GEOTECHNICAL GEOPHYSICS: THE BRIDGE BETWEEN TWO EARTH SCIENCE DISCIPLINES
Geophysics is the application of physics principles to the study of the Earth.
The Earth is comprised of materials that have different physical properties. Clay and granite, for example, have different densities, acoustic velocities, elastic moduli, electrical conductivities, magnetic susceptibilities, and dielectric constants.
Geotechnical geophysics is the application of geophysics to geotechnical engineering problems; such investigations normally extend to a total depth of less than several hundred feet but can be extended to thousands of feet in some instances. Geotechnical geophysical surveys are performed on the ground surface, within boreholes, and from the water and air.
Geotechnical geophysics is routinely used for many types of highway engineering investigations, including:
groundwater flow, locating fractures, weak zones, expansive clays.
MAGNETIC SURVEY METHOD
Magnetic geophysical surveys measure small, localized variations in the Earth’s magnetic field. The magnetic properties of naturally occurring materials such as magnetic ore bodies and basic igneous rocks allow them to be identified and mapped by magnetic surveys.
Magnetometers are highly accurate instruments that measure local magnetic fields to a high degree of precision. Magnetometer systems used for commercial applications include proton precession, cesium vapour and gradiometer magnetometers.
Data acquisition for magnetic surveys involves taking a series of point readings at regular intervals on a survey grid. The spacing between grid lines and reading stations is dependent upon the application. Generally smaller targets require higher resolution surveys and denser survey grids. Modern cesium vapour magnetometers and gradiometers are more sophisticated, allowing data to be collected either in continuous mode or as a set of point readings.
Data is stored digitally on site, and later downloaded on to a PC (personal computer) for post-survey processing and interpretation. Various interpretation techniques are applied to the data using specialist interactive software to identify the targeted anomalies.
Magnetic surveys usually also conducted from an aircraft, to which a magnetometer is attached. This usually measures only the magnetic field intensity. Regional magnetic maps are usually produced by flying the magnetometer at a low level in regular parallel passes over the ground.
The magnetic maps are usually studied alongside geology maps to help define the origin of the magnetic features, and any interesting departures from expected structure and geology. Most magnetic texture is the result of differences in the abundance of magnetite. Thus a magnetic interpretation will be closely related to geology defined by other parameters, but can be markedly different in some aspects
September 23, 2019