Investigating the effect of the device parameters and geometric characteristics of the sample on the pressure wave velocity in travertine rock samples
The importance of estimation of mechanical and physical characteristics of rocks is not negligible for designing and analyzing rock structures such as rock slopes, deep trenches, and caverns. Also, these characteristics are necessary for studying rock bursts in underground mines, designing pillars and evaluation of rock failures. There are not any direct methods by which rock characteristics can be determined without a laborious, costly and time-consuming process. Therefore, a simple but reliable method is needed to determine the mechanical and physical properties of rocks in an indirect way. Non-destructive methods of measurement of physical and mechanical characteristics of rocks are considered as indirect methods. Among these, ultrasonic methods, as a low cost, simple and quick approach, are employed to determine the characteristics of rocks. In the ultrasonic method, it is possible to determine other indicator parameters and the quality of a rock using the measurement of wave velocity in the rock. The measurement of the P-wave velocity can be carried out in both field and laboratory environments. The P-wave velocity of rock is closely related to the intact rock properties and measuring the velocity in rock media interrogates the rock structure and texture. Pulse velocity measures the passing time of an ultrasonic pulse within a material, and hence it measures the pulse velocity of the mediumIn the ultrasonic method, some of the electro-mechanical transducers are used for transmitting and receiving elastic waves. The velocity of wave is obtained by dividing sample length to wave transmit time which is measured for the sample that has placed between transmitter and receiver transducers. In this research, in order to find the influence of these parameters on the pressure wave velocity, it was attempted to fulfill a series of laboratory experiments on eighteen samples of travertine rocks by using an ultrasonic device which was able to transmit and receive pressure wave. These experiments were focused on four parameters, i.e. the sample length to diameter ratio, frequency and amplitude of the wave, and pulse repetitions per unit time. An ultrasonic device used in this research had the ability to transmit and receive ultrasonic pressure waves by using two types of transducers 75 and 125 KHz. Other features of this device can be cited as the ability to change the frequency, amplitude and repetition of pulse per unit time of the transmitted pressure wave. Ateach stage of the testing process, only one parameter was changed, and the rest of the parameters were considered constant to make the assessment of the impact of evaluated parameter possible.For each transducer, the sample length to diameter ratio was changed at six levels from 0.5 to 3, the submitted wave frequency from 50 to 210 Hz, the amplitude from 0.1 to 4 volts and the wave pulse repetition from 0.5 to 16 Hz. The results were recorded.It is worth mentioning that the results of each test were the outcome of averaging three tests at each level. The purpose of averaging was to minimize the effect of different errors during the tests.To analyze the results, a software developed code was used to obtain the possibility of determination of the wave velocity in the samples by plotting the wave envelopeIn general, it was shown that by increasing the length to diameter ratio and amplitude and decreasing the frequency, the pressure wave velocity decreases. However, with changes of repetitions per unit time, the pressure wave velocity was maximized at a certain value of this parameter.
Iranian Journal of Geophysics, Volume:9 Issue:4, 2016
126 - 142
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