Friday, June 10, 2011

And Then There are the Earth Tides...

I blogged earlier about the difficulties of measuring groundwater levels here in Northwest Kansas (here) and (here).  What with the effect barometric pressure has on the water levels, and the time-critical date-of-measurement relative to recovery stage, it'd seem a careful hydrologist wouldn't have a chance at an accurate measurement!  But I have not yet discussed Earth Tides - yet another influence on water levels.

Earth tides are caused by the same mechanism that causes ocean tides - the gravitation forces of the sun and the moon on the rotating earth.  The elasticity of the earth allows it to deform as it rotates through the gravitational pulls of the sun and the moon causing pressure differentials that affect groundwater levels.  The data shown to the right are from a deep well in Oklahoma and show the 6-hour and 12-hour earth tidal effects on the groundwater just over a half mile below the earth's surface.  While the ocean tides are affected normally by about 3 feet, the earth deformation is typically only about 8 inches.

The good news is that the earth tides are pretty difficult to pick up in shallow wells because the effect is really quite small and all the other influences (temperature changes, barometric pressure, pumping wells, local recharge events, etc.) are considerably larger and almost always drown them out. They are also very variable as the tilt of the earth and the constantly changing positions of the sun and moon cause different gravitational forces on the earth throughout the year.  Of course, the moon, being considerably closer, has more gravitational affect on the earth than does the sun.  But when both these bodies line up (or are fully opposite each other) their gravitational forces become cumulative and we experience stronger tides.

Science has in several places connected the tidal forces on earth to earthquakes, but the details are not fully understood.  The earthquakes would have happened anyway because of the shifting plates, but the daily waxing and waning tidal forces push and pull just a bit harder it seems. There is a definite correlation to Spring tides and increased earthquake activity in several places on earth.

Bottom line - I thought you might be interested in earth tides and we should all know they exist, but I'm not going to get too exited about trying to coax out their affect on my water level measurements!  

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