It is based on the principle that electromagnetic waves will reflect when encountering different object interfaces during underground propagation. It is a geophysical exploration method based on the difference in dielectric constants of different underground media. It transmits high-frequency electromagnetic pulses to the underground through the transmitting antenna. When this pulse encounters the changing interface of the stratum during the underground propagation, it will generate reflected waves. After the reflected waves of the illuminance meter propagate back to the surface, they are received by the receiving antenna, and their waveforms are transmitted to the host for recording and display. After analyzing and processing the abnormal information on the radar image, the location distribution, burial depth and other information of the target body can be obtained by inversion. This method is used to measure the pore depth and size of the soil layer, the bedding of the concrete pipe and the range of saturated water seepage, as well as the foundation under the pipe. However, its output image is relatively complex, and rich experience is required to make accurate judgments.

Impact echo method When a heavy object or a heavy hammer hits the pipe wall, a stress wave will be generated. The stress wave propagates through the pipe, and the underground microphone can detect the cracks inside the pipe and the reflected waves generated on the outer surface. When the waves propagate at different speeds and scatter into the soil outside the pipe through different paths, the special surface wave analyzer is used to separate the waves into components of different frequencies, and relevant information about the pipe structure and external soil can be obtained.

The principle of the infrared thermometer recorder method is that the leakage point of the drainage pipe forms a temperature difference with the surrounding soil. The infrared temperature recorder is used for measurement and recording, and the extremely small temperature changes are measured and an automatic temperature image is generated. This method can detect pores and leakage on the surface of the pipe wall and the surrounding soil layer, but it cannot find out the pore size. This method is usually used to detect large-diameter empty concrete pipes and brick pipes

Surface wave spectral analysis method This method uses auxiliary sensors and a spectrum analyzer for analyzing surface waves, so it is easy to distinguish problems caused by the pipe wall and the surrounding soil, and can detect the pipe wall and soil conditions at the same time. The above-mentioned external pipeline detection technologies are all for the detection of drainage pipe defects through instruments. The advantages are non-destructive detection of pipelines and avoiding the danger of manual inspection in the well, but there are disadvantages such as single detection content, great environmental impact, non-intuitive collected data, and the need for rich experience to make accurate judgments.