Glossary of GPR Terms Glossary of GPR Terms
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Glossary of GPR Terms

Term Common symbol Typical units Description
3 dB bandwidth   MHz or GHz the range of frequency spectrum over which the GPR signal amplitude remains above a value equal to the (peak amplitude /√2)
3D View     the term used when displaying data covering a surface area with the third dimension being time or depth or time. The data are binned into voxels (small cubic volumes) and the data values are displayed with volume rendering tools that render cross sectional slices or variable transparency of the 3D volume.
6 dB bandwidth   MHz or GHz the range of frequency spectrum over which the GPR signal amplitude remains above a value equal to the (peak amplitude /2)
Acquisition Mode     a term used to describe the method used to initiate GPR trace acquisition. Common methods are:

·         Distance – trace collection occurs at spatial locations (usually equispaced steps) with position determined from odometer or other distance measuring device.

·         Free run – traces are collected one after the other as fast as the system will permit.

·         Time – trace collection is controlled by defined time interval either between traces or delay time after completion of previous trace collection.

·         External Trigger – trace collection controlled by external trigger such as manual button pushes or electronic input signal.

aggregate     any hard, inert, mineral material used for mixing in graduated fragments. It includes sand, gravel, crushed stone, or slag. Materials often used in the creation of concrete or asphalt.
air waves     GPR systems emit radio frequency energy that travels in all directions from the transmitter and is received from all directions by the receiver.  GPR is used to detect subsurface signals.  GPR signals that travel in the air above the ground and reflect from above-ground objects produce undesired, coherent clutter signal called air waves.
antenna     Device used to transform electrical signals into propagating electromagnetic waves. With GPR there is usually a transmitting antenna and receiving antenna.  When combined with the transmitting or receiving electronics, the term transducer is used (see transducer).
antenna separation     GPR normally uses separate antennas to transmit and receive.  The linear distance between the physical center of the antennas is referred to as the antenna separation.
array   the term applied in advanced GPR systems containing a multiplicity of transmitting and receiving antennas deployed over a spatial volume with fixed spatial locations defined for each element.
attenuation α nepers or dB/m the term applied to the reduction in signal amplitude of a propagating signal caused by energy dissipation in the media.  For GPR, the attenuation is commonly associated with the electrical conductivity of the media.
Automatic Gain Control AGC   a gain function that attempts to equalize the amplitudes of all GPR signals by applying a gain which is inversely proportional to the signal strength.   This type of gain is most useful for defining continuity of reflecting events.
Average Frequency Spectrum AFS   term used to describe the process of computing and then averaging all the amplitude spectra for traces in a GPR data file or section.  The plot shows the frequency content of a signal so it is often used to determine parameters for frequency filtering, such as applying a band-pass filter.
Average Trace Amplitude ATA   a plot that displays the average rectified signal amplitudes for an entire GPR line. This plot has proven to be a powerful way to display how rapidly the GPR signal amplitude decays and to assess ambient noise levels, thus giving an idea of the maximum depth of GPR signal penetration.
Background Average Subtraction BAS   The process of computing the average of all traces in a GPR data set and then subtracting this average trace from all traces in the data set.  Most often used in poor quality GPR data to suppress coherent system noise that appears as constant bands in a GPR image.  This process makes weaker, spatially-varying events (such as hyperbolas from point targets) more visible by removing the time constant responses common to all traces in the GPR line. It is also used to remove the direct air and direct ground waves (transmit pulse) visible at the time zero; the top of the line which are virtually invariant in common offset profiling data.
Background Subtraction BSUB   This process is similar to background average subtraction (see above) but uses the running average over a localized set of traces centered at the discrete processed trace to obtain the average background trace to subtract. This process enhances localized events (such as hyperbolas from point targets) and suppresses horizontal or slowly varying events. This can be very useful for removing localized flat-lying events.  It is also used to suppress the direct air and direct ground waves (transmit pulse) visible at time zero on the trace; the top of the line which are virtually invariant in common offset profiling data.
