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GEOPHYSICAL METHODS

Geophysical methods are low-cost, non-destructive subsurface characterization and monitoring techniques that work through applying a specific kind of energy to the ground, to measure the variations in the physical properties of subsurface materials.These measurements are then processed and converted into a continuous subsurface 2D/3D model. Geophysics is one of the quickest site investigation techniques, however, it requires a lot of data processing. It is always recommended to apply at least two subsurface investigation methods for each application.

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VALLCEBRE LANDSLIDE 3D MODEL

Geotechnical 3D model using borehole data and geophysical data (ERT & SR)

ELECTRICAL RESISTIVITY

The Electric Resistivity Tomography (ERT)  works by injecting an electric current into the ground and then measuring the resulting potential field using electrodes set on its surface. ERT is a very powerful method for characterizing the sub-surface materials by the variation in their electrical properties which represents variations in lithology, porosity, permeability, water content, and conductivity ...etc.

Applications

  • Locating the extent and depth of buried landfills

  • Mapping and monitoring leachate plumes

  • Mapping and monitoring groundwater pollution

  • Determination of depth to bedrock

  • Locating dissolution features

  • Stratigraphic mapping

  • Locating buried channels, dykes, ore bodies, and other geological features

  • Slope stability assessments

  • Cross-hole tomography

  • Time-lapse infiltration studies

  • Assessment of aquifer heterogeneity

SEISMIC REFRACTION

Seismic survey methods are widely used for investigating the geological structure and rock properties. The technique involves introducing artificial seismic energy into the subsurface (using a hammer, free-fall weight,...etc.) and measuring the refracted waves between layers of contrasting seismic signals. The results of the seismic refraction survey are usually presented in the form of seismic velocity boundaries on interpreted cross-sections.

Applications

  • Stratigraphic mapping

  • Geological mapping

  • Estimation of depth to bedrock

  • Locating sinkholes & channels

  • Predict rippability of specific rock types

  • Landfill investigations

  • Estimation of depth to the water table

  • Geotechnical investigations

MAGNETIC SURVEY

The magnetic survey technique is based on mapping variations in the earth's magnetic materials, which range from natural magnetic minerals to man-made objects, which opens a wide range of applications in civil engineering, Archeology, and mineral investigation, the magnetic surveys can be applied on small scale or large scale. It is carried out using a portable instrument that measures the earth's magnetic field to very high precision.

Applications

  • Archeological studies

  • Mapping underground storage tanks.

  • Detecting piles and reinforced concrete

  • Detection of underground pipes and other utilities

  • Locating landfill boundaries

  • Detection of buried infill

  • Mineral exploration

  • Geological studies

  • Mapping landmines and unexploded ordnance

GPR

Ground Penetrating Radar (GPR) is a non-destructive testing method that penetrates an electromagnetic signal into the subsurface and measures the amplitude and the travel time of the returned signal. GPR is a very common technique as it provides high-resolution cross-sectional images of the scanned surface, especially for shallow investigation.

Applications

Geotechnical

  • Location of rebar/post-tension cables

  • Utility and conduit mapping

  • Slab thickness/concrete cover

  • Concrete integrity/void identification

Environmental

  • Site assessment

  • Location of buried drums and tanks

  • Landfill and trench boundary mapping

  • Archeological investigations

  • Map contaminant plumes

  • Water-table mapping

Transportation

  • Asphalt thickness

  • Base-layer evaluations and void detection

  • Bridge condition assessments

  • Bridge concrete cover and deck thickness

GRAVITY

The gravity survey works by collecting surface measurements of the variation in the Earth's gravitational field which identifies the anomalies

Applications

  • Cavity detection (sinkholes, old mine workings, including bomb shelters, hidden tunnels/adits, and dissolution features)

  • Mineral exploration

  • Density determination

  • Grouting investigations

  • Shallow pipe and sewer detection

ELECTROMAGNETIC

The electromagnetic survey method is a rapid method that is based on measuring the electrical conductivity of the ground as a function of depth and/or horizontal distance. Mapping the variations in electrical conductivity can identify the anomalies that may represent different formations, buried structures,... Etc.

Applications

  • Locating dissolution features and subsurface voids

  • Locating underground storage tanks and drums

  • Mapping buried foundations

  • Mapping buried utilities

  • Locating the boundaries of landfills

  • Mapping leachate migration

  • Mapping and monitoring groundwater pollution

  • Mapping saline intrusions

  • Determination of layer thickness and conductivities

INDUCED POLARIZATION (IP)

Induced Polariazation method (IP) is a geophysical method used to identify the electrical chargeability of subsurface materials, and therefore the chargeability distribution using the surface electromagnetic data, IP surveys provide information about the spatial variation in lithology and grain-surface chemistry. It has been widely used for mineral exploration.

Applications

  • Metal exploration

  • Porosity estimation

  • Mining industry

  • Hydrogeophysical surveys

  • Environmental investigations

  • Geotechnical engineering projects

SELF-POTENTIAL

This method measures naturally occurring electric fields generated by weathering or corrosion of ferrous minerals or the flow of groundwater. It consists in the passive measurement of the distribution of the electrical potential at the ground surface. The electric potential field is very small, but can be measured over small or large areas.

Applications

  • Map the flow of groundwater through porous materials

  • Map preferential flow paths for seepage through embankment dams or levees

  • Map the path of flow for seeps observed at the ground surface downstream of cut-off walls, naturally occurring springs, or abandoned mines

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