Seismics

Seismic methods measure acoustic velocities (the speeds at which sound waves propagate) in earth materials.  There is a direct relationship between acoustic velocity and material density (dense materials propagate sound faster than loose materials); therefore, seismic velocities can be interpreted to determine the subsurface conditions below a survey area.

Example seismic profile.

Seismic refraction

The seismic refraction method is based on the measurement of the travel time of seismic (sound) waves refracted at the interfaces between subsurface layers of different velocity.  A seismic wave is introduced into the subsurface via a shot point using explosives (blank shotgun cartridge), hammer blow, dropped weight or an elastic wave generator.  Energy radiates out from the shot point, either traveling directly through the upper layer (direct arrivals), or traveling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface at a series of receivers (geophones) spaced at regular intervals.  Beyond a certain distance from the shot point, known as the cross-over distance, the refracted signal is observed as a first-arrival signal at the geophones (arriving before the direct arrival).  A seismograph records the travel time for the energy to travel between source and receivers.  In most refraction work only the first P-wave arrivals are recorded, providing depth information of interfaces.  However, techniques can be used to record the arrival of the shear (S-) waves, which provide additional data about engineering properties of the subsurface media.

Schematic showing aspects of a seismic survey.

Seismic refraction methods rely on the tendency of acoustic velocities to increase with depth, which can make it insensitive to low velocity layers in the subsurface.  Travel times are interpreted to compute velocities of, and depths to, materials at various interfaces.  Refraction data are presented as cross-sectional plots representing P-wave path(s), velocities and depths to various interfaces.

Seismic Software Graphic.

Seismic refraction has many applications.  In geotechnical engineering and mining applications, it is used to determine depth to and rippability of bedrock for design and cost estimates.  Groundwater applications include mapping bedrock channels, identifying faults and fracture zones, and delineation of geologic boundaries to constrain hydrogeologic models.

Refraction Seismics results plot.

Seismic reflection

Reflection seismic methods are similar to the refraction method described above; travel times are recorded for an induced seismic wave reflected from subsurface interfaces to reach an array of geophones placed at known distances from the source.  Reflection of the transmitted energy will only occur when there is a contrast in the acoustic impedance (product of the seismic velocity and density) between these interfaces.  Since we are recording reflections the seismic waves travel a much shorter distance in the subsurface. Consequently the seismic waves possess higher frequencies, potentially leading to higher resolution.  Seismic energy is introduced using the same techniques as for the refraction surveys, and reflected travel times to various interfaces are recorded on a seismograph.

Travel times are a result of seismic velocities of all subsurface materials between the surface and a particular interface, but not their differences; therefore reflection techniques can be used to find depths to less dense materials beneath denser strata.  Further, because reflected waves occupy a shorter horizontal distance, the reflection method can reach greater depths with less energy than the refraction method.  However, seismic reflection is more sensitive to interference and so the method is not always suitable at noisy sites.

Seismic reflection surveys are commonly used for groundwater investigations, faults studies, landslide investigations, and resource assessments.