## 3 Results and Discussion
The technique can monitor the soil's health in terms of relative content of air, water (effective elasticity), and foreign objects (Appendix A). The significant advantage of the EBME void detection is the requirement of compact equipment set up on the ground. Almost all the existing sonic soil void detecting techniques use a bistatic setup. To place the emission source and receiver on the ground required more space. Some techniques require either sound wave emission source array or detector array on the ground, which the preparation is time-consuming. In addition, the alignment of the array is important to obtain accurate results. The non-flat ground condition would introduce non-negligible uncertainty to the detection. Since the EBME technique uses a high penetrating low-frequency sound wave, the probe is not necessary to contact the ground. The air layer between the probe surface and soil does not decrease the signal-to-noise ratio a lot. In this way, the limitation from the non-flat ground surface condition could be overcome. A sound wave array system would be preferred as it provides a 2D surface scan. Using EBME, the 2D surface scan could be carried out through a raster scan since it is independent of the ground surface condition and does not require any contact with the ground surface. Most seismic methods apply transversal mode sound wave or radiational stress, which approaches a limitation of wave propagation in the fluid such as underground water. Since most fluids do not have a shear modulus to transmit transverse mode vibration, the existing seismic methods have difficulty determining the underground structure and properties once the underground water layer exists. The EBME technique uses longitudinal mode wave, which can propagate in both solid and fluid media to overcome the limitation of the underground fluid propagation issue compared to existing seismic methods. Ground-penetrating radar is another non-destructive method in soil void detection. The radar emitted an MHz microwave into the soil and collecting reflection. Since the electromagnetic wave has a shorter wavelength, the resolution of ground penetrating radar is usually appreciable. However, the increased resolution by small wavelength introduces large noise as a tradeoff. The wavelength of the low-frequency EBME technique can minimize noise since it is employing a long-wavelength sound. Instead of using a long-wavelength sound wave to detect the comparable size of soil void, the EBME technique uses a relatively scaled effective bulk modulus and effective density to estimate the volume fraction of the void inside the soil by using a rapid non-contact raster scan. Once the porous soil's target area was determined, a higher frequency sound wave detector [22], acoustic lens/collimator [23; 24; 25; 26], or electromagnetic radar could be applied in the region to find detailed information of the specific voids for further interest. This work showed the initial feasibility of underground acoustic detection. Real experiments will be further performed as future works in the lab scaled-down condition and further in real condition with the influence factors studies such as vegetation on the ground surface and unknown solid objects in the soil layer.