## 2 Borehole Multi-Source Data Acquisition Technology  
### Borehole Optical Imaging Technology  
The borehole optical imaging technology can directly obtain the texture image information of the borehole wall, which is a direct observation technology. Borehole optical imaging technology is a kind of detection technology that places the panoramic camera probe inside the borehole to obtain the panoramic image of the borehole wall [18; 19]. The light source of the probe in the borehole illuminates the camera area on the borehole wall. The panoramic camera simultaneously captures the panoramic image of the borehole and the compass azimuth image, and transmits the captured image to the video distributor located on the ground through a special cable. One cable enters the video recorder to record the whole process of detection, and the other cable enters the capture card in the computer for digitization. The measuring wheel on the winch measures the position of the probe in the borehole in real time, and places the depth value in the special port in the computer through the interface board. The probe is lowered in the borehole until the end of the whole detection process. Through continuous capture, the panoramic image of the borehole can be quickly restored to the plane expansion diagram and displayed in real time. The borehole camera image and the plane expansion diagram are shown in Figure 1. \"\(W\)\", \"N\", \"E\" and \"S\" in Figure 1 represent the west, north, east and south of the geography. The face represented by ABCD represents the intersection of joint and borehole. \"A\", \"B\", \"C\" and \"D\" represent several typical joint locations on the borehole. The blue and red lines in the figure have no special meaning, although they are convenient to explain the orientation correspondence of the borehole expansion image. A borehole detection environment has the particularities of small space and small size of target body, so the conventional stereo image pair imaging method is not applicable in the borehole. Therefore, an in-hole stereo image pair acquisition method based on combined binocular panoramic imaging is proposed. The main technical points include the following: (1) Use of the upper and lower conic mirrors with different inclination angles to reflect the 360\({}^{\circ}\) image information of the borehole wall into the imaging component. (2) An imaging component is set directly above the biconical mirror to collect the panoramic images of the borehole wall from two different observation angles in real time, so that a single imaging component can be obtained from the panoramic stereo image of the hole. (3) A precondition for the image formed by the reflection of the biconical mirror to be an effective image pair image is that the image contains overlapping areas. Therefore, the parameters of the conic mirror and imaging components are designed accordingly to enable it to obtain the best stereoscopic image pair image. Imaging principle of biconical mirror stereo image pair is shown in Figure 2.  
Figure 1: Plane expansion diagram. The core problem of the panoramic image pair image solution is to obtain the borehole wall stereo image, which mainly includes two aspects: (1) image pair image registration and (2) the distance from any point on the borehole wall to the imaging center axis. For image-to-image registration, the method of automatic search and recognition of registration points is adopted. The specific location is determined as follows: take any point P in the circular band between C1 and C0 or C0 and C2, and its azimuth line OP is the line between the point and the center of the circle. Then, along the OP, search for the same point P', that is, the registration point, in another ring (that is, the ring between C0 and C2 or C1 and C0), and calculate the image coordinates of the registration point P'.