Interpretation of earth tide responses of three deep, confined aquifers

The response of a confined, areally infinite aquifer to external loads imposed by earth tides is exam ined. Because the gravitational influence of celestial objects occurs over large areas of the earth. the confined aquifer is assumed to respond in an undrained fashion. Since undrained response is controlled by water compressibility, earth tide response can be directly used only to evaluate porous medium compressibility if porosity is known. Moreover, since specific storage S. quantifies a drained behavior of the porous medium, one cannot directly estimate S. from earth tide response. Except for the fact that barometric changes act both on the water surface in the wei) and on the aquifer as a whole while stress changes associated with earth tides act only in the aquifer, the two phenomena influence the confined aquifer ill much the same way. In other words, barometric response contains only as much information on the elastic properties of the aquifer as the earth tide response does. Factors such as well bore storage, aquifer transmissivity, and storage coefficient contribute to time lag and damping of the aquifer response as observed in the well. Analysis shows that the observation of fluid pressure changes alone, without concurrent measurement of external stress changes, is insufficient to interpret uniquely earth tide re sponse. In the present work, change in external stress is estimated from dilatation by assuming a reasonable value for bulk modulus. Earth tide response of geothermal aquifers from Marysville, Mo