Feasibility of acoustic imaging for in-situ characterization of subsurface soil injected with fresh mortar

This paper investigates the potential of acoustic imaging technologies to determine the spatial geometry of the soil–fresh mortar interface (water–cement ratio > 2). The proposed study is first devoted to the characterization and monitoring of the acoustic properties of this evolving material. They were analysed in the laboratory using a specifically-designed experimental device for eight hours, in order to assess the evolution of the mixed material as a function of its maturity. The measured seismic dataset shows the very low P wave velocities and their evolution as a function of time (hardening), their large attenuation and low dispersion characteristics in the [0.5–6 kHz] frequency range. In particular, seismic attenuation remains the lowest when mortar mixture is younger than 90 min, which consequently constitutes the optimum time window for imagery purposes. A comparison between the measured properties and existing theoretical models shows that the material can be considered as a fluid mixture with solid particles in suspension, which exhibits poorly compressible elastic behaviour during the first 8 h. In a second part, numerical seismograms are generated to assess the reflected P wave properties as a function of offset for a mortar/soil interface. Such study permitted to establish the optimum source–receiver configuration, which results from a compromise between acquiring large amplitudes for signal to noise ratio issues (reflectivity, attenuation), while to correctly analyse the P wave velocity using the hyperbola of the reflected wave. A correct assessment of the in-situ velocity, which depends on both the mixture characteristics (i.e. the soil conditions) and the mortar maturity, is a necessary step towards a correct 2D image of the interface.

Research Highlights
► This paper investigates the potential of acoustic imaging technologies to determine the spatial geometry of the soil-fresh mortar interface (water cement ratio > 2).
► The first part was dedicated to assessing the seismic characteristics and behaviour of fresh mortar mixture and its time-dependence after mixing.
► In a second part, we focused on the search for the best source-receiver configuration to detect and characterize the interface between in situ soil and mortar, via reflected waves.