Your Questions About GPR Answered

This page is designed as a basic introduction to some of the key concepts of ground-penetrating radar (GPR).

GPR Equipment

A GPR system is made up of three main components:

1) Control unit, 2) Antenna and 3) Power supply as seen in Figure 1 below.

Figure 1: Complete GPR System

GSSI GPR equipment can be run with a variety of power supplies ranging from small rechargeable batteries to vehicle batteries and normal 110-volt current. Connectors and adapters are available for each power source type. The unit in the photo above can run from a small internal rechargeable battery or external power.

The control unit contains the electronics that produce and regulate the pulse of radar energy that the antenna sends into the ground. It also has a built in computer and hard disk to record and store data for examination after fieldwork. Some systems, such as the GSSI SIR-20, are controlled by an attached Windows laptop computer with pre-loaded control software. This system allows data processing and interpretation without having to download radar files into another computer.

The antenna receives the electrical pulse produced by the control unit, amplifies it and transmits it into the ground or other medium at a particular frequency. Antenna frequency is a major factor in depth penetration. The higher the frequency of the antenna, the shallower into the ground it will penetrate. A higher frequency antenna will also ‘see’ smaller targets. Antenna choice is one of the most important factors in survey design. The following table shows antenna frequency, approximate depth penetration and appropriate application.

GPR Method

GPR works by sending a tiny pulse of energy into a material and recording the strength and the time required for the return of any reflected signal. A series of pulses over a single area make up what is called a scan. Reflections are produced whenever the energy pulse enters into a material with different electrical conduction properties (dielectric permittivity) from the material it left. The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants of the two materials. This means that a pulse which moves from dry sand (diel of 5) to wet sand (diel of 30) will produce a very strong, brilliantly visible reflection, while one moving from dry sand (5) to limestone (7) will produce a very weak reflections. Materials with a high dielectric are very conductive.

While some of the GPR energy pulse is reflected back to the antenna, energy also keeps traveling through the material until it either dissipates (attenuates) or the GPR control unit has closed its time window (Figure 2). The rate of signal attenuation varies widely and is dependant on the dielectric properties of the material through which the pulse is passing.

Figure 2: GPR Emits a Pulse of Energy

Materials with a high dielectric are very conductive and thus attenuate the signal rapidly. Water saturation dramatically raises the dielectric of a material, so a survey area should be carefully inspected for signs of water penetration. Radar surveys should never be conducted through standing water, no matter how shallow. Depth penetration through a material with a high dielectric will not be very good.

Metals are considered to be a complete reflector and do not allow any amount of signal to pass through. Materials beneath a metal sheet, fine metal mesh, or pan decking will not be visible.

Radar energy is not emitted from the antenna in a straight line. It is emitted in a cone shape (Figure 3). The two-way travel time for energy at the leading edge of the cone is longer than for energy directly beneath the antenna. This is because that leading edge of the cone represents the hypotenuse of a right triangle.

Figure 3: Radar Energy is Emitted in a Cone Shape

Because it takes longer for that energy to be received, it is recorded farther down in the profile. As the antenna is moved over a target, the distance between them decreases until the antenna is over the target and increases as the antenna is moved away. It is for this reason that a single target will appear in a data as a hyperbola, or inverted “U.”  The target is actually at the peak amplitude of the positive wavelet (Bottom Figure 3).

Data are collected in parallel transects and then placed together in their appropriate locations for computer processing in a specialized software program such as GSSI’s RADAN. The computer then produces a horizontal surface at a particular depth in the record. This is referred to as a depth slice, which allows operators to interpret a planview of the survey area.

Data Processing

In many situations, a GPR operator will simply note the location of a target so that it can be avoided. For these clients, it may only be necessary to use a simple linescan format in order to mark the approximate area of the target on the survey surface. Other clients may require detailed subsurface maps and depth to features. These situations will require the operator to use GSSI GPR processing software, which applies mathematical functions to the data in order to remove background interference, migrate hyperbolas, calculate accurate depth and much more.