Bandpass Filter     The process whereby a range of frequencies are retained in a GPR data and all other frequencies are suppressed. GPR are ultra-wideband recording devices and can contain noise signals that are not created by the GPR transmitter.  Judicious selection of the frequencies to retain and suppress can enhance the interpretability of GPR images.  Bandpass filtering is most commonly achieved by Fourier analysis and spectrum weighting, but it can also be achieved by temporal convolution of the GPR signal with the suitable temporal filter impulse response.
bandwidth     The range of frequencies over which a given device transmits or detects signals above a specified amplitude or power.
center frequency     A term specifically defined for ultra-wideband devices.  An upper and lower cut off in terms of signal amplitude (such as 3dB bandwidth) is defined.  The average of the upper and lower cutoff frequencies is defined as the center frequency.  Often the center frequency is very close to the frequency where the spectral amplitude peaks for GPR systems.  As a result, peak frequency and center frequency terms are often used interchangeably although this is not strictly correct.
channel     the term used to describe the GPR signal created by a single TX-RX antenna pair which have fixed parameters such as frequency, separation, orientation etc. (Also see definition of array element)
color bar     a bar of colors which display the color palette and usually labelled to show the range of data values (usually GPR signal amplitudes) indicated by a specific color in a color image.
color palette     The term used to refer to the mapping of a data value to a color.  Sometimes referred to as the color table. Most often used in GPR to assign a data attribute (like amplitude) value to a color in the creation of cross-sections, depth or time slice images and 3D images.
common mid-point CMP   common mid-point (CMP) is a GPR survey type where the transmitter and receiver antenna positions are varied but the movement is carried out in a manner that makes the mid-point remain constant.
common-offset reflection configuration COR   Term used when data are collected with fixed antenna geometry which is moved from location to location.  Most often both the transmitter and receiver antenna orientation as well as the separation are fixed.  See also Line Profiling.
Concurrent receiver operation     Term used when multiple receivers are acquiring data samples at the same time. Data acquisition is synchronized such that all receivers acquire data at the same time after only a single transmitter pulse emission.  The timing accuracy of the individual receivers need to be in the 10’s of picoseconds to achieve successful data acquisition.  This speeds up GPR data acquisition greatly when only a single receiver is employed. (Historically GPR had to use multiplexed receiver operation to achieve the same result making for much slower data acquisition).
conductivity σ milliSiemens/meter or mS/m or millimhos/m (historical) The ability of a material to conduct electrical current. In isotropic materials the reciprocal of resistivity. Sometimes called specific conductance.
cross-section     Image that results from side-by-side display of a number of traces which are from adjacent spatial measurement positions.
decibel dB   The unit used in expressing a ratio in the form of 20 log10 of the ratio.  A ratio of 10 equals 20 decibels.
depth variable gain     The term applied to a multiplicative factor that changes with depth that a trace is multiplied by.  The purpose is to change the relative signal amplitude versus depth. See also time variable gain.
Depth or Depth-Section Image     The term used when GPR data traces are plotted side-by-side to create an image of the ground and where the vertical axis is depth often obtained by translating signal travel time to depth.  Quite often the term is shortened to depth-section or section.
Depth Slice (Image)     The term used to describe the image obtained by rendering a slice through a volume of data as a color or contour display.  Normally generated by a computer process.   This term is normally shortened to depth slice.
dewow     The process of removing a very low frequency component from GPR data. These low frequency data components are associated with either inductive phenomena or possible instrumentation dynamic range limitations. The GPR trace baseline slowly undulates up and down; in the early days of GPR, this was called ‘wow’.  Removal of this effect came to be called ‘de-wow’.
dielectric constant K   See dielectric permittivity.
