Effect of volatiles on attenuation (<i>Q</i>⁻¹) and velocity in sedimentary rocks

Small amounts of water have previously been found to have a dramatic effect on the value of the seismic dissipation factor Q −1 . This suggests the importance of controlling the environment in which Q −1 measurements are made. Since the effect of moisture is so large, studying the variation in Q −1 with partial pressure of water and other volatiles can provide important clues for understanding the attenuation mechanisms in ‘room dry’ rocks. Thus measurements have been made of both Q −1 and velocity for several limestones (all &gt;99% CaCO 3 ) and sandstones. The Q −1 of both limestones and sandstones is affected by the presence of water vapor, but at any given partial pressure of water ( P H2o ) the Q −1 of the limestones is lower than the Q −1 of the sandstones. Velocities of the sandstones decreased by as much as 30% as the relative humidity was changed from 0 to 80%. Velocities of limestones of comparable porosity decreased only 2% over the same range. Simultaneously with the Q −1 and velocity measurements, adsorption isotherms (∼23°C) were measured on a companion sample. Generally, limestones adsorbed less water at a given partial pressure than the sandstones. The amount of water adsorbed, at most 2–3 mg/g,does not increase the density enough to account for the decrease in velocity, particularly in the sand‐stones. The Brunauer‐Emmett‐Teller equation was used to calculate surface areas from the adsorption isotherms. Surface areas ranged from 0.5 to 3.4 m 2 /g. Limestones had lower surface areas than the sandstones. The larger surface areas in the sandstones can probably be attributed to varying amounts of clays.A plot of Q −1 versus mass adsorbed shows that most of the increase in dissipation is associated with the adsorption of the first two monolayers of water. A fixed amount of water causes a greater change in the Q −1 of the sandstones than of the limestones. One sandstone was measured by using anhydrous benzene as the adsorbate. In contrast to the action of the water, Q −1 changed very little with increased benzene pressure for small P/P 0 , even though significant amounts of benzene were adsorbed. For P/P 0 greater than 0.7, there were significant increases in Q −1 . It appears that there are two mechanisms of attenuation in ‘dry’ rocks: one is observed at low P/P 0 and is associated with surface effects that are strongly dependent on the nature of the adsorbate, and the other is observed at high P/P 0 and is associated with fluid flow in fine pores.