Multi-Channel Analysis of Surface Waves (MASW)
The MASW method is a seismic technique that is commonly used to evaluate the in-situ S-wave velocity distribution of overburden soils and the underlying bedrock. The method utilizes surface seismic wave (Rayleigh/Love waves) energy to derive a dispersion curve (phase velocity versus frequency) which can be mathematically inverted to generate a one-dimensional S-wave profile at each measurement station. Two dimensional profiles can be generated from multiple linear sounding locations.
Newer MASW developments include using multiple components of geophone orientations (vertical, in-line radial horizontal, and transverse horizontal). Other advances include generating radial particle motion surfaces, and/or horizontal to vertical curves for joint inversion.
A land streamer spread can be substituted for land geophones to perform MASW surveys on road pavement or other hard surfaces.
Vs contrast between gravel deposit and host material
Seismic Refraction Survey
The seismic refraction survey method is commonly used to measure variations in the speed of acoustic compressional wave, or P-wave, propagation in layered earth.
For the seismic refraction method, an acoustic wave that is typically generated at ground surface propagates into the subsurface soil and rock. Upon encountering boundaries having contrasting mechanical properties (i.e., density, elasticity and, consequently, seismic wave propagation speed) the acoustic wave pulse is partially refracted and partially transmitted into underlying strata. The ray-path of the incident-transmitted pulse is bent, or refracted, at the boundary in accordance with Snell’s law.
By measuring the elapsed time between initial pulse generation at the shot-point and the arrival of the refracted waves at surface, layer speed and thicknesses can be determined.
The seismic refraction method generally requires an increase in seismic velocity with depth. A high velocity layer overlying lower velocity layers results in a “velocity reversal”; thicknesses and velocities of the underlying low velocity layers may not necessarily be accurately determined by seismic refraction analysis. Velocity reversals can result from a relatively dense layer (e.g., clays of high plasticity) overlying less dense layers (e.g., silt, sand, etc.).
Overburden thickness and rock P wave velocity