dielectric permittivity K   A fundamental physical property that describes the electrical polarizability of a material.  Free space or vacuum has a permittivity of 8.89 x10-12 Fd/m.  Most often the term is applied to the relative dielectric permittivity where the material permittivity is divided by the free space permittivity.  Most natural materials have a relative permittivity in the range of 1 -80 in the radio frequency range. The dielectric permittivity is often referred to as relative permittivity or dielectric constant.
display unit     The term applied to a device that controls, records and displays the GPR data.  This device is sometimes a personal computer (PC) but can also be a custom-built, fit-for-purpose computer system (see digital video logger DVL).
distance measuring instrument DMI   A device for accurately measuring elapsed distances along a survey line or transect. The term is common in roadway surveys to measure longitudinal distance from an intersection or other visible geographic location.  See also odometer. In GPR applications the output of the device may be used to trigger the acquisition of GPR at fixed distance (step) intervals.
DynaQ DynaQ   is an advanced, Sensors & Software-patented technology that dynamically adjusts stacking as system movement speed varies. Stacking is a means of improving signal to noise so being able to adapt the stacking to sensor movement speed creates a dynamic quality data acquisition technique.
electromagnetic EM   is the term applied to methods which use electric and magnetic fields to transmit signals or make sensing observations. The term is a general one that encompasses the whole field of science involved in electric and magnetic fields – particularly when the fields are time varying and therefore coupled.
envelope     formed from a pair of traces that uniquely bracket the extremes of an oscillatory signal.  The signal is assumed to have a zero average or baseline, so the upper and lower traces are identical but of opposite sign.  The envelope is obtained by using the method referred to as the Hilbert transform to calculate the positive and negative traces; the positive bounding trace is normally referred to as ‘the envelope’.  The envelope does not have the oscillatory nature of the original signal.  The envelope is more indicate of the data resolution. Enveloping can also simplify the GPR section display making it easier to interpret. Creating depth slices of the envelope is a powerful way of displaying GPR data that cover an area.
falling weight deflectometer FWD   A non-destructive testing device that is used to perform structural testing for pavement rehabilitation projects, research, and pavement structure failure detection.  It is used for conventional and deep-strength flexible, composite and rigid pavement structures.  The FWD applies a dynamic load to the pavement surface, similar in magnitude and duration to that of a single heavy moving wheel load.   The response of the pavement system is measured in terms of vertical deformation, or deflection, using seismometers.
file size   bytes The actual or physical limit on the amount of data stored in a computer file.
first break time     Time of onset of the direct air signal at the GPR receiver, referenced to the start time of trace recoding. When plotting GPR data this time offset establishes the zero point on the time scale axis. In practice, this offset time on a GPR trace is determined by sensing when the receiver signal rises above a defined level for the first time from start of recording.
frequency f Hz, MHz, GHz is the term applied when a signal is repetitive at regular intervals of time.  Frequency measures the number of repetitions that occur in a time interval such as pulse per second or cycles per second.  (Frequency can also be used in other contexts such as spatial frequency which indicates a signal that repeats with distance or length such as cycles per meter).  The term is also used to indicate the center or peak frequency of a GPR antenna response although it is somewhat a mis-use of the term.
Fresnel zone     For a reflection from a surface, there is a finite area on the surface for which signals travelling from the source to the receiver are indistinguishable based on the time of travel or length of travel path.   This area is defined to be the Fresnel zone or zone of influence.  The Fresnel zone is defined by differences in path length being a faction of the wavelength when the signal is sinusoidal excitation.  The zone of influence is the same concept but expressed in terms of differences in travel time being less than a fraction of the pulse-width of an impulsive transient signal.
gain     Process of amplifying signals to match recording device or the dynamic range of the display.  See time gain.
global positioning system GPS   GPS is a satellite-based approach to obtaining georeferenced coordinates.  The satellites transit a defined timing signal and the GPR receiver, usually on or near the Earth’s surface, records the signals from many satellites and uses the signals to triangulate the best estimate of the receiver location in 3D space with reference to a standard earth geoid shape.  The result is normally latitude and longitude and elevation as well as a very precise time.
