Application of the Shallow Refraction Seismic Method for Analysis of Block Divisibility of Carpathian Sandstones in the Górka-Mucharz Deposit

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1 PUBLS. INST. GEOPHYS. POL. ACAD. SC., M-29 (395), 2006 Application of the Shallow Refraction Seismic Method for Analysis of Block Divisibility of Carpathian Sandstones in the Górka-Mucharz Deposit Jarosław BADERA, Sławomir NIEMCZUK and Radosława TOMASZEWSKA University of Silesia, Faculty of Earth Sciences, Department of Applied Geology ul. Będzińska 60, Sosnowiec Abstract The application of the shallow refraction seismic method for analysis of block divisibility of natural building stone is presented for an example of the Górka-Mucharz sandstone deposit (Polish Carpathians). Geological block divisibility is a relative participation of the block material in a deposit, profile etc. The applied method reflects the main fracture directions to the satisfactory degree but the estimation of fracture linear density has only an approximate character. The authors suggest the possibility to set the volume of average block as an additional (geophysical) index of the block divisibility. 1. Location and Geological Structure of the Deposit The Górka-Mucharz deposit (Fig. 1) is located in the Mucharz village, south of Wadowice, in the eastern part of the Beskid Mały Mts. (Western Carpathians). It consists of the Paleogene sandstones of the Lower Krosno beds belonging to the Silesian nappe (Outer Carpathians). There are four sandstone complexes in the deposit (Znańska 1974). Their total thickness is about 60 m and they dip slightly toward the NE at an angle of 0-6º. The raw material (a natural building stone for blocks and slabs) is excavated by a quarry with five exploitation levels which reflect the complexes mentioned above. 2. Block Divisibility The block material is defined as blocks of raw material with a subrectangular shape and volume higher than a minimal limit. For example, the marginal size for sandstones is 0.5 m 3 (according to the Polish standard PN-B-11200).

2 Fig. 1. Schematic geological map of the Carpathians in the Skawa valley region (Książkiewicz 1972). The block divisibility can be considered as a geological or mining one (according to the Polish standard PN-88/B-04120). The geological block divisibility is a relative participation of block material in a deposit, profile etc. The mining block divisibility is a participation of extracted blocks in the total excavation. For economic deposits of block sandstones, the geological block divisibility should be equal to at least 20% (Nieć 1994).In the case of geological documentation, the block divisibility index is the most often used measure. It is calculated according to the formula (Bromowicz and Karwacki 1982): b= PxiPyiPzi 100%, where P x, P y and P z are probabilities that x-, y- and z-dimensions are higher or equal to the limit. They are estimated on the basis of the cumulated frequency diagrams. In the case of non-orthogonal systems, the minimum values of x, y and z must be increased by a linear correction coefficient: k = f( d, β ), where d is the form index and β is the angle deviation. 3. Structural Analysis Structural measurements made by means of a compass in a fragment of the quarry and then analysed within the RockWorks program reveal the presence of two distinct sets of fractures (Fig. 2). The resultant vector of strike is 101 for the sublongitudinal set (A) and 41 for the subtransversal one (B). The linear density of fractures was calculated for measuring lines traced along an exploitation wall and differing in the azimuth (Table 1). In the studied fragment of deposit, the block divisibility is significant. It is 96% for x-, y- and z-dimensions equal to at least 0.8 m.

3 Fig. 2. Orientation of the fractures in the Górka-Mucharz deposit: left contour scheme, right fracture rose (strikes); the main set of fractures: A sublongitudinal, B subtransversal; diagrams made within the RockWorks program. Table 1 Linear density of fractures according to different directions of profiles in the Górka-Mucharz deposit Azimuth of profiles Linear density of fractures [fr./m] 0 1, , , , , , , , , ,61 4. Seismic Analysis Seismic measurements were done in accordance with the widely known methodology (Reynolds 1997) using an Terraloc MK6 (ABEM) along profiles, azimuths of which changed every 20º around a common central point (Fig. 3). Seismic waves were induced in the central point and at the ends of measuring lines. The total length of profiles was 40 m and geophones were situated every 4 m. A similar scheme of measuring station has already been applied during other researches of anisotropy in rock massives (for example Idziak 1992). The measuring data were afterwards converted with the WinSism 9.0 GeoSoft program.

