GeoSonics/Vibra-Tech’s seismic reflection gives you deep insight when you need it most. Rely on us when you need to know more of what’s in store below the surface.
The seismic reflection technique is similar to the seismic refraction technique in that a sound wave is induced into the subsurface and recorded along a traverse line, resulting in depths to different reflectors. The advantage of seismic reflection over seismic refraction is that there is no assumption that velocity must increase with depth; therefore, low-velocity layers can be mapped beneath high-velocity layers, and deeper targets can be seen with greater detail at short spread lengths. Typical applications of seismic reflection surveys include depth to bedrock, depth to water table, fault location, and mapping buried stream channels and other irregular bedrock topography in more detail than the seismic refraction method.
Our low-frequency testing methods get high marks when it comes to locating underground resources.
A VLF survey uses the magnetic components of the electromagnetic field generated by existing radio transmitters broadcasting in the VLF (10-30 kHz) band. These transmitters are used for long-distance military communication throughout the world. Conductive bodies such as water-bearing fractures, buried metal pipes and certain ore bodies affect the direction and strength of the field generated by the transmitted radio signal, producing large secondary electromagnetic and magnetic responses that enable targets to be located at depths up to 100 feet. Features such as fractures or faults should strike toward a transmitting station so that the receiving antenna is oriented perpendicular to the potential targets, maximizing signal strength.
Need a sound way to improve your subsurface imaging? GeoSonics/Vibra-Tech takes our seismic reflection technique to the next level with seismic tomography.
Tomography is the iterative inversion of a matrix of material properties used to reconstruct a cross section or slice through a region. During a seismic tomography survey an array of sensors is deployed in a borehole or along the ground surface. Acoustic energy from individual source points located at multiple stations in another borehole or along the ground surface is recorded. The compressional or shear waves’ arrival times measured on the recorded seismograms define the material properties between the source and receivers. Variations in the material properties of the region caused by weathered bedrock, fracture zones, voids or solution activity will result in corresponding variations in the seismic velocity. The result is a tomogram – an image of the internal velocity structure based on the distribution from a set of array paths obtained at different viewing angles.
Want a fast, cost-effective, low-involvement imaging solution? GeoSonics/Vibra-Tech’s seismic refraction delivers more bang for your buck.
The seismic refraction technique induces a sound wave into the subsurface and measures the velocity of sound at intervals along a traverse line to obtain depths and velocities of various subsurface strata. The velocity of the strata provides an indication of the ease with which the material can be excavated. One advantage of the seismic refraction technique is that it allows you to determine subsurface conditions inexpensively over large general areas. Typical refraction survey applications include depth to bedrock; depth to water table; dig, rip, or blast assessment; fault location; mineral exploration; sand and gravel reserve assessment; and subsurface sinkhole and pinnacle location.
Electrical resistivity testing is the answer to a wide range of subsurface problems and objectives, and is becoming a logical, reliable tool for everyday use in engineering applications.
The electrical resistivity survey is based on the principle that the earth material being tested acts as a resistor in a circuit. After inducing an electrical current into the ground, we measure the ability of that material to resist the current. Since various earth materials exhibit characteristic resistivity values, they can be distinguished using this method. Factors that affect the resistivity of earth materials include degree of saturation, porosity, pore-fluid content, temperature, salinity, and thicknesses of clay or sand layers. Applications of the electrical resistivity method include locating aquifers, saltwater intrusions, and other groundwater contamination problems. We can also characterize bedrock by locating weathered zones, fractures, and voids attributed to solution activity, or determine depth to bedrock, and thickness of clay or sand layers. Electrical resistivity can also aid in evaluating soils for corrosively or their potential grounding capabilities.
Ground Penetrating Radar
Ground Penetrating Radar (GPR) is GeoSonics/Vibra-Tech’s most widely used and broadest range solution for seeing virtually anything below the surface.