GPS Files     When GPS is used with GPR, the GPS receiver data is recorded in a data file and recorded information tied to the GPR data.  When the GPS record is available and synced to the GPR data, Latitude and Longitude, UTM coordinates, and GPS elevation can be provided for every GPR trace in a data set.  A GPS file is created by attaching a GPS system to the GPR system during data collection. GPS files contain lines of standard GPS positional output text (called NMEA strings) and the associated GPR trace number. When the GPS file is attached, latitude, longitude, and GPS elevations for every GPR trace can be saved into the GPR file.
GPZ .gpz   A .gpz file is a Sensors & Software standard data file for GPR data recording and interchange.  The .gpz is used with the PC-based GPR software called EKKO_Project™.  The .gpz file contains a wide variety of data files including GPS files and .dt1 GPR data files as well as proprietary data from Sensors & Software instruments and auxiliary sensors.
Grid     A square or rectilinear set of straight lines which cover an area. Acquiring data on a grid means acquiring data along each line forming the grid.   Acquiring data on a grid at regular spatial steps is the optimal way to obtain a data set suitable for advance signal processing, such as the creation of 3D volume renderings and depth or time slice images. Conventional notation is to use a first quadrant Cartesian coordinate system with X and Y axes.
Grid Resolution     is the term used to describe the XY spatial resolution attained in a grid and is controlled by the largest line or trace spacing.
grid size     term used to describe a real extent of a rectilinear grid (i.e. 5m x 10m, 20ft x 50ft., 24″ x 24″, 600 mm x 600mm).
grid survey     is the term to describe the process of acquiring data on a grid over an area with the end goal to create 3D, depth or time slice images.
ground-penetrating radar GPR   Ground penetrating radar is a geophysical method for mapping subsurface structure using electromagnetic energy.  There are several deployment methods, but all approaches involve a transmitter creating radio frequency signals typically in the 1 to 5000 MHz frequency range and a receiver which detects similar signals.  The goal is to measure the impulse response or the transfer function of the surrounding medium with the view of reconstructing the material property structure.  In its simplest form, a GPR system with a transmitter and receiver are moved over the ground surface and reflections returning from subsurface objects are detected, recorded and displayed to the user.
Herringbone effect     Term applied to distortion in a map image derived from a grid survey when there are measurement system errors. The effect is most pronounced when data are collected in alternating directions along grid lines with an improperly calibrated odometer.  In the case of GPS or similar positioning techniques, the systematic positioning error can be created by a finite temporal response lag so that the measured response is delayed with respect to the device (GPR system) position.
horizontal stretch     A term used to describe stretching the horizontal (position) axis of a cross-section image.  If GPR data positioning is distorted, this is a means of regularizing the spatial position so that distance is a linear-axes on a GPR cross section image.  Sometimes the process is referred to as rubber banding or rubber sheeting since the effect is to stretch or squeeze the spatial axis of the data is achieve uniform spatial presentation.
hyperbola     Characteristic inverted “U” GPR response from a point target. (Mathematical form of the position-travel time response from a point target).  The apex of the hyperbola (top of the inverted U) represents the location of the GPR system’s closest approach to the object.
hyperbola velocity estimate     A point source GPR reflection appears as a hyperbola in the cross-section image. One of the parameters controlling the shape of the hyperbola is the ground velocity.  Hyperbolic fitting enables the ground velocity and target depth to be estimated.
hyperbolic fitting     Process of fitting a hyperbolic shape to a local GPR response in the space-time domain. The fitting process yields a velocity estimate for the material (media) above the target and a depth estimate of the target.
lateral resolution length     The minimum lateral separation between two objects required so a GPR clearly detects two side-by side responses.
Line Profiling or Common-offset profiling     is the term to describe collecting data along one or more GPR lines for immediate site assessment using cross-section images.  A series of lines can be used to help define site conditions prior to a grid survey.  Also see Common Offset Reflection (COR).
