Abstract
Geological exploration plays a fundamental and crucial role in geological engineering. The most frequently used method is to obtain borehole videos using an axial view borehole camera system (AVBCS) in a pre-drilled borehole. This approach to surveying the internal structure of a borehole is based on the video playback and video screenshot analysis. One of the drawbacks of AVBCS is that it provides only a qualitative description of borehole information with a forward-looking borehole video, but quantitative analysis of the borehole data, such as the width and dip angle of fracture, are unavailable. In this paper, we proposed a new approach to create a whole borehole-wall cylindrical panorama from the borehole video acquired by AVBCS, which provides a possibility for further analysis of borehole information. Firstly, based on the Otsu and region labeling algorithms, a borehole center location algorithm is proposed to extract the borehole center of each video image automatically. Afterwards, based on coordinate mapping (CM), a virtual coordinate graph (VCG) is designed in the unwrapping process of the front view borehole-wall image sequence, generating the corresponding unfolded image sequence and reducing the computational cost. Subsequently, based on the sum of absolute difference (SAD), a projection transformation SAD (PTSAD), which considers the gray level similarity of candidate images, is proposed to achieve the matching of the unfolded image sequence. Finally, an image filtering module is introduced to filter the invalid frames and the remaining frames are stitched into a complete cylindrical panorama. Experiments on two real-world borehole videos demonstrate that the proposed method can generate panoramic borehole-wall unfolded images from videos with satisfying visual effect for follow up geological condition analysis. From the resulting image, borehole information, including the rock mechanical properties, distribution and width of fracture, fault distribution and seam thickness, can be further obtained and analyzed.
Highlights
Borehole camera technology (BCT) is an important optical signal acquisition and processing technology to image the interior structure of boreholes
To enhance the efficiency and accuracy of the video image matching, we propose a template matching algorithm based on the sum of absolute difference (SAD) and gray projection transformation (GPT), which is referred to as the projection transformation SAD (PTSAD) algorithm
The resolution of video sequences obtained by the axial view borehole camera system (AVBCS) was about 0.1 mm and the acquired images were both of 320 pixels × 240 pixels
Summary
Borehole camera technology (BCT) is an important optical signal acquisition and processing technology to image the interior structure of boreholes. It has been widely used in geological detection [1], investigating the structure of glaciers [2] and hydrogeological investigation [3]. Sci. 2019, 9, 3437 a borehole-wall cylindrical panorama can be produced by image mosaic technology. This complex and expensive equipment can only be applied to vertical drilling and the diameter for the test borehole should be in the range from 48 to 180 mm. It can be applied to the vertical, horizontal and oblique holes and can be applied in wider drilled holes whose diameters range from 28 mm to 250 mm
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