## 2 Borehole Multi-Source Data Acquisition Technology
### Borelole Radar Technology  
Borehole radar technology can effectively detect the rock mass structure information around the borehole, and is an effective technical method for the detection of unfavorable surrounding rock geological bodies [22; 23; 24; 25; 26]. Borehole radar technology is a technology used to detect the rock mass structure around the borehole by determining the distribution of broad-spectrum electromagnetism in the underground medium. It can penetrate a certain distance in the geotechnical medium, and the frequency used is usually 50-250 MHz. The borehole radar technology uses one antenna to transmit high-frequency broadband electromagnetic waves, and the other antenna to receive the transmitted waves from the underground rock and soil media. Because the propagation of the radar wave is affected by the electromagnetic properties and geometric shape of the rock and soil, the strength and waveform of the electromagnetic wave at the receiving end will change accordingly. According to this, the structural characteristics of underground rock and soil media can be inferred from the two-way travel time, amplitude and waveform data of the electromagnetic wave at the receiving end. The detection schematic diagram is shown in Figure 6.  
The strength of the reflected signal of the electromagnetic pulse is related to the reflection coefficient of the interface and the absorption degree of the wave penetrating the medium. Generally, if the electromagnetic parameters of the medium vary greatly, the reflection coefficient is large, and therefore the energy of the reflected wave is also large. This is the prerequisite for the detection of borehole radar. The relationship formula is as follows:  
\[PR=\frac{\mathrm{P_{T}G^{2}\lambda_{0}^{3}RSL}}{(4\pi^{3})\mathrm{H^{4}}} \mathrm{RSLe^{-4\alpha R}} \tag{7}\]  
Among them, \(\mathrm{P_{T}}\) and PR are the transmitting and receiving power of borehole radar, respectively. \(\mathrm{G}\) is antenna gain. \(\mathrm{R}\), \(\mathrm{S}\) and \(\mathrm{H}\) are the reflectivity, scattering surface section and depth of the underground target, respectively. \(\alpha\) is the attenuation rate of rock and soil. \(\mathrm{L}\) is the scattering loss of radar wave from transmission to reception. \(\lambda_{0}\) is the wavelength of radar wave in the medium. The basic principle of borehole radar is similar to that of ground radar. Generally, it uses a common transceiver antenna. The antenna moves along the direction of the borehole and transmits broadband high-frequency pulse to the surroundings of the borehole. It can penetrate a certain distance in the rock and soil and transmit and obtain signals through the transceiver antenna. The probe is connected with the ground control unit through optical fiber for data acquisition and signal triggering. There are three modes for borehole radar measurement, namely single-hole reflection mode, cross-hole mode and VRP mode. Because the probe is located underground, the borehole radar can detect deeper geological targets than the conventional geological radar. The radial detection radius depends on the physical properties of the rock, and can sometimes reach hundreds of meters. Borehole radar is widely used in engineering geological surveys and environment-related fields due to its special capabilities. Borehole radar, as a geophysical exploration method, is based on radar wave propagation through the underground medium. It can penetrate a certain distance in rock or soil, and the frequency is usually 10-1000 MHz. The propagation of the radar wave depends on the dielectric constant \(\varepsilon\) (=\(\varepsilon_{F}\varepsilon_{0}\)), permeability \(\mu\) (=\(\mu_{F}\mu_{0}\)),  
Figure 6: Schematic diagram of borehole radar detection. conductivity \(\sigma\) and angular frequency w (=2\(\pi\)f). With regard to parameter Q = w\(\varepsilon\)/\(\sigma\), it determines whether a certain medium supports the propagation of radar waves. If for a certain medium Q >> 1, the energy of electromagnetic wave mainly propagates in the form of wave.