Ground Penetrating Radar

    Parts of the discussion of GPR techniques are taken directly from a set of course notes written in 1992 by A.P. Annan,President of Sensors and Software Inc. Information: SENSORS AND SOFTWARE  (Email radar@sensoft.on.ca).

    GPR -- High Frequency Electromagnetic Energy
    Ground penetrating radar (GPR) is the general term applied to techniques which employ high frequency (20-2,000 MHz) electromagnetic waves to map lithologic structures or buried objects within the ground.

    Earth scientists have used radar for several years in experimental and research environments. Recently the practical and successful application of these geophysical techniques to engineering and geologic has grown rapidly.
    The application of GPR to an engineering or geologic problem can be analyzed with respect to the following principles and characteristics:

    MICROGEOPHYSICS CORPORATION has completed radar surveys of objects at depths from 2 inches to 20 feet. A variety of modern computer driven instruments are used depending on the circumstances and availability. MicroGeophysics Corporation (MGC) has access to the two-dimensional software of Dr. Mike Powers and would be happy to model the geometry of your problem to assist you in designing your program.

    GPR profiles are spectacular when successful (detection of plastic gas pipes in sandy soil, shallow lithology, and buried drums). GPR is not recommended by MicroGeophysics Corporation (MGC) in many areas of the country. High attenuation in conductive soils renders GPR impractical in these areas. An example is the Denver metro area where the near surface is dominated by heavy clay soils. This unpredictability of performance usually demands the use of a test phase over a known situation.

    Electrical Properties
    Ground penetrating radar systems make use of electromagnetic waves which propagate into the earth. The manner in which the electromagnetic waves propagate is determined by the physical properties of the earth materials.
    In most geologic settings the electrical properties are the dominant factor in wave propagation; magnetic properties are significant in some rare cases, but those will not be considered here.

    An electric field applied to earth materials gives rise to the movement of electrical charges. Charge movement is accomplished through two current mechanisms: conduction and displacement. The rate of change of the applied electric field dictates which mechanism will dominate.

    In general, displacement currents dominate at high frequencies and conduction currents dominate at low frequencies. A transition frequency is defined as the frequency that lies between the conductive current region and the displacement current region. The transition frequency varies depending on the conductivity and dielectric constant of the host materials.

    Conduction currents result from the flow of free charges in the presence of an electric field. As charges move through the conducting material, they dissipate energy in the form of heat, because of interaction with their surroundings. Therefore, electromagnetic waves propagating in a conductive medium are dissipated.

    Displacement currents are associated with charges that are constrained from moving long distances, such as electrons in a non-conductive medium, or molecular dipoles. When an electric field is applied, the constrained charges move to a new equilibrium position and return to their original position when the field is removed.

    Charge separation (charge movement) is described in terms of a dipole moment density, which is directly proportional to the applied electric field with a proportionality constant that is called the dielectric permittivity. It is common to normalize the permittivity of a material by the permittivity of free space (a vacuum). The normalized value is called the dielectric constant or the relative permittivity of the material.

    Water is the dominant variable that controls electrical properties of most geologic materials. Water molecules have a natural dipole moment which corresponds to a high dielectric constant. Water is capable of holding a large amount of dissolved ions which gives it a higher conductivity; therefore, most natural waters with some amount of dissolved ions strongly attenuate radar waves.

    Radar Wave Propagation and Reflection
    If an impulsive current is applied to a small dipole antenna, an electromagnetic field is generated which propagates through the earth as a function of time and distance.

    The dominate wave types found in GPR are spherical waves and lateral waves (similar to a refracted wave along a boundary). The velocity and attenuation of propagating waves change as a function of frequency. Below the transition frequency, conductive mechanisms dominate and the electromagnetic field diffuses (by electromagnetic induction) into the host material. Conventional time domain and frequency domain methods operate in the inductive realm.

    At frequencies above the transition frequency, the electromagnetic waves propagate within the earth. Ground penetrating radar systems are designed to work in the high frequency, propagating realm; however, highly conductive media ( > 100 mS/m) raise the transition frequency to the range of some commercial GPR systems. In this case the GPR systems undersirably operate as time-domain electromagnetic instruments.

    The distance a radar wave can travel before its amplitude decreases by 100 dB is a useful measure of the permissible attenuation of a radar impulse, because 100 dB is representative of the dynamic range for commercial radar systems. For a conductivity of 200 mS/m the distance at which the wave can be detected is less than 1 meter. Attenuation is more dependent on the conductivity than on the dielectric constant.

