Fig. 3
a and b Typical results for third series of experiments. a Currents for combination of gabbro/CH4 or N2 at ~ 160 °C, which are enlarged the signals in the middle of Fig. 2e; b electric potential for gabbro/N2 at ~ 160 °C. c and d Typical results for fourth series of experiments. c Current and vibration for combination of granite/CO2 at ~ 25 °C and d induced current signals at the pipe electrode with and without non-conductive resin coating buried in a container filled with granite grains and soil. Arrows show in a–d the final rupture point when the gas started to flow in the crack gap. e Gamma-ray equivalent dose rate for granite, gabbro, and quartz diorite in 30-min measurements. f Comparison number Nγ>0.04 of events for the γ-ray equivalent dose rate greater than 0.04 μSv/h and average peak currents for several test runs of quartz diorite, gabbro, basalt, and granite with CO2 at S = 1.2 × 10–4 m2 and ~ 25 °C; Nγ>0.04 for the background in open air is also included. Arrows pointing to the left and right indicate the vertical axis showing the units of the blue and orange bar graphs