| Coded excitation technology (CET) is an effective measurement method to enhance the penetrability and resolution of ultrasound. However, due to the high complexity of rock structure, which leads to significant influence on pulse compression performance of coded signals. To verify the effectiveness and applicability of this technology in rock ultrasonic testing, and also to provide some guidelines for its application in seismic exploration, the propagation of several different coded signals in rock, and the change of pulse compression performance of them are investigated and discussed。The present developments of this technology, all over the world, has been analyzed, the principles and methods are also summarized and demonstrated, and three performance indexes (mainlobe width, integrated sidelobe level, and gain in SNR) are selected to evaluate various coded signals. Meanwhile, the GSNR of coded excitation technology are compared with that of superposition averaging. It turned out that CET could suppress the colored noise, as well as more effectively suppress the random noise.To more exactly determine the impact of rock properties on the performance of the coded signals, it is necessary to exclude the influences of noise and instrument response on it before the rock ultrasonic testing with CET. So the effects of random noise, colored noise, and ultrasound transducer on it are analyzed and discussed by simulation and transducer docking. The results show that the random and colored noise has less effects, of which on GSNR can be alleviated by filtering; and the transducer has the most impact on coded signals with minimum impact on BTL signal.Since there exists no absolute uniform rock without attenuation, the influence of ultrasound waveform throughout the homogeneous rock, without decaying, on the performance of pulse compression of coded signals thus are not easy to study. Hence, the propagation of coded excitation ultrasound in the homogenous rock is simulated with finite difference method, which is compared with the simulation of single pulse.The experimental results show that the change of waveform has minimum influence on performance of pulse compression. In addition, coda wave measurement is an important method for rock ultrasonic testing, the waveform of coded signals throughout the random media is also simulated, which manifested that the waveform of the compressed phase coded signals is consent with that of single pulse, meaning that CET can be used in the coda wave detection of rock.On the basis of the simulation, the detectability of several coded signals are studied and compared. The experimental results indicate that the porosity, heterogeneity, and attenuation could decrease the GSNR and broaden the mainlobe width. According to the results, TLFM has the maximum GSNG loss and lowest resolution loss. However, on the contrary, BS has the least GSNG loss and maximum resolution loss, and overall, BTL has a higher GSNR and narrower MLW, which means that BTL has the most stable performance. In conclusion, the results, which can be used as a reference for parameters setting of coded signals applied in rock ultrasonic testing and seismic detection, verified the validity of CET.To further verify the practical effect of CET, in this paper, BS signal are used to automatically measure the velocity of P wave, which is compared with the measurement using single pulse; the results showed that the measurement of coded signals is more closely to the manual measurement. Moreover, coda wave measurement experiment of rock was also made, the results of which showed that the detection result of coded signals is better than that of the single pulse. This also verifies the effectiveness and reliability of coded signals used on coda wave measurement. |
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