Radio Wave Generation by a Collision or Contact between Various Materials

Tuesday, 16 December 2014
Tadashi Takano1, Rikuya Hanawa1, Kenji Saegusa1 and Hirokazu Ikeda2, (1)Nihon University, College of Science and Technology, Funabashi, Japan, (2)JAXA Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, Sagamihara, Japan
In fracture of rock, radio wave emission was found experimentally [1]. This phenomenon could be used to detect a rock fracture during an earthquake or a volcanic activity [2]. The cause of the radio wave is expected to be micro-discharges, which are generated by an inhomogeneous potential distribution around micro-cracks. In order to better understand the phenomena and clarify the cause of radio wave emission, we carried out experiments to detect the emission in the cases of a collision or contact between various materials. We used receiving systems with great sensitivities and sufficient frequency bandwidths at 1 MHz-, 300 MHz-, 2 GHz-, and 18 GHz-bands. The specimen materials are as follows:
  1. Steel (2) Brass (3) Copper (4)Small coin (5)Celluloid.

We obtained the following results:

  1. The signal was detected for the specimen of (1) to (4), but not for (5).

  2. The signal is composed of intermittent spikes which include waves with a frequency close to the center frequency of each frequency band.

  3. The power is strongest at the lower frequencies among all frequency bands.

The more details will be given in the presentation.

The origin of radio wave emission from the metal is supposed to be discharges between materials in these experiments. It is surprising that even a small coin can generate a significant amount of radio wave. Accordingly, it is inferred that all amount of charges are discharged through a conductive metal. On the other hand, celluloid did not generate radio wave, though the specimen was sufficiently charged by brushing. It is inferred that a quite localized charge was discharged but the remaining charges were blocked due to poor conductivity. Extending this hypothesis, large-scale contact should have occurred between broken fragments for the radio wave generation in the aforementioned rock fracture experiments. Turbulence of the fragments is a candidate for the explanation.

[1] K. Maki et al., "An experimental study of microwave emission from compression failure of rocks" (in Japanese), Jour. of the Seismological Society of Japan, vol.58, no.4, pp.375-384, 2006.

[2] T. Takano al., “Detection of microwave emission due to rock fracture as a new tool for geophysics: A field test at a volcano in Miyake Island, Japan”, Journal of Applied Geophysics, 94, pp.1-14, 2013.