About Geophysical Logging

"Geophysics -n. The physics of the earth and its environment; geology that uses physical principles to study properties of the earth"

Geophysical logging is the practice of acquiring a range of physical properties of the earth by lowering measuring instruments into bore holes.

Geophysical Logging was pioneered by the Schlumberger brothers in 1927 for oil exploration and has since developed into a multi-billion dollar global industry serving the petroleum, mining and groundwater industries. It is also a widely used tool for regolith and environmental studies.

Traditionally, information in regard to subsurface structures and resources was gained through drilling and sampling, an expensive, invasive and costly process. Geophysical logging can sample a greater volume of material than the recovery from one borehole whilst providing iive in-situ measurements of properties that would otherwise require access to a laboratory to attain the same information from drill hole chip or core samples.

Geophysical logging tools are typically electrically powered probes containing a number of measuring devices or sensors which are lowered into a borehole on a conductive wireline. The tool is powered from the surface and data samples transmitted back to the surface and stored on a computer for subsequent interpretation and presentation in both printed and digital form.

Typical logging tools include:

• Density: Gamma radiation from a radioactive source is backscattered by the formation (Compton effect) and is measured by a detector where the relative count-rates provide a measure of formation bulk.
• Calliper: A mechanical device used to determine the size of a borehole.
• Magnetic Susceptibility: An electrical log where a magnetic field is generated to determine the ease with which a substance may be magnetised.
• Televiewer: An acoustic or optical imaging device that is used to image the borehole wall. Very useful in determining structural features of the formation.
• Resistivity: An electrical device used to measure changes in fomation resistance which indicate the presence or absence of minerals such as clays, sulphides, oxides and graphite.  Useful for straitgraphic correlation, identification of mineralised zones and alteration zones and determination of water quality.
• Sonic: Uses high frequency sonic impulses to enable the determination of formation mechanical properties. Under suitable acquisition conditions shear wave and slower (e.g. Stonely) arrivals can be detected
• Neutron: Utilises neutron radiation to determine the formation porosity and water content