Abstract:
An innovative method based on the capability to measure temporal changes of gas flow such as Rn-222 and CO2 in deep boreholes, led to the clear discovery that both gases are affected by underground activity and could be associated with the regional geodynamic pre-seismic evolution along the Dead Sea Fault Zone (DSFZ) in northern Israel.
Long-term monitoring of natural gases in deep boreholes along seismogenic active fault zones, based on passive measuring systems (avoiding pumping and gas circulation that disturb the local equilibrium) enables to eliminate from the acquired time series, the climatic-induced periodic contributions caused by temperature and barometric pressure, and to expose the remaining portion of the signals that may be associated with the underground tectonic preseismic activity.
It was highlighted that the radon present in country rock formations as measured by gamma radiation detectors at different depths, is propelled by the surface temperature gradient to flow downward, up to a proven depth of 100 meters, revealing a daily periodicity similar to the diurnal cycle of surface temperature. The gamma detectors at each depth present very sharp, clear, and accurate peaks as a result of a high counting rate and low error, with a specific time lag between each other. It was found that the time lag depends on the downward radon velocity within the bedrock type.
The amplitudes of the radon periodic signals are controlled by the intensity of the climatic driving force, in linear dependency with the pressure gradient according to the existing physical model, and with the largest non-linear variations induced by the ambient temperature gradient, that according to the ratio between the radon level in winter to summer, varies by a factor of 3-10 while the temperature varies only within 10% span (280 C change versus an average of 2850 K).
Now, monitoring radon at a depth of several dozens of meters, substantially attenuates the climatic contribution and increases the possibility of resolving from the radon temporal spectrum the preseismic radon signals that are not periodic and are independent of the atmospheric driving forces.
In parallel, it was observed that CO2, within the internal airspace of a borehole, follows the radon measured by an alpha detector at 40m, as well as the radon temporal variations at the surrounding bedrock measured by gamma detectors up to 88m, and both are driven by the same driving forces.
The plausibly preseismic local movement of the two gases at depth is identified by the appearance of discrete, random, non-cyclical signals, wider in time duration than 20 hours and clearly wider than the sum of the width of the periodic diurnal and semidiurnal signals driven by ambient meteorological parameters. These non-cyclical signals may precede, by one day or more, a forthcoming seismic event with magnitude > 4.5.
Thus, deep gas monitoring technology may become a useful tool for the investigation of seismic precursors since similarly to radon and CO2, the existence of any natural gas such as nitrogen, oxygen, methane, hydrogen sulfide, carbon monoxide, and helium within deep subsurface media can serve as a proxy for pre-seismic precursory phenomena.
Since natural disaster events are relatively rare, and thus it's going to take a very long time to establish statistically the ability of this approach, it seems essential to verify our selected monitoring technology of gas flow in the geological medium, by independent physical methodology, such as latest Tensor Optical Fiber Strainmeter designed and being deployed over the past three years. It will be used as an orthogonal sensing proof of the non-periodic, Physico-chemical parameter variations.
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