Ground penetrating radar (GPR) is an established technology used for investigating shallow geologic and hydrologic features. GPR is extremely useful in locating man-made features such as buried drums, tanks, pipes and other metallic objects. Another popular use is locating rebar in concrete or detecting voids beneath concrete or asphalt. GPR operates on the simple principle that electromagnetic waves emitted from a transmitter antenna are reflected from buried objects having different electrical properties than the host material. The signals detected at a receiver antenna and recorded provide a detailed cross section of the subsurface that is similar in appearance to a seismic reflection record. The depth of penetration of the radar pulse is controlled by site conditions and the frequency of the antenna chosen.
Electromagnetic surveys are a simple solution for shallow underground imaging, with applications from archaeological and environmental to highway infrastructure conditions.
Electromagnetic conductivity is a method to locate buried materials having a high conductance. During a survey, alternating electromagnetic waves generated at the surface are induced into the ground. When the waves pass through a conducting body, they induce an alternating electrical current in the conductive materials. These currents become the source of secondary magnetic fields which can be detected at the surface. The strength of the field is directly proportional to the average conductivity of the subsurface materials. Typical electromagnetic applications include location of buried pipes, tanks, drums and metallic objects, as well as sludge wastes, leachate plumes, saltwater intrusions, acid mine drainage and other groundwater contamination problems. Other applications include quick and economical site evaluation of areas with variable bedrock topography such as those found in karst terrain, clay layer mapping, fault detection, and mine or quarry location assessment.
Magnetic methods, including magnetometry and microgravity surveys, are a popular, effective approach for near-surface metal detection – and GeoSonics/Vibra-Tech is synonymous with accuracy, quality and reliability in the field.
Magnetometry surveys are used to measure disturbances in the earth’s magnetic field generated by buried ferrous objects such as pipes, drums or a UST. Geologic structures such as igneous dikes or ore bodies can also cause local deviations in the earth’s magnetic field. The shape of the magnetic anomaly is an indication of the feature’s location and approximate depth of burial. The ease of data acquisition and low cost of such surveys make them ideal for delineating old landfills or buried debris, locating abandoned well heads, investigating the source of coal burns or acid mine drainage, detecting faults, and archaeological investigations.
Microgravity surveys are used to detect very small variations in the earth’s gravity field, such as those caused by near-surface features of engineering significance, including faults, buried river channels, fissures and solution cavities. Depth to bedrock and approximate fill thickness can also be measured if density values can be assumed for the material. The microgravity method is particularly useful in urban areas where other geophysical methods fail because of electrical interference or traffic noise.
(MASW) Multi-Channel Analysis of Surface Waves
New problems need new solutions. GeoSonics/Vibra-Tech’s multi-channel analysis is a modern solution to the growing problem of subsurface imagery in today’s noisy construction sites and urban environments.
Multi-channel analysis of surface waves (MASW) is a seismic method for the characterization of the shear wave velocity of the subsurface. The MASW technique is similar to seismic refraction in that a sound wave is induced into the subsurface and recorded along a traverse line that contains an array of sensors. However, MASW utilizes the dispersive properties of Rayleigh-type surface waves to determine the variation of shear wave velocity with depth. The obtained shear wave results can be used to determine the IBC Vs100 of the study area. Based on the 2000 International Building Code, IBC Vs100 involves classification of the upper 100 feet of an area’s rock and soil package into an average shear wave velocity value. In addition to IBC Vs100 site classification, typical MASW survey applications include foundation engineering, void detection, landfill and in-fill investigations, and stratigraphic and lithological studies.
Cross-Hole Seismic Surveys
Cross-hole seismic surveys have proven value for non-invasive investigations of geological structures and have become the most common method of measuring and detecting hazards beneath the surface.
In a cross-hole seismic survey, acoustic waves are generated in a borehole, then wave arrivals are sensed by geophones in at least two other boreholes at the same elevation. By determining the arrival of the compressional and shear wave, we can calculate their propagation velocities. A velocity log of the materials between the boreholes can be constructed by taking measurements at various depths. Knowledge of the site-specific compressional and shear wave velocities is used to determine the dynamic elastic moduli for the various layers. This method is typically used to characterize the elastic properties of subsurface materials for dynamic structural analysis. Other applications include evaluating anomalous conditions between two or more boreholes, such as abandoned mine workings, sinkholes, sand channels and other stratigraphic discontinuities.