インパルス音波による地下埋設管の探査実験

抄録

The authors devised a detection method of pipes buried underground by applying a pulse-echo method using impulsive sound and studies on its practical use are proceeding. In this method, an electromagnetic induction type sound source is used to radiate an impulsive sound with wide directivity into the ground. The echo signals from buried pipes are received by four receivers arranged symmetrically with respect to the sound source. A proper delay time is given to each of the four received outputs. These outputs are processed by a technique similar to a polarity correlation. Then, the output signals give an image along a line in a particular direction. Two-dimensional image pattern of pipes are drawn by varying the direction of directivity and scanning the ground. In this paper, the results of experiments performed in two model sand baths are described. Fig. 1 (a) and (b) show model sand baths (I) and (II) together with the location of the buried pipes. In the model sand bathes, a small electromagnetic induction type sound source was buried at 1. 0 m in depth. Using the buried sound source and a receiver placed on the ground surface, the values of sound velocity in the model sand baths (I) and (II) were measured at about 153 m/s and 155 m/s respectively. Fig. 2 shows the arrangement of the sound source and four receivers on the ground surface and a block-diagram of the measuring equipment. In the measurement for the detection of pipes, the sound source was placed just above the pipes and the four receivers were arrayed perpendicular to the pipes and symmetrically with respect to the sound source at intervals of 50 cm. The outputs of the four receivers were recorded in a magnetic tape. The signal processing and image pattern display were made at our laboratory. Delay time given to each receiver output was set so as to compensate the transmission time of sound from an imaginary position of the buried pipes to the four receivers. Then, the four delayed received outputs were divided into two pairs and summed in each pair, leading to two outputs, and the signal processing was carried out. The output of the signal processing circuit was memorized once in the WAVE MEMORY and reproduced at a time conversion of 0. 0l by a pen-recorder on a recording paper. Fig. 4 and 6 show a bock-diagram of the signal processing device and the displaying process of output waveform after signal processing respectively. In Fig. 7, (b) shows the output waveforms of four receivers which are arrayed perpendicular to the pipe axis. The measurements were made at point C in Fig. 1(a). The sound source was placed just above the poly-vinyle chloride (P. V. C. ) pipe of 8 cm in diameter buried at the depth of 1 m. In Fig. 7, (c) shows four output waveforms after signal processing obtained by setting delay times corresponding to four different depths as shown in Fig. 7, (a). The figure illustrates that the amplitude of the output signal from the pipe buried 1 m deep decreases only a little when setting delay times corresponding to the depths of 0. 8 m and 1. 2 m, but the amplitude decreases by about one-half when setting them corresponding to the depth of 0. 6 m. This fact explains that by setting the delay times corresponding to the position of 1 m in depth, the received output represents reflective wave pattern of pipe buried 0. 8 m to 1. 2 m deep. Accordingly, in order to obtain the entire image patterns over a wide range of depth, it is necessary to synthesize some displayed patterns in each effective range. Fig. 8 through 19 show underground cross sectional patterns for detection of buried pipes in model sand bathes. Fig. 10 and 11 show image patterns obtained by setting delay times corresponding to the depth of 0. 6 m and 1. 0 m respectively. Fig. 12 shows the synthesized pattern of these two patterns in each effective range of 0. 8 m to 1. 2 m in depth and of 0. 5 m to 0. 8 m in depth. The figure illustrates that the clear image patterns of a steel (S. A. ) pi