Line     is the term used to identify the location of a single GPR data set.  A line is normally straight, and data are recorded from the start to the end of the line.
Lowpass Filter     removes frequencies above a cut-off frequency – useful for removing high frequency noise in GPR data.  The filter can be a zero-phase or causal type filter.  It can be applied in the frequency domain via Fourier methods or in the time domain by convolution.
magnetic permeability u   A fundamental physical property that describes the magnetic polarizability of a material.  Free space or vacuum has a permittivity of 12.57 x10-7 H/m.  Most often the term is applied to the relative permeability where the material permeability is divided by the free space permittivity.  Most natural materials have a relative permeability.  For most materials relative permeability is in the range of 1 +/- .00001 in the radio frequency range.  For this reason, permeability has no impact on GPR signals.
Megahertz MHz   A measure of frequency equal to 1 million repetitions per second.  See frequency.
microsecond     microsecond =10-6s = 1000ns; occasional unit of GPR time.
Migration     The process of collapsing the response of a point target back to the source point.  Common term in reflection seismic and similar to synthetic aperture image reconstruction. Can be visualized by thinking of the signal on a hyperbolic response being summed and placed at the apex of the hyperbola.
multiplex     the process of creating a single stream of data obtained from a multiplicity of receivers or detectors.  (The sequencing is normally fixed- i.e. for four channels named 1, 2, 3, and 4, the sequence of the samples in this serial stream might be … 1a, 3a, 2a, 4a, 1b, 3b, 2b, 4b, 1c, 3c, 2c, 4c, where a, b, c refers to each cycle of selected channels…)
multi-polarization configuration (multi-polarization)     Describes a multi-channel GPR configuration where there is more than one polarization of the antenna elements. The terminology will take on two forms namely, one when single lines are being surveyed in which case the PR-BD, PL-BD, PR-EF, PL-EF, X-POL terms are used whereas if an area is being covered we will have the XX, XY, YX, YY terminology being used. Refer to the pulseEKKO manual for more details.
nanosecond ns ns 10-9s (One Billionth of a second)
odometer calibration value   ticks/m Odometers and distance measuring indicators (DMI) are based on encoders that generate a defined number of electrical signals (normally referred to as ticks) per revolution of the encoder.  When attached to a wheel, the wheel diameter dictates the distance travelled per revolution.  To calculate an accurate measure of distance travelled, an “odometer calibration” is required which is usually attained by measuring the number of ticks that are observed when the wheel traverses a known distance.  Calibration is the result of this measurement process converted to a value for the number of measured ticks per unit distance such as ticks/m.
penetration depth     The maximum depth for buried targets for which reflected signals can be detected. When a GPR pulse travels into the ground, it decreases in amplitude because of geometrical spreading of the energy and absorption of the energy by the medium.  At some point the signal amplitude gets so small that it is no longer detectable.  The depth that the signal amplitude falls below detectability defines the penetration depth or depth of exploration.
picosecond ps   A picosecond is 10−12 seconds (one trillionth of a second).
point stacking     Term used to describe stacking (averaging of repetitive observations) at a single time point.  Often done when using a Digital Equivalent Time Sampling (DETS) receiver. Stacking is carried out sequentially for all the points in a GPR trace.
Points per Trace (N) N   the number of sample points in a GPR trace.
Power Cable Detector PCD   A sensor that detects the alternating current in electrical cables using the magnetic field intensity produced. The sensor is normally attuned to detect the magnetic fields that oscillate at 50 or 60 Hz which are the standard oscillation rate for power lines carrying electric current.
pulse repetition frequency PRF kHz, MHz acronym for pulse repetition frequency. For a GPR system, it is the number of times the transmitter emits a pulse in a second.
pulse repetition period PRP ms, us acronym for the pulse repetition period.  For a GPR system, this is the time interval between transmitter firings and is equal to 1/PRF.
