# Fine Characterization Method of Concrete Internal Cracks Based on Borehole Optical Imaging
## 2 Crack Image Acquisition  
### Borehole Camera Technology  
A borehole camera system is a kind of in situ optical measurement technology. This technology records the structural information of the borehole wall by continuous photography or video recording, and the measurement results are intuitive and low-cost. The equipment used in this paper is the digital panoramic borehole camera system (DPBCS) developed by the Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. The system mainly includes a control box, optical probe, flexible cable, depth recorder, rigid push rod (used in nonvertical downward boreholes), and power supply components. The structural diagram of the DPBCS is shown in Figure 1.  
The optical probe is the most critical component of the system, mainly including a data-processing module, electronic compass, LED light source, glass window, camera, and conical mirror, as shown in Figure 2a. The optical probe has three sizes, with diameters of 31 mm, 51 mm, and 73 mm, respectively, and corresponding lengths of 471 mm, 462 mm, and 587 mm, respectively. The functions of the three types of probes are the same, and the probe closest to the borehole diameter is usually selected for use. The use of conical mirror technology is creative, so that the 360\({}^{\circ}\) borehole wall reflects into a planar panoramic image. The advantage of this technology is that the camera can capture a video image of the 360\({}^{\circ}\) borehole wall at the same time without rotating the probe. The LED light source illuminates the borehole wall through the glass window, and the light is reflected into the camera by the conical mirror. The conical mirror is located in the focal length range of the fixed focus lens. The electronic compass is fixed coaxially with the camera, with an accuracy of 0.1\({}^{\circ}\). Even if the probe shakes or rotates in the borehole, the electronic compass can determine its orientation.  
Figure 1: Digital panoramic borehole camera system (1—Control box; 2—Optical probe; 3—Flexible cable; 4—Depth encoder; 5—Rigid push rod; 6—Power supply).  
Figure 2: Principle of borehole camera system. (**a**) Measurement process. (**b**) Borehole wall image.  
The depth encoder is fixed at the orifice to record the depth of the optical probe in the borehole with an accuracy of 0.1 mm. The flexible cable descends the optical probe to the bottom of the borehole at a constant speed through the depth encoder, and the lowering speed shall not exceed 1.5 m/s. In addition, the flexible cable is also a channel for power supply and data transmission between the control box and the optical probe. The control box superimposes the video data, orientation data and depth data in real time to ensure that the depth and orientation of each frame of the image are determined.  
Figure 2a shows the measurement process. As the optical probe moves in the borehole, the video image, orientation information, and depth information of the borehole wall structure are collected and saved. Because the conical mirror technology reflects the panoramic strip image, the image must be restored to a form convenient for viewing through a certain algorithm. As shown in Figure 2b, the borehole wall image collected by the borehole camera system is shown. The first picture is the plan expansion of the borehole wall along the due north direction, and the other four are borehole histograms from different perspectives. In the image, the aggregate and cracks on the borehole wall can be clearly seen. The top of the image is the orientation data, and the left is the depth data; that is, the coordinates of each pixel on the borehole wall are determined.