ACOUSTIC MONITORING OF SEDIMENT-PORE FLUID INTERACTION

Acoustic and chemical measurements were made on water-saturated synthe tic sediments undergoing diagenesis from uncompacted glass beads into a cemented glass bead, sandstone-like material. The sediment-water int eraction was investigated as a function of time by monitoring the acou stic properties, water electrical conductivity, pH,and ion concentrati on. A reduction in the transmitted acoustic wave amplitude was correla ted with a decrease in the total number of ions in the pore water. The lowest acoustic wave amplitude occurred when the ionic strength in th e pore water had reached a minimum value. Calculation of the electrost atic potential showed that the force of repulsion between beads was st rongest when the acoustic wave amplitude was at its minimum and the do uble layer electrostatic interaction potential between the beads was a t a maximum value. At longer times the acoustic wave velocity of the t ransmitted wave increased as the cementation progressed. These results suggest that the chemical interaction between the pore fluid and grai n alters the strength or stiffness of grain-to-grain contacts, which i n turn alters the elastic modulus of the sediment frame. The important implications of this work are the potential use of acoustic or seismi c techniques to monitor ion exchange between pore-fluids and sediments insitu.