Ground Penetrating Radar
Terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Antenna Radiation Pattern:
A plot of the intensity of the radiation received at a given radial distance from an antenna versus angle, relative to a given reference axis. The pattern is a three dimensional measure of the energy at a fixed radial distance from the antenna.

Attenuation:
A measure of the loss of radiated signal amplitude or signal energy as it progresses through a lossy medium. The loss can be due to a spreading loss as the wave expands out into the medium and also due to an ohmic loss, due to the finite conductivity of the medium.

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B

Backscatter:
A portion of a radar's transmitted energy that is intercepted by a target, or other object and reflected (scattered) back in the radar's direction.

Bandwidth:
The band of frequencies occupied by the central lobe of the spectrum of an electromagnetic signal. Bandwidth is usually defined so that it includes most, but not all of the signal power. Generally, it includes the portion lying between the points at which the power has dropped to half that at the center of the band.

Beamwidth:
The angular width of a slice through the main lobe of the radiation pattern of an antenna.

Bias:
The amount by which the average of a set of values departs from a reference value.

Bistatic:
The survey method that utilizes two separate antennas at a constant distance. One antenna has a transmitter and the other contains a receiver sampler. With this method it is possible to transmit a higher power signal and receive reflections with a greater time delay.

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C

Common Depth Point Method:
A survey method that can be performed to calculate the electromagnetic velocity of a material. This is conducted by transmitting from one antenna and receiving from a second antenna at several known offsets (surface horizontal distance).

Clutter:
Unwanted reflections from the ground, from within the ground or from above the ground. In the case of ground penetrating radar (GPR), clutter may be produced by boulders, soil interfaces and other scatterers that are not of interest. Clutter is also produced within the radar system.

Conductivity:
The electrical conductivity of a dielectric material is a measure of the ease with which an electrical current can be made to flow through it. In the MDS system, the unit of conductivity is the Siemens per meter (S/m). Conductivity is the reciprocal of resistivity. The higher the conductivity of the subsurface materials, the greater the attenuation of the radar signal.

Control Unit (C/U):
An electronic instrument that interfaces a transducer (s) to recorders, processors, displays, survey wheel, power supply, etc. It also has controls to allow radar functions such as range, gain and filtering to be adjusted. A C/U can be analog, digital, or hybrid.

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D

Decibel (db):
A unit of measure for gain. A logarithmic unit used to express power ratios. One decibel equals 20 LOG (P2/P1). Decibels are also used to express voltage ratios.

Deconvolution:
A digital signal processing function designed to attenuate multiples and improve the recognition and resolution of reflected events. A process that restores a waveshape to the form it had before it underwent a linear filtering action (convolution).

Delay Time:
The amount of time the radar wave propagates through a material, reflects off an interface and returns to the receiver. Time lag introduced by either hardware (cable length, etc.) or software ( filter averaging, etc.).

Depth of Penetration:
In any medium, the radar wave is attenuated as it progresses due to lossesthat occur. At radar frequencies in a conductive material (sea water, metallic materials, clay soils, etc.) the rate of attenuation is very great and the wave may penetrate only a short distance (<1m) before being reduced to a negligibly small value. In a resistive dielectric earth material (fresh water, granite, quartz sand, etc.), where the losses are low, the depth of penetration can be quite great (>30m).

Dielectric Constant:
See Dielectric Permittivity.

Dielectric Permittivity:
Dielectric Permittivity is a property of an electrical insulating material (a dielectric) equal to the ratio of the capacitance of a capacitor filled with the given material to the capacitance of the identical capacitor filled with air. Earth materials are classified generally as conductors, semiconductors and insulators (dielectrics). A dielectric material is a poor conductor of electric current. The specific capacitance of a vacuum is Eo = 8.85 x 10- 12 Farads per meter. The relative dielectric constant, Er for air is 1 and is approximately 81 for fresh water.

Dielectric Interface:
A place in the subsurface of a dielectric material where the dielectric permittivity changes.

Diffraction:
The phenomenon that causes electromagnetic waves in the beam of a direction antenna to spread out. The bending of wave energy around obstacles without obeying Snell's Law. An event that occurs at the termination of curved topped, or steeply dipping reflectors that is characterized by a distinctive curved alignment.Geophysical Survey Systems, Inc. RADAN 6.5

Diffuse:
To break up and distribute the energy in an incident electromagnetic wave in many Directions

Dipole:
A simple antenna having two elements driven from the center of the antenna by a balanced source.

Directivity:
Ability of an antenna to concentrate transmitted energy in a given direction and to emphasize the returned energy received from that direction.

