Real-time ultrasound process tomography in pipelines

The primary descriptor of the interaction of ultrasound with a material is its acoustic impedance. This is related closely to density and hence variations in density can be sensed, e.g. a piped liquid containing gas bubbles. The greater the difference in acoustic impedance at the interface, the greater will be the amount of energy reflected. Conversely if the impedances are similar most of the energy is transmitted. Reflections may provide a basis for process tomography where the materials of interest have distinctly different acoustic impedance values; water and air, for example, fulfil this requirement. The complete interaction behaviour is complex and depends not simply upon the differences in acoustic impedance, but on the size and shape of the interface. These various interactions lead to three basic modes of ultrasound tomography: transmission mode; reflection mode; and diffraction mode