Effect of Partial Water Saturation on Attenuation Characteristics of Low Porosity Rocks

Abstract

Keywords Wave attenuation Partial water saturation Compressional wave Longitudinal wave Low porosityrock1 IntroductionWith the advance of geophysical exploration techniques, insitu measurements on elastic wave velocities have beensuccessfully employed for investigating near-surface anddeep geological structures. However, in situ techniques thatmeasure the attenuation of elastic waves are still in theirinfancy due to the difficulty of calibration, the lack oftheoretical models, and the inaccuracy of field scale mea-surements. While attenuation itself is strongly related toimportant physical characteristics of porous media, such aspore fluid composition, stress states, and internal hetero-geneity (Johnston et al. 1979), the understanding of theattenuation characteristics of porous media is still limited.The rocks are frequently exposed to water through rainor ground water. The presence of pore water not only playsan important role from an engineering perspective, but alsosignificantly alters acoustic wave propagation. Compres-sional wave velocity changes little until the pore spaces arefully saturated with water because the air in partially sat-urated pore fluids diminishes the stiffness of the pore fluidsand hardly contributes to strengthening the rock frame. Onthe other hand, when there is an increase in water satura-tion, the compressional wave attenuation—that is related toenergy dissipation—tends to increase more sensitively thanvelocity (Gardner et al. 1964; Toks}oz et al. 1979; Mavkoand Nur 1979; Murphy 1982; Winkler and Nur ;Cadoret et al. 1998).Several previous researchers have investigated the fluideffect on the wave velocity and attenuation, mainlyfocusing on high porosity rocks (mostly greater than 20%of porosity), such as limestone (Cadoret et al. 1998) andsandstone (Murphy 1982; Winkler and Nur ). Thisstudy, therefore, examines how water saturation affects theattenuation characteristics of low porosity rocks. The effectof partial water saturation on the attenuation is explored byrecapitulating Biot model and by conducting a series oflaboratory tests. Then, the results of Biot model and lab-oratory tests are compared and some implications arediscussed.2 Recapitulation of Biot Model: Partial WaterSaturation Effect on AttenuationBiot model (Biot 1956a, b) can be applied to porous media,particularly for modeling wave velocity and attenuation ofa rock. Biot model assumes that pore fluids flow only in thedirection parallel to the direction of the wave propagationand that the viscous fluid motion follows Poiseuille flow ina cylindrical tube. No chemical, electrical, or thermal