The crack systems which exist in rocks decide about their physical properties. Especially, they have an essential influence on velocity of seismic waves propagating through the fractured rock mass. The aim of the present study was to verify if fracturing is an important factor affecting the elastic properties of rock masses, particularly, generating the anisotropy of dynamic elastic moduli in sedimentary rocks from the southern part of Poland. The seismic anisotropy of the rock mass were measured using P.A.S.I. Seismograph (Mod. 16S24-N). The research was made along precise oriented radial seismic profiles. P-waves and S-waves velocities were established from recorded seismograms and then the values of dynamic elastic moduli were calculated for all profiles. The diagrams of azimuth distribution of elastic moduli showed an anisotropy of elastic properties of the investigated rocks. The maximal values of moduli agree with orientation of main crack sets or potential directions of weakening of the rock mass. Seismic methods, enabling the determination of dynamic elastic modulus are a useful tool to study the degree of fracturing and rock massifs quality assessment for engineering purposes. An important element is the fact that they are relatively inexpensive and fast.

1 Introduction

Fracturing is very common feature of the rock mass which influences physical properties of rocks. The main consequence of the existence of cracks for the elastic wave propagation is the development of seismic anisotropy due to the anisotropic distribution of crack orientation. There are a lot of theoretical and experimental papers which concern the relationship between fracture parameters and seismicwave velocity (e.g., Bamford & Nunn 1979, Idziak 1992, Gueguen & Schubnel 2003, Stan-Kłeczek 2010). Preferred orientation of crack systems involves a velocity anisotropy. The velocity of the P-wave is maximum for a wave propagating parallel to the crack strike and is minimum for a wave propagating perpendicular to it. If two or more systems of parallel cracks exist in rocks, the minimum value of velocity is observed for the wave propagating diagonally to crack systems. This dependence allowed to develop a method of seismic anisotropy measurements to study the crack pattern in a rock mass. Velocity of P- and S-waves can be also used to assign dynamic elastic moduli of rock mass. On the other hand, it is possible to calculate the effective moduli of cracked rocks from crack tensor describing geometry of crack sets (Oda 1986, 1993, Stan-Kłeczek & Idziak 2008).

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