Processing and interpretation of ground-penetrating radar (GPR) data for detection of cavities, investigation of bedding and grain sizes and also estimation of clay content in shallow subsurface sediments



Ground penetrating radar (GPR) method as a high resolution non-destructive geophysical method is used for detection of shallow subsurface targets. This method is based on the transmission of electromagnetic waves inside the earth and recording the reflected waves from the subsurface. As the method uses high-frequency electromagnetic waves in the frequency ranges of 12.5 to 2500 MHz (called GPR waves), it can only be used for shallow subsurface investigations. Using this method, continuous images of the reflections of GPR waves from the interfaces of subsurface media with different electrical properties are obtained. Shallow cavities, due to their different electrical characteristics from the background, are among the targets which can be detected by this method. Since the depth of penetration of GPR waves in an area is controlled by the electrical conductivity and permittivity of the ground of the survey area, the depth of penetration of GPR waves, where fine-grained sediments are present, is relatively lower due to higher electrical conductivity of fine sedimentary grains compared to coarse sedimentary grains. Thus, the relative grain sizes and clay content of the subsurface sediments can be investigated by the GPR method. In general, shallow subsurface structures, having different materials and thicknesses, can be detected by the method as the structures and their host media normally have different electrical (namely, conductivity and permittivity) properties. In this research work, the GPR data acquisition has been carried out using 250 MHz shielded antenna along 5 lines in Darkhanyab area near Mojen Town, which is located at the distance of approximately 25 km northwest of Shahrood City. The purpose of this GPR survey is to detect shallow subsurface structures such as the water Qanat in the area. Due to the low distance between the GPR transmitter and receiver as well as the electrical properties, especially the conductivity of the ground, and also, to remove the unwanted low frequency signals or reflections while preserving the high frequency signals, the Dewow filter was applied before any other processing to all GPR data sets. Short time intervals between the transmitted GPR pulses and the pulses received directly from the air-ground surface, and also, the existence of reflections from the shallow subsurface targets, cause signal saturation in the receiver. For this, the Dewow filter is applied on the GPR data to correct for signal saturation or Wow in the data. Different types of gains are also among the processing methods applied on the data to reduce the attenuating effect of the GPR waves as the depth increases. To demonstrate the effects of different gains and to select the optimum gain, we applied different gains on the GPR data. To convert the trace from a wavelet with both positive and negative components (i.e. sine or cosine nature) to a mono-pulse wavelet with positive components, we used the envelope filter. This process removes the oscillatory nature of the radar wavelet and shows the data in its true resolution, making it easier to interpret. In this research, for processing the two-dimensional (2-D) GPR data or sections, Win_Ekko_Pro software was used. In addition, for three-dimensional (3-D) processing and modelling of the GPR data, EKKO-Mapper and EKKO-3D software programs were used. To display the output data from the Win_Ekko_Pro and EKKO-3D software programs, we also used T3D software. These software programs have been developed by Canadian Sensors & software Company.
The results of this research work indicate that using the characteristics of GPR waves in the 2-D GPR sections; we can detect the subsurface targets such as cavities and discriminate coarse-grained sediments from fine-grained sediments, and also, determine the electrical properties of subsurface layers with high success. High resolution of the GPR data in this research have enabled us to determine the water qanat interfaces with its surroundings such as soil-concrete, concrete-air, air-water and water-concrete interfaces in the subsurface. Furthermore, the high conductive clayey soils above the water qanat canal in some places cause high attenuation of the GPR waves, and thus, highly limit the depth of penetration of the GPR waves. This phenomenon is also seen in the surrounding zone of the water qanat canal that mainly occurs due to the seepage of water to the ground. The soil bedding can also be easily observed in the obtained GPR sections. The horizontal soil layers, having thicknesses of several centimeters, have covered the surface of the ground in the survey area. A high resistive subsurface zone in the GPR sections, characterized by the ringing phenomenon, is interpreted as a cavity. In general, the relative high conductivity of the ground in the area causes to have a limited depth of penetration of the GPR waves that rarely exceeds 2 meters. The location of the water qanat in the shallow subsurface of the area was evident from the 2-D and 3-D GPR modeled sections. However, the detection of the water qanat in depths greater than 1 or 2 meters was difficult or even impossible from the GPR results due to the limited depth of penetration of the GPR waves in the area. Overall, it was possible to discriminate coarse-grained sediments from fine-grained sediments, and to some extent, to determine the amount of clay content and moisture in the subsurface from processing, modeling and interpretation of GPR data.