Glossary of Geophysical Terms
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Fan shooting
A seismic refraction technique where the sensors (geophones) are deployed on a segment of a circle centered on the seismic source. Variations in the time of arrival are caused by radial variations in the velocity structure. Could be used, for example, to search for low velocity anomalies caused by buried waste.
Ferrimagnetic
Substances having positive and relatively large magnetic susceptibility as well as generally large hysteresis and remanence. This is due to the interaction of atoms and the coupling of magnetic moments aligned in opposition, which result in non-zero net moments. Ferrimagnetic minerals have this property.
Field
That space in which an effect, such as gravity or magnetism, is measurable.
Field print
A copy of a log obtained at the time of logging that has not been edited or corrected.
Filtering
a) The attenuation of a signal's components based on a measurable property (usually frequency). Filtering usually involves a numerical operation that enhances only a portion of the signal. b) Fluid passage through a material that retains particles or colloids above a certain size.
First reading
The depth at which logging began at the bottom of the hole.
Flexural Waves
Flexural waves occur in bars and refers to the flexing, or bending, of a bar. Thus they can be created in shafts by impacting the side of a shaft. The velocity of flexural waves depends on their wavelength.
Flowmeter
A logging device designed to measure the rate, and usually the direction, of fluid movement in a well; most are designed to measure vertical flow.
Fluid sampler
An electronically controlled device that can be run on a logging cable to take water samples at selected depths in the well.
Flushed zone
The zone in the borehole wall behind the mudcake that is considered to have had all mobile native fluids flushed from it.
Focused log
A resistivity log that employs electrodes designed to focus the current into a sheet that provides greater penetration and vertical resolution than unfocused logs.
Formation
Used in well-logging literature in a general sense to refer to all material penetrated by a drill hole without regard to its lithology or structure; used in a stratigraphic sense, formation refers to a named body of rock strata with unifying lithologic features.
Formation-resistivity factor
Formation factor (F) is the ratio of the electrical resistivity of a rock 100 percent saturated with water (Ro) to the resistivity of the water with which it is saturated (Rw). F = Ro/Rw.
Frequency domain
In geophysics, refers to measurements analyzed according to their constituent frequencies. The usual alternative is time domain measurements.
