30Hz～1MHz周波数領域における岩石の誘電率について

岩石の誘電的性質に関する研究 (第1報)

抄録

The study of dielectric properties of rocks are usefull for improvement and development of the existing electric prospecting method and interpretation of field data. In order to attend these objectivies, it is necessary to accumulate data on the dielectric properties of rocks.
22 core samples of basalt, andesite, dacite, liparite, sandstone, tuff and mudstone taken from drill holes in Kosaka mine area were used in this study. Dielectric constant and loss tangent (tan δ) were measured in the frequency range from 30 Hz to 1 M Hz at 28°C.
The following results were obtained.
(1) Dielectric constant of rocks was decreased in proportion to the increase of frequency and were tended to approach the characteristic constant of rocks.
(2) Dielectric constant of rocks was increased on account of increasing water content in this frequency range.
(3) When the water content in rocks exceeds 0.3%, dielectric constant becomes large, e. g. up to the order of 10² at 30 Hz, compared to the value for pure water, which is about 80.
(4) Dried rocks or pure water alone did not exhibit the dielectric dispersion effect in this frequency range and these dielectric constants were constant.
(5) The dielectric constant theory of heterogeneous mixture applies to dried rocks, but not to water bearing rocks.
(6) Dielectric constant of rock, which was dried and then moisturized with pure water (wt. 0.47%) showed the different value from the natural state rock (wt. 0.41%) and the dielectric characteristics of the moisturized rock was different from that of the natural state rock.
(7) Water bearing rocks had the maximum tan 5 at certain frequency, basalt had at about 1 KHz, andesite at about 5 KHz, mudstone, tuff and sandstone at about 100 KHz. The frequencies obtained seemed to be the characteristic properties of rocks, but dried rocks had not the maximum.
For water bearing rocks, phenomena showing large dielectric constant in low frequency would be the effect of dielectric dispersion due to interfacial polarization. The effect of such rocks could be of important in relation to ground noise as experienced in the induced polarization method.