Abstract
A state-of-the-art data acquisition system has been built at Delft University of Technology which allows rapid acquisition of reflection, pulse-echo and transmission records during hydraulic fracturing experiments on cement or rock samples. Recent experiments show that it is possible to observe clear acoustic signals from a hydraulic fracture that is very thin compared to the wavelength of the illuminating elastic waves. Imaging with reflected compressional signals produces a clear image of the fracture. The signals diffracted from the fracture tip enable accurate calculation of the radius-time diagram. The variation of the amplitude of the compressional waves transmitted through the fracture is mainly caused by the interference between transmitted and scattered wave-fields. However, a subtle difference between numerically calculated and observed wavefields suggest that it might be possible to infer the length of the non-penetrated zone. The material type has a major influence on the fracture visibility after shut-in.
