HGI is adept at providing 2D and small scale 3D seismic reflection surveys. We typically employ the seismic reflection technique for surveys requiring higher resolution or greater imaging depths than seismic refraction can provide, or where the survey area is constrained. Although, this increase in resolution and imaging depth is typically associated with increased costs.
Seismic reflection surveys are commonly used for stratigraphic mapping, fault and fracture zone studies, landslide investigations, void and underground mine mapping, and resource assessments.
Reflection seismic methods are similar to refraction method as 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 used 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.
We have also partnered with large scale 3D seismic reflection providers, such as Bird Seismic Services, Inc., to provide data QC, logistical, and data collection services for surveys involving hundreds of channels of data. The figure below is from a recent HGI partnership with Bird Seismic for a large 3D seismic reflection survey. This survey was performed to map stratigraphy and faults at the PG&E El Diablo Canyon Nuclear Power Plant site in order to update their seismic hazard analysis. The survey consisted of about 800 channels of both wired and wireless geophone sensors using several vibroseis trucks as sources.