Dynamic Range:
The spread between the minimum signal at the input of a system, which produces a discernible change in the output and the maximum input that the system can handle without saturating. Measured in decibels (db).

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E

Electromagnetic Wave:
A wave that is propagated by the mutual interaction of electric and magnetic fields. Radiant heat, light and radio waves, are electromagnetic waves.

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F

Frequency:
The number of positive and negative voltage amplitude cycles that a pure unmodulated sine wave completes per second. The unit of frequency is Hertz (Hz).

Frequency Domain:
Mathematical realm in which the amplitudes of signals are expressed as functions of frequency, rather than time. The frequency spectrum of a time varying signal is obtained by translating the expression for the signal from the time domain to the frequency domain.

Frequency Domain (FK) Filter:
Also called a velocity filter. A digital signal processing function that discriminates on the basis of apparent velocity. Coherent arrivals with certain apparent velocities are attenuated.

Fresnel Zone:
The portion of a reflector from which reflected energy can reach a detector within one-half wavelength of the first reflected energy.

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G

Gigahertz:
A unit of frequency. One GHz equals 109 Hz.

GPR:
Acronym for Ground Penetration Radar.

GPS:
Acronym for Global Positioning System.

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H

High Pass Filter:
A filter that passes without significant attenuation frequencies above some cutoff frequency while attenuating lower frequencies. The same as low-cut filter.

Hilbert Transform:
A digital signal processing function that determines the magnitude envelope, instantaneous phase and instantaneous frequency of the received signal.

Horizontal Filter:
A digital signal processing function that attenuates signals outside the filter function across adjacent scans.

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I

Interface:
The common surface separating two different media in contact. A location in the subsurface where the dielectric constant changes.

Isotropic Radiator:
An antenna that radiates equally (both in amplitude and in phase) in all directions. The imaginary source of the radiation used as a reference for the gain of a directional antenna.

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L

Low Pass Filter:
A filter that passes frequencies below some cutoff frequency while substantially attenuating higher frequencies. Same as a high-cut filter.

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M

Megahertz:
A unit of frequency. One MHz equals 106 Hertz.

Migration:
A digital signal processing function that rearranges data so that reflections and diffractions are plotted at the locations of the reflectors and diffracting points rather than with respect to observation points on the profile. Migration by computer is accomplished by integration along diffraction curves (Kirchhoff migration), by numerical finite-difference downward-continuation of the wave equation and other algorithms.

Monostatic:
A survey method that utilizes a single transducer with either a single antenna with transceiver or a dual antenna transducer with separate electronics.

Multiple:
Also known as ringing. Wave energy that has been reflected more than once. In radar data multiples may occur when there is a large change in the dielectric permittivity or conductivity of the medium. A multiple can be identified by signals that have the same time delay as between the surface and the first reflector.

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N

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P

Phase:
Degree of coincidence in time between a repetitive signal, such as a sine wave and a reference signal, having the same frequency. The angle of lag or lead of a sine wave with respect to a reference. Generally expressed in degrees. 360 degrees corresponds to the period of the signal.

Point Reflector:
A subsurface feature with electromagnetic properties different from its surroundings, whose dimensions are approximately the same as the fresnel zone of the radar wave.

Polarization:
The orientation of the electric and magnetic fields of an electromagnetic wave, such as a radio wave. By convention, the polarization of the wave is the direction of the electric field. If the polarization does not change as the wave propagates, the polarization is said to be linear.

Power:
A measure of the quantity of electric energy, commonly expressed in Watts. One watt equals one Joule per second.

Profile:
A graph showing the depth measurements as a vertical cross section of the medium along a horizontal line.

PRF (Pulse Repetition Frequency):
The number of pulses per second transmitted by a pulsed radar.

Propagation:
The outward spreading, or travel, of an electromagnetic wave, such as a radio wave.

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R

Radar Cross Section:
A factor relating the power of the radio waves that a radar target scatters back in the direction of the radar, to the power density of the radar's transmitted waves at the target's range. Takes account of the cross sectional area of the target, as viewed by the radar, the target's reflectively and its directivity.

Radiation:
Energy in the form of an electromagnetic wave emitted by an antenna, in which free electrons are accelerated. Radiant heat, light and radio waves are electromagnetic radiation. They differ only in wavelengths.

Range:
The radial distance from a radar to a target or other object. A user adjustable setting on a control unit to determine the amount of time displayed and recorded (0 - thousands of nanoseconds).

Range Gain:
Also known as time gain control or time varying gain. Control for varying the amplification or attenuation of an amplifier, used to compensate for variations in input signal strength over time.

