I want to know the specific method and principle of urban underground cavity detector to detect underground cavities. Who knows?

Ground penetrating radar (GPR) is an effective shallow target detection equipment, which uses electromagnetic waves to detect the interface between two underground media with different electrical properties. Ground penetrating radar detection uses three basic methods: reflection, velocity ranging and tomography. The transmitting antenna emits high-frequency electromagnetic waves to the underground in the form of broadband short pulses on the ground, and the incident waves generate reflected waves at the interface (stratum interface or target body) between two media with dielectric differences. The receiving antenna can receive and record the waveform, amplitude and arrival time (two-way travel time) of the reflected wave, and display the detection results in the form of radar images. According to the measured bidirectional travel time and wave velocity, the depth of the target body is calculated; Continuously measure the reflected wave at each point of the profile to form a radar image.

Underground cavity detector reflects the characteristics of underground targets in the form of reflected wave waveform. When there is enough electrical difference (conductivity and dielectric constant) between the target body and the surrounding soil medium, reflected waves will be generated. The reflected waveforms of targets with different shapes have different characteristics. Because the dielectric constant of air is 1 and the dielectric constant of surrounding soil is about 9, there is obvious difference between the dielectric constants of underground cavity and surrounding soil, which can produce obvious reflected waves at the interface between them, so the underground cavity detector can effectively detect underground cavities.

The waveform characteristics of underground cavities are as follows: (1) the in-phase reflection wave group of the stratum which should be continuous is interrupted and discontinuous; Due to the filling of air, the intensity of reflected waves in the cavity becomes weak, and even the reflected waves in the cavity disappear; The characteristics of reflection wave group between karst cave and surrounding strata are obviously different.

The recent practice in Beijing, Deyang, Sichuan, Shanxi and other places proves that LD9000 urban underground cavity detector can effectively detect the distribution range and depth of underground cavities under suitable conditions (the soil around the cavities is relatively dry and uniform, and the surface is relatively flat), which is an effective method to detect underground cavities.