Determining Material Changes on the Centimeter and Meter Scales with Diffuse Elastic Waves

We present two experiments that use repeatable sources and permanent sensors installations to monitor changes in the propagation medium that are introduced in a controlled way during the experiment. In the laboratory experiment with a concrete sample of about 0.5m^3 volume, ultrasound around 100 kHz is used to excite diffuse elastic waves. Medium changes are introduced by careful drilling of holes with increasing depth and diameter. In the second experiment the excavation of a new gallery in a mine is monitored by permanently installed and precisely controlled vibroseismic sources that act around 1 kHz and a set of permanently installed three component geophones. The excavation process allowed multiple measurements during the progression of the gallery. In both experiments the alteration of the wave field is larger in the coda of scattered waves than in the early ballistic part. This effect is modeled under the assumption of diffuse wave propagation. Despite the length scales of the experiments the differing scattering strength plays a major role in fitting the data.