A servo-controlled 1500 kN testing machine (MTS) was used to compress the specimens. The experiment was carried out under ordinary room conditions, with no special noise shielding or other measures. Figure 1 shows a schematic diagram of the experimental arrangement. EMR events were detected using a loop antenna (80.0 dB gain at 500 kHz) and radio frequency interference (RFI) field intensity meter (KNM2401, Kyoritsu), and time-dependent changes were observed on an oscilloscope (DL708, Yokogawa Electric). Figure 1(b) shows an impulse input and the corresponding response curve produced by the RFI field intensity meter, together with the transfer function. The meter has an integrated filtering stage, allowing a short pulse similar to a delta function to be detected at a moderate sampling speed.
The sampling interval in the experiments was set nominally at 0.01 s, but this was adjusted in certain cases to intervals of up to 0.2 s, particularly when the time to failure was expected to be long. However, an interval of 0.5 s is still shorter than the response time of the RFI field intensity meter. A noise reading taken in the laboratory before the experiment indicated an electromagnetic field intensity of 65-68 dBÊV/m at the 500 kHz level.
Seven types of rock were examined in this study: Inada granite, Honkomatsu andesite, Kuzuu dolomite, Akiyoshi marble, Kimachi sandstone, Sanjome andesite and construction mortar (Okubo and Nishimatsu, 1985). Table 1 lists the characteristics of each rock. The Inada granite is from Ibaragi Prefecture, and is Mesozoic in age, consisting of biotite, feldspar and quartz (porosity: 1%). The Honkomatsu andesite is from Kanagawa Prefecture, and is harder than most andesites. The Kuzuu dolomite is quarried in Tochigi Prefecture, and is Mg-rich, with nearly equal proportions of Ca and Mg. The Akiyoshi marble is Paleozoic in age, quarried in Yamaguchi Prefecture, and features highly developed particle cleavage. The Kimachi sandstone is from Shimane Prefecture. The Sanjome andesite is from Fukushima Prefecture, is gray-white in color, highly porous, and contains pyroxene and plagioclase phenocrysts of 0.1 mm to several millimeters in size in a chlorite and quartz matrix. The mortar specimens were over 20 years old, and were dry mortar originally supplied by KFC, containing cement, sand, and water in respective proportions of 1:0.5:0.5.
The cylindrical test specimens were 25 mm in diameter and 50 mm in length, with ends parallel to within }0.02 mm. After the ends were ground, the specimens were air-dried in the laboratory for at least two weeks to ensure air-dryness as one of the standard procedures for laboratory rock testing (Okubo and Nishimatsu, 1985). Experiments were conducted at 20 } 5C and 70 } 15% relative humidity.