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| Anomaly |
A variation or perturbation in the measured quantity which can be quantified in various ways, generically any variation from the normal or whatever is expected. It is always an indirect indicator of something else that is usually described as a feature.
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| Apparent Resistivity |
The array-specific value of resistivity that can be calculated from electrical resistance data. Conversion to apparent resistivity allows surveys with different probe spacings to be compared as long as they are of the same general form. The general equation is given below where AM, BM, AN & BN are the distances between the current probes A & B & the potential probes M & N. R is the measured resistance.
rhoa = 2 pi R / [ (1 / AM) - ( 1 / BM) - (1 / AN) + ( 1 / BN) ]
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| Background |
The component of a set of data that lacks any signal or anomaly of interest. It may be visible as a region of data without variation or as a texture that continues through the entire set of data & upon which everything else is superimposed.
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| Base Station (GPS) |
A GPS receiver set up on a known point from either previous survey or a DGPS position & used to send a correction signal to other GPS receivers so that the position of these is accurately known relative to the location of the base station.
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| Base Station (Magnetometer) |
A stationary magnetometer used to record temporal variations (diurnal, micropulsations & local 'noise') throughout a survey so that these can be removed from magnetic survey data later. Base stations may transmit their readings to a centralised logging system or to individual magnetometers via a radio link but it is more common to record each reading with a timestamp & this used for correlation with survey data. The sensor of a base station magnetometer should be positioned on a mast to lessen the effects of features & discontinuities in its immediate vicinity.
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| Chemical Remanent Magnetisation |
CRM
As crystals form they can acquire an overal magnetic field as the crystals expand beyond their blocking volume. Below this thermal randomisation is predominant.
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| Curie Temperature |
The temperature above which thermal agitation is sufficient to randomise the orientation of magnetic grains & destroy any previous common alignment. At this point any remanent magnetisation is destroyed.
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| Detrital Remanent Magnetisation |
DRM
As small particles of magnetic material sink in water they can realign with the direction of the ambient magnetic field & therefore preserve this in the accumulated deposits on the bottom.
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| Differential Global Positioning System |
DGPS
A GPS system that receives correction signals from land-based beacons or from relay satelites to improve the position accuracy of the GPS. In Europe EGNOS will be the primary source while WAAS covers the Americas.
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| Diurnal Variation |
Both magnetic & gravity data experience a recurring variation each day called the diurnal which is caused in the case of gravity by changes in the relative position of the Earth & the moon. Within magnetic data the principal component is solar & this is manifest as both the absolute value of the magnetic field & sometimes the amount of high-frequency variation.
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| Drift Geology |
The component of the ground that rests on the solid crustal rock & includes weathered material, alluvial, fluvial & glacial gravel etc.
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| Fiducial Mark |
A signal inserted into a stream of data to denote the surveyor has passed a particular (known) position or that a certain amount of time has elapsed. They are most commonly used during survey to mark the data at equal intervals of perhaps 5m or 10m or whatever is suitable. Upon interpolation of that line of data the process is constrained within marks to guarantee accurate location.
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| Fourier Transform |
A mathematical procedure for expressing data sampled in time or space as a set of waves which when summed together reconstruct the data. It is a core element of many data filters because it allows funtions that are dependent upon radial distance from a point to be applied to the whole data set.
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| Fluxgate Magnetometer |
This class of instrumet was invented during the 1930s & had prominent use in wartime as a means of detecting submarines & as a proximity detonator for bombs. It was subsequently used for geological & military applications, eventually migrating into archaeological studies in the 1970s. There are three common makes available now, the Bartington Instruments 601, the Foerster Ferex & the Geoscan Research FM36 & FM256. Fluxgate instruments for field survey are always differential magnetometers, i.e., have two electronically & magnetically linked fluxgate elements, & have sensitivities that approach 0.1nT. This can, however, be offset by low thermal stability.
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| Frequency Domain Electromagnetic (survey) |
FDEM
Electromagnetic survey where a wave is injected into the ground & the response is a measure of the electromagnetic coupling at the particular frequency used. It is the operating principle of instruments seeking to measure the electrical conductivity of the ground. Examples are the Geonics EM38 & EM31.
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| Global Positioning System |
GPS
A system based on a constellation of satelites around the globe for determining 3D position. Each satelite broadcasts a time signal based on extremely accurate atomic clocks & (in basic terms) these are processed at a receiver to determine how long the signal took to reach it. From this it is possible to triangulate the position of the receiver. There are currently two constellations in use, the American GPS & the Russian GLONASS. There will soon be a third, the European Union Galileo constellation which will integrate with EGNOS to provide sub-meter precision.
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| Heading Error / Variation |
Called an error because it is normally an unwanted element of the data but it is a normal result of surveying in varying directions, e.g. zigzag traverses. For all combinations of instruments & surveyor there is usually a small overall magnetic field or measurement bias that will be evident in the data as a linear (DC) offset applied to each line. For zigzag surveys it should be constant per direction walked & is therefore easily reduced by statistical approaches, however, for GPS tracked surveys a more sophisticated approach is necessary.
