What is Ground Penetrating Radar?

Ground penetrating radar is an effective method for detecting underground targets developed in recent decades. It is a non-destructive detection technology. Compared with other conventional underground detection methods, it has fast detection speed, continuous detection process, high resolution, With the advantages of convenient and flexible operation and low detection cost, it is increasingly used in the field of engineering survey. ^[1]

Ground Penetrating Radar (GPR) is a geophysical method that uses antennas to transmit and receive high-frequency electromagnetic waves to detect the characteristics and distribution of material inside the medium. In the early days of GPR, there were various names. For example, Ground-probing Radar, Sub-surface Radar, Geo Radar, Impulse Radar, Surface Penetrating Radar, etc. An electromagnetic wave method for exploring targets and detecting the internal structure of geological targets using high-frequency pulsed electromagnetics. ^[2]

Ground penetrating radars often use antennas to emit high-frequency pulsed electromagnetic waves to detection targets for detection. Detecting target depth usually satisfies far-field conditions, which can be approximated as plane wave propagation. The polarization of a plane wave refers to the time-varying characteristics of the direction of the field vector at a given point in space. Generally can be divided into three types of linear polarization, circular polarization and elliptical polarization. Wave polarization is an important characteristic of electromagnetic waves. Waves with different polarizations have different engineering applications. When anisotropy exists in the subsurface medium, the plane wave is incident in a linearly polarized manner. Its reflected echo may be transformed into elliptical polarization. Therefore, by studying the changes in the polarization mode of radar waves, information related to the physical properties of the underground medium can be obtained.

The processing method of GPR data is similar to that of seismic data. The more common and traditional methods are generally zero drift removal, gain processing, band-pass filtering, and channel equalization. Deeper people can use two-dimensional filtering, offset homing, deconvolution and other methods for processing. With the development of wavelet technology and neural network. This method has also been satisfactorily applied in the processing of GPR data.

The ground penetrating radar is mainly composed of a host (main control unit), a transmitter, a transmitting antenna, a receiver, and a receiving antenna. Others may include positioning devices (such as GPS, odometer or mark (MARK)), power supplies, and trolleys. Transmitting and receiving antennas appear in pairs for transmitting and receiving radar waves reflected from the ground. The host is an acquisition system that sends and receives control commands (including start-stop timing, transmission frequency, number of repetitions, etc.) to the transmitter. The transmitter transmits radar waves to the ground according to the host command. The receiver starts data acquisition according to the control command. After sampling and A / D conversion, the received reflected signal is converted into a digital signal and displayed and stored. ^[3]

The kinematics of high-frequency electromagnetic waves are similar to those of elastic waves. Therefore, the data collection method of seismic survey is also used in the field acquisition of ground penetrating radar. Includes reflection, refraction, and transmitted wave methods. The refracted wave method is currently used less frequently. Here are just a few of the commonly used reflection and transmitted wave methods. The high-frequency radar antennas of some radar systems have fixed spacings between the transmitting and receiving antennas in a box. It is not possible to perform variable center offset (CMP) or transmission method measurements, and only profile measurements can be used. Other types of systems, especially low-frequency radar antennas (50, 100, 200 MHz), mostly use discrete plate antennas, which can flexibly adopt variable offset or transmission measurements.

Ground penetrating radar profile

Profile method is the most commonly used ground penetrating radar observation method. It is similar to the common offset acquisition method in seismic exploration. That is, the transmitting antenna and the receiving antenna are moved along the measurement profile in the same interval with a fixed antenna pitch and a certain measurement step (point distance). Data to obtain radar records over the entire profile. This is the observation method commonly used by most radar systems at present, only two channels are needed for transmitting and receiving, and the system design is relatively simple. The advantage of the section method is that the section results can be used for interpretation without the need or simple processing, and the measurement results can be obtained intuitively, which is very suitable for the occasions where the measurement results are urgently needed.

Ground Penetrating Radar Wide Angle Method

The wide-angle method has two working methods: one method is that one antenna is fixed at a certain point (regardless of the transmitting or receiving antenna); the other antenna moves along the survey line and collects data at equal intervals, and the obtained record is equivalent to seismic exploration Chinese Communist Party Records (CSP). Another method is to use a certain point on the ground as the center point, and the transmitting antenna and the receiving antenna are symmetrically located on both sides of the center point, and move along the survey line to both sides in a certain interval and collect data. The obtained record is similar to the Communist Central Point Record (CMP) of the seismic survey, and it is similar to the Common Depth Point Record (CDP) at the local subsurface level.

The purpose of using the wide-angle method is to obtain the radar wave velocity of the underground medium to provide data for time-depth conversion and data interpretation. The second is to achieve multiple levels of superposition to improve the signal-to-noise ratio. Multi-point measurement along the profile using this measurement method is similar to seismic exploration. It can obtain high signal-to-noise ratio radar data through dynamic and static correction and horizontal superposition processing, and at the same time, it can increase the exploration depth.

Ground Penetrating Radar Transmitted Wave Method

The transmission wave method mainly measures the arrival time of the direct wave passing through the measurement object to calculate the radar wave velocity, and judges the quality of the measurement object by the difference of the radar wave velocity passing through the measurement object. Therefore, the transmitted wave method requires the transmitting and receiving antennas to be separated on both sides of the measurement object. Since only the earliest direct wave is interpreted and calculated, waveform identification and calculation are relatively simple. The transmission wave method is mainly used for the quality inspection of walls, columns, pier and piles in engineering and radar measurement in wells. In-well radar surveys require two well bores to be arranged in advance, similar to seismic cross-bore surveys. The transmission wave method can also work with the observation method of tomography, so as to obtain more fine imaging of the velocity of the medium between the holes.

Ground Penetrating Radar 3D Measurement

With the improvement of the requirements of exploration targets, the information such as the burial depth and range of anomalous targets on the section that can be given by the 2D profile measurement can no longer meet the industry's requirements for detailed information such as the extension direction and spatial variation of the detection target. The scale of archeological targets is relatively small. It is difficult for the two-dimensional profile method to make the survey line just cross the detection object, and the interpretation of profile anomalies is also a problem. Therefore, 3D radar exploration is the trend and direction of archeological and geophysical applications. Some commercial radar systems can support 3D radar exploration from hardware equipment to processing software.

At present, GPR 3D survey is a pseudo 3D survey design, that is, a combination of multiple 2D sections is used to form an area 3D data volume, which is then processed and displayed by software. For radar systems with only one transmitting antenna and one receiving antenna. This pseudo 3D design is also a good alternative. With the development of electronic technology, the emergence of multi-channel instruments and equipment will bring about a revolution in 3D radar exploration technology.

In terms of efficiency, the low efficiency of profile point measurement also restricts the application of 3D radar. Some companies, such as SSI, use SMARTCART (trolley) equipped with an odometer or GPS positioning system, which can achieve fast mobile acquisition. Greatly improve the efficiency of 3D data collection.