Ground Penetrating Radar, also known as Geo radar or GPR, uses Radar technology to obtain a continuous profile of the subsurface. The basic principle of GPR is simple- An electromagnetic pulse is sent into the medium under investigations. Part of energy gets reflected from an interface across which there is a contrast in dielectric constant, and remaining energy travels further into the medium. The travel time and amplitude information of the returning signal are recorded. Almost continuous transmission of these pulses and recording of returning signal, provides a real time continuous profile of the subsurface. The Georadar is of immense application in areas requiring high resolution information of relatively shallow subsurface. Depth of penetration of radar signal depends on the frequency used and material properties. The application areas of GPR technology include:

• Detection of underground utilities- Pipes, cables (metallic and non-metallic)
• Soil-bedrock interface, shallow geological investigations
• Mining Development
• Mineral Exploration
• Water Table determination
• Glaciology
• Archaeology & Forensics
• Cavities & Voids (Structures- Dams, Bridges, Weirs, Barrages etc.)
• Ground Contaminants (Environment).
• Road Investigations (Layer thickness, Subsidence)

Seismic technique is one of the most developed geophysical techniques, providing vital information on subsurface, crucial for most of the engineering projects. GeoTech specializes in Seismic Refraction surveys, routinely carried out for assessment of subsurface conditions prior to engineering projects. The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between layers of different velocities. The seismic energy is generates at shot point using a source (hammer, weight drop, explosive). The energy radiates from shot point, and first arrivals are detected using geophones. The first arrivals can be direct arrivals or refracted arrivals, on different geophones, depending on velocities and thickness of subsurface layers. The time-distance curve is used to determine velocities and thickness of various subsurface layers. The velocity depends on elasticity and density of material through which the energy is traveling, and hence provides information on the material strength, and helps in classification of the material. The seismic refraction surveys are used to determine:

• Bedrock profile, rock quality and depth
• Thickness of overburden
• Fractures and weak zones
• Topography of ground water
• Rippability assessment in mines
• Slope stability studies
• Pipeline route studies

The electrical methods, used for measurement of subsurface resistivity, involve passing an electrical current into the ground using two electrodes, and measuring resultant potential using two potential electrodes. Resistivity sounding involves gradually increasing the spacing between the current and/or potential electrodes to obtain deeper penetration. Under profiling, the electrode spacing is kept constant and the entire arrangement is moved along profile lines, to obtain lateral variation in subsurface resistivity. Electrical resistivity techniques are based on the principle that the resistivity varied depending on the material encountered. Resistivity can then be used to identify different geological units by their electrical properties. If a material’s resistivity value drops it could mean that the rock is water saturated and one can expect to find fractured bedrock. The variation in resistivity will correspond to a geological variation along a investigated line. By calibrating the resistivity results with known geologic materials on site, valuable geotechnical information can be obtained for the dam. This type of survey becomes interesting in determining the weathered bedrock layers and the presence of sand/ moraine under the clay. It also corroborates the information from the refraction results for better geological interpretation. 2D resistivity imaging uses an array of electrodes connected by multicore cable to provide a linear depth profile of the variation in resistivity both along the survey line and with depth. The acquisition system consists of two units connected to a laptop computer. One unit is a resistivity meter which injects the electrical current into the ground and measures the voltage between the electrodes. The other unit is a control switch box which selects both the injection electrode pair and the potential electrode pair for each datapoint . The data are presented as pseudo-section in which the spatial distribution of the electric properties of the investigated material can be qualified. To qualify the data , a modeling routine is applied to the dataset according to the Zohdy method. This new section can be correlated to a depth section since the Zohdy method computes from the datapoint a series of continuous vertical electrical profiles. The Electrical surveys of used in civil engineering, water resources, mining and environmental projects to:

• Determine the underground water resources
• Bedrock quality and depth measurements
• Mineral prospecting
• Dam structure analysis
• Landfill
• Contamination source detection

Borehole seismic tomography is used to derive an image of seismic velocity between two or more boreholes by measuring travel time between source and receivers along various raypaths. The technique is used for detailed and targeted evaluation of material. One hole is used as source hole, and for wave generated at various depths in this hole, travel times are measures at number of receivers in the other hole, also known as receiver hole. This yields a network of overlapping raypaths, which are used in computations to create a velocity image, also known as tomogram. The tomogram helps identifying anomalous zones, which are visible as velocity contrasts. The application areas of seismic tomography are:

• Identification of features like fault zones and voids.
• Mapping of loose zones, sinkholes, anomalous zones in structures like dams
• Detailed study of old foundations