pulser voltage   volts the peak amplitude of the voltage pulse applied to a GPR transmitting antenna.  Impulsive GPRs are characterized by this voltage.  This is a simple alternative means of defining the energy available to be emitted.  Energy is stored in a source power supply and released when the transmitter is triggered. The GPR signal at a distance is directly proportional to the peak voltage.
radio wave     Electromagnetic fields that travel through a material as waves. Radio signals which are commonly in the frequency range of 1 MHz to 1,000 MHz and electromagnetic waves in this frequency range are generally referred to as radio waves.  (Electromagnetic signals in the 10, 000 MHz range are similarly referred to as microwaves. Light waves are just higher frequency electromagnetic waves.)
range resolution length     The minimum radial distance separation between two objects required so a GPR clearly detects two vertical responses.
receiver Rx   General term for electronic device used to detect electromagnetic field strength and translate the signals into electrical voltages or currents to be recorded or displayed.  Modern receivers generally convert the signals into digital values (numbers) for recording and display.
reflection coefficient     normally named “Fresnel reflection coefficient”.  Quantifies GPR signal reflection amplitude from a flat interface between two materials.
reflectivity     measure of signal amplitude returned by a target.
relative permittivity K   See dielectric permittivity.
resolution     The minimum separation of two objects before their individual responses merge into a single response.
ringing Impulsive GPR signals can give rise to reverberating responses that oscillate for a much longer time than the GPR pulse or wavelet. Such a response is referred to as a “ringing” response or “ringing” for short.
sample point     Signal amplitude measured at specific point in time.
sample time interval     the time between success sample points on a trace which is usually a constant fixed value and is selected to assure that the Nyquist sampling criteria are met.
signal amplitude     The value assigned to amplitude of the radio wave signal at a point in time.
Signal-to-noise ratio     The ratio of GPR signal amplitude to the average noise amplitude. A large ratio results in a larger penetration depth or the ability to detect weaker signals.
Slice     Describes the GPR data in a time or depth slice when the data is displayed as a computer-generated image.
Spatial Filter Spatial filters act on GPR data in the spatial (or positional) direction. These filters use adjacent traces during the filtering procedure and alter the shape of the trace through various mathematical manipulations designed to enhance or eliminate certain responses.  For example, Background Subtraction is a spatial filter that removes flat-lying responses in the GPR data.
Spreading & Exponential Compensation Gain SEC   SEC gain is a composite of a linear time gain and an exponential time gain which attempts to compensate for the spherical spreading losses and the exponential ohmic dissipation of energy with depth in the GPR Line.
stacking     term used to describe recording many repetitions of a signal and computing the average value.
stacks     The number of repeated measurements averaged to get a resulting measurement.
station interval     Spatial distance between observation points along a survey traverse line or mesh points on a grid.
step size     See station interval.
system performance Q Measure of system exploration depth indicated by the ratio of transmitter output power or voltage to receiver noise power or voltage.
time gain GPR signal amplitudes decrease rapidly in amplitude versus delay time after the transmit pulse is emitted.  Time gain is the term applied to the operation of applying an amplification, which increases with time, to the signal, attempting to compensate for large differences in signal amplitude.  This is a non-linear operation.  See gain.
time sampling interval delta-t or Δt) ns or ps The time interval between sample points on GPR trace; usually a constant interval.  The interval is usually adjusted to match the GPR frequency of operation and often set automatically by the system based on GPR frequency.
Time Slice     is the term to describe the data acquired between two times – top of time slice and bottom of time slice.  Most often a grid survey has the maximum time subdivided into a number of time slices of equal thicknesses.
Time Window   ns The maximum recording time selected for a GPR measurement.  Normally set in the field during measurement but can be reduced when viewing processing data after acquisition.
Topography Files A topography file is a text file containing GPR line positions and the elevations at those positions. When a topography file is attached to a GPR line, elevations for every GPR position are interpolated saved into the elevation field of the GPR trace header.
trace     Sequence of sample points from a single GPR channel that indicate the time variation of signal amplitude.
