Wyznaczanie modułu statycznego skal metodami sejsmiczno-akustycznymi

Abstract

DETERMINATION OF THE STATIC MODULUS OF ROCKS BY MEANS OF SEISMIC-ACOUSTIC METHODS

Summary

A good characteristic of the rock medium which is to be used as a site for building purposes requires not only the knowledge of the geological structure of that medium but also the ability of determining the strength parameters of the medium concerned on such a scale as is needed for the structure designed. This can be done by means of geophysical methods and, first of all, by engineering seismic methods combined with lab analyses of rock samples. By means of the seismic method it is possible to determine the modulus of elasticity in a dynamic way, while for the purpose of designing a structure the modulus of deformation or elasticity is determined in a static way. The studies that have so far been made show considerable differences between the values of the moduli determined by means of the two methods. These differences arise from the very fact that the moduli are determined in different ways and, above all, they usually result from differences in the volume .of the medium to which the moduli refer. The obvious conclusion is that we can compare only those moduli that have been determined for media of the same scale.

In the present paper the authors describe the method of determining the static modulus of deformation by means of seismic-acoustic tests. First a correlation is established between the velocity of longitudinal waves and the static modulus in the case of the same rock samples and in laboratorial conditions. The correlation thus defined is then used for determining the static modulus, on the basis of measurements of the velocity of longitudinal waves made by means of the seismic method in natural conditions. In this method, the influence of the heterogeneity and anisotropy of the medium is, as if automatically, taken into consideration.

If we also consider the fact that measurements of the velocity of the waves by means of the seismic method refer to media of considerable dimensions, then the modulus thus determined represents the mean value for the given medium on the scale on which the measurements have been made.

The authors describe the method as it was used in the case of limestone rocks in theLublinregion. First the authors determined the correlation between the static modulus and the velocity of longitudinal waves of ultralsonic frequency (Fig. 1). Then, on the basis of seismic measurements in those places where rock samples had been collected, the authors established the value of the static modulus of deformation for the rocks in the area concerned (Fig. 2).

The author deals with the problem of accuracy in determining the static module by means of the method described here. This method is expected to have considerable practical importance in geophysical studies made for the purpose of investigating the character of rock media considered as sites for the construction of large hydrotechnical, industrial and mining structures in areas where solid rocks occur at a small depth.