    As GPR wavelengths are short in most earth materials, resolution of interfaces and discrete objects is very good. However, penetration of less than a meter can be expected in highly attenuative media. Water and clay soils increase the attenuation, decreasing penetration. A conservative rule-of-thumb is to state that radar will be ineffective if the actual target depth is greater than 50 percent of the system's maximum range.

    Radar waves propagate into the earth and are partially reflected at electrical impedance boundaries. Electrical impedance changes are dominated by changes in the dielectric constant. A portion of the radar wave is reflected at electrical impedance boundaries and another portion is transmitted through the boundary. The amplitude of reflected and transmitted energy is dependent on the reflection coefficient. Reflecting interfaces may be soil horizons, voids, soil/rock interfaces, man-made objects or any other interface possessing a contrast in dielectric properties. The dielectric properties of materials correlate well with many of the mechanical and geologic parameters of materials.

    Ground Penetrating Radar Instrumentation
    Ground penetrating radar uses a high-frequency (20 to 2000 MHz) electromagnetic pulse transmitted from a radar antenna through the earth. The radar signal is propagated into the ground by an antenna that is in close proximity to the ground. The transmitted radar pulses are reflected from various interfaces within the ground and return to be detected by a radar receiver antenna. The reflected signals are processed and displayed on a graphic recorder. As the antenna is moved along the surface, this display results in a cross-section record (depth vs. time) of the reflected radar signatures of the earth.

    The center frequency of the input radar signal's bandwidth is referred to as the system's operating frequency. The bandwidth is generally the same as the operating frequency: i.e., a 120 MHz GPR system will have a center frequency of 120 MHz with a total bandwidth of 120 MHz (from 60 MHz to 180 MHz). The lower frequencies propagate with less attenuation in general. In MGC's experience, the frequency chosen is often too low, resulting in poor resolution.

    Interpretation of Ground Penetrating Radar Data
    The objective of GPR surveys is to map near-surface interfaces or anomalous objects. For many surveys, the location of objects such as tanks or pipes in the subsurface is the objective. The dielectric properties of materials are not measured directly, thus the method is most useful for detecting changes in the geometry of subsurface interfaces. Geologic problems conducive to solution by GPR methods are numerous and include the following: bedrock configuration, location of pipes and tanks, location of water table, borrow investigations, and others.

    The geologic and geophysical objectives determine the specific field parameters and techniques. Delineation of the objectives and the envelope of acceptable parameters are specified in advance. However, as the results cannot be foreseen from the office, considerable latitude is given to the field geophysicist to incorporate changes in methods and techniques. The useful item of interest recorded by the GPR receiver is the train of reflected pulses. An analogy to the seismic reflection method is appropriate because two reflection methods used in seismic reflection, common offset and common midpoint, are also used in GPR. The most common mode of operation is the common-offset mode where the receiver and transmitter are maintained at a fixed distance and moved along a line to produce a profile. Energy does not necessarily propagate downwards. Reflected energy will be received from the sides of objects. Therefore, the reflected image from a buried object may be laterally larger than the actual buried object. An added complication of GPR is the fact that some of the energy is radiated into the air and, if reflected off of nearby objects like buildings or support vehicles, will appear on the record.

    A primary focus of GPR investigations is the search for buried objects, as opposed to the delineation of stratigraphic features. Buried objects often appear to be point scatterers to the propagating radar wave. Using simple geometry it is possible to determine that reflected energy from a point scatterer will spread out in the form of a hyperbola on the GPR record. The buried object is generally located at the horizontal position of the apex of the hyperbola. The time to the apex of the hyperbola is indicative of the depth to the object. An absolute depth calculation requires that the velocity of radar waves above the buried object must be known, which is often not the case. It is also possible to locate a buried object by a lack of reflected energy. It is possible that during installation of the object, shallow stratigraphic layers were disturbed. A GPR profile would possibly show a horizontal reflection over the undisturbed ground, and then a lack of reflected energy over the area of disturbed ground fill. In this case the object would not have been directly detected, but evidence for the existence of the object would be strong.

    MicroGeophysics Corporation (MGC) can model your geologic situation (if necessary) and assist you in selecting the optimum frequency for depth penetration and for resolution of your target.Questions are welcome! Call 1-800-GEOPHYSics or fax 303-424-0807 or E-mail microgeo@aol.com.