Trace Plot Process of plotting traces side-by-side to create an apparent cross section of the ground. Trace number, which is normally equivalent to horizontal position, increases in the horizontal direction while data points on the trace are plotted in the vertical direction representing increasing signal delay time or depth.
Trace Repetition Rate    traces/second the number of GPR traces that can be collected in a given time interval normally determined as 1/ (PRP x Points per trace x stacks) + delay time)).
Trace Stacking     term used to describe stacking (recording and averaging) of complete traces.
Trace Time Interval     Time between acquisition of sequential traces in free run data acquisition mode.
transducer     Name used where GPR antenna, electronics, and shield are combined into one physical unit.
transmitter  Tx   General term used for electronics devices used to create propagating electromagnetic fields
Transmitter Output Voltage   Volts (V) the peak output amplitude of the transmitter electronics that is fed to the antenna.
Two-dimensional (2D) array     GPR array elements can be distributed over a flat surface to form a two-dimensional array of elements. (Note in some rare cases arrays may also be 3D)
Universal Transverse Mercator (UTM) UTM UTM is a geographic coordinate system that uses a 2-dimensional Cartesian coordinate system to give locations on the surface of the Earth. It is a horizontal position representation, i.e. it is used to identify locations on the Earth independently of vertical position but differs from the traditional method of latitude and longitude in several respects.
UTM Letter Each UTM zone is segmented into 20 latitude bands. Each latitude band is 8 degrees high and is lettered starting from “C” at 80°S, increasing up the English alphabet until “X”, omitting the letters “I” and “O” (because of their similarity to the numerals one and zero). The last latitude band, “X”, is extended an extra 4 degrees, so it ends at 84°N latitude, thus covering the northernmost land on Earth. Latitude bands “A” and “B” do exist, as do bands “Y” and “Z”. They cover the western and eastern sides of the Antarctic and Arctic regions respectively.
UTM Zone The UTM system divides the Earth between 80°S and 84°N latitude into 60 zones, each 6° of longitude in width.
UTM Zone Number Zone 1 covers longitude 180° to 174° W; zone numbering increases eastward to zone 60 that covers longitude 174 to 180 East.
velocity v m/ns or m/us is the term used to characterize the speed at which GPR signals travel in a media.  Velocity is a critical parameter when creating depth slice images and estimating depths of targets since velocity is used to convert travel-time to depth.
Vertical Filter Applies a running average filter vertically (down-the-trace) to a GPR trace plot. The signal is averaged by replacing the data value at a given point by the average data value over a window centered about that point. Its primary purpose is to reduce random or high frequency noise by acting as a low pass temporal filter.
wavelet or EM pulse Impulsive GPRs emit an oscillatory electromagnetic pulse which is short in time and space and is often referred to as a wavelet.
X Line Spacing the term used to refer to the spacing between X lines when a grid is covered by equispaced X lines.
X Line A line oriented in the X direction. (i.e. Y = constant while X position varies).  This is a Sensors & Software convention when collecting GPR data in a grid.
X Slice time or depth slice image created from X lines in a grid.
XY Axes X and Y are the names given to the two orthogonal directions of a grid. When positioned at the specific corner of the grid which is selected to be the origin of the coordinate system and facing diagonally across the grid, the positive X direction runs to the right along the edge of the grid and the positive Y direction runs straight ahead.
XY Slice time or depth slice image created from combining both X and Y lines in a grid.
Y Line Spacing the term used to refer to the spacing between Y lines when a grid is covered by equispaced Y lines.
Y Line A line oriented in the Y direction. (i.e. X = constant while Y position varies). This is a Sensors & Software convention when collecting GPR data in a grid.
Y Slice time or depth slice image created from Y lines in a grid.
zero time The time of first emission of signal by the transmitter.  This time should be equated to the first break time unless the transmitter-receiver separation is zero.
zone of influence     The size of an area on a reflecting feature that can be uniquely resolved. (See Fresnel zone definition and lateral resolution length).