• Physical 12, 39
Seismic measurements reveal the influences of lunar gravitational forces and solar heat on the properties of rocks.
If you want to measure how the mechanical properties of complex natural rock formations change under physical stress, neither the laboratory tests on small samples nor the measurements in the field can do a complete job. Researchers now report that a new technique can provide information on rock properties from the long-term recording of environmental seismic noise in a single monitoring station. The data reveal the effects of the periodic stresses resulting from the gravitational forces of the Moon and the Sun, as well as solar heating. The monitoring of the subsoil conditions in this way could be important to follow the natural geological processes and to safeguard the construction and mining operations.
Heterogeneous materials, such as rocks and minerals, tend to have elastic properties that change when the material is distorted (distorted) by external forces. This change in elastic properties alters the speed at which seismic waves travel through the rock. For several decades, researchers have tried to use variations in seismic velocity as a diagnostic of geophysical conditions, but experiments have proved difficult. Measurements of active sources, in which researchers unleash seismic disturbances and observe waves at several stations, have yielded good results, but their cost and organizational requirements make them impractical for routine monitoring. Other researchers have measured the environmental seismic noise in the networks of existing seismic stations, but have not obtained accurate information.
Christoph Sens-Schönfelder of the German Geoscience Research Center GFZ in Potsdam and Tom Eulenfeld of the Friedrich Schiller University in Jena, Germany, tested a new observation method in which they badyzed the value of 11 years of data from a single source, the Patache seismic station in the Atacama desert in the north of Chile. The measured vibrations came from the natural seismic noise produced, among other things, by the impact of ocean waves on the nearby Pacific coast.
The researchers badyzed the seismic recordings with the so-called autocorrelation technique, a procedure that revealed seismic waves that hit the detector and then reflected in the environment and hit it again, a bit like listening to echoes in a church, says Schönfelder. The mathematical badysis of the echo patterns allowed the researchers to see how the speed of the seismic waves changed from one moment to another.
A graph of the variations in velocity during the entire 11-year observation period showed a clear annual oscillation with an amplitude that represents approximately half of a percentage change in seismic wave velocity. This signal was punctuated by some larger jumps that coincided with the earthquakes. A more detailed examination of the data revealed smaller oscillations with periods of exactly one day and approximately half a day; the latter consisted of two separate signals, with periods of approximately 12.42 and 12.65 hours. The period of 12.42 hours, which showed a seismic velocity variation of 0.026%, is exactly that of the main influence of the lunar tide, the periodic distortions of the Earth attributable to the gravitational force of the Moon. The longest period corresponds to a secondary tidal effect that comes from the slight ellipticity of the Moon's orbit.
To verify these badociations, the researchers estimated the tension in the rocks produced by the forces of the lunar and solar tides. They discovered that the amplitudes of the predicted voltage oscillations were proportional to the amplitudes of the oscillation signals of the seismic velocity of approximately 12 hours, further supporting the idea that tidal tension affected the properties of the rock.
But the daily oscillation was different. Although there is a solar tidal effect linked to the daily rotation of the Earth, it is much smaller than the lunar effect and insufficient to explain the seismic signal that had a period of one day. The researchers concluded that the daily variation is badociated with the warming and cooling of the Earth's surface, which also has a period of one day. Interestingly, the badysis also showed small signals grouped around frequencies that are sums and differences of lunar and solar signals, a clbadic sign that two effects with different natural frequencies are interacting in some way.
Tom Daley, a geoscientist now retired from Lawrence Berkeley National Laboratory in California, says the use of a single station to monitor background noise at the station is novel and that the precision he achieves is "the best I've seen." The method could be said to be used almost anywhere, and a potential next step would be to look for differences between various types of rock material. Sens-Schönfelder and Eulenfeld suggest that the technique could even be incorporated into planetary probes, since only one location is needed.
This research is published in. Physical revision letters.
David Lindley is an independent scientific writer in Alexandria, Virginia.