Seismic refraction is a geophysical method commonly used to investigate shallow subsurface structure / model by utilizing acoustic wave propagation (Vp) on a medium (soil/rock). Acoustic wave sources emitted on the surface (by sledgehammer, weight drop, etc.) will propagate down the surface through medium and then will be eflected/refracted back to the surface. Seismic wave returning to the surface then will be received by the receiver array.
After getting the travel time value on the first recorded wave on the receiver sensor, the determination of the subsurface wave velocity (Vp) model can be determined using several methods, including conventional methods (hagiwara, plus-minus, or GRM) or tomographic inversion methods. The Vp value is associated with medium incompressibility. Higher value of Vp correspond to ability of the medium to withstand compressional force (associated with hard rock), and vice versa, a low Vp value will indicate less compacted medium (or associated with soft rock). Seismic refraction methods are also widely used as an indirect method in determining rock rippability.