Various methods can characterize microstructural properties of reser- voir rocks with the ultimate goal of relating microstructure to seismic properties. Scanning electron microscopy, transmission electron microscopy, and optical microscopy have traditionally been used for such studies. They have identified lithol- ogy, pore space, interconnectivity of pores, grain size, and cementation as the most important factors control- ling seismic wave velocity and atten- uation. However, these techniques provide qualitative descriptions only. The acoustic techniques presented here, scanning acoustic microscopy (SAM) and acoustic sounding (AS), can map and, more importantly, quantify microstructure as variations in acoustic impedance. Ultrasonic stress waves are sen- sitive to local variations in elastic properties and are therefore particu- larly suited for characterizing microstructural properties of reser- voir rocks. Reflections from imped- ance boundaries in grains and between interfaces in the sample are used to construct the microstructural image. This paper will show that acoustic microscopy can be a powerful tool for studying internal structure and pore geometry of reservoir rocks. Working principles. Acoustic microscopy’s basic principle is almost identical to that of reflection seis- mology. Images of surface and sub- surface microstructures are prepared on the basis of reflected acoustic waves—that is, on the impedance changes in the sample. Acoustic waves on a sample are mode con- verted, partly transmitted into the sample, and partly reflected. The reflection coefficient and with it the signal intensity received by the trans- ducer are determined by the elastic constants of the material. Changes in acoustic impedance in the sample that influence wave reflection char- acteristics can be studied by map- ping the reflected waves. Because the working frequency of the acoustic waves can be varied, their penetration depth into the sam- 172 THE LEADING EDGE FEBRUARY 2001 FEBRUARY 2001 THE LEADING EDGE 0000 Mapping impedance microstructures in rocks with acoustic microscopy MANIKA PRASAD, Stanford University, Stanford, California, U.S. Scanning system Monitor Image storage Receiver HF pulse generator switching device coupling fluid sample Pulse generator & receiver unit Acoustic & scanning unit Image handling & stage control unit sapphire rod transducer X Z Y plane wave fronts acoustic ray Figure 1. Diagram of an acoustic microscope. The Acoustic & Scanning Unit consists of a transducer, the acoustic lens, and a movable sample stage. The Pulse Generator & Receiver Unit emits short pulses to excite the transducer and receives and amplifies the reflected acoustic signals regis- tered by the transducer. The Image Handling & Stage Control Unit controls stage motion and transforms the transducer signals to color scaled images with 512 pixels in x-y(z) directions with 8-bit resolution (256 colors). The transducer acts as a generator and receiver of acoustic waves. Acoustic waves reflected from sample surface and subsurface features carry imped- ance information. An acoustic image is made by registering the acoustic information as the acoustic lens carrying the transducer is scanned over the sample in x and y directions. Z or t Y X C - scan B - scan A - scan A E B C F G D a a’ Figure 2. Types of scans obtained from acoustic imaging. An A-scan (a-a’) is an x-t scan similar to a seismic trace. B-scan “seismograms” (ABCD) are made with a moving (scanning) source and receiver. These scans are made by recording several A-scans through a time window with a fixed width along a line, similar to a seismic profile. In the figure, the dashed lines are schematic traces of layers, which can produce signals in the A-scan. A com- posite 3-D seismic image is created by combining profiles (B-scans) made at various locations (y positions). C-scans (ABEF) are x-y scans made at dif- ferent, user-selectable time windows. These time windows correspond to depths (z positions) in the sample.