Enviroscan performs bedrock depth profiling or mapping for a variety of geotechnical and environmental purposes - to facilitate design of balanced grading plans, to allow estimation of rock excavation volumes, to aid in proper pre-mobilization selection of excavation or drilling equipment, to provide data on the mechanical properties of soil and rock, to delineate the bases of buried valleys for well siting, and for detection of karst-related bedrock depth variations.

At shallow depths (i.e. less than approximately 100 feet), seismic refraction is typically the method of choice.  This method utilizes the “first arrival” of seismic energy from a hammer blow or explosion (“shot”) to an array of ground motion sensors or geophones.  The speed at which the seismic energy travels varies with (primarily) the density of the subsurface materials.  Therefore, the pattern of travel times from several shot points to the geophone array can be mathematically inverted to provide an accurate profile of seismic velocity (i.e. primarily density) variations beneath the geophone array.

If travel times are measured using both compressional or primary (P) and shear or secondary (S) waves, the elastic properties (i.e. Young’s modulus, bulk modulus, shear modulus, and Poisson’s ratio) of the material can also be determined.  For known soil or sediment types (e.g. sand vs. clay vs. silt), S wave data can also be converted to standard penetration test (SPT) “N” values for use in estimating bearing capacities.  For known rock types, P wave data can be used to estimate ease of excavation or rippability.

At greater depths (i.e. greater than approximately 100 feet), or in marine or water-saturated environments, seismic reflection may be preferred. Seismic reflection involves analysis of the entire wave train of seismic energy arriving at an array of geophones from a shot point.  As with seismic refraction, both P and S waves can be employed, allowing profiling of subsurface variations in density and/or elastic parameters, N values, and/or rippability.

Where boreholes are available, hole-to-hole or hole-to-surface seismic tomography data can also be collected (using downhole geophones and a sparker or airgun source) to improve the resolution of either seismic refraction or reflection surveys.

In selected situations, it may be preferable to perform bedrock depth profiling or mapping using microgravity measurements or electrical imaging.  Microgravity measurements are sensitive to the subsurface mass distribution, and can therefore discriminate between locations underlain by dense rock at shallow depths, and those where there is a significant thickness of less dense sediments. Where electrical imaging is used for rock depth profiling, it is important to remember that the method is actually sensitive to variations in electrical conductivity of subsurface materials, and while rock is often less conductive than soil or sediment, there are rocks (e.g. clay-rich, mineralized or water-saturated) that may be more conductive than some soils (e.g. dry gravel, sand or silt). Note that neither microgravity nor electrical imaging methods are capable of providing the elastic parameters and rippability information that are available from seismic data.