Geophysical Mapping: Method Details
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Method Name: Tunnel Seismic Prediction (TSP)
Method Type:   Seismic Techniques
Assigned Problems:
+ Cavity detection Civil Engineering
+ Fault zones Civil Engineering
+ Fracture zone Civil Engineering
+ Soil / rock quality Civil Engineering
+ Young's / shear modulus, Poisson's ratio Civil Engineering
   '+' = Technique applicable
Principle:   Tunnel Seismic Prediction (TSP) is a technique that measures seismic waves ahead the tunnel face
Keywords:   Seismic Techniques; Tunnel seismic Prediction (TSP); high resolution 3-component seismic section; Seismic velocity contrast; Tunnel referenced sections; Prediction models, Geological models
  • Intermediate to hard rock mass conditions
  • Targets must be characterized by a sufficient acoustic impedance contrast
  • Ambient seismic noise (e.g., blasting, mucking, TBM drivage) may reduce data quality significantly
  • 1.5-2.0 m long receiver and shot holes of 32-45 mm diameter should be available
  • Small explosive charges and detonators are required.
Resolution:   Tunnel ahead resolution depends on seismic velocity and the dominant signal frequency. Because shot points and high-frequency sensors are located in the rock mass of an underground building, high frequency signals of up to 3kHz can be obtained according to absorption of rock mass condition. Typical range of investigation ahead the tunnel face varies between hundred to several hundreds of m (engineering-scale applications). Typical resolution in tunnelling direction varies between one to few m.
Expected Results:  
  • Measured parameter: Acceleration of ground motion in three directions and signal travel time.
  • Data analysis: Processing of reflection seismic data yields an image of reflectors ahead the tunnel face in distance and an image of velocity distribution in a longitudinal view. Further analysis of velocity data yields to the evaluation of rock mechanical parameters such as Young's modulus and Poisson's ratio.
  • Interpretation: Seismic interpretation assumes that the resolved reflector segments represent true lithological interfaces and do not change dipping and striking when interpolating them to the tunnel axis. Additional geotechnical data such as compressive strength, densities or probe drilling information may help for more reliable interpretation.
Combination with other Methods:  
  • Desirable additional information: Geological information from Core logging and / or petro-physical logs for interpretation, density log for better calibration.
  • Related add-on information: Reflection seismic data, Refraction seismic data, Surface-based tomographic data, Sonic logs (synthetic seismograms), Surface reflection seismic data: lateral extension of information, Geological constraints on fracture zones / fault planes
  • Independent additional information: electrical and /or electromagnetic data, georadar data, gravity data, borehole logs
Operating Expense:  
  • Crew size: 1 key person. Receiver and shot holes should be prepared by tunnelling staff.
  • Acquisition speed: approx. 1.5 hours each measurement (24 shots) yielding a prediction range between hundred and several hundreds of m.
  • Processing: Requires 3-4 hours per measurement.
  • Service costs: moderate
  • System rental costs: intermediate
  • System purchase costs: medium to low in relation to tunnel project size
Parameters to specify:  
  • Source type / Source parameters in borehole: explosives.
  • Geophone type: triaxiale accelerometer.
  • Seismograph: Channel number 12, dynamic range 120 dB.
  • Source-point spacing: usually 1.5 m
  • Sampling rate: usually 62.5 μs for high resolution.
  • Record length: usually around 450 ms.
QC Documents:  
  • Coordinates and map of shot and receiver locations.
  • Receiver and shot hole information (reference, azimuth, tilt).
  • Accuracy of travel time picks.
  • Acquisition checks: noise level; impedance of sensors and cables; dynamic range and gain adjustment of seismograph.
  • Field notes (e.g., kind of explosives and detonators, effective time schedule, present personnel).
  • Raw data and geometry files.
  • First-arrival times and / or amplitudes of seismic signals.
  • Reflector Extraction
  • Tunnel ahead prognosis with reference to tunnel axis (2-D and / or 3-D cube models).
  • Rock Parameter (P-wave / S-wave velocity ratio, Poisson's ratio, dyn. Young's Modulus, etc). Interpretation.
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