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The commonly used geophysical method for site exploration is ________
Gravitational method
Electrical resistivity
Magnetic method
All of the mentioned
Electrical resistivity
The electrical resistivity method is the most widely used geophysical technique for soil exploration because it is non-destructive, cost-effective, and sensitive to soil moisture and clay content. It involves injecting an electrical current into the ground and measuring the potential drop between potential electrodes to determine the subsurface electrical resistivity.
The electrical resistivity method is the most widely used geophysical technique for soil exploration because it is non-destructive, cost-effective, and sensitive to soil moisture and clay content. It involves injecting an electrical current into the ground and measuring the potential drop between potential electrodes to determine the subsurface electrical resistivity.
╧БaтАЛ=2╧АaIVтАЛ тАФ Apparent resistivity for Wenner configuration where 'a' is the uniform spacing between electrodes.
The principle is based on Ohm's Law and the variation of resistivity among different soil types. An external current (I) is passed through two outer electrodes (A and B), and the potential difference (V) is measured between two inner potential electrodes (M and N). The apparent resistivity (╧Б_a) is calculated using the measured V/I ratio and the specific geometry of the electrode arrangement, such as the Wenner or Schlumberger configuration.
Resistivity depends on porosity, saturation, and clay content.
Clayey soils exhibit low resistivity compared to dry, granular soils.
The method is excellent for locating the water table and depth to bedrock.
It is categorized as an 'indirect' method of subsoil exploration.
Non-invasive and non-destructive
Rapid data acquisition
Economical for large area surveys
Sensitive to buried metallic objects and power lines
Ambiguity in interpretation of data
Effectiveness decreases with increasing depth
Groundwater exploration
Mapping bedrock topography
Detection of pollution plumes in groundwater
While gravitational and magnetic methods are used in deep geological prospecting (e.g., oil and mineral exploration), electrical resistivity is preferred for geotechnical shallow site exploration due to higher contrast sensitivity in soil materials.
The depth of investigation in resistivity surveys is approximately proportional to the electrode spacing.
B is correct тАФ Electrical resistivity is the most common geophysical method for engineering site exploration due to its sensitivity to soil moisture and stratigraphy.
Always remember that geophysical methods are preliminary tools; they must be validated with direct methods like boreholes and standard penetration tests (SPT) for final engineering design.