Summary

   The TGS 360 Pro seismic recorder is intended for seismic surveying of 3D rock massif in front of a tunnel face based on signals of reflected waves at a distance of 5 to 200 m from the chest of the tunnel face in order to:

   -detect subvertical integrity failure zones of the massif (disintegration zones of and geodynamic types, karst cavities, contrast rock boundaries of different genesis);

   -estimate the elastic moduli of the massif (Young modulus, shear modulus, uniaxial compression modulus, Poisson ratio);

   -estimate the relative values of the rock pressure gradient modulus (abnormal areas with low and high stresses in the rock massif related to its integrity failures and capable of causing dangerous effects upon encounter with mine workings);

   - evaluate the rock massif stability criterion in the range from I (stable) to V (unstable) over the entire prognosis distance.

   The software and hardware facilities of the recorder provide time-efficient support for tunnelling operations with a minimum amount of time required for one seismic survey session (no more than 1 hour of operation at the tunnel face) and are intended for use by one standard-qualified engineer with geological and geophysical education.

Method

   The major factors that influence the sudden change in the massif stability and development of other dangerous situations (rock falling, water inflow, gas-related phenomena, rock bump, etc.) within the tunnel face impact zone during tunnelling operations are stress-strain parameters of the rock massif with a discrete (block) structure.

   The division process of rock of any genesis (sedimentary or magmatic) under the influence of Earth’s non-stationary geodynamic field forms a discrete hierarchy of blocks at all levels. Fig. 1 shows the main elements of rock massif model with a typical block structure within a sedimentary basin. Such pattern of multi-order block structures is repeatedly detected and independently confirmed in various applications in geology, geophysics, geodynamics, and in particular geomechanics.

   The active blocks Bk are formed at the upper level of the sedimentary rock, with massif disintegration (crushing) zones along the boundaries between them. At the next level, blocks Bn of smaller size are formed, and so on until a limit size di is reached, after which the division process stops. Essentially, these are the massif disintegration zones along the boundaries between blocks of different levels that pose the main danger in the development of hazardous phenomena when encountering the mine working (tunnel collapse, water or gas flows, etc.).

Fig 3. Scheme for polarization reception of reflected waves

   In application conditions for polarization method of reflected wave detection, two main types of surveillance systems are used in tunnel construction: on the chest or on boards of the tunnel face.

The scheme for surveillance on the tunnel face chest is shown in Fig. 4. In this scheme 3C-geophones (GP) are installed in 0.5 m deep boreholes along two parallel lines 2 m apart from each other. The distance between the lines is 2 m as well. The points of elastic wave excitation are shown as SP-points. Therefore, the reflected waves are detected from 10 excitation points, which provides for multiple accumulations of reflected wave signals at reflection points uniformly distributed within the massif with a sufficient coverage density.

   In case the access to the tunnel face chest is unavailable, the second option shown in Fig. 5 is used. Two groups of 3C-geophones are symmetrically installed at tunnel boards in the closest proximity to the chest. In this option, 10 points of elastic wave excitation are applied.

Fig 4. Scheme for3C-registration of reflected waves

on the chest of the tunnel face

SGD-SMT/FDU3 Receiver Units

SGD-SMT/FDU3 Receiver Units

Fig. 5. Scheme for 3C-registration of reflected waves on the board of the tunnel

   It should be kept in mind that the first surveillance system option (at the chest) is the major option that is most effective in terms of quality of reflected wave field detection.

       The software-aided detection and processing of seismic signals provides the operator with two predetermined scenarios of a surveillance system depicted in Fig. 4 or Fig. 5. These scenarios provide for automatic detection and processing of seismic data with minimal control functions for surveillance process in a tunnel.

   The process of seismic signal detection in the tunnel is described in detail in Appendix 1 (Hardware description.pdf), and the initial wave field processing is described in Appendix 2 (текст Зудилина).

Analysis of processing results and drawing up a Conclusion

on rock massif stability forecast

   Process the recorded data based on the results of seismic surveillance at the tunnel face, following the recommendations of Appendix 2. Upon completion of this process, select the Results option and proceed to the final process of drawing up recommendations on the results of evaluation of the structure and elastic properties of rock in front of the tunnel face by combination of a series of three-dimensional arrays of parametric data comprised of Stress, Water, Vp, Vs, Vp/Vs, Poisson, Young, Category.

   In the Results window, the buttons for operating the resultant data arrays are then selected in

any sequence:

   -type of the parametric array;

   -type of image (Mode) on the computer screen (1-Map or 2-Cube);

   -export of the array in the selected file format.

