Inversion scheme for multichannel impact echo measurements

Abstract

Conventional imaging techniques from medical ultrasound and seismic imaging are not suitable for multilayered domains where there is no separation between received reflection events. This happens in layered structures where the thickness of the individual layer is small with respect to the induced wavelengths. An example of these structures is the wall of a typical blast furnace. Imaging these domains require a method based on the interference pattern in the frequency domain. This method is already widely used for imaging concrete structures. However, these consists of only one or two layers. This study introduces a FullWaveform Inversion scheme which extends the usability of themethod to multilayered domains. Itminimizes the error between measurements and a forward model. To reduce the computation load, a simplified forward model based on a plane wave response in a vertically layered medium is adopted. Pre-processing steps are necessary to remove the effect of the source wavelet and to enhance the peaks and troughs positions. A sensitivity analysis provides information about the usability of the method. This shows only layers with standing wave frequencies in the range of the induced source wavelet can be imaged. Furthermore, results show it is impossible tomeasure the wave velocity and thickness of a single layer simultaneously given our simplified forward model. Multichannel measurements over an aperture of 500 mm are required to transform a spherical wave response to its plane wave equivalent in order to fit the forward model. Tapering the sides of the aperture is essential to reduce simulation artefacts. The inversion scheme is successfully tested on synthetic data of a blast furnace wall in the acoustic and elastic domain using realistic simulations of the measured data.