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Description
Internal erosion can produce localized losses of material inside fluid-saturated porous structures and detecting such hidden defects from outside the structure is difficult. This work studies, theoretically and experimentally, the transmission of normally-incident ultrasonic waves through a fluid saturated porous medium in which a thin solid layer is embedded at the midplane to stand in for the missing material. The thickness of the solid layer is treated as a control parameter. Each porous layer is described either within Biot's single-porosity framework or within the Berryman–Wang double-porosity extension, and the multilayer boundary-value problem is solved using a global matrix method. Transmission spectra are computed as functions of frequency over a range of solid-layer thicknesses.Inserting the solid layer changes the transmission response in two ways: the frequencies of the transmission extrema are shifted and the amplitudes across the band are modified. A peak-frequency sensitivity is introduced to quantify the frequency shift and the amplitude change gives a complementary indication of the internal discontinuity. Measurements on water-saturated glass-bead (single-porosity) and Robu sintered borosilicate glass (double-porosity) samples, with and without a PVC plate inserted at mid-height of the column, show good agreement with the model within the transducer bandwidth. The combined dependence of peak positions and transmission amplitudes on the internal-layer thickness suggests that such measurements can be used as a practical indicator of internal discontinuities in saturated porous structures.