4 Fig. 3. Scheme of the measuring station in the Górka- Mucharz deposit. The velocities of seismic waves calculated on the basis of non-cracked rock samples from Mucharz (Dziedzic 2003) are 0P = 4031 m/s for longitudinal wave (P) and 0S = 2698 m/s for transversal wave (S). The velocities of P and S seismic waves calculated in situ for the studied massive are obviously lower and change according to the azimuth of profiles (Table 2). They have been recounted in relation to the velocities in non-cracked samples and raised to the second power to emphasize differences in particular directions. Table 2 Anisotropy of velocities of seismic waves in the Górka-Mucharz deposit Azimuth P [m/s] P / 0P ( P / 0P ) 2 S [m/s] S / 0S ( S / 0S ) 2 0 o o o o o o o o o Seismic Analysis vs. Structural Data and Block Divisibility The phenomenon of seismic wave propagation through fractured rock-masses and models describing this process have been the subject of investigation for many

5 years (Crampin 1976, Oda 1984, Marcak and Mortimer 1986, Idziak 1988 and 1992). In the examined case the comparison of structural and geophysical data shows (Fig. 4) that seismic waves reach maximum velocities in the directions of minimum density of fractures (maximum density of fractures clockwise rotated by 90º) i.e. in accordance with strikes of the main fracture sets. It is consistent with the results of some previous investigations (for example Idziak 1988, 1992). Fig. 4. Azimuthal distribution of velocities of seismic waves and linear density of fractures. P velocity of longitudinal wave, S velocity of transversal wave, γ linear density of fractures clockwise rotated by 90, A and B the main fracture sets. Functional interdependences between the linear density of fractures and the velocities of P and S seismic waves were calculated by means of the empirical data (Fig. 5). Involution functions approximate the empirical data in the best possible degree but absolutely high determination (R 2 ) and statistical validity (α = 0.95) were obtained only for the P waves. Fig. 5. Relationships between velocities of seismic waves and linear density of fractures. One can assume that the most significant minimums of P-wave velocity ( Pmin1, Pmin2 ) correspond with the highest densities of fractures (γ max1, γ max2 ), i.e., the average densities of the main fracture sets (γ A, γ B ). Thus, the volume of an average subrectangular block limited by three fracture sets can be characterized by the following function:

6 b = f( γ, γ, γ, k), vg A B C where (after transformations of formulas from Fig. 5): γ = A(max1) γ = B(max 2) P , P min P , P min 2 γ C is the thickness of rock beds valued from drill-cores or outcrops, k is a linear correction coefficient. However, there is no direct translation on the block divisibility index, and that is why the authors suggest the possibility of setting the volume of an average block (b vg ) as an additional (geophysical) index of the block divisibility. In the case of the Górka -Mucharz deposit, b vg amounts to 4.1 m 3 therefore, it is considerably more than the limit. 6. Conclusions 1. The refraction seismic method reflects the main fracture directions in Carpathian sandstones to a satisfactory degree. elocities of seismic waves are the highest in the directions of strike of the main fracture sets. 2. The estimation of fracture linear density shows only an approximate character. However, it seems that empirical functions calculated on the basis of P seismic wave are more precise and can be useful in the case of a simple orthogonal fracture system. 3. The described method allows calculations of sizes of the average block, but there is no direct translation on the block divisibility index. The authors suggest the possibility of setting the volume of an average block as an additional (geophysical) index of the block divisibility. References Bromowicz, J., and A. Karwacki, 1982, Metodyka badań bloczności złóż budowalnych materiałów kamiennych (English summary), Zeszyty Nauk. AGH, Geologia 8, 2, Crampin, S., 1978, Seismic wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic, Geophys. J. R. Astr. Soc. 53, Dziedzic, A., 2003, Structural control on fracture toughness (brittle cracking) in the Krosno sandstone of Mucharz, southern Poland, Geol. Quart. 47, 1, Idziak, A., 1988, Seismic wave velocities in fractured sedimentary carbonate rocks, Acta Geophys. Pol. 36, 2,

7 Idziak, A., 1992, Anizotropia prędkości fal sejsmicznych i jej związek z orientacją systemów spękań masywów skalnych (English summary), Prace Nauk. Uniw. Śląskiego 1328, Katowice. Książkiewicz, M., 1972, Budowa geologiczna Polski t. I, Tektonika cz. 3, Karpaty, Wydawnictwa Geologiczne, Warszawa. Marcak, H., and Z. Mortimer, 1986, Modele związków prędkości fal sejsmicznych ze stopniem szczelinowatości górotworu, Publ. Inst. Geophys. Pol. Acad. Sci. 191, Nieć, M. (ed.), 1994, Zalecane kryteria bilansowości złóż kopalin, Min. Środ., Kom. Zasobów Kop., Warszawa. Oda, M., 1984, Similarity rule of crack geometry in statistically homogenous rock masses, Mech. Mater. 3, Reynolds, J.M., 1997, Seismic refraction surveying, In: An Introduction to Applied and Environmental Geophysics, John Wiley & Sons, Chichester. Znańska, M., 1974, Dokumentacja geologiczna złoża piaskowców krośnieńskich Górka- Mucharz w kat. B+C 1, Przedsiębiorstwo Geologiczne, Kraków. PN-B-11200, 1996, Materiały kamienne. Bloki, formaki, płyty surowe, PKN, Warszawa.PN- 88/B-04120, 1988, Kamień budowlany. Podział, pojęcia podstawowe, nazwy i określenia, PKN, Warszawa. Accepted 10 April 2006