Variations in ultrasonic wave velocities of Miocene carbonate and clastic sedimentary rocks under dry and fully water saturated conditions

Abstract

This study investigates the variations in P and S wave velocities (Vp, Vs) under dry and saturated conditions and attempts to develop empirical equations estimating Vs based on Vp. In this context, 48 intact core specimens extracted from the Miocene sedimentary rocks were examined. A significant decrease was observed in Vp under saturated conditions (Vp-sat) for several specimens. Thus, selected samples were further examined by thin section, X-ray diffraction, and scanning electron microscopy equipped with an energy dispersive spectrometer. Correlation analyses were performed to reveal the causes of reduction in Vp-sat in terms of porosity, microcrack density, and mineralogical content. Vs was also observed to have a decreasing trend under saturated conditions. Strength and elasticity loss after saturation were not well correlated with the reduction in Vp-sat. The measured velocity values were also compared with the ones derived from Gassmann’s theory. Despite the discrepancy in the literature, a new parameter explaining the negative difference between the values of saturated and dry Vp was suggested. The mudstone texture and the abundant microporosity were firstly declared to be indicative of a negative velocity difference among the same rock group having similar mineralogy. Afterward, two empirical equations estimating Vs were derived from the ultrasonic wave velocities measured in this study and the data compiled from published literature. These equations were applied to a different dataset and compared with the ones suggested by various researchers. Proposed equations were determined to be more widely applicable and valid for limestone, mudstone, and sandstone samples according to several accuracy metrics.