The following is a useful collection of geophysical terms. Search through using the alphabetical shortcuts or the “Find” function in your web browser.


fan shooting

A seismic refraction technique where the sensors (geophones) are deployed on a segment of a circle centred 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.

fiducial, or fid

Timing mark on a survey record. Originally these were timing marks on a profile or film; now the term is generally used to describe 1-second interval timing records in digital data, and on maps or profiles.

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.

Figure of Merit (FOM)

A sum of the 12 distinct magnetic noise variations measured by each of four flight directions, and executing three aircraft attitude variations (yaw, pitch, and roll) for each direction. The flight directions are generally parallel and perpendicular to planned survey flight directions. The FOM is used as a measure of the manoeuvre noise before and after compensation.

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.

fixed-wing

Aircraft with wings, as opposed to “rotary wing” helicopters.

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.

focussed 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.

footprint

This is a measure of the area of sensitivity under the aircraft of an airborne geophysical system. The footprint of an electromagnetic system is dependent on the altitude of the system, the orientation of the transmitter and receiver and the separation between the receiver and transmitter, and the conductivity of the ground. The footprint of a gamma-ray spectrometer depends mostly on the altitude. For all geophysical systems, the footprint also depends on the strength of the contrasting anomaly.

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.

forward modelling

Forward modelling means calculating a data set that would occur if a survey were gathered over a known model of the Earth. This usage of the word “modelling” is essentially the reverse of the definition above, and this often causes confusion for new users of geophysics.

free-air gravity field

The gravity field after the free-air correction. This correction is applied to observed or “raw” gravity readings to correct for the change in gravity due to the difference in elevation of the gravity station relative to datum elevation (usually sea level). The change in gravity with elevation is inversely related to the change in distance between the meter’s centre of mass (meter elevation) and the Earth’s centre of mass.

frequency domain

1. An electromagnetic system which transmits a harmonic primary field that oscillates over time (e.g. sinusoidal), inducing a similarly varying electrical current in the ground. These systems generally measure the changes in the amplitude and phase of the secondary field from the ground at different frequencies by measuring the in-phase and quadrature phase components. See: time-domain

2. A domain is where a mathematical function (the independent and dependent variables x and y and maybe z and perhaps more) exists. In the frequency domain, the independent variable has been transformed from a distance such as miles (seconds in the case of seismic) to frequency like cycles/mile (a spatial frequency versus a temporal frequency like cycles/second). The dependent variables then become the strength and phase of that frequency.

full-stream data

Data collected and recorded continuously at the highest possible sampling rate. Normal data are stacked (see stacking) over some time interval before recording.