Response of well-aquifer systems to Earth tides

Abstract

The water level in a well open to an artesian aquifer responds to pressure‐head fluctuations caused by the dilatation of the aquifer. Based on hydrologic considerations, it is shown that (1) most well‐aquifer systems respond to disturbances with periods of less than several days as if the well were drilled into a medium of infinite or partially bounded extent, and (2) the representation of an aquifer as a finite cavity is unrealistic for most well‐aquifer systems. The amplitude of tidal water level fluctuations in well‐aquifer systems depends on the dilatation and the specific storage of the aquifer. Analysis of the dilatation caused by the earth tide is based on the assumptions that (1) the latitudinal and longitudinal strains caused by the earth tide are determined by the elastic properties of the earth as a whole and are largely independent of the elastic properties of a near surface aquifer, and (2) the vertical strain of a near surface aquifer depends on Poisson's ratio (or the Lamé constants) for the aquifer and the latitudinal and longitudinal strains. The tidal dilatation can be computed from equilibrium tide theory provided that Poisson's ratio is known. The amplitude of the tidal dilatation produced by the large semidiurnal wave, M2, is approximately 1 × 10−8. It is not unusual to have earth‐tide fluctuations in wells corresponding to M2 with an amplitude of 1 to 2 cm. The fact that tidal water‐level fluctuations depend upon the specific storage of the aquifer explains the variation in amplitude which troubled others. The specific storage and the porosity of the aquifer can be computed from an analyses of earth‐tide fluctuations if Poisson's ratio for the aquifer is known. Computations of specific storage and porosity are presented for three artesian wells for which tidal harmonic analysis of hydrograph data was published by Melchior. The results of these calculations appear to be better than one might reasonably expect.

Estimación de algunas propiedades geohidrológicas en un conjunto de pozos de monitoreo en el Valle de Mexicali, B.C., México.

“…La marea terrestre actúa directamente sobre la matriz de los materiales que conforman el acuífero, por lo que la respuesta del nivel del agua dentro de un pozo a la acción de la marea terrestre es controlada principalmente por las propiedades elásticas de la matriz porosa; mientras que, la presión barométrica actúa en la superficie libre del agua dentro del pozo y en la superficie del terreno de manera simultánea (Bredehoeft, 1967;Van Der Kamp y Gale, 1983;Rojstaczer y Agnew, 1989); la Figura 1 ilustra estos mecanismos actuando en una sección de acuífero idealizado.…”

Pahute Mesa Well Development and Testing Analyses for Wells ER-20-8 and ER-20-4, Nevada National Security Site, Nye County, Nevada, Revision 0

“…They note that including compressibility would lead to lower specific storage in general (the term 1 -K/K s would always be less than 1). Similar to the approach applied by Merritt (2004), this study uses the first form S s expression (Equation [D-1]) as given by Bredehoeft (1967).…”

“…Bredehoeft (1967) showed that the fluctuations in head in an open well produced by tidal dilatation were a function of the specific storage of the aquifer. This analysis assumed incompressible grains, a simplifying assumption that was later relaxed by various authors.…”

Groundwater Anomalies Associated with the 1995 Hyogo-ken Nanbu Earthquake.

“…The reasons why we utilized the questionnaire survey of hot or mineral spring waters are as follows: 1) hot or mineral spring water is generally confined, and confined groundwater is much more sensitive to volumetric strain changes than unconfined groundwater (Bredehoeft, 1967); 2) hot or mineral spring water comes from deeper sources than normal groundwater; 3) hot or mineral spring water usually flows up along cracks in basement rock in a fault zone and brings some information about such zones, and 4) people in general, are more interested in hot or mineral spring water than normal groundwater and this should result in a higher volume of quality data on the survey. The number of reported groundwater anomalies was 56, while 186 springs reported no anomalies.…”