This article delves into the fundamental principles of GPR (Ground Penetrating Radar) technology and the intricacies of data processing, providing essential awareness for professionals in the field.
Ground Penetrating Radar (GPR) consists of a pulse transmitter and a receiver. The transmitter emits an impulse into the ground and the receiver immediately captures the (under)ground response. The reflected pulse arrives in the form of an EMW (Electro-Magnetic Wave) with phase and amplitude variations.
Wave changes the Phase (commonly represented by colors) only on Permittivity or Conductivity (or both) boundary. The interface properties, such as abrupt change (e.g. rock to air) or gradual transition (e.g. wet to dry sand), can be extracted from the shape and form of the electromagnetic wave. Thus, making the penetrated medium to be assessed for further excavation works, etc. The wave can be displayed on computer screen real-time during the measurements.
The wave record is represented by a narrow vertical data-column with amplitudes converted into colors; plotting these columns one after another forms the GPR section, this is the actual RADARGRAM.
Principally, a Radargram is a color image into which the reflected signals are converted using preprocessing routine for enhancing the image readability. To point out the specific structures in the subsurface investigation, various filters can be used.
The colors don’t indicate specific materials or objects. Shapes of the reflection, a contrast and color intensity are important for the interpretation.
PERMITTIVITY, CONDUCTIVITY and COLOR CONTRAST in RADARGRAMS
The more the permittivity (or conductivity) of two media varies depending on how bigger the color contrast is. The air has a permittivity of 1, soil and rock between 3 and 9, water around 81. The upper soil layers and the bedrock in fissures and cracks, are usually well saturated with water and hence it is easy to differentiate between them due to the significant variance in permittivity values.
Various ground disturbances such as trenching or disruptions for excavations are well illustrated on radargrams. Rock crevices or fissures, cracks, and sub-horizontal layering that comes with humidity changes are easily detectable and measurable.
CAPTURING REFLECTED WAVES by GPR
GPR waves reflect from horizontal structures easily. However, circular shaped objects (e.g. cavities, pipes) are more difficult to detect compared to horizontal structures, as GPR can capture the reflections at a maximum of 35-40 degrees, thus it is possible to only see the spherical cap of such an object in radargrams.
The floor of an underground chamber (tunnel, adit, cellar etc.) may occur too, but in the form of a series of strong replication stripes. These stripes are caused by multiple individual reflections, originating in the space between the ceiling and the bottom of a chamber. They are projected under the object’s floor because the receiving antenna obtains the reflection later and only after the first wave travels through cavity.
HYPERBOLIC and OTHER REFLECTIONS
Reflections from significant objects (in terms of permittivity/conductivity, e.g. metal objects, cavities, tunnels, etc.) are depicted in the shape of a Hyperbola with a Vertex on top which is caused by a Wide Antenna Radiation Pattern or Array, and a data logging method.
The hyperbola width is usually several times larger than the source object. The thickness of the hyperbola is biggest on its vertex and fades towards the edges (where it can still cover other reflections). Hyperbolas flatten with increasing depth. Hyperbolic reflections do not originate in the high wave attenuation environment.
Radargrams don’t show underground tunnel as a sharply bordered rectangle, though the ceiling will be well recognizable and sometimes sidewalls may be indicated and only rarely a bottom is shown. Moreover, the Georadar image under the ceiling becomes deformed as reflections deflect upwards (because electromagnetic wave is several times faster in free-air than in solid media). After passing through the ceiling, the wave arrives to the tunnel bottom much sooner than through the rock. This helps making the subsurface investigation object more apparent.
Hyperbola reflections occur on the edge of significant objects because of the Conical Antenna Radiation Pattern; meaning the GPR begins to receive reflections even before it is directly over the object. However, the wave travels a longer path and therefore it takes a longer period of time. The object is projected right under the transmitter/receiver position. Consequently, the reflection is recorded into greater depth than is the actual depth of the object – and a hyperbola is being plotted.
When the GPR is approaching to the object, a hyperbola reflection grows upwards until the antenna almost reaches the object (not always straight over the object). Then a hyperbola vertex is logged. While GPR continues in move, hyperbola arm decreases and so the reflection intensity diminishes.
- Vertical irregularities perpendicular to the profile direction are usually indicated by straight, sharp, inverted “V” shape (hyperbola).
- Loosened subsoil shows a number of tiny, individual reflections induced by various inclined surfaces and crevices or cracks.
- Homogeneous materials with higher water content slows the wave down. When the wave is propagated more slowly, single reflections thicken in a vertical direction (and hence form stripes).
- The upper layers of soil are primarily horizontal, sometimes slightly wavy and the reflections are clearly apparent.
- The rock is illustrated by a series of minor, short reflections. The intensity gets mitigated with increase in depth.
DEPTH DETERMINATION
Depth axes are lining both sides of the radargram. The GPR (Ground Penetrating Radar) records time of wave reflection arrival. Values in Nanoseconds occupy the left side of the radargram. Distance values, automatically calculated based on characteristic values for the velocity of wave propagation in model materials, are given in meters as depths on the right side of the radargrams. The wave velocity can also be estimated from the shape of hyperbola reflection (if they are present in the image).
AIR REFLECTIONS in RADARGRAMS
GPR (Ground Penetrating Radar) antennas transmit a signal not only to the ground, but also to the surrounding environment. Reflections from ground objects (fencing, poles and sticks, metal construction etc.) are called Air and Ground Reflections. They are comparatively easy to identify in the radargrams.
When approaching or moving away from the object, its reflection always forms a straight line. While passing the object, a Protracted Hyperbola arises. Those objects can be back-located in the terrain, depending on the recorded time of reflection and the calculated distance. It is also possible to partially filter them out from the radargram.
RADARGRAM ENHANCING SOFTWARE
Radargram processing softwares are generally equipped with various filters to suppress the above-mentioned difficulties in a radargram interpretation. The target frequencies can be emphasized and the disturbing ones can be suppressed. Multiple reflections can be eliminated like air reflections filtered out, etc. Such processing helps to expose minor changes in wave behavior and uncover irrelevant but essential and deep-located objects.
Compiled by:
Technical & Service Manager with experience in the Subsurface Utility Engineering (SUE) and Utility Locating/Mapping Surveys.
Dated: 10-08-2025