Estimation of geological structure around underground tunnel based on cross-correlation analyses of random continuous signals from small scale core drilling

• Published in: BUTSURI-TANSA(Geophysical Exploration) Vol. 66; no. 2; pp. 69 - 83
• Main Authors: Soma, Nobukazu, Oikawa, Yasuki, Hirano, Toru, Matsui, Hiroya, Asanuma, Hiroshi
• Format: Journal Article
• Language: Japanese
• Published: The Society of Exploration Geophysicists of Japan 01.04.2013
• Subjects: continuous random signal; cross-correlation analysis; downhole three-component seismic measurement; drilling noise; passive measurement
• ISSN: 0912-7984; 1881-4824
• DOI: 10.3124/segj.66.69

We have studied a reflection imaging technique in which passive seismic signals from other construction activity such as drilling noise are used as a source signal in order to develop a low-cost measurement method inside underground gallery. In this paper, we show an application to data sets from the Mizunami Underground Research Laboratory of Japan Atomic Energy Agency. The drilling was conducted with a main purpose of extraction rock core samples for rock mechanics research. The three-dimensional reflection imaging results and comparison with borehole observation are shown in this paper. Firstly, we show a concept and advantages of utilization of drilling noise inside narrow underground gallery, and explain analytical technique based on correlation analysis of three-component waveforms by which we can theoretically obtain three-dimensional imaging results even with only one single point observation. Next, we describe an outline of the data acquisition at Mizunami, and characteristics of observed drilling noise waveforms. Then, we show results of reflection imaging by using drilling noise signals, and compare those with fractures based on borehole observations. We discuss relationship between the estimated three-dimensional reflection images and possible spatial distribution of fractures. These results infer that the proposed technique can be useful tool when we need additional geological information in order to achieve maximum safety at reasonable low-cost in case at a narrow space such as for a high level radioactive waste repository development.