## 2 Photo-Acoustic Combined Measurement Technology in Boreholes  
The borehole photo-acoustic combined measurement technology is a comprehensive fine detection technology of rock mass structure in the borehole that combines optical image technology and acoustic circumferential scanning technology [24]. The optical image information of the borehole wall can be acquired by the optical measurement component. The digital image processing of the image information can finely reflect the optical texture characteristics of the borehole rock mass structure. The acoustic measurement component can capture the full pulse information reflected by the acoustic wave pulse after encountering the rock wall. The digital signal processing of the full pulse echo information can finely reflect the acoustic response characteristics of each scanning point on the borehole wall. On the basis of combining the acoustic time signal processing, it can effectively reflect the geometric characteristics of the borehole profile. The instruments and equipment corresponding to the borehole photo-acoustic combined measurement technology are mainly composed of five parts: in-hole probe, transmission cable, depth encoder, integrated control box and computer. The schematic diagram of the test principle of applying the borehole photo-acoustic combined measurement technology to the geological borehole is shown in Figure 1.  
The detection process of weak intercalation in geological borehole is as follows: first, lower the probe in the hole to the upper edge of the AB area to be detected in the geological borehole, and complete the optical image and acoustic scanning signal acquisition of the A  
Figure 1: Schematic diagram of working principle. area. Then, lower the probe in the hole at a constant speed. After the optical and acoustic signal acquisition in this area is completed, continue to lower the probe in the hole, and repeat this process until the probe in the hole has acquired the optical and acoustic signals at different depths in the AB area. Finally, through post analysis and processing, the optical image signal and acoustic detection signal of rock wall at each depth section are synthesized to realize the fine detection of the geological borehole. In the aspect of data processing, the borehole photo-acoustic combined measurement technology mainly uses digital image expansion technology to realize optical image processing. In the captured video image, the image near the center reflects the rock wall image at the upper end of the drilling rock wall, and the image far away from the center reflects the rock wall image at the lower end of the drilling rock wall. The hole wall image closer to the center in the video image is more blurred, and the hole wall image farther away from the center in the video image is clearer, which is mainly caused by the principle of cone mirror imaging. For this reason, in image processing, the image far from the center of the video image is mainly used as the analysis data source. First, cut the video image along the geographical north direction. In the process of cutting, the center point selects the center point position of the video image to build a polar coordinate system. The tangential unfolding process can be realized by the following formula:  
\[\left\{ \begin{array}{l}X_{\theta}(\rho)=X_{o}+\rho*\cos(\theta)\\ Y_{\theta}(\rho)=Y_{o}+\rho*\sin(\theta)\end{array} \right. \tag{1}\]  
where \(X_{\theta}(\rho)\) and \(Y_{\theta}(\rho)\), respectively, represent the image coordinates of the borehole wall when the angle is \(\theta\) and the radius is \(\rho\). (\(X_{0}\), \(Y_{0}\)) is the center coordinate of the borehole \(\theta\in[0,\,2\pi]\). When \(\rho\) changes from small to large, the pixel points on each circumference are sampled iteratively on \(\theta\in[0,\,2\pi]\). Then, the radial unwrapping technology of fish eye image is used to expand the image by interpolation.