Seismic methods involve the measurement of elastic waves traveling through the subsurface.  Stratigraphy, structure, and material properties can be assessed with seismic methods.  Three of the most useful seismic methods for shallow applications include refraction, reflection, and multi-channel analysis of surface waves (MASW).

Seismic Refraction is a method to determine the P-wave velocity structure of the subsurface and the depths to layers with a significant change in P-wave velocity (e.g. sediment to rock).  The P-wave velocity Vp is dependent upon the bulk modulus, the shear modulus and the density (ASTM D5777-00).  Seismic P-waves are generated on the surface by an energy source such as a simple sledge hammer. The P-waves propagate through the soil and rock, and when the seismic waves encounter interfaces between materials of different seismic velocities, the waves are refracted according to Snell’s Law. The seismic wave will travel along the interface with a velocity of the underlying (faster) layer. An array of geophones is placed along the survey line and a seismograph is used to record the travel-times of the seismic signals.

Applications

• Primary application is for determination of depth to bedrock
• Mapping geologic strata and anomalous conditions
• Rippability and weathering of the rock can be determined from P-wave velocity measurements
• If compressional (P-wave) and shear (S-wave) velocities are measured, in-situ elastic moduli of soil and rock can be determined

Advantages

• Provides data to depths of 100 feet or more
• Provides a 2D cross-section of P-wave velocity
• Resolves up to 3 or 4 layers
• The source of seismic energy can be as simple as 8-pound sledgehammer

Limitations

• The survey line length (source to farthest geophone) may be 4 to 5 times the desired depth of investigation
• Sensitive to acoustic noise and vibration
• Seismic velocity of layers must increase with depth (will not resolve low velocity layers below high velocity layers)
• Will not detect thin layers
• Deep measurements may require explosives as an energy source

The Seismic Reflection technique measures the two-way travel time of seismic waves from the ground surface downward to a geologic contact where part of the seismic energy is reflected back to geophones at the surface (ASTM 7128-05).  Reflections occur when there is a contrast in the density and velocity between two layers.  The reflection method provides a high resolution cross section of soil/rock strata along a profile line.  For geotechnical and environmental work, reflection measurements are typically made from about 50 to 1,000 feet deep on land.  More detailed (higher-frequency) and shallower data can be obtained on water.

Applications

• Primary application is for determination of depth and thickness of geologic strata
• Mapping structural and anomalous geologic conditions
• Recent applications have attempted to use higher frequencies to identify smaller targets such as mines, tunnels, and caves

Advantages

• Provides a high resolution cross-section of soil/rock along profile line
• Depth range as shallow as 50 feet to greater than 1,000 feet
• Both P-waves and S-waves can be measured with the appropriate equipment

Limitations

• Slower production rate than most geophysical methods
• Requires extensive processing
• Land measurements generally not suitable for imaging upper 50 feet
• Sensitive to acoustic noise and vibration

MASW uses the dispersive characteristics of surface waves to determine the variation of shear wave (S-wave) velocity with depth.  Data are acquired by analyzing seismic surface waves generated by an impulsive source and received by an array of geophones.  A dispersion curve is calculated from the data that shows the phase velocity of the surface waves as a function of frequency.   A shear wave velocity profile (1-D profile of velocity as a function of depth) is then modeled from the dispersion curve.  The resulting shear wave profiles from multiple locations along a survey line are combined and contoured into a 2-D cross-section of shear wave velocity.  Seismic shear-wave velocity is a key parameter for determining the elastic properties of soil and rock for geotechnical investigations.

Applications

• Determining the depth and thickness of stratigraphic layers
• Identifying low-velocity (weak) zones such as voids and sinkholes
• Determining soil and rock elastic properties

Advantages

• Provides a cross-section of shear-wave velocity along a profile line
• Can identify low-velocity (soft) layers below higher velocity (hard) layers
• Data can be acquired with a “landstreamer” where the geophones are towed along the ground for fast acquisition
• Multiple geophones provide greater signal-to-noise ratio than more traditional SASW methods.
• Measurements can also be made in water-covered areas

Limitations

• Maximum depth limited to approximately 100 feet in typical conditions
• Lateral resolution is typically 25 to 50% of the geophone spread length
• Poor quality in areas with shallow, sharp velocity contrasts (better in areas with gradational velocity changes)