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| Interference |
Interference is loosely some sort of signal that adversely affects the measurement process. It may be stationary in time or variable. Within geophysics there are numerous sources, including buried cables & pipes, mobile phones etc.
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| Magnetic Domains |
Magnetic behaviour in ferrimagnets & anti-ferromagnets is dependent upon magnetic grain size, not the same as particle size but related. These grains are grouped into domains & it is the relative orientation of these that governs the type of magnetisation that a material exhibits.
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| Magnetic Susceptibility |
MS
See Magnetic susceptibility for more information.
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| Noise |
Noise is similar to interference but is usually inherent to the measurement process rather than due to external sources. It could be an instability in an instrument for example & in geophysics is usually of a high frequency. Sometimes highly variable regions in 2D data are called noise if they are considered to interfere with what the viewer wishes to see.
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| Permanent Grid / Ground Marker |
PGM
A marker, usually of metal, that is driven or otherwise set into the ground to provide a long-lived point from which measurements can be made.
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| Post-detrital Remanent Magnetisation |
PDRM
Material that has accumulated can undergo a slow realignment of magnetic grains which increases the volumetric magnetic susceptibility & preserves the ambient field direction at the time. Compare this with DRM which forms as particles of the material settles (usually from suspension in water).
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| Potential Field |
PF
A class of physical phenonema including magnetic, gravitational & electrical fields that are manifest as forces from point sources, the strength of which falls exponentially with increasing distance.
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| Profile |
A vertical slice of geophysical data through the ground or an object. It is also sometimes used as an alterative to 'traverse', a line of survey.
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| Relative Dielectric Permittivity |
RDP
See Electrical Permittivity
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| Sampling Interval |
The distance between two adjacent measurements in the line of motion through a survey. Assuming the survey is traversed along parallel lines the orthogonal quantity is the line separation.
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| Solid Geology |
The crustal rock on which everything else rests.
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| Stacking |
A process of automatically averaging readings at a measurement station to increase the signal-to-noise ratio. Most instruments support some sort of stacking. This works best when the instrument is stationary, however, most of the time the instrument is continuously moving & therefore the resulting averaged reading is a combination of temporal & spatial variations. The length of time taken for a reading must always be less than the time taken to move between adjacent measurement positions along the survey line otherwise the data will be smeared.
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| Static Test |
A method of assessing & quantifying instrument behaviour by allowing it to collect a set of data while stationary & then analysing the amount of variation in what should theoretically be a constant measurement.
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| Thermo-remanent Magnetisation |
TRM
When materials are heated & allowed to cool back through their blocking temperature (around the Curie point) the energetic agitation associated with heating ceases & magnetic grains become aligned with the ambient field. This is the basis of archaeomagnetic dating & also the increase in susceptibility associated with burnt & heated soil.
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| Tie Lines |
These are a small number of lines surveyed across the main survey so that they intersect with as many survey lines as possible, each survey line being crossed by at least one line. The purpose is to permit removal of a model of the temporal variation of a property from the survey data by assuming that in the time taken to complete the tie lines the temporal variation will be insignificant. At each intersection the difference between the measurement on the tie line & that in the survey data can be considered indicative of the amount of temporal variation. By calculating these & interpolating in time a model of the temporal variation can be formed & subtracted from the survey data. The process is effectively a high pass filter as the duration of temporal variation removed is a function of the intersection spacing in time.
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| Time Domain Electromagnetic (survey) |
TDEM
A principle of operation of certain electromagnetic instruments that responds to the electrical eddy currents induced in buried objects. It is most commonly used for metal detection while especially sensitive versions are Pulse Induction Meters capable of measuring the magnetic viscosity of materials.
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| Time Slice |
A plan view of data in a 3D stack from radar or seismic surveys. It equates to the responses received at the surface at the same time & thus considered to be from materials at similar depths.
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| Total Field |
TF
Literally, all the field. Within magnetics caesium & proton magnetometers measure the total field while fluxgates measure one directional component of the field.
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| Traverse Interval |
Alternatively, line separation, the distance between parallel & adjacent traverses walked by the surveyor.
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| Vertical Component |
That directional component of a field, usually magnetic or gravimetric, that is directed straight upwards from the ground.
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| Vertical Gradient |
A description used to describe the vertical change in the strength of a measured magnetic field. For many magnetic surveys in archaeology the data is called a gradient but is not because the separation of the sensors has not been taken account of.
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| Viscosity, magnetic |
A measure of how magnetically sticky a material is. Materials with large magnetic grains rotate only slowly if the direction of the ambient magnetic field is changed but smaller grains can react much more quickly. The property has considerable importance in determining whether magnetic grains developed naturally (large ones) or through thermal effects (smaller ones).
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