Abstract
Seismic imaging is the most effective geophysical method and has been extensively implemented to detect potential geological hazards in tunnels during construction. The coupling of geophones and the design of geometry in tunnels are the two major challenges. To ensure successful coupling, a high-sensitivity semi-automatic coupling geophone with a broadband was designed. In practice, this geophone is attached with a wheel and two springs. Once inserted into the borehole, an automatic coupling action occurs. This semi-automatic coupling design within the geophone not only guarantees good coupling, but reduces the time and costs usually required to install a traditional geophone. In the use of geophones for tunnel seismic detection, we propose two new two-dimensional (2D) seismic geometries based on the two commonly used geometries. A test to assess the effectiveness of the qualities of imaging from four geometries was completed by comparing the results of the forward modeling of sandwich models. The conclusion is that the larger the horizontal offset of the layout geometry, the higher the resolution of the imaging; the larger the vertical offset, the weaker the mirror image. The vertical offset is limited due to the narrow tunnel condition. Therefore, the mirror effect cannot be entirely eliminated; however, it can be further suppressed by constructing 2D geometry. The two newly proposed 2D geometries caused the imaging arc of the inter-layer, but suppressed the mirror image. The mirror image added a significant number of errors to the data, which could misguide tunnel construction; therefore the new 2D geometries are more reasonable than the two most commonly used. We applied one of the two new 2D geometries that was more practical to an actual project, the Chongqing Jinyunshan Tunnel in China, and acquired high-quality seismic data using two semi-automatic coupling geophones. The detection results were essentially consistent with the excavation conclusions.
Highlights
Infrastructure and energy projects, such as transportation, water resource transportation, municipal pipelines, and mining, all involve tunnel construction
The processing flow resembles the Vertical Seismic Profile (VSP) method [8,9]; the coupling of the geophone differs from oil seismology to some degree
First, a 2-m-deep hole is drilled into the tunnel wall and an anchoring agent is pushed into the hole with a push rod before a special-made steel pipe is pushed into the borehole
Summary
Infrastructure and energy projects, such as transportation, water resource transportation, municipal pipelines, and mining, all involve tunnel construction. The third method is the direct coupling method, wherein the geophone is directly attached to the tunnel wall with quick-drying cement [12] This method acquires a large amount of noise, such as acoustic waves and surface waves, and fails to avoid the influence of the tunnel surface low speed circle caused by excavation. To address these coupling issues, we designed a wide-band and high-sensitivity semi-automatic coupling geophone. We applied the semi-auto coupling the vertical offset and the horizontal (tunneling direction) geophone and geometry in the Jinyunshan. We applied the semi-auto coupling geophone and geometry in the Jinyunshan Tunnel, China
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