Quantitative Acoustic Microscopy

Abstract

Rocks are composites made of grains that are cemented together by different materials. The strength and seismic properties of the rock depend on the stiffness of the cement. Traditional techniques, such as scanning and transmission electron and optical microscopy can help identify variations in type of cement. However, they provide qualitative descriptions only. We demonstrate here applications of scanning acoustic microscopy (SAM) to quantify microstructure as variations in acoustic impedance. The acoustic maps can then be used to understand the seismic response of the rock. For example, ultrasonic measurements on kerogen shales have shown that the elastic modulus increases with increasing kerogen maturity. SAM of these shales shows a stiffening of the shale framework with increasing kerogen maturity. The load-bearing material changes from low impedance kerogen to high impedance shale. The impedance values determined from SAM are comparable with those obtained from ultrasonic time-of-flight data. In contrast, the cement is load bearing in sandstones where high impedance quartz grains are cemented together by low impedance clay. The cement determines not only the total impedance of the rock but also its pressure dependence.