We use the magnitude and centroid period of Rayleigh wave along with the amplitude of fluctuations of water level in a well to calculate effective porosity of a karst aquifer at the site scale. The radial and vertical displacements of Rayleigh wave are first related to the confining pressure of rock, which is then related to fluid pressure via the Gassmann equation. Three seismograms recorded at station 633A of the USARRAY and the induced responses of Well J-17 in the Edwards Aquifer (Texas) allow the calculation of an effective porosity between 17.0 and 24.4 percent, the average of which is close to the total porosity of core samples determined by geophysical well logs. This paper provides an innovative method to measure effective porosity in aquifers. Because of the long wavelengths of Rayleigh wave, the interdisciplinary approach is advantageous in that the resulting effective porosity is at the site scale which includes large conduits or voids.
Za izračun efektivne poroznosti kraškega vodonosnika na lokalni ravni smo uporabili velikost in centroidno časovno vrednost Rayleighjevega valovanja, skupaj z amplitudo nihanja vodne gladine v vrtini. Radialni in vertikalni premiki Rayleighjevega valovanja so najprej povezani s tlakom neprepustne kamnine, ki je nato z Gassmannovo enačbo povezan s tlakom tekočine. Trije seizmogrami, evidentirani na postaji 633A USARRAY, in inducirani odzivi vrtine J-17 v vodonosniku Edwards (v Teksasu) omogočajo izračun efektivne poroznosti med 17,0 in 24,4 odstotka, povprečje teh vrednosti pa je blizu poroznosti jedra v vzorcih, ki so bili določeni z geofizikalnimi meritvami v vrtinah. V prispevku je predstavljena inovativna metoda za merjenje efektivne poroznosti v vodonosnikih. Zaradi dolgih valovnih dolžin Rayleighjevega valovanja je interdisciplinarni pristop ugoden, saj efektivna poroznost obravnavanega območja vključuje velike kanale ali praznine.
Keywords: Gassmann equation; Poisson’s ratio; Rayleigh wave; displacements; well head.