Geophysical Survey Methods

Types of Engineering Geophysical Surveys

Electrical Resistivity, Soil Resistivity, or Earth Resistivity Methods

Electrical resistivity methods map detectable variations in the apparent resistivity of soils, rock, and fluids, by injecting an electrical current into the ground . Electrical methods include soundings, profiling, multi-electrode resistivity cross-sections, and capacitance coupled resistivity. These methods utilize various arrays. These arrays include but are not limited to wenner, dipole-dipole, pole-dipole, pole-pole, schlumberger, self potential, and induced polarization techniques.

Seismic Refraction, Seismic Reflection, and Multi-Channel Analysis of Surface Waves MASW Methods

Seismic methods are based on the propagation of seismic waves. Seismic waves vary in velocity, amplitude, wavelength, frequency,  and path traveled. A geophysical interpretation of variations in these properties often assists with determining the depth and type of rock, soil characteristics, and engineering properties (e.g., clays, sands and gravels, and shear modulus), and structural geology (e.g. faults and bedrock valleys). Click here for help with a MASW seismic survey.

Electromagnetic Frequency Domain (electromagnetic terrain conductivity) and Time Domain Methods

Electromagnetic conductivity methods map detectable variations in apparent conductivities, much like electrical resistivity methods (when conductivities increase resistivities decrease) . However, electromagnetic methods are often more suitable for electrically conductive soils because EM methods transmit an electromagnetic signal into the ground’s near surface. Variations between the transmitted and received electromagnetic signal reflect geologic conditions at depth. While time domain methods offer good vertical resolution, frequency domain surveys using Geonics EM-31, EM-38, or EM-34 are often used to geophysically map lateral variations over spatially large areas, 100’s or 1000’s of acres.

Ground Penetrating Radar GPR

transmits an electromagnetic pulse that travels through the ground and returns back to a receiver, much like a fish locator. The GPR instrument measures the time it takes for the transmitted pulse to return to the receiver. GPR instruments have various configurations for different types of GPR surveys. Lower frequency antennas generally yield greater depths of penetration but have lower resolution. With good site conditions, GPR results are often spectacular. However, under poor conditions the results can be confusing and difficult to appreciate. Click here for help with a ground penetrating radar survey.

Magnetic Methods

map variations in the earths magnetic field. In small areas, buried iron and even historic fire pits are some of the targets for a magnetic survey.

 

 

 

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