Effect of volatiles on seismic attenuation and velocity in sedimentary rocks

Abstract

Small amounts of water can have a dramatic effect on the seismic quality factor, Q; this suggests the importance of controlling the environment in which Q measurements are made. Because the effect of moisture is so large, studies of the variation in Q with partial pressure of water (P[subscript H[subscript 2]O]) and other volatiles can provide important clues for understanding the attenuation mechanisms in "room dry" rocks. Measurements of both Q and velocity, as a function of P[subscript H[subscript 2]O], were therefore made for several limestones and sandstones. Although the Q of both limestones and sandstones was affected by the presence of water vapor, at any given pressure the Q of limestones was higher than the Q of sandstones. Velocities in the sandstones decreased by as much as 30% as the relative humidity (R.H.) was increased from 0% to 85%; this was caused by softening of the clay between the grains. Velocities in limestones decreased by only 2% as water was added. At the same time the Q and velocity were measured, adsorption isotherms were also measured on a companion sample. In general, limestones adsorbed less water at a given pressure than did sandstones. The amount of water adsorbed, which was at most 2-3 mg/g, was not enough for changes in density to account for the decrease in velocity. Surface areas, which can be calculated from the adsorption isotherms, ranged from .5 m²/g to 3.4 m²/g; limestone samples had lower surface areas than sandstone samples. Clay content of the sandstones probably determines their surface areas. Although most of the increase in attenuation occurred when the first two monolayers of water were adsorbed, there was no correlation between the surface area and the magnitude of the change in Q⁻¹. Sites where the water molecules are adsorbed, rather than the amount of water, must determine the magnitude of the change in Q⁻¹. ...