Sonar or ultrasound is a method that uses the reflection and refraction behavior of acoustic waves at layer boundaries and inhomogeneities. The elastodynamic wave emitted by the actor propagates through the rock and ultrasound sensors record the waves that are reflected at material boundaries. The contrast of the acoustic impendances of two different media, which are a product of the wave‘s velocity and material density, determine wave‘s the reflection and transmission behavior.
Compared to seismic waves, the significantly higher frequencies of ultrasonic waves yield a much higher resolution facilitating the detection of small scale inhomogeneities like fissures and fractures in a material. At the same time the attenuation of high frequency signals reduces the penetration depth to few meters to decimeters (depending on the material).
Besides the application for the detection of air- or gas-filled inhomogeneities ultrasonic waves can also pass through zones of increased moisture almost without attenuation which is why this method is suitable for the analysis of areas within or behind zones moisture.