Receiver:
The portion of the antenna used, to intercept the radio waves reflected from the subsurface and convert them back into electrical impulses (receiving antenna).

Reflection:
The degree to which an object returns incident radio waves.

Reflection Coefficient:
A description of the reflected field strength from an infinite interface between two media 1 and 2. The reflection coefficient r is defined by: r= Z2-Z1 / Z2+Z1where Z is the impedance of the respective media.

Refraction:
The bending of an electromagnetic wave that occurs when the wave passes obliquely from one medium into another whose dielectric constant is different from that of the first medium. The bending results from the speed of the propagation being different in one medium than in the other. Refraction may also occur in a single medium whose dielectric constant gradually changes in a direction normal to the wave's direction of propagation.

Resistivity:
The reciprocal of the electrical conductivity is the electrical resistivity. The electrical resistivity of a substance is a measure of the difficulty an electrical current can have flowing through it. In the MKS system the unit of resistivity is the ohm-meter ( -m).

Resolution:
The ability to separate two features that are very close together. The minimum separation of two bodies before their individual identities are impossible to interpret. The smallest change in input that will produce a detectable change in output.

Running Average:
A digital signal processing function that averages a finite number of scans with a user designated function to produce a single output scan. This method can be operated in continuous or static modes.

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S

Scan:
One discrete sequence of events such as a sampling at all time points the amplitudes at a receiver.

Scatter:
The irregular and diffuse dispersion of energy caused by imhomogeneities in the medium through which the energy is traveling.

Signal Position:
The relative delay between the time when the Radar system sends out a transmit pulse and when the Radar systems sends out a receiver pulse.

Signal-to-Noise Ratio:
Ratio of the power or energy of a received signal to the power or energy of the accompanying noise.

SIR:
Acronym for Subsurface Interface Radar.

Spectrum:
The distribution of the power or energy of a signal over the range of possible frequencies is commonly represented by a plot of amplitude versus frequency. If the amplitude is a voltage, a plot of the square of the amplitude is the power spectrum. The area under the power spectrum corresponds to the signal's energy.

Specular Reflection:
Mirror-like reflection occurring when an electromagnetic wave strikes a flat surface, the irregularities (roughness) in which are small compared to the wavelength of the incident wave.

Static Correction:
Corrections applied to data to compensate for the effects of variations in elevation, weathering thickness, weathering velocity, or reference to a datum.

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T

Transducer:
An antenna or antennae with built-in or plug-in transmitter and receiver electronics.

Transducer Input Connector:
The connector on the control unit that connects the cable from the transducer/antenna.

Trans-illumination:
This a method of surveying where a transmitting antenna transmits one way through a material to a receiving antenna. Example; crosshole investigations. Transect: The line along the surface that a profile is acquired.

Transmit Pulse:
The impulse of radar energy from the transmitting antenna as seen at the receiving antenna. This is shown on the recorded data at the top of the display. When the transducer is moved on the ground surface the first arrival of the transmit pulse is interpreted as the surface.

Transmitter:
The electronics, which after receiving a trigger pulse from the control unit, sends an impulse of electromagnetic energy to the attached antenna.

Travel Time:
The amount of time (nanoseconds) that the radar signal takes to travel from the transmitting antenna to a target or receiving antenna. This is used for transillumination methods (also called Transit Time).

Trigger Pulse:
Pulse generated in the control unit that is sent through the cable to the transmitting antenna.

Two-way Travel Time:
The amount of time (nanoseconds) that the radar signal takes to travel from the transmitting antenna, reflect off a target and return to the receiving antenna. This is used for most standard GPR field methods.

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V

Vertical Filter:
A digital signal processing function that attenuates signals outside of the filter function for each individual scan

Vertical Resolution:
The ability to separate two feature within one scan that are very close together. The minimum separation of two bodies before their individual identities are lost on the resultant map or cross-section. A function of transducer frequency, sampling interval and range.

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Some of the above definitions are from the following sources:
Encyclopedic Dictionary of Exploration Geophysics, Third Edition, R.E. Sheriff, Society of Explorations Geophysicists, Tulsa, OK, 1991.

EEE Standard Dictionary of Electrical and Electronics Terms, John Wiley and Sons, Inc., New York, NY, 1972.

Van Nostrand's Scientific Encyclopedia, Fifth Edition, D.C. Considine, Van Nostrand Reinhold Company, New York, NY, 1976

Courtesy of GSSI Geophysical Survey Systems, Inc. "RADAN 6.5 User's Manual "