When this sample
is eliminated from the plot (Fig. 6), the difference between structurally deformed and
undisturbed coals becomes more apparent.
It is worthwhile to mention that the behaviour of sample L11 differs from other
undisturbed coal samples, which might be attributable to the fact that it was collected
at other point (Fig. 1). Madograms of the samples L12, L13 and L14 are particularly
50
L3
L4
L6
L9
L11
L12
L13
L14
2g1(h, a)
40
30
20
10
0
100
200
h [mm]
300
400
Fig. 5. Madograms obtained for coal samples collected from the outburst zone
in the “Zofiówka” Colliery
199
50
L3
L6
L9
L11
L12
L13
L14
2g1(h, a)
40
30
20
10
0
100
200
300
400
h [mm]
Fig. 6. Madograms obtained for coal samples collected from the outburst zone
in the “Zofiówka” Colliery, except the sample L4
interesting. Apparently the sills in these madograms tend to decrease as the sample
collection point is nearing the fault. The madogram range display a similar behaviour,
although it is less noticeable. It is reasonable to expect that as the gate approached the
fault, the coal structure would be changed and this change could be well detected by
the proposed methods. Still, further research is fully merited to reliably confirm these
observations.
5. Conclusions
These analyses of coal structure are indicative of the presence of structurally deformed coal in the direct vicinity of tectonic disturbances in the loading gate D-6 on
the coal seam 409/4 in the “Zofiówka” Colliery. The authors tend to believe that the
presence of this coal, with fissured structure, became a major cause of the rock and gas
outburst occurrence and determined the course of the whole process.
This study outlines two methods used for detecting deformed coals. The conventional stereological approach yields the determine the interrelations between structurally
deformed and undisturbed coals and allows for finding the volume fraction of the fissures
in the analysed coal sample. A certain drawback of this method is that coal microsections
have to be specially prepared, followed by time-consuming and cumbersome microscopic
measurements. That is why the stereological method, though yielding good results, is
rather rarely used. In this study a new method is proposed for evaluating the deforma-
200
tions of coal structure using profile measurements. The method consists in finding the
roughness components of the sample surface and the analysis of madograms reveals the
coal structure deformations, enabling us to reliably distinguish between deformed and
structurally undisturbed coals. A major advantage of the new method over the stereological approach is that it does not require specially prepared coal samples. Furthermore, the
method can be well employed both in situ and in the laboratory conditions.
As the occurrence of deformed coals presents a major hazard to miners in gassy
mines, the existing methane detection and control methods have to be constantly improved
and new methods developed. The authors hold the opinion that the methods outlined in
this study might vastly contribute to research on methane and outburst hazard.
The investigations were supported by the Ministry of Science and Higher Education, Project No R09 027 02.
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Received: 21 January 2009