Abstract
For tunneling in complex geological conditions, effective and accurate advanced prospecting techniques are required to detect unexpected geological heterogeneities in front of the tunnel face. Reverse time migration (RTM) method is a promising method to image the geological changes based on seismic forward-prospecting data acquired in tunnels. However, conventional tunnel-based RTM images suffer interference of “trailing” artifacts. Beamforming method can obtain a focused wave front through the stack of wavefield, resulting in improved data quality and RTM results. In this study, we incorporate the beamforming method in RTM imaging procedure, and propose a “sweep and stack” mode RTM method as well as its calculation scheme. Three tunnel-based models with different kinds of geological interfaces are designed to generate synthetic seismic records. Wavefield extrapolation is achieved by an acoustic staggered-grid finite-difference algorithm and zero-lag cross-correlation imaging condition is applied to present RTM results. Analysis and comparison of conventional RTM results and “sweep and stack” mode record-side beamforming RTM results illustrate that, both methods can successfully identify the geological interface ahead of the tunnel face, while record-side beamforming RTM images present more concentrated energy arcs with higher amplitude, which is preferred for geology interpretations. Moreover, the synthetic test in a noisy environment demonstrates that record-side beamforming RTM has better anti-noise capability than the conventional RTM approach. A field test in a highway tunnel construction site is performed to show the good application effects of “sweep and stack” mode RTM in practical seismic detections.
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Abbreviations
- \(d\) :
-
Vibrator interval of seismic source array
- \(\theta\) :
-
Propagation direction of beamforming wavefield
- \(k\) :
-
An attenuation coefficient in \(x\) or \(z\) direction
- \(n\) :
-
Time step for iterations
- \(l{\text{ }}(0 \leqslant l \leqslant L)\) :
-
Distance from a certain PML to the inner boundary of the PML area
- \(L\) :
-
Thickness of the whole PML area
- \(m\) :
-
Order of the polynomial function
- \(p\) :
-
Pressure
- \(R\) :
-
Theoretical reflection coefficient of PML layers
- \(R(x,z,t)\) :
-
Receiver wavefield
- \(S(x,z,t)\) :
-
Source wavefield
- \(\sigma\) :
-
Wave attenuation coefficient for calculating \(\varPsi\) and \(\varOmega\)
- \(t\) :
-
Time
- \({t_{\text{max} }}\) :
-
Maximum recording time of received seismic record
- \(\tau\) :
-
Delay time for beamforming
- \(u\) and \(w\) :
-
Particle velocity components in \(x\) and \(z\) direction, respectively
- \(v\) :
-
Wave velocity of the media
- \(\varPsi\) and \(\varOmega\) :
-
Added terms denoting wave attenuation in \(x\) or \(z\) direction
- \(x\), \(z\) :
-
Spatial coordinates
- TBM:
-
Tunnel boring machine
- RTM:
-
Reverse time migration
- SNR:
-
Signal-to-noise ratio
- SFD:
-
Staggered-grid finite-difference
- PML:
-
Perfectly matched layer
- CFS:
-
Complex frequency shifted
- PAS:
-
Phased array source
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Acknowledgements
This work was supported by the National Program on Key Basic Research Project of China (nos. 2014CB046901 and 2015CB058101), the National Key Scientific Instrument and Equipment Development Project (no. 51327802), the National Natural Science Foundation of China (nos. 51739007, 51479104 and 41502279), the National Key Research and Development Plan (nos. 2016YFC0401801, 2016YFC0401805 and 2016YFC0801604), the Royal Academy of Engineering under the UK-China Industry Academia Partnership Programme scheme (UK-CIAPP\314), the Key Research and Development Plan of Shandong Province (nos. 2016ZDJS02A01 and 2016GSF120001), the Major Science and Technology Special Projects of Henan Province (no. 161100211100), the Fundamental Research Funds of Shandong University (no. 2017JC002). The above supports are greatly acknowledged.
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Li, S., Ren, Y., Liu, L. et al. Reverse Time Migration of Seismic Forward-Prospecting Data in Tunnels Based on Beamforming Methods. Rock Mech Rock Eng 52, 3261–3278 (2019). https://doi.org/10.1007/s00603-019-01763-2
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DOI: https://doi.org/10.1007/s00603-019-01763-2