Acoustic tomographic difference imaging of dynamic stress fields

Acoustic tomographic imaging was conducted on a complex stress field generated during indentation into a Berea sandstone core sample. The imaging process used 130 ultrasonic raypaths and 97 pixel elements to construct tomograms perpendicular to the indentation direction. 'Zeroed velocity' acoustic difference tomograms were made by subtracting the initial velocity tomogram from the seven successive indentation tomograms up to stresses of 110 MPa. The tomograms illustrate a marked development of a high velocity zone beneath the indenter. The high velocity maximum continues to increase with increasing application of indentation stress. At the highest indentation stress levels the rate of change in velocities with added deformation is lower than at low stress levels. This observation is attributed to the closure of cracks and pore spaces at high stress levels. These results indicate that it may be possible to image changes in stress fields created during petroleum extraction and production operations.