Principle of GPR and data processing

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 electromagnetic wave with phase and amplitude variations. Wave changes the phase (represented by colours) only on permittivity or conductivity (or both) boundary. 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 of the electromagnetic wave. Therefore, the penetrated medium can be assessed. The wave can be displayed on computer screen real-time during the measurement.

The wave record is represented by a narrow vertical data-column with amplitudes converted into colours; plotting these columns one after another forms the GPR section, an actual radargram.

Basically, a radargram is a colour image into which the reflected signals are converted using preprocessing routine for enhancing the image readability. Various filters are used then to point out specific structures in subsurface.

The colours don’t indicate specific materials or objects. Shapes of the reflection, a contrast and colour intensity are important for interpretation.

Permittivity, conductivity, and colour contrast in radargrams

The more the permittivity (or conductivity) of two media varies, the bigger the colour 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, therefore it is easy to distinguish between them due to the significant difference in permittivity values. Various ground disturbances such as trenching or disruptions for excavations are well depicted on radargrams. Rock crevices, cracks, and subhorizontal layering coming with humidity changes are easily detectable.

Capturing reflected waves by GPR

GPR waves reflect from horizontal structures easily. Circular shaped objects (e.g. cavities, pipes) are more difficult to detect as GPR can capture the reflection in the angle of 35­-40 degrees at maximum, so it is possible to see only spherical cap of such an object in radargrams.

The floor of underground chamber (tunnel, adit, cellar etc.) may occur, too, but in the form of a series of strong reflection stripes. These stripes are caused by multiple individual reflections, originating in the space between a ceiling and a bottom of the 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…) are depicted in the shape of a hyperbola with a vertex on top, which is caused by a wide antenna radiation pattern and a data logging method. The hyperbola width is usually several times larger than the source object. The thickness of hyperbola is biggest on 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 won’t show underground tunnel as a sharply bordered rectangle; the ceiling will be well recognizable, sometimes sidewalls will 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 the 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 subsurface object more apparent.

Hyperbola reflections occur on the edge of significant objects because of conical antenna radiation pattern: GPR is not yet over the object but already receives a reflection from it. However, the wave travels longer path so it takes longer time. Nevertheless, the object is projected right under the transmitter/receiver position. Thus, 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 is ‘growing’ 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 reflection intensity lowers.

• Vertical anomalies 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.

• Homogeneous materials with higher water content slow the wave down. When the wave is propagated more slowly, single reflections thicken in vertical direction (and form stripes).

• The upper layers of soil are mainly horizontal, sometimes slightly wavy, reflections are clearly apparent.

• The rock is depicted by a series of minor, short reflections. Its intensity is attenuated with increasing depth.

Depth determination

Depth axes are lining both sides of the radargram. GPR 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 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 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, thanks to the recorded time of reflection and the calculated distance, back-located in the terrain. It is also possible to partially filter them out from the radargram.

Radargram enhancing software

Radargram processing software is equipped with various filters to suppress the above-mentioned difficulties in radargram reading. Target frequencies can be emphasized and disturbing ones can be suppressed. Multiple reflections can be eliminated, air reflections filtered out, etc. Such a processing helps to reveal minor changes in wave behaviour and uncover insignificant, but important and deep-located objects.