   The scheme and principle of drawing up the recommendations for evaluation of the stability forecast for a rock massif are further demonstrated with a practical example for analysis of results of method application in one of the tunnels.

1.Evaluation of the structure and stress-strain parameters of a rock massif.

   The Stress button is activated in the 1-Map mode with the horizontal section of the massif at the tunnel floor level (H=0). The following image is displayed:

   The values along the horizontal axis in this image indicate the distance from the central axis of the tunnel (H=0) in meters, and along the vertical axis, the distance from the tunnel face chest in the forward direction up to a distance of 200 meters. The colour scale of the parameter corresponds to the relative gradient of rock pressure (Stress). Red indicates high gradient level, blue indicates low one. High values of the pressure gradient must correspond to the massif disintegration zones (tectonic rock boundary, high-fracture zones, karst, etc.).

   In this particular example, a tunnel is being constructed in a horizontally layered rock massif of carbonate composition (limestones, salt, anhydrite), and the red zones indicate the probability of finding vertical high-fracture zones, which can develop into water-flooded karst cavities.

In the 2-Cube mode, the following 3D image of the same massif is displayed for the Stress parameter:

   The left window displays a 3D Stress massif, and the right window displays its horizontal (LINE) and vertical (CROSS) cross-sections. The section position in the cube is adjusted with ← → (CROSS), ↑ ↓ (LINE) buttons.

   In this example, the stable and strongly pronounced position of disintegration zone is within the range of 60-80 m from the tunnel face chest (marked as a "risk zone"). Further reflections at distances over 100 m are considered as preliminary and require further confirmation during next monitoring cycles after 30-40 m of tunnel penetration.

2.Water content forecast.

   Tap the Water button in 1-Map mode. The horizontal section of the massif is displayed, and water content value in % at the floor level of the tunnel (H=0) is given. Here, abnormal water content values are detected within the same "risk zone" within the range of 60-80 m. In the 2-Cube mode, this zone is viewed as stable in the massif volume and, in combination with its position on the Stress images, it should be concluded that the position of a rock massif disintegration zone is very likely to be at this distance with a risk of high water content.

3.Elastic properties.

   The elastic properties of the rock are analysed based on a series of parameter arrays Vp, Vs, Vp/Vs, Poisson, Young.

   In specific geological conditions, these arrays may be used to calculate the rock stability criteria for various calculation schemes of rating systems (ссылка на статью Rock_Mass_Classification_Systems).

   In this example, the horizontal Vp and Vp/Vs cross-sections and the Vp/Vs images in 2-Cube mode are of interest. Here we note the contrast change boundary for these parameters, which additionally confirms the position of the "risk zone" at a distance of 60-80 m.

4.Stability categories.

   The Category parameter array is the transformation of the Stress array under the assumption of a fixed method for grading the range of pressure gradient values. The standard software version applies a uniform division of the actual pressure gradient range into 5 intervals, each of which is assigned with a category from 1 to 5 according to the degree of hazard: from non-hazardous (1) to hazardous (5).

   The Category parameter images in 1-Map and 2-Cube modes show the "risk zone" position based on category 5 (red colour) more clearly.

   Therefore, a final conclusion about the position of the risk zone is made in this example, in which the hazardous rock massif disintegration zone with a high probability of water content can be met at a distance of 60 to 80 m ahead of the tunnel face.

   Hence, it is necessary to repeat the seismic surveillance cycle after tunnelling forwards by 30-40 meters. In case the position of the "risk zone" is confirmed, a horizontal well must be drilled at the relevant distance from the tunnel face.

   In conclusion, it should be noted that the use of TGS 360 Pro in specific geological conditions requires a certain adjustment of seismic signal detection and processing parameters, which must be performed by relevant specialists from a service company under the supervision (remote or direct) of the design company.

References

   1.Biot, M.A., 1965. Mechanics of incremental deformations. New York, 430 p.

   2.Pisetski, V., 1998. Method for Determining the Presence of Fluids in a Subterranean Formation, US Patent, No. 5,796, 678.

   3.Pisetski, V., Kormilcev V., Ratushnak A., 2002. Method for predicting dynamic parameters of fluids in a Subterranean reservoir. US Patent, No. 6, 498, 989 B1.

   4.Galperin, E. I., 1977. The Polarization Method of Seismic Exploration, Moscow, Nedra Publ. House; search.rsl.ru/ru/record/01007719597; English